Marshall W. Carpenter MD

• Staff Perinatologist
• St. Elizabeth's Medical Center
• Boston, Massachusetts, USA

Females have 23 homologous pairs prostate mri cheap 50mg casodex with visa, while males have 22 "autosomal" pairs and two different sex chromosomes that share a region of homology known as the pseudoautosomal region prostate 44 cheap casodex online american express. One of each pair is contributed by each parent in the egg and sperm prostate-7 review casodex 50 mg online, respectively androgen hormone video order genuine casodex line. At fertilization prostate juice remedy cheap 50mg casodex with amex, haploid gametes fuse to form a zygote prostate irritation buy casodex 50mg, restoring the diploid chromosome number of 46. In plants, the haploid phase is represented by gametophytes, which produce ovules and pollen. In most fungi, such as yeasts, haploid and diploid forms are alternate phases of the life cycle, and both can propagate by mitosis. Meiosis changes the genetic makeup of offspring relative to parents in two ways: the first round of meiotic segregation produces novel combinations of M chromosomes from the two parents in each gamete, and recombination between parental chromosomes produces novel chromosomes. Each haploid gamete is endowed with a random set of the homologous chromosomes from the two parents. Thus each daughter cells ends up with just one of each pair of homologous chromosomes. This differs from mitosis where the duplicated chromatids separate, so both daughter cells get the full set of homologous chromosomes from both parents. Because it culminates in daughter cells carrying just one set of chromosomes instead of two, meiosis I is also known as the reductional division. Meiosis: An Essential Process for Sexual Reproduction Sexual reproduction is an important survival strategy that offers organisms an accelerated mechanism for altering the genetic makeup of offspring. Meiosis I produces random combinations of homologous maternal and paternal chromosomes. For each pair of homologs, orientation on the spindle is random during meiosis I (ie, each homolog has two equivalent options for the direction to migrate). Thus, for humans (with 23 pairs of homologous chromosomes), each gamete has one of 223 (more than 8 million) possible complements of maternal and paternal chromosomes. This process does not create new versions of genes, but it guarantees the offspring will have novel combinations of subtly different (due to polymorphisms) chromosomes. This occurs because to segregate from one another, each pair of homologous chromosomes must first find each other. They do this by undergoing reciprocal recombination (crossover) events that then hold them together until anaphase of meiosis I. These include not only polymorphisms (differences of single base pairs), but also thousands of longer insertions, deletions, and rearrangements. Recombination events that result in a crossover and exchange chromosomal segments produce new chromosomes that are a patchwork of segments from the maternal and paternal homologs. The combined effects of recombination and random assortment of homologs in meiosis I yields a vast number of genetically different gametes. This genetic diversity increases the ability of eukaryotic populations to adapt to changing environmental conditions. This reduces the process to only three essential key terms: pairing, homologous recombination, and segregation. This article discusses each step in detail, so they are defined only briefly here. Pairing is a two-step alignment of homologous chromosomes with one another in the nucleus. In many organisms, early events of recombination drive the homologous pairing process. In the second stage, synapsis, the paired homologous chromosomes become intimately aligned along their entire lengths with one another separated by approximately 100 nm. A specialized scaffolding structure called the synaptonemal complex mediates this process. Recombination drives the pairing process in many organisms and can occur without synapsis under certain circumstances. Crossover recombination sites are detected by microscopy as chromatin structures called chiasmata (singular: chiasma, from the Greek, meaning "X-shaped cross"). Segregation of homologous chromosomes in meiosis I differs from the segregation of sister chromatids during mitosis (Box 45. When homologs orient at the metaphase plate of the meiosis I spindle, centromeres belonging to the two sister chromatids are fused to form a single kinetochore that binds microtubules. At anaphase I the distal cohesion is released from chromosomes allowing the chiasmata to separate, and the two sister chromatids (at least one of which has undergone a crossover exchange) move as a single unit toward the same spindle pole while the sister chromatids from other parent move to the other daughter cell. As a result, the two daughter cells produced in meiosis I have a haploid number of chromosomes derived randomly from the two parents, each with two sister chromatids. Proper orientation and segregation of homologous chromosomes is achieved thanks to the pairing, synapsis (synaptonemal complex formation), and recombination that occur in a lengthened prophase during meiosis I. In humans, prophase in mitosis takes an hour, whereas meiotic prophase lasts many days in males and many years in females. The recombination rate is 100- to 1000-fold higher in prophase I of meiosis than in mitosis. The process has two main consequences: the formation of chiasmata and the introduction of genetic variation. Chiasmata are structures that physically link the homologous chromosomes after crossover and play an essential role in meiotic chromosome segregation. During meiosis I, kinetochores of sister chromatids attach to spindle microtubules emanating from the same pole. Sister chromatid cohesion is essential for orientation of bivalents (paired homologous chromosomes) on the metaphase I spindle. During anaphase of meiosis I, cohesion is destroyed between sister chromatid arms, and chiasmata are released to allow segregation of homologs. Spo11-induced double-strand breaks are not required for synapsis of homologous chromosomes in the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. How these organisms pair their homologs without recombination is still mysterious. Rad51 and Dmc1 are related to the Escherichia coli RecA protein used for homologous recombination in bacteria. Recombination occurs between homologous chromosomes rather than between sister chromatids. At this point, the pathway splits in two, one outcome leading to a crossover and the other to a noncrossover. However, here are a number of terms used by geneticists that will assist in the understanding of the discussion of genetic recombination and its role in meiosis (also see Box 6. The genotype