Normal somatichuman cells room diploid possessing a 2Namount of DNA in the kind of 46 chromosomes arranged in 23homologous pairs. One chromosome in every homologous pair originates from eachparent. Of this chromosomes 44 room autosomal and 2are sex chromosomes. Somatic cellsreproduce by regular cell department known together mitosis,which returns daughter cells additionally with a 2N quantity of DNA.The daughter cells developed by mitosis room genetically identical.

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Gametes (oocytesand spermatozoa) space the progeny of primordial germ cells that originate in the wall the the yolk sac in the embryo and also migrate to the gonadalregion. Gametes are committed haploidreproductive cell possessing 1Namount of DNA in the form of 22 autosomal chromosomes and one sex chromosome fora total of 23 chromosomes.

Mitosis and also Meiosis

Primordial germ cells distinguish into gametes through aspecialized two-phase cell division process well-known as meiosis, i m sorry produces four haploid (1N) cell from one diploid(2N) germ cell. Replication that DNAand crossover occur throughout meiosisI. Centromeric division(and reduction of chromosome number) occurs during meiosis II. The randomdistribution that chromosomes in between the resulting daughter cells in this processresults in the independent assortment ofchromosomes, and in addition to crossover room mechanisms because that ensuring hereditary variability amongoffspring.

Female Gametogenesis (Oogenesis)

In females, many of gametogenesis wake up duringembryonic development. Primordialgerm cell migrate right into the ovaries in ~ week 4 of development and differentiateinto oogonia (46,2N).Oogonia go into meiosis I and undergo DNA replication to type primary oocytes (2N,4C). Allprimary oocytes room by the fifth month of fetal life andremain dormant in prophase of meiosis I until puberty.

During a woman�s ovarian cycle one oocyte is selectedto complete meiosis i to kind a secondaryoocyte (1N,2C) and a first polar body.After ovulation the oocyte is arrested in metaphase of meiosis II until fertilization.At fertilization, thesecondary oocyte completes meiosis II to kind a mature oocyte (23,1N) and a secondpolar body.

Male Gametogenesis (Spermatogenesis)

In males, gametogenesis begins at puberty and continuesinto advanced age. Primordial germcells (46,2N) migrate into the testes in ~ week 4 of advancement and remaindormant. In ~ puberty, primordialgerm cells distinguish into type Aspermatogonia (46,2N). Form Aspermatogonia division by mitosis toform one of two people more kind A spermatogonia (to maintain the supply) or typeB spermatogonia.

Gamete Transport


Under the influence of estrogen released throughout the first half the the menstrual cycle,three alters take location in the uterine tubes come facilitate its capture of theegg:

1.The uterine tubes relocate closer to the ovaries (physicalapproximation)

2.The fimbriae top top the end of the tube beat much more rapidly (increasedfluid current)

3.The number of ciliated cell in the epithelium that the fimbriae rise (increasein ciliation)

Transport that Sperm in Female

Sperm room deposited in the top vagina and also must overcomeseveral obstacles to with an egg in the ampulla of among the uterine tubes.

Sperm shed their capability to fertilize one egg after3 - 3� days. The egg itself is viable because that only around 24 hours.

Table 1 - Obstacles come Sperm Transport



Low pH of upper vagina

The alkaline seminal fluid temporarily neutralizes the normal acidity (pH 4.3 � pH 7 � 7.2) to permit the sperm to endure in the upper vagina.

Cervical mucus

The ingredient of cervical rubber changes during menstrual cycle. Sperm can most easily penetrate the diluent E-mucus that predominates during the last few days prior to ovulation, as opposed come the thicker G-mucus.

Cervical canal, uterus

Two settings of transport:

Rapid � some sperm take trip from the vagina come the upper 1/3 the the uterine pipe in as little as 30 minutes. Since sperm usually swim only 2-3 mm/hr, it is thought that castle are proactively transported by smooth muscle contractions that the mrs or some various other mechanism.

Slow � the remainder of the sperm swimming their method up the last part of the cervical tube, space stored in cervical crypts (folds of the cervix), and are gradually released into the uterus end 2-3 days.

Table 2- Karyotypes the Germ Cells and also Gametes



Primordial germ cell





Primary oocyte


Secondary oocyte


Mature oocyte



Type A spermatogonium


Type B spermatogonium


Primary spermatocyte


Secondary spermatocyte




Clinical Correlations


Aneuploidy is an abnormal number of chromosomes that canresult from one of two people unbalanced chromosomaltranslocations or nondisjunctionduring meiosis II. Mostchromosomal abnormalities room incompatible v life, however, some combinationsdo result in live offspring, and trisomies including chromosomes 13, 14, 15, 21and 22 (groups D and G chromosomes) are reasonably common bear defects.

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Down syndrome outcomes from trisomy21 that occurs in about 1/500live births, and also is characterized by development retardation, mental retardation, andspecific somatic abnormalities. Aneuploidyof the sex chromosomes can likewise occur, and particular karyotypes space associatedwith characteristic syndromes.


Table 3- Syndromes linked with Aneuploidy of the Sex Chromosomes





45,X (XO)

Turner syndrome

1/5000 woman live births

Phenotypic female, gonadal dysgenesis and sexual immaturity after puberty, infertility


Klinefelter�s syndrome

1/1000 male live births

Phenotypic male, gonadal dysgenesis and also sexual immaturity after puberty, infertility


XYY syndrome

1/1000 male live births

Phenotypic male, behavioral abnormalities