Pre-fertilization Events Embryology Questions

Pairing of homologous chromosomes (synapsis) is a unique event that occurs only during meiosis I in the production of gametes. Synapsis is necessary so that crossing over can
occur.

A normal gamete (sperm in this case) contains 23 single chromosomes. These 23 chromosomes consist of 22 autosomes plus 1 sex chromosome.

The secondary oocyte is arrested in metaphase of meiosis II about 3 hours before ovulation,
and it remains in this meiotic stage until fertilization.

A normal fertile male will ejaculate about 3.5 mL of semen containing about 100 million
sperm/mL (3.5 mL × 100 million = 350 million).

The structure formed is a duplicated chromosome. DNA replication occurs, so that the
amount of DNA is doubled (2 × 2N = 4N). However, the chromatids remain attached to the centromere, forming a duplicated chromosome.The structure formed is a duplicated chromosome. DNA replication occurs, so that the amount of DNA is doubled (2 × 2N = 4N). However, the chromatids remain attached to the centromere, forming a duplicated chromosome.

At birth, a male has primordial germ cells in the testes that remain dormant until puberty, at which time they differentiate into type A spermatogonia. At puberty, some type A spermatogonia differentiate into type B spermatogonia and give rise to primary spermatocytes by undergoing DNA replication.

Type B spermatogonia give rise to primary spermatocytes by undergoing DNA replication,
thereby doubling the amount of DNA (2 × 2N = 4N) within the cell.

The Barr body is formed from inactivation of one X chromosome in a female. All somatic cells
of a normal female will contain two X chromosomes. The female has evolved a mechanism for
permanent inactivation of one of the X chromosomes presumably because a double dose of
X chromosome products would be lethal.The Barr body is formed from inactivation of one X chromosome in a female. All somatic cells
of a normal female will contain two X chromosomes. The female has evolved a mechanism for permanent inactivation of one of the X chromosomes presumably because a double  dose of X chromosome products would be lethal.

Gametes contain 23 chromosomes and 1N amount of DNA, so that when two gametes fuse at fertilization, a zygote containing 46 chromosomes and 2N amount of DNA is formed.

A sperm contains 22 autosomes and 1 sex chromosome. The sex chromosome in sperm may  be either the X chromosome or the Y chromosome. The sex chromosome in a secondary oocyte is only the X chromosome. If an X-bearing sperm fertilizes a secondary oocyte, a genetic female (XX) is produced. Therefore, sperm is the arbiter of sex determination.

During early fetal life, oogonia undergo mitotic divisions to populate the developing ovary.
All the oogonia subsequently give rise to primary oocytes by month 5 of fetal life; at birth, no oogonia are present in the ovary. At birth, a female has her entire supply of primary oocytes to carry her through reproductive life.

Over her reproductive life, a woman will ovulate approximately 480 oocytes. A woman will
ovulate 12 primary oocytes per year, provided that she is not using oral contraceptives, does not  become pregnant, or does not have any anovulatory cycles. Assuming a 40 year reproductive period gives 40 × 12 = 480.

Primary oocytes are formed by month 5 of fetal life and remain dormant until puberty, when
hormonal changes in the young woman stimulate the ovarian and menstrual cycles. From 5 to 15 oocytes will then begin maturation with each ovarian cycle throughout the woman’s reproductive life.

Primordial germ cells, the predecessors to gametes, are first seen in the wall of the yolk sac at
week 4 of embryonic development, and they migrate into the gonads at week 6.

All primary oocytes are formed by month 5 of fetal life, so no oogonia are present at birth.

Synapsis (pairing of homologous chromosomes) is a unique event that occurs only during
meiosis I in the production of gametes. Synapsis is necessary so that crossing over, whereby
large segments of DNA are exchanged, can occur.