of an organism is the combination of genes present on the chromosomes of that organism. The phenotype is the physical manifestation of the action of these gene products (ie, the appearance and macromolecular composition of the organism). In discussing recombination, scientists typically refer to the presence or absence of specific genetic markers. A heterozygous organism has different forms of the genetic marker on the two homologous chromosomes. Although the physical events of genetic recombination occur in both homozygotes and heterozygotes, they are most readily detected in the latter. Two genetic markers located on different chromosomes will separate from one another in the anaphase of meiosis I 50% of the time as a result of the random distribution of chromosomes to the two spindle poles. If they are on the same chromosome, they will be linked to one another unless the chromosome undergoes a genetic recombination event between them. The greater the separation of two markers along one chromosome, the more likely it is for such an intervening recombination event to occur. The first of these-noncrossover events (frequently referred to as gene conversion)-may involve the loss of one or more genetic markers. In recombination by crossing over, the makeup of genetic markers remains constant; it is the linkage between different markers that changes. The normal separation of chromosomes or chromatids is referred to as disjunction (disjoining). Chromosome synapsis defects and sexually dimorphic meiotic progressioninmicelackingSpo11. Mutants lacking Dmc1 are defective in homologous chromosome pairing and interhomolog recombination. These mechanisms, collectively known as achiasmate segregation, allow the segregation of chromosomes that have not undergone crossover recombination. One model for the achiasmate segregation in flies proposes that nonrecombined chromosomes remain paired at the end of meiotic prophase owing to stickiness of heterochromatin and, as a result, segregate properly during anaphase I of meiosis. This might be regarded as a cruel joke of evolution by those students who find all the Greek terms of meiotic nomenclature to be daunting. However, meiosis without recombination is clearly the exception, and in most species meiosis depends on recombination in both males and females. Tracking the Homologous Chromosomes Through the Stages of Meiotic Prophase I Pairing and recombination of homologous chromosomes take place during prophase of meiosis I. Leptotene (from the Greek, meaning "thin ribbon") starts with the first visible condensation of the chromosomes. This axis consists of proteins that play a role in mitotic chromosome structure as well as proteins specialized for meiotic chromosomes. I, Three-dimensional models of the nuclei (nuclear periphery[red dots],telomereposition[green stars]). In fission yeast dynein motors and microtubule dynamics in the cytoplasm move the telomere cluster from one end of the cell to the other every 10 minutes or so. During the transition from leptotene to the zygotene (Greek, "yoke ribbon") stage of prophase, clustering of chromosome ends at the nuclear envelope reaches its peak, with the "bouquet" arrangement of chromosomes. This protein scaffold forms part of the synaptonemal complex when pairing is complete. The final resolution of these recombination intermediates into crossovers occurs close to the time of synaptonemal complex disassembly, dispersal of the bouquet of chromosomes and exit from pachytene. The crossovers then mature into structures called chiasmata that link homologous chromosomes through meiosis I metaphase. The duplicated sister chromatids remain closely associated, and chiasmata hold the homologous chromosomes together, although their axes tend to drift apart in the absence of synaptonemal complex. This part of meiotic prophase may last for days or years, depending on the sex and organism (up to 45 years or more in female humans). Oocytes (immature eggs) actively transcribe their chromosomes during diplotene, as they store up materials for use during the first few divisions of embryonic development. Diakinesis (from the Greek, meaning "across movement") is the prometaphase of meiosis I. Following nuclear envelope breakdown, homologous chromosomes shorten and condense. The two homologs (each a pair of tightly linked sister chromatids) are attached to opposite poles of the meiotic spindle, which applies force, attempting to pull them apart. The homologs separate and move to opposite spindle poles during anaphase I when the cohesion along the chromosome arms is released. The sister chromatids move together to one pole, because they remain linked by cohesion at their centromeres, where the cohesion complex is protected by a shugoshin protein (see later). The second meiotic division is mechanistically similar to mitosis except that the number of chromosomes is reduced by half. The earliest pairing events in meiosis involve a tendency of homologous chromosome territories to move together in the nucleus even before leptotene chromosome condensation. Genetic analysis in budding yeast revealed that mutants defective in the earliest stages of recombination are also defective in homolog pairing. Homologs are paired in nonmeiotic (somatic) cells in a few organisms, such as the fruit fly D. Thus, recombination has important roles both in the exchange of genetic material and in the mechanics of chromosome behavior during meiotic prophase. Pairing is reduced but not absent in yeast meiotic cells lacking both Rad51 and Dmc1, and homologous chromosomes still pair in some systems that lack recombination (eg, certain D. Homolog pairing initiated during leptotene becomes much more intimate during synapsis as the chromosomes are linked by transverse fibers to form the synaptonemal complex. Each of the two outer rails, 90 to 100 nm apart, is the axis of a pair sister chromatids. They are traditionally termed lateral elements, but for the sake of simplicity, we refer to them as axial elements. Thin transverse filaments oriented perpendicular to the axial elements appear to connect homolog axes to each other and to the central element (the "third rail"). Synaptonemal complex formation begins during zygotene at a limited number of sites along the paired homologous chromosomes where recombination events will mature into crossovers. It was once thought that the synaptonemal complex aligns homologous chromosomes in preparation for recombination, but it is now clear that homolog pairing and (in many organisms) the initiation of recombination precedes synapsis. Thus, synapsis is a downstream consequence of early steps in recombination in some well-studied organisms including yeast and mammals. However, under certain artificial circumstances, even nonhomologous chromosomes can undergo synapsis.

