Describe in brief about pre-fertisation event
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Gametes
Normal somatic human cells are diploidpossessing a 2N amount 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 2 are sex chromosomes. Somatic cells reproduce by normal cell division known as mitosis, which yields daughter cells also with a 2N amount 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 DNA in the form of 22 autosomal chromosomes and one sex chromosome for a total of 23 chromosomes.
Mitosis and Meiosis
Primordial germ cells differentiate into gametes by a specialized two-phase cell division process known as meiosis, which produces four haploid (1N) cells from one diploid (2N) germ cell. Replication of DNA andcrossover 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 among offspring.
Female Gametogenesis (Oogenesis)
In females, most of gametogenesis occurs during embryonic development. Primordial germ cells migrate into the ovaries at week 4 of development 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 month of fetal life and remain dormant in prophase of meiosis I until puberty.
During a woman’s ovarian cycle one oocyte is selected to complete meiosis I to form asecondary oocyte (1N,2C) and a first polar body. After ovulation the oocyte is arrested inmetaphase of meiosis II until fertilization. Atfertilization, the secondary oocyte completes meiosis II to form a mature oocyte (23,1N) and a second polar body.
Normal somatic human cells are diploidpossessing a 2N amount 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 2 are sex chromosomes. Somatic cells reproduce by normal cell division known as mitosis, which yields daughter cells also with a 2N amount 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 DNA in the form of 22 autosomal chromosomes and one sex chromosome for a total of 23 chromosomes.
Mitosis and Meiosis
Primordial germ cells differentiate into gametes by a specialized two-phase cell division process known as meiosis, which produces four haploid (1N) cells from one diploid (2N) germ cell. Replication of DNA andcrossover 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 among offspring.
Female Gametogenesis (Oogenesis)
In females, most of gametogenesis occurs during embryonic development. Primordial germ cells migrate into the ovaries at week 4 of development 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 month of fetal life and remain dormant in prophase of meiosis I until puberty.
During a woman’s ovarian cycle one oocyte is selected to complete meiosis I to form asecondary oocyte (1N,2C) and a first polar body. After ovulation the oocyte is arrested inmetaphase of meiosis II until fertilization. Atfertilization, the secondary oocyte completes meiosis II to form a mature oocyte (23,1N) and a second polar body.
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