Saturn Medic
Embryology (Prefertilization Events)
MCQ
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Fetal sex can be diagnosed by noting the presence or absence of the Barr body in cells obtained from the amniotic fluid. What is the etiology of the Barr body?
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.
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A normal somatic cell contains a total of 46 chromosomes. What is the normal complement of chromosomes found in a sperm?
A normal gamete (sperm in this case) contains 23 single chromosomes. These 23 chromosomes consist of 22 autosomes plus 1 sex chromosome.
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A young woman enters puberty with approximately 40,000 primary oocytes in her ovary. About how many of these primary oocytes will be ovulated over the entire reproductive life of the woman?
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.
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Which of the following is a major characteristic of meiosis I?
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.
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Which of the following describes the number of chromosomes and amount of DNA in a gamete?
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.
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Where do primordial germ cells initially develop?
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.
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Approximately how many sperm will be ejaculated by a normal fertile male during sexual intercourse?
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).
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During meiosis, pairing of homologous chromosomes occurs, which permits large segments of DNA to be exchanged. What is this process called?
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.
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In the production of female gametes, which of the following cells can remain dormant for 12 to 40 years?
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.
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All primary oocytes are formed by
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.
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Which of the following chromosome compositions in a sperm normally results in the production of a genetic female if fertilization occurs?
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.
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Concerning maturation of the female gamete (oogenesis), when do the oogonia enter meiosis I and undergo DNA replication to form primary oocytes?
All primary oocytes are formed by month 5 of fetal life, so no oogonia are present at birth.
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When does formation of primary spermatocytes begin?
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.
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In the process of meiosis, DNA replication of each chromosome occurs, forming a structure consisting of two sister chromatids attached to a single centromere. What is this structure?
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.
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During ovulation, the secondary oocyte resides at what specific stage of meiosis?
The secondary oocyte is arrested in metaphase of meiosis II about 3 hours before ovulation, and it remains in this meiotic stage until fertilization.
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How much DNA does a primary spermatocyte contain?
Type B spermatogonia give rise to primary spermatocytes by undergoing DNA replication, thereby doubling the amount of DNA (2 × 2N = 4N) within the cell.
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