The most commonly observed and characteristic form of the disease is its subacute phase androgen hormone network cheap casodex 50 mg on-line. This produces a microgranulomatous interstitial pneumonitis that is centrilobular in distribution reflecting the aerogenous deposition of antigen (Table 6 prostate 09 casodex 50mg without prescription. The small airways can show lymphohistiocytic inflammation or organizing pneumonia man health solution buy casodex 50 mg without prescription. One can identify small aggregates of epithelioid macrophages prostate cancer vasectomy casodex 50 mg with mastercard, some of which contain cholesterol clefts in at least 85% of cases androgen hormone excess trusted 50mg casodex. It is also worth noting that most cases do not show increased tissue eosinophilia healthy prostate usa laboratories purchase casodex 50mg on line, as the response is not IgE driven but instead appears to represent a combination of immune complex and cell-mediated immunity. Effective management at times may require the patient to change residences, which they understandably may be reticent to do. There is an increasing tendency among radiologists to suggest the presence of air trapping, in the presence of interstitial disease of prima facie evidence of hypersensitivity pneumonitis. This observation has some merit, particularly if there is clear upper lung zone predominance but also has limitations as other interstitial disorders that involve the small airways can produce comparable changes. It is composed of necrotic alveolar lining cells and fibrin that remain tightly apposed to the injured alveolar surfaces. They are most prominent identified along the alveolar ducts where oxygen tension is the highest. After 72 h, the lung shows a wide number of changes related to the healing process. A chronic inflammatory infiltrate composed predominantly of lymphocytes and macrophages is seen, together with a marked increase in interstitial mast cells and eosinophils that peaks around 2 weeks postinjury. The source of these cells is uncertain but may be an epithelial stem cell or cells from the damaged small airways. For this reason, perfusion pressures should be maintained with near normal wedge pressures if possible and overdiuresis, which can potentially deplete intravascular volume and decrease right heart filling pressures, avoided. The pulmonary alveolar vascular bed is markedly reduced and can fail to fill due to intrapulmonary pressures due to the application of positive endexpiratory pressures. The proximal pulmonary vessels undergo intimal proliferation and show extension of the vascular smooth muscle into precapillary arterioles. The massive release of tissue procoagulant due to cellular injury leads to intravascular and extravascular thrombus formation. Patients can die late in the disease from ventilatory complications with high peak airway pressures, increased dead space, and hypercarbia. Histological evidence of end-stage lung can be detected as early as 10 days after the onset of disease and must be distinguished from an exacerbation of an occult chronic interstitial disorder, at times with substantial difficulty. At the present time, mechanical ventilation and hemodynamic support remain the mainstays of treatment. However, extracorporeal membrane oxygenation may be used successfully to support these patients or as a possible bridge to lung transplantation. Typically, the clinical presentation includes a viral-type prodrome and weeks to months of progressive dyspnea on exertion with cough accompanied by diffuse lung infiltrates. However, the prognosis is overall poor with the majority progressing to respiratory failure. Intravascular thrombi are common and should not be misinterpreted as thromboembolic disease. Both presented with a "viral" syndrome, diffuse pulmonary infiltrates, and acute lung injury that ultimately required extracorporeal membrane oxygenation. The cause of the delayed re-epithelialization is uncertain and is currently being investigated. This pathological entity is not currently recognized as a separate entity by the American Thoracic or European Respiratory Societies. Despite having a long and controversial history with respect to causation, it remains idiopathic. The most common presentation is a relatively young individual, more commonly women than men, with a high prevalence in African Americans and Scandinavians. Currently, treatment is reserved for patients with symptomatic disease, lung function abnormalities, progressive radiographic findings, or extrapulmonary symptoms, including uveitis, hypercalcemia, congestive heart failure, cardiac arrhythmias, bony, neurological, and cutaneous disease. Although considered a disease of exclusion, once mycobacterial and fungal infections have been excluded, there are histological features of sarcoidosis, both clinical and pathological, that suggest the diagnosis. Indeed, the incidence of positive stains for mycobacteria or fungi is vanishingly small, when the patient is not immunosuppressed for other reasons and there is no evidence of necrosis in the granulomas. One problem that invariably arises in some cases is whether the presence of necrosis within granulomas is inconsistent with the diagnosis of sarcoidosis and should trigger a more extensive work-up to exclude infection. However, occasional cases show more extensive necrosis that suggests infection, but if diligent efforts fail to yield an infectious etiology, one is left to conclude that the changes most likely represent sarcoidosis, and the patient should be followed at regular intervals. Other diseases besides infection can at times show nonnecrotizing granulomas as a feature and may be confused with sarcoidosis. These include malignancies in the lung, where regional lymph nodes and lung tissue can develop nonnecrotizing granulomas that likely represent a hypersensitivity reaction to tumor. Generally, these granulomas are not extensive, confluent, or hyalinizing, but their presence can lead to the misdiagnosis of sarcoidosis. Hypersensitivity pneumonitis generally shows microgranulomas that are loosely arranged but at times their frequency can produce confusion with sarcoidosis. Bronchoalveolar lavage has been used to both diagnose sarcoidosis in the correct clinical setting and to monitor its progression. When there is concern for lymphoid malignancy or pulmonary tumor, one must interpret the biopsy with some caution based on what has already been discussed. The technique most widely adopted by the pulmonologist is the transbronchial biopsy. This is a sensitive and specific technique for sarcoidosis because granulomas tend to be distributed along the airway lymphatics in this disorder. The number of biopsies taken increases the yield and five interpretable biopsies should yield a diagnosis in >90% of cases. Otherwise, in the absence of these findings, the diagnosis by the pathologist should be descriptive and the final diagnosis hinges on clinical and radiographic features. It is furthermore worth noting that sarcoidal granulomas that are identical to sarcoidosis in appearance and even distribution may be seen in a variety of other autoimmune disorders, in response to certain drugs, including interferonalpha, and there appears to be an increased incidence of sarcoidosis complicating a variety of malignancies. Sarcoidosis can progress to produce extensive upper lobe scarring in a minority of cases. Areas of traction bronchiectasis, usually in the upper lobe, may be prone to colonization by Aspergillus and other hyphate molds. The lung shows irregular scarring and one may be hard-pressed to identify residual