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U. Ur-Gosh, M.B. B.CH., M.B.B.Ch., Ph.D.

Program Director, University of Arizona College of Medicine – Tucson

Because the strands of the parent duplex are antiparallel, and because both new strands are synthesized more or less simultaneously in the direction of movement of a replication fork, one daughter strand is synthesized in the 5 -to-3 direction while the other is synthesized from the 3 direction. Chromosomal replication is a complex process requiring a number of different proteins. Only one half of this region is shown; in this half, the strands of the duplex will continue to separate in a left right direction. Following synthesis of the 1st primer, the replication fork has moved further to the right ­ opening up the duplex to the extent shown in the diagram. As the replication fork moves even further to the right additional template will become available, and the 3rd, 4th, 5th. Each primer is elongated in the 5 -to-3 direction to form an Okazaki fragment (1000­2000 nt in length) that extends to the 5 end of the previous primer. In this model, a section of lagging-strand template (liberated by ongoing leading-strand synthesis) forms a loop at the replication fork. Thus, the complex synthesizes both leading and lagging strands concurrently as it moves along with the replication fork. It appears that coupling is also needed between the helicase and the polymerases ­ loss of such coupling being associated with a drastic reduction in efficiency of the helicase [Cell (1996) 84 643­650]. Moving in opposite directions, the two replications forks meet at the ter site on the chromosome, 180° from oriC. This might be expected to result in a relatively high frequency of errors because tautomerism in the bases can interfere with the accuracy of base-pairing. In some bacteria a linear chromosome is replicated bidirectionally from an internal origin of replication. Plasmid replication, and the partitioning to daughter cells, seems likely to involve some kind of association between the plasmid and the cytoplasmic membrane of the host cell [Mol. Primers are labelled with fluorescent dyes, a different dye being used for each of the four reactions. The products from all four reactions are combined and then subjected to electrophoresis from a single well in a polyacrylamide gel. On excitation by an argon laser, fragments from each reaction mixture can be distinguished from those of other reaction mixtures by their dye-specific emission characteristics; emissions from the fragments of all four reaction mixtures are detected automatically and stored. When base-paired to the template strand, the primer is extended (5 -to-3) by the sequential addition of nucleotides as dictated by the template. In sequencing, there are four separate reaction mixtures (G, C, T, A), each containing all the constituents mentioned above (including millions of copies of the unknown sequence, and of the primer). Thus, primer extension will stop (= chain termination) at any location where a dideoxyribonucleotide has been incorporated. For example, with ddG (see diagram) the three products are of different lengths because, during extension of the primers, ddG has paired with cytosine residues at three different locations in the template; note that, in this case, the length of a given product strand is related to the location of a particular cytosine residue in the unknown sequence. At the end of the reaction, product strands are separated from templates by formamide. Each reaction mixture is then subjected to electrophoresis in a separate lane of a polyacrylamide gel. Short product strands move further than longer ones, in a given time; products that differ in length by only one nucleotide can be distinguished, the shorter product moving just a little further. The gel contains urea; this inhibits base-pairing between product strands and templates. The therapeutic effect of the Hsp65 vaccine (manifested by a marked decrease in the numbers of live tubercle bacilli in lung and spleen) was associated with a switch from a Th-2-dominated to a Th-1-dominated response in the mice. DnaA was earlier reported to be needed for the replication of certain plasmids. The locations of bands indicate the relative lengths of product strands ­ shorter products having moved further down the gel (from top to bottom in the diagram). Note that the first unknown 3 nucleotide (C) is identified by (i) the shortest product strand (which has moved the furthest), and (ii) the fact that this product is from the ddG mixture ­ indicating a base that pairs with G, i. Similarly, the next unknown (G) is the next shortest product strand which came from the ddC mixture, thus indicating G. The central portion of the septum is thickened and barrel-shaped, the septal pore forming a small tunnel (ca.

