what happens to the chromosome number as a result of fertilization
Gametogenesis
Gametes
Normal somatic human being cells are diploid possessing a 2N corporeality of DNA in the form of 46 chromosomes arranged in 23 homologous pairs. One chromosome in each homologous pair comes from each parent. Of these chromosomes 44 are autosomal and two are sex chromosomes. Somatic cells reproduce by normal cell segmentation known equally mitosis, which yields daughter cells also with a 2N corporeality of DNA. The daughter cells produced by mitosis are genetically identical.
Gametes (oocytes and spermatozoa) are the descendants of primordial germ cells that originate in the wall of the yolk sac in the embryo and migrate to the gonadal region. Gametes are specialized haploid reproductive cells possessing 1N amount of Deoxyribonucleic acid in the form of 22 autosomal chromosomes and ane sex chromosome for a full of 23 chromosomes.
Mitosis and Meiosis
Primordial germ cells differentiate into gametes by a specialized two-stage cell division procedure known equally meiosis, which produces 4 haploid (1N) cells from one diploid (2N) germ cell. Replication of Dna and crossover occur during meiosis I. Centromeric division (and reduction of chromosome number) occurs during meiosis Ii. The random distribution of chromosomes between the resulting daughter cells in this process results in the independent assortment of chromosomes, and together with crossover are mechanisms for ensuring genetic variability amid offspring.
Female person Gametogenesis (Oogenesis)
In females, most of gametogenesis occurs during embryonic development. Primordial germ cells migrate into the ovaries at week four of evolution and differentiate into oogonia (46,2N). Oogonia enter meiosis I and undergo DNA replication to form primary oocytes (2N,4C). All primary oocytes are formed by the fifth calendar month of fetal life and remain dormant in prophase of meiosis I until puberty.
During a woman�s ovarian cycle 1 oocyte is selected to complete meiosis I to form a secondary oocyte (1N,2C) and a beginning polar body. After ovulation the oocyte is arrested in metaphase of meiosis II until fertilization. At fertilization, the secondary oocyte completes meiosis II to course a mature oocyte (23,1N) and a 2nd polar torso.
Male Gametogenesis (Spermatogenesis)
In males, gametogenesis begins at puberty and continues into advanced age. Primordial germ cells (46,2N) migrate into the testes at calendar week 4 of development and remain dormant. At puberty, primordial germ cells differentiate into blazon A spermatogonia (46,2N). Type A spermatogonia split by mitosis to class either more type A spermatogonia (to maintain the supply) or type B spermatogonia.
Gamete Transport
Ovulation
Under the influence of estrogen released during the first one-half of the menstrual wheel, three changes take place in the uterine tubes to facilitate its capture of the egg:
1. The uterine tubes move closer to the ovaries (physical approximation)
2. The fimbriae on the ends of the tubes beat more than chop-chop (increased fluid current)
3. The number of ciliated cells in the epithelium of the fimbriae increment (increase in ciliation)
Send of Sperm in Female
Sperm are deposited in the upper vagina and must overcome several obstacles to achieve an egg in the ampulla of 1 of the uterine tubes.
Sperm lose their power to fertilize an egg after three - three� days. The egg itself is viable for merely about 24 hours.
Tabular array 1 - Obstacles to Sperm Send
| Obstacle | Adaptation |
| Low pH of upper vagina | The alkali metal seminal fluid temporarily neutralizes the normal acidity (pH 4.3 � pH 7 � 7.2) to allow the sperm to survive in the upper vagina. |
| Cervical mucus | The composition of cervical mucus changes during menstrual cycle. Sperm can nigh easily penetrate the thinner East-mucus that predominates during the last few days earlier ovulation, as opposed to the thicker G-mucus. |
| Cervical canal, uterus | Two modes of transport: Rapid � some sperm travel from the vagina to the upper 1/3 of the uterine tube in as piffling as xxx minutes. Since sperm usually swim merely ii-3 mm/hr, information technology is thought that they are actively transported past smooth muscle contractions of the female person or some other machinery. Slow � the rest of the sperm swim their fashion up the last office of the cervical tube, are stored in cervical crypts (folds of the cervix), and are slowly released into the uterus over 2-3 days. |
Tabular array 2 - Karyotypes of Germ Cells and Gametes
| Cell | Karyotype |
| Primordial germ jail cell | 46,2N |
| Female | |
| Oogonium | 46,2N |
| Master oocyte | 46,4N |
| Secondary oocyte | 23,2N |
| Mature oocyte | 23,1N |
| Male person | |
| Type A spermatogonium | 46,2N |
| Type B spermatogonium | 46,2N |
| Primary spermatocyte | 23,2N |
| Secondary spermatocyte | 23,1N |
| Spermatid | 23,1N |
Clinical Correlations
Aneuploidy
Aneuploidy is an aberrant number of chromosomes that can consequence from either unbalanced chromosomal translocations or nondisjunction during meiosis Ii. Most chromosomal abnormalities are incompatible with life, however, some combinations practice result in live offspring, and trisomies involving chromosomes 13, 14, fifteen, 21 and 22 (groups D and Thousand chromosomes) are relatively common birth defects. Down syndrome results from trisomy 21 that occurs in approximately ane/500 live births, and is characterized by growth retardation, mental retardation, and specific somatic abnormalities. Aneuploidy of the sexual practice chromosomes can besides occur, and certain karyotypes are associated with characteristic syndromes.
Table 3 - Syndromes Associated with Aneuploidy of the Sexual practice Chromosomes
| Karyotype | Syndrome | Frequency | Description |
| 45,10 (XO) | Turner syndrome | 1/5000 female live births | Phenotypic female, gonadal dysgenesis and sexual immaturity later on puberty, infertility |
| XXY | Klinefelter�s syndrome | one/1000 male live births | Phenotypic male, gonadal dysgenesis and sexual immaturity later puberty, infertility |
| XYY (XXYY) | XYY syndrome | 1/yard male live births | Phenotypic male person, behavioral abnormalities |
Source: https://www.med.umich.edu/lrc/coursepages/m1/embryology/embryo/02prefertilization.htm
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