sarcoidal granulomas. The chief concern is the development of a malignant lymphoma, although this is infrequent. Cases should be checked for evidence of monoclonal light chain production and patients monitored radiographically without repeated biopsies unless there has been a very significant change in the radiographic appearance or clinical course. Many excised upper lobes of lungs show apical fibroelastotic scars or so-called apical caps. In more recent years, as the prevalence of these infections has diminished, alternative explanations, including chronic apical ischemia, have been suggested to account for this change. One sees dense subpleural scars with large numbers of elastic fibers associated with distorted small airways, and with a small component of surrounding interstitial scar. However, some patients show severe interstitial disease associated with subpleural scars. The prognosis in these patients depends on the extent of disease, but it tends to be progressive and may require lung transplantation. In the lung, changes may occur within the proximal airways, in the lung parenchyma, and the pleura. They are characterized by active storiform fibrosis with a loose edematous stroma associated with dense lymphoplasmacytic inflammation. American Thoracic Society/European Respiratory Society international multidisciplinary consensus classification of the idiopathic interstitial pneumonias. A recent review that explains the spectrum of diseases associated with this immune defect. Organizing pneumonia and pulmonary lymphatic architecture in diffuse alveolar damage. A recent review of the associations of autoimmune disease and interstitial lung disease. Chapter 7 Nonneoplastic Smoking-Related Disorders A variety of benign pulmonary disorders have been causatively linked to cigarette smoking. These range from those producing pulmonary nodules to scarring of the lung interstitium. Smoking consumption is an important aspect of the clinical history in pulmonary medicine. Although it is unusual to encounter substantial pathological changes in patients who have less than a 20 pack-year cigarette consumption history, the epidemiological literature suggests that even low levels of consumption increase the risk of disease. The same may be said of "second-hand" smoke, which has been estimated to increase the risk of developing lung cancer by 20e30%. It is commonly observed in lung cancer resections from smokers and serves as a biological marker of smoking. Iron stains can be weakly positive but dense hemosiderin deposits should not be seen. For reasons that are not evident, these changes generally produce neither symptoms nor radiographic abnormalities in most patients. If questions persist as to whether the patient is being disingenuous with respect to smoking cessation, cotinine level can be obtained for confirmation. Unlike other smoking-related disorders, radiographic ground glass opacities are accentuated in the lower lung zones as opposed to the upper lung zones. Since the link to cigarette smoking was established, the first step in treatment has been smoking cessation. It has upper lobe predominance and a distinct propensity to spare the costophrenic angles. The classical radiographic pattern of this disease is a centrilobular nodular and cystic disorder with upper lung zone accentuation. Patients may be symptomatic with cough, dyspnea, pneumothorax, or hemoptysis, but most cases are incidentally discovered on chest imaging, which shows upper lobe centrilobular nodules with cyst formation performed for other reasons. Although the vast majority of patients have disease limited to the lung, the literature notes rib involvement, in 10e15% of cases, and rarely, the disease may involve visceral organs. The latter change is commonly seen in heavy smokers whose lungs are resected for neoplasm, and in the absence of areas of active nodular disease is clinically insignificant and should not prompt interventions that exceed smoking cessation. However, some patients develop progressive disease leading to respiratory failure with end-stage honeycomb lung. In such cases, cytotoxic therapy may be required, although there is little evidence to support its efficacy. In addition, as the lesions tend to be centrilobular in distribution, transbronchial biopsy may suffice to establish the diagnosis but sensitivity of the approach is limited due to sampling error. HandeSchullereChristian disease is a systemic disorder that can at times affect the lung. Patients have systemic disease including ossifying bone lesions, skin, and central nervous system disease. Common thoracic involvement includes the regional lymph nodes, pleura, and lung interstitium. Patients with areas of bullous emphysema were recognized to have peri-bullous foci of scarring, and old eosinophilic granuloma could at times cause "bridging" forms of fibrosis linking centrilobular regions but interstitial disease was not recognized by most pathologists. In recent years, it has been noted by several groups that patients with heavy smoking histories show a form of subpleural interstitial fibrosis both radiographically and microscopically. It may be noteworthy that the latter disorders are also commonly seen in ex-smokers, suggesting that smoking-related injury may play a role in the pathogenesis of a spectrum of interstitial fibrotic disorders. Nonneoplastic Smoking-Related Disorders Chapter 7 135 A paper outlining the problems distinguishing smoking related fibrosis radiographically from pulmonary asbestosis in exposed workers. Disease in the pulmonary vessels can be caused by (1) intraluminal thrombosis or thromboemboli that increase vascular resistance and promote vascular remodeling, (2) inflammatory angiitis, (3) loss of microvasculature, as is seen in emphysema and other interstitial lung disorders, (4) chronic elevation of postcapillary pressures due to congenital heart disease or chronically elevated pulmonary venous pressures, (5) chronic hypoxemia, which is a stimulus for pulmonary vasoconstriction and vascular remodeling, (6) circulating vasoactive factors, The changes in the pulmonary vascular bed that result from injury are largely stereotypic. One observes (1) thickening of the pulmonary vascular media, (2) thickening of the pulmonary intima, (3) reduplication of elastic fibers in the vessel wall, and (4) extension of muscular arterioles to the precapillary level. One of the characteristic features of pulmonary vascular disease is its heterogeneity. In the first three orders of arterial branching, atheromatous changes may be due to hyperlipidemia as in systemic arteries; however, when seen in the more distal pulmonary arteries, it virtually always indicates chronically elevated pulmonary arterial pressures. As in systemic atherosclerosis, luminal dilation and even aneurysmal changes can develop. They present with dyspnea on exertion and late in the disease may succumb to right heart failure. The physical examination may be unremarkable or there may evidence of splitting of the second heart sound and a right ventricular heave. Right heart catheterization confirms the diagnosis and excludes elevated pulmonary capillary wedge pressures due to pulmonary venous hypertension. Lung biopsy is rarely pursued to establish the diagnosis, as there is an elevated risk of anesthesia complications and death associated with lung biopsy in patients with severe pulmonary hypertension. Exceptions include atypical presentations and cases where pulmonary hypertension is observed only with exercise. Foci of hemorrhage with hemosiderin deposition are also seen reflecting elevated microhemorrhage and fibrosis.