Causes of reduced testosterone concentrations include temperature extremes, exogenous estradiol, stressful events, and induced hypothyroidism. The prognosis for return to breeding soundness is generally good if the stress is not prolonged. Practitioners should expect to discover other morphologic abnormalities in the spermiogram following a more prolonged stressful event. The spermiogram will not recover for 6-8 weeks following a prolonged stressor to the bull. Abnormal midpiece There are many morphologic abnormalities of the midpiece that can placed in this generalized category. The other defects reported that can be placed under this category are the "corkscrew" or bent midpiece. Abnormalities of the midpiece are characterized by major disruptions in the axonemal fibers and mitochondrial sheath. This segment of the cell develops within the seminiferous tubule and is important to fertilization because the mitochondria is housed here. Abnormalities of the mitochondria are considered compensable as these sperm lack motility and therefore the ability to traverse the female reproductive tract. The Dag defect named after a Jersey bull whose ejaculate contained 100% of the specific abnormality is believed to have a genetic predisposition. The dag defect is characterized by a figure eight appearance of the midpiece and principal piece. Some forms of this group of defects can be caused by gossypol toxicosis if fed in high quantities. Gossypol is a phenolic compound found at different levels in whole cottonseed that causes damage to sperm structure during spermatogenesis. Gossypol induced defects of the midpiece can be reversed 28 days post elimination from the diet. Proximal Droplets these spherical cytoplasmic condensations should move down the tail during the epididymal transit and eventually will be shed when the sperm cell is exposed to seminal plasma during the ejaculatory process. The droplets should diminish as the bull matures and carries a good prognosis for gaining satisfactory semen quality. The justification is that the cell has the ability to traverse the female tract and compete with normal sperm at the level of the uterine tube for the right to bind with the oocyte. In vitro studies indicate poor zona binding and cleavage rates in ejaculates containing significant numbers of proximal droplets. The detached but otherwise normal heads are likely due to prolonged storage in the epididymis or ductus deferens during extended periods of senescence. This defect is often noted in young bulls approaching puberty or bulls that have recently endured a stressful event. Detached heads are considered a compensatory defect because of the lack of the tail to provide linear motility. It is suggested that bulls be recollected within the same visit if a significant amount of detached heads are noted on the initial spermiogram. Some bulls may have to be collected multiple times in one day to deplete the epididymal reserve and achieve a satisfactory sample. The key is that on subsequent samples the number of detached normal heads become less with each collection to give the practitioner a clue that they are seeing a so called "rusty load". If the defect is determined to be due to stressful event the bull can be deferred and rechecked in two to three weeks if the insult is deemed to be eliminated at the time of the exam. Coiled principal pieces this defect appears like a rolled up rope at the very distal end of the principal piece. This defect is commonly discovered concurrently with defects such as the distal midpiece reflection indicating an epididymal origin. Barth reported this defect in several breeds, but found that Herefords were commonly represented. The classic beaded appearance of the knobbed acrosome defect 17can have other variations characterized by either a flattened or indented apex region. It is considered a heritable condition in cases where no other significant sperm abnormalities exist and the defect is discovered in a significant percentage of the sample.

Anomalies of position, form and orientation Renal ectopia (pelvic kidney), renal fusion (horseshoe kidney) and persistent foetal lobation. Cystic lesions in the kidney may occur at any age, extending from foetal life (detected on ultrasonography) to old age. Renal dysplasia is the most common form of cystic renal disease in the newborn and infants. The kidney or its affected part is replaced by disorderly mass of multiple cysts resembling a bunch of grapes. M/E the characteristic feature is the presence of undifferentiated mesenchyme that contains smooth muscle, cartilage and immature collecting ducts. The cysts in the mass represent dilated tubules lined by flattened epithelium which are surrounded by concentric layers of connective tissue. The prognosis of unilateral renal dysplasia following removal of the abnormal kidney is excellent. Though the kidneys are abnormal at birth, renal function is retained, and symptoms appear in adult life, mostly between the age of 30 and 50 years. The cut surface shows cysts throughout the renal parenchyma varying in size from tiny cysts to 4-5 cm in diameter. The contents of the cysts vary from clear straw-yellow fluid to reddish-brown material. The renal pelvis and calyces are present but are greatly distorted by the cysts and may contain concretions. The most frequent and earliest presenting feature is a dull-ache in the lumbar regions. It is transmitted as an autosomal recessive trait and the family history of similar disease is usually not present. The age at presentation may be perinatal, neonatal, infantile or juvenile, but frequently serious manifestations are present at birth and result in death from renal failure in early childhood. G/A the kidneys are bilaterally enlarged with smooth external surface and retained normal reniform shape. Cut surface reveals small, fusiform or cylindrical cysts radiating from the medulla and extend radially to the outer cortex. Since the cysts are formed from dilatation of collecting tubules, all the collecting tubules show cylindrical or saccular dilatations and are lined by cuboidal to low columnar epithelium. In severe form, the gross bilateral cystic renal enlargement may interfere with delivery. Almost all cases of infantile polycystic kidney disease have associated multiple epithelium-lined cysts in the liver or proliferation of portal bile ductules. The condition occurs in adults and may be recognised as an incidental radiographic finding in asymptomatic cases, or the patients may complain of colicky flank pain, dysuria, haematuria and passage of sandy material in the urine. G/A the kidneys may be enlarged, normal or shrunken in size depending upon the extent of secondary pyelonephritis. On cut surface, the characteristic feature is the presence of several, small (less than 0. M/E the cysts are lined by tall columnar, cuboidal, transitional or squamous epithelium. It is classified 431 Chapter 20 the Kidney and Lower Urinary Tract 432 into infantile, juvenile and adolescent type depending upon the age at presentation. G/A the kidneys are moderately reduced in size and granular and have narrow cortices. Cut surface reveals minute cysts, majority of which are present at the cortico-medullary junction. There is widespread nonspecific chronic inflammatory infiltrate and interstitial fibrosis. A firm diagnosis, however, can be established by examination of renal biopsy under light, electron and immunofluorescence microscopy. Following six major glomerular syndromes are commonly found in different glomerular diseases: I. The underlying causes of acute nephritic syndrome may be primary glomerulonephritic diseases (classically acute glomerulonephritis and rapidly progressive glomerulonephritis) or certain systemic diseases. In children, primary glomerulonephritis is the cause in majority of cases of the nephrotic syndrome; most frequent being lipoid nephrosis (65%). The most common primary glomerular disease in adults is membranous glomerulonephritis (40%).