Two other important milestones were the complete genome sequences of two "model" organisms that are widely used by cell and developmental biologists: the nematode worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster mens health 9 best teas purchase casodex 50 mg on-line. These metazoan sequences revealed many important organizational differences from fungi man healthfitness buy 50 mg casodex free shipping. Although its genome is eight times larger than that of budding yeast (103 million bp distributed in six chromosomes) prostate ablation casodex 50mg visa, the nematode has only about three times more genes mens health 7 day workout best casodex 50mg. Surprisingly man health style cheap 50 mg casodex with amex, the fly prostate cancer is discount 50 mg casodex fast delivery, with an even larger genome and more complex body plan and life cycle, has about one-third fewer genes than the worm. The "finished" sequence of the human genome published in 2004 (and which still contains a number of unresolved "gaps") revealed an even lower gene density. Humans have far fewer genes than the up to 100,000 that had been predicted (current total 20,296, although this is subject to change) (Table 7. Various repeated-sequence elements and pseudogenes occupy approximately 50% of the genome, as is discussed in a later section. As a result of this organization, a common strategy is to sequence the exome of an individual (the 1. However this strategy misses many mutations in noncoding regulatory regions that cause disease. Most human protein-coding genes have introns separating an average of nine exons averaging only 145 bp each, but the variability is enormous. The average intron is a bit over 3000 bp long, but the human genome has more than 3000 introns that are greater than 50,000 bp and nine that are greater than 500,000 bp long. The distribution of protein-coding genes along chromosomes is also highly variable. There are many types of these elements, but for purposes of simplicity, they can be divided into two overall classes. Transposons generally move by a cut-and-paste mechanism in which the starting element cuts itself out of one location within the genome and inserts itself somewhere else. There is currently no evidence for active transposons in humans, but in Drosophila, transposition by transposons, such as the P element, accounts for at least half of spontaneous mutations. Even though humans no longer have active transposons, we still use at least two functional vestiges of these elements. Thus, on completion of a transposition event, the original retrotransposon remains in its original chromosomal location, and a newly generated element (which may be either full-length or partial) is inserted at a new site in the genome. Apparently, the reverse transcriptase usually falls off before it completes copying the entire element. Transposition can be harmful, as along the way, genes can be disrupted, deleted, or rearranged. Because of their tendency to insert into gene-rich regions of chromosomes, Alu elements are one of the most potent endogenous human mutagens, with a new Alu insertion occurring once in every 100 births. In contrast, mice apparently have many more active L1 elements (~3000), and L1 transposition causes approximately 2. Alu transcripts accumulate under conditions of cellular stress such as viral infection. Thus, it has been suggested that Alu transcripts might be natural regulators of protein synthesis under conditions of cellular stress. As discussed at the end of this chapter, the structure of telomeres (the ends of chromosomes) is in part maintained by telomerase, a specialized form of reverse transcriptase, whose mechanism is closely related to that of the L1 reverse transcriptase. Pseudogenes One surprise that emerged from analysis of the eukaryotic genome sequences was the presence of pseudogenes: more than 14,000 in humans. They also lack sequences that regulate transcription initiation and termination (see Chapter 10), so they are not expressed. The duplications can initially create bona fide functional gene copies that may become pseudogenes as they accumulate mutations that render their transcripts nonfunctional. Segmental Duplications in the Human Genome Approximately 5% of the human genome is composed of regions of segmental duplication that have formed relatively recently in evolutionary time. If the deleted region contains genes important for human health, then the result can be disease. Because of the highly complex organization of this region and the large size of the duplications, this turned out to be the most difficult region of chromosome 7 to sequence. Given the large number of genomes sequenced to date, it makes sense to talk of a "typical" genome and how this differs from the reference. The largest number of affected base pairs are in 2100 to 2500 "structural variants" (changes involving >50 bp). Other variations occur in genes, with a typical genome having approximately 165 mutations that truncate proteins, approximately 11,000 mutations that change protein sequences, and a staggering 520,000 mutations in regions thought to be involved in regulating gene expression. In mitotic chromosomes of most higher eukaryotes, the centromere forms a waist-like stricture or primary constriction where the two sister chromatids are most intimately paired. In other organisms, including the fission yeast Schizosaccharomyces pombe, centromere sequences require an activation event to nucleate kinetochore formation. Iftheplasmidislost,thecoloniesbecomeredasaresult of the accumulation of a metabolic by-product. If the plasmid is capable of replication but lacks a centromere, the colonies will be mostly red, reflecting the inefficient segregation of the plasmid at mitosis(B). Kinetochores assembled on regional centromeres bind multiple microtubules (two to four in the case of S. Epigenetic mechanisms also play an essential role in the assembly of centromeres in higher eukaryotes, including humans. Regional centromeres are typically organized around a core region that nucleates kinetochore formation during mitosis. Constitutive heterochromatin is characterized by the presence of special modifications of the histone proteins and other proteins that "read" (bind to) those modifications. Surprisingly, this centromere region contains at least four genes that are actively transcribed. These centromeres are thought to be evolutionarily new, and may have originated from neocentromeres (see later). The rice centromere is not evolutionarily new, having had its present organization for at least the last 10,000 years (since the indica and japonica cultivars of rice were separated) and appears to be intermediate between a canonical metazoan centromere and a neocentromere. This resembles the situation in plants; however, in Drosophila, no sequences were found in this region that are unique to the fly centromeres; all sequences found at centromeres could also be found on the chromosome arms. In addition to point centromeres in budding yeast and regional centromeres found in most metazoans, many plants and insects as well as in the nematode C. These holocentric chromosomes have binding sites for about 20 microtubules distributed along the whole poleward-facing surface of the chromosome during mitosis rather than a disk-like kinetochore at a centromeric constriction, as in humans. If a holocentric chromosome is fragmented, every piece can bind microtubules and segregate in mitosis. Perhaps surprisingly, the proteins of the holocentric kinetochore are the same as those found at disk-like regional kinetochores (see Chapter 8). One possibility is that in these chromosomes, any chromatin with the right transcriptional profile can serve to nucleate kinetochore assembly-perhaps the requirement for special epigenetic marks has been relaxed. For example, the centromere of chromosome 21 (the smallest human chromosome at ~48 million bp) has been estimated to encompass more than 5 million bp. The organization of higherorder repeats varies greatly from chromosome to chromosome, and numerous repeat patterns, comprising 2 to 32 monomers, have been described. These satellites occur in blocks more than 20,000 bp long that are immediately adjacent to the centromeres of a number of chromosomes and may be found at lower levels near most centromeres. There is an interesting corollary of this role of epigenetic modifications in assembly of a functional centromere. Rare individuals have a chromosome fragment that segregates in mitosis, despite loss of the normal centromere. Such chromosomes have acquired a new centromere or neocentromere in a new location on one of the chromosome arms. The epigenetic mark that defines an active centromere can be lost as well as gained. This has been seen most clearly in naturally occurring human dicentric chromosomes. As shown in the figure, one of these lost the ability to assemble a kinetochore even though it retained its -satellite. What is the elusive epigenetic mark and how does it "magically" mark a region of the chromosome as a centromere This is remarkable, because transcription is supposed to be entirely shut off during mitosis, and indeed, it seems that centromeres are the only region of the genome that is transcribed at that time. In the human, roughly 650 to 2500 copies of this sequence are found at the ends of each chromosome, a total length of approximately 4000 to 15,000 bp (this varies in different tissues). The telomeric repeat is organized in a unique orientation with respect to the chromosome end. Thus, the end of every chromosome has one G-rich strand and one complementary C-rich strand. Furthermore, the end of the chromosome is not a blunt structure; the G-rich strand ends in a single-stranded overhang 30 to 400 bp long. It regulates telomerase activity and also "invades" the double helix of telomeric repeats, base-pairing and causing the ends of chromosomes to form large loops, called T loops that protect chromosome ends (see later discussion). In addition, approximately 90% of cancer cells express active telomerase and abnormal expression of telomerase has been linked to cancer. This telomerase is thought to enable the cancer cells to grow indefinitely without undergoing erosion of the ends of the chromosomes. In cells that lack telomerase, a second pathway can help maintain the telomeric repeats at chromosome ends. In the fly, a few bp are lost from the end of the chromosome at every round of replication. This erosion of the chromosome ends is remedied by the occasional transposition of specialized transposable elements to the chromosome end. They protect the end of the recessed C-rich strand at telomeres, and this strand is rapidly degraded if these proteins are missing, with lethal consequences for the cell. Shelterin appears to both regulate telomerase activity and play an essential role in protecting chromosome ends. If shelterin is lost, chromosomes fuse with one another, and many abnormalitiesareseen. It thus appears that the breakage repair machinery recognizes chromosome ends, but the shelterin complex somehow changes its function from an end-joining role to an end-blocking protective role. Loss of shelterin results in a loss of the G-strand overhangs and a dramatic increase in the tendency of chromosomes to fuse end to end. Telomeres may also direct chromosome ends to their proper location within the cell. In budding yeast (and many other species), telomeres prefer to cluster together at the nuclear periphery. Mutants in telomere-binding proteins, or in regions of the histones with which they interact, disrupt this clustering in yeast. This results in activation of genes that are normally silenced when located in close proximity to telomeres. Thus, positioning of the telomere within the nucleus may be used to sequester genes into compartments where their transcriptional activity is repressed. A,The chromosomesofawild-typefemaleDrosophila melanogasterseenat mitotic metaphase (see Chapter 44). B, the Caravaggio mutant is characterized by a "train" of chromosomes generated by telomeretelomerefusions. Chromosomes of older individuals have shorter telomeres, and gametes have longer telomeres. This suggested the interesting possibility that chromosomes might lose telomeric sequences during the life of an individual. The relationship between telomere length and aging can be studied in cultured cells. Normal cells in culture grow for only a limited number of generations (often called the Hayflick limit) before undergoing senescence (this involves permanent cessation of growth, enlargement in size, and expression of marker enzymes, such as -galactosidase). Because normal somatic cells lack telomerase activity, their telomeres shorten and eventually reach a critically short threshold before the cells senesce. In some cases, it is possible to force senescent cells to resume proliferation (eg, by expressing certain viral oncogenes). These "driven" cells continue to divide and their telomeres continue to shorten until a crisis point is reached. In crisis, cells suffer chromosomal instability (chromosomal fusions and breaks can occur) and cell death. In populations of human cells in crisis, very rarely (in approximately 1 in 106 cases), cells appear that once again grow normally. These observations with cultured cells led to the suggestion that senescence might occur in cells when the telomeric repeats of one or more chromosomes are reduced to a critical level. If correct, this model suggests very interesting (and controversial) implications for the regulation of cell life. Suppose that telomerase is active in the germline, so that all gametes have long telomeres. Now, if the enzyme were inactivated in somatic cells, this would effectively provide every cell lineage with a limitation on how many times it could divide before loss of telomeric sequences caused it to become senescent. Provided that the starting telomeres were sufficiently long and that telomerase was expressed in stem cells of tissues like testis and intestine, in which rapid division occurs throughout the life of the individual, this lack of telomerase in most cells would have no deleterious effect on the life span of the organism. In fact, such a mechanism might provide an important advantage by minimizing the chances that a clone of cells would escape from the normal regulation of growth control and become cancerous. These mice were healthy and fertile for six generations in the complete absence of telomerase but then subsequent generations became sterile as a result of cell death in the male germline.

This tethering of kinetochores to disassembling microtubules is essential for chromosome movements during mitosis prostate position order casodex discount. Correcting Errors in Chromosome Attachment to the Spindle the goal of mitosis is to partition the replicated chromosomes accurately between two daughter cells prostate lymph nodes effective 50 mg casodex. Three other sorts of attachment are seen: (a) chromosomes with one kinetochore lacking attached microtubules (known as monotelic attachment; this is a normal intermediate) androgen hormone urinary buy discount casodex line, (b) chromosomes with both sister kinetochores attached to the same spindle pole (known as syntelic attachment) prostate cancer 30s purchase casodex in united states online, and (c) chromosomes with a single kinetochore attached simultaneously to both spindle poles (known as merotelic attachment) mens health blog casodex 50 mg fast delivery. When syntelic attachments occur prostate otc medication safe 50 mg casodex, one or both kinetochores must detach for the chromosome to achieve a bipolar orientation. Chromosome attachment to opposite spindle poles is more stable than attachment to a single pole, because the tension generated by bipolar attachment (where forces pull a chromosome simultaneously toward opposite spindle poles) preferentially stabilizes microtubule connections to both kinetochores. Merotelic attachments are more dangerous, as the kinetochore is under tension and the attachments are therefore stable. Merotelic attachments are the most common cause of chromosome segregation errors in cultured mammalian cells. The other subunits target Aurora B to its various sites of action during mitosis and regulate the kinase activity. The complex concentrates at inner centromeres (the heterochromatin beneath and between the two sister kinetochores) during prometaphase and metaphase. Inset in A, Distribution of kinetochores (red), and borealin (green) in a prometaphase cell. Along the way it contributes to the correction of chromosome attachment errors and to the operation of the checkpoint that delays the cell cycle in response to those errors. Aurora B phosphorylation strongly inhibits Ndc80 binding to microtubules, causing the kinetochore to release attached microtubules. Finding Time to Fix Chromosome Attachment Errors: the Spindle Assembly Checkpoint Segregation of replicated chromosomes into daughter cells is extremely accurate. For example, budding yeasts lose a chromosome only once in 100,000 cell divisions. The frequency of chromosome loss may be 20-fold to 400-fold higher for human cells grown in culture. To achieve even this level of accuracy, most cells delay entry into anaphase until all chromosomes have achieved amphitelic attachment to the spindle. Mps1 phosphorylation of Knl1 creates a binding site that results in Mad1 recruitment to the kinetochore. A loop on Mad2 wraps around Mad1 like a safety belt making the complex particularly stable. When microtubules bind, cytoplasmic dynein motors actively strip checkpoint components from the kinetochore, dragging them away toward the centrosomes. In yeast, access of Mps1 to its target sites on Knl1 is physically blocked when microtubules bind. However, the network of interactions is very complex and details are still being worked out. Experimental inactivation of the spindle checkpoint causes a catastrophic, premature entry into anaphase, regardless of the status of chromosome alignment. This leads to an unequal distribution of sister chromatids and genetic imbalance between daughter cells known as aneuploidy. Yeasts can live without the checkpoint genes, but their loss is lethal for mice, which die early during embryogenesis. Humans with mutations in BubR1 have mosaic variegated aneuploidy syndrome (extra copies or loss of various chromosomes in a variety of tissues), which is associated with microcephaly (decreased brain size) and an increased cancer risk. The compact grouping of chromosomes at the middle of the spindle is referred to as the metaphase plate. In many cells, even though chromosomes remain, on average, balanced at the middle of A. Degradation of cyclin A begins earlier, at the entry into prometaphase, and is largely complete before metaphase. Loss of securin initiates a process leading to the separation of sister chromatids and the onset of anaphase. First, there is constant net addition of new tubulin subunits (approximately 10 subunits per second) to the plus end of the microtubules, where they are attached to the kinetochore. Second, a comparable number of tubulin subunits is continuously lost from the minus end of the kinetochore tubules at the spindle poles. This subunit flux or treadmilling in kinetochore microtubules is caused by microtubule depolymerization at the poles driven by kinesin-13 family members. In addition, tubulin moves toward the poles as by microtubules are transported towards the pole (many nucleated by the augmin complex) within the kinetochore fiber. A,Cells entering mitosis were injected with tubulin subunits modified chemically by attachment of a caged fluorescent dye. This can occur if new subunits are added to the microtubule at the kinetochore and old subunits are removed at the pole. Sister chromatids move to opposite spindle poles (anaphase A), and the poles move apart (anaphase B). Anaphase is also the time when the mitotic spindle activates the cell cortex in preparation for cytokinesis. Biochemical Mechanism of Sister Chromatid Separation Separation of sister chromatids is regulated by the chromosomes themselves, not by the mitotic spindle. Under certain circumstances, sister chromatids can separate in the absence of microtubules, ruling out a requirement for forces from the spindle in the process. Cells with mutations in cohesin components separate sister chromatids prematurely in mitosis, resulting in chaotic chromosome missegregation. A variety of evidence suggests that cohesin forms a ring with a diameter of 35 nm, large enough to encircle two sister chromatids like a lasso. Cohesin accumulates at preferred sites on the chromosomes, often near centromeres in budding yeast or in regions of heterochromatin in fission yeast. In vertebrates, most cohesin dissociates from the chromosome arms by late metaphase, owing to the action of the protein kinases Plk1 and Aurora B. Importantly, a critical fraction remains associated with heterochromatin flanking centromeres where it is protected from cleavage by shugoshin until the onset of anaphase (see following paragraphs and Chapter 45). Sequential cleavage of two key proteins triggers sister chromatid separation at anaphase. After the last chromosome forms an amphitelic attachment to the spindle, the spindle checkpoint is silenced. When securin levels fall below a critical threshold, separase is unleashed to cleave the Scc1 subunit of cohesin. Efficient Scc1 cleavage requires that the protein be phosphorylated near its cleavage site. This inhibits its cleavage and protects cohesin until shugoshin is released following amphitelic attachment of the chromosome. Securin can act as an oncogene in cultured cells and is overexpressed in some human pituitary tumors. Overexpression of securin may disrupt the timing of chromosome segregation, leading to chromosome loss and, ultimately, contributing to cancer progression. Mitotic Spindle Dynamics and Chromosome Movement During Anaphase Anaphase is dominated by the orderly movement of sister chromatids to opposite spindle poles brought about by the combined action of motor proteins and changes in microtubule length. Anaphase A, the movement of the sister chromatids to the spindle poles, requires a shortening of the kinetochore fibers. The poles separate partially because of interactions between the antiparallel interpolar microtubules of the central spindle and partially because of intrinsic motility of the asters. Most cells use both components of anaphase, but one component may predominate in relation to the other. Several kinesin "motors" influence the dynamic instability of the spindle microtubules. Kinetochores are remarkable in their ability to hold onto disassembling microtubules. In straight (growing) microtubules, the Ndc80 complex is mostly responsible for microtubule binding. This could allow it to redistribute onto straight sections of the lattice and thereby move away from the curved protofilaments at the disassembling end. If a laser is used to bleach a narrow zone in the fluorescent tubulin across the spindle between the chromosomes and the pole early in anaphase, the chromosomes approach the bleached zone much faster than the bleached zone approaches the spindle pole. In these cells, subunit flux accounts for only 20% to 30% of chromosome movement during anaphase A, and this flux is dispensable for chromosome movement. In Drosophila embryos, in which subunit flux accounts for approximately 90% of anaphase A chromosome movement, the chromosomes catch up with a marked region of the kinetochore fiber slowly, if at all. During the latter stages of anaphase B, the spindle poles, with their attached kinetochore microtubules, appear to move away from the interpolar microtubules as the spindle lengthens. This movement of the poles involves interaction of the astral microtubules with cytoplasmic dynein molecules anchored at the cell cortex. Within the central spindle, an amorphous dense material called stem body matrix stabilizes bundles of antiparallel microtubules and holds together the two interdigitated half-spindles. How can protein kinases such as Aurora B continue to function during anaphase while protein phosphatases are removing phosphate groups placed there by Cdks and, indeed, Aurora B during early mitosis One answer is that the phosphatase activity is highly localized, controlled by specific targeting subunits. Some further anaphase B movement may still occur, but the most dramatic change in cellular structure at this time is the constriction of the cleavage furrow and subsequent cytokinesis. The pore subsequently matures as various peripheral components and elements of the permeability barrier are added. In cells where the nuclear membrane is absorbed into the endoplasmic reticulum during mitosis, reassembly involves lateral movements of membrane components within the membrane network and their stabilization at preferred binding sites at the periphery of the chromosomes. Lamin subunits disassembled in prophase are recycled to reassemble at the end of mitosis. Later during telophase when nuclear import is reestablished, lamin A enters the reforming nucleus and slowly assembles into the peripheral lamina over several hours in the G1 phase. If lamin transport through nuclear pores is prevented, chromosomes remain highly condensed following cytokinesis, and the cells fail to reenter the next S phase. Early cytokinesis New membrane inserted Actomycin Actomycin contractile ring forms Midbody begins to form B. A, A classic experiment in which a sand-dollar egg is caused to adopt a toroid shape. Right, In a profilin mutant, no central spindle forms, and the cell fails to form a contractile ring. Signals from the mitotic spindle and cell cycle machinery control the position of this ring (ie, the relative sizes of the two daughter cells) and the timing of its constriction. Protozoa, animals, fungi, and plants use an evolutionarily conserved set of components to implement different strategies to separate daughter cells. In animal cells, contractile ring constriction provides the force that remodels the cortex to generate the two daughter cells. In contrast, in yeasts, which have a cell wall, contractile ring constriction is thought to guide the orderly centripetal growth of the cell wall septum, which contributes force to overcome turgor pressure and invaginate the plasma membrane. These differences reflect the fact that widely divergent eukaryotes use variations of similar themes for cytokinesis. Cytokinesis in prokaryotes is genuinely different, since completely different proteins are involved (Box 44. Although cytokinesis has been studied for more than 100 years, it has posed a number of challenges due to its complexity at the molecular level. For example genetic analysis of fission yeast revealed more than 150 genes that contribute to cytokinesis. Cytokinesis research typically employs living cells, although progress is being made toward reconstituting some aspects of the process in cell-free systems. We now know that the central spindle does emit a positive signal directing a cleavage furrow to form above it, while the poles contribute by focusing that furrow at a point on the cortex midway between them. The molecular nature of the cleavage stimulus is now beginning to be understood in animals. During anaphase, overlapping microtubules between the separating chromatids establish an ordered array known as the central spindle. A key protein component of this array is a protein heterodimer known as centralspindlin. Signals from the poles of the mitotic spindle contribute, particularly in large invertebrate embryos, by confining the zone of active RhoA to a narrow equatorial band between the separating sister chromatids. In addition, a signal emitted by the bundled microtubules of the Assembly and Regulation of the Contractile Ring Exposure of the cell cortex to the cleavage stimulus culminates in the assembly of a contractile ring consisting of a very thin (0. The plasma membrane adjacent to this actinmyosin ring undergoes alterations in its lipid composition that may help recruit proteins important for the function of the contractile ring. In their absence, furrowing begins, but the cleavage furrows ultimately regress, producing binucleated cells. Thus, they appear to contribute to the cleavage stimulus, and indeed, both require microtubules to localize to the site of cleavage furrow formation as originally shown for the cleavage stimulus. In fission yeast, with closed mitosis, the nucleus determines the position of cleavage. During interphase, assemblies of proteins called nodes form on the inside of the plasma membrane around the middle of cell. Prior to mitosis, an anillin-like protein leaves the nucleus and joins these nodes. Contractile ring assembly in animal cells shares many properties with fission yeast, but is less completely understood. The decline in the activity of cell cycle kinases at the onset of anaphase is part of the trigger, since they inhibit centralspindlin components through metaphase. Formins and profilin polymerize some new actin filaments, but preexisting actin filaments are recruited into the contractile ring from adjacent areas of the cortex.

Purchase cheapest casodex and casodex. Human Body Quiz.