BRS Embryology - Prefertilization Flashcards Preview

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Flashcards in BRS Embryology - Prefertilization Deck (18)
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1

Which of the following is a major characteristic of meiosis I?

(A) Splitting of the centromere

(B) Pairing of homologous chromosomes

(C) Reducing the amount of DNA to 1N

(D) Achieving the diploid number of chromosomes

(E) Producing primordial germ cells

B.

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

2

A normal somatic cell contains a total of 46 chromosomes. What is the normal com- plement of chromosomes found in a sperm?

(A) 22 autosomes plus a sex chromosome

(B) 23 autosomes plus a sex chromosome

(C) 22autosomes

(D) 23 autosomes

(E) 23pairedautosomes

A.

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

3

Which of the following describes the number of chromosomes and amount of DNA in a gamete?

(A) 46 chromosomes, 1N

(B) 46 chromosomes, 2N

(C) 23chromosomes,1N

(D) 23 chromosomes, 2N

(E)  23 chromosomes, 4N

C.

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.

4

Which of the following chromosome compositions in a sperm normally results in the production of a genetic female if fertilization occurs?

(A) 23 homologous pairs of chromosomes

(B) 22 homologous pairs of chromosomes

(C) 23 autosomes plus anXchromosome

(D) 22 autosomes plus a Y chromosome

(E) 22 autosomes plus an X chromosome

E.

A sperm contains 22 autosomes and 1 sex chromosome. The sex chromosome in sperm may be either the X 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.

5

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?

(A) A duplicated chromosome

(B) Two chromosomes

(C) A synapsed chromosome

(D) A crossover chromosome

(E) A homologous pair

A.

The structure formed is a duplicated chromosome. DNA replication occurs, so that the amount of DNA is doubled (2

6

All primary oocytes are formed by

(A) week 4 of embryonic life

(B) month 5 of fetal life

(C) birth

(D) month 5 of infancy

(E) puberty

B.

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.

7

When does formation of primary spermatocytes begin?

(A) During week 4 of embryonic life

(B) During month 5 of fetal life

(C) At birth

(D) During month 5 of infancy

(E) At puberty

E.

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 sperma- tocytes by undergoing DNA replication.

8

In the production of female gametes, whichofthefollowingcellscanremaindor- mant for 12–40 years?

(A) Primordial germ cell

(B) Primary oocyte

(C) Secondaryoocyte

(D) First polar body

(E) Second polar body

B.

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.

9

In the production of male gametes, which of the following cells remains dormant for 12 years?

(A) Primordial germ cell

(B) Primary spermatocyte

(C) Secondaryspermatocyte

(D) Spermatid

(E) Sperm

A.

Primordial germ cells migrate from the wall of the yolk sac during the week 4 of embry- onic life and enter the gonad of a genetic male, where they remain dormant until puberty (about age 12 years), when hormonal changes in the young man stimulate the production of sperm.

10

Approximately how many sperm will be ejaculated by a normal fertile male during sexualintercourse?

(A) 10 million

(B) 20 million

(C) 35million

(D) 100 million

(E) 350million

E.

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

11

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?

(A) 40,000

(B) 35,000

(C) 480

(D) 48

(E) 12

C.

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

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?

(A) Inactivation of both X chromosomes

(B) Inactivation of homologous chromosomes

(C) InactivationofoneYchromosome

(D) Inactivation of one X chromosome

(E) Inactivation of one chromatid

D.

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.

13

How much DNA does a primary sperma- tocyte contain?

(A) 1N

(B) 2N

(C) 4N

(D) 6N

(E) 8N

C.

Type B spermatogonia give rise to primary spermatocytes by undergoing DNA replica-

tion, thereby doubling the amount of DNA (2

14

During meiosis, pairing of homologous chromosomes occurs, which permits large segments of DNA to be exchanged. What is this process called?

(A) Synapsis

(B) Nondisjunction

(C) Alignment

(D) Crossing over

(E) Disjunction

D.

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

15

During ovulation, the secondary oocyte resides at what specific stage of meiosis?

(A) Prophase of meiosis I

(B) Prophase of meiosis II

(C) MetaphaseofmeiosisI

(D) Metaphase of meiosis II

(E) Meiosis is completed at the time of ovulation

D.

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

16

Concerning maturation of the female gamete (oogenesis), when do the oogonia enter meiosis I and undergo DNA replication to form primary oocytes?

(A) During fetal life

(B) At birth

(C) At puberty

(D) With each ovarian cycle

(E) Following fertilization

A.

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

17

Where do primordial germ cells initially develop?

(A) In the gonads at week 4 of embryonic development

(B) In the yolk sac at week 4 of embryonic development

(C) In the gonads at month 5 of embryonic development

(D) In the yolk sac at month 5 of embryonic development

(E) In the gonads at puberty

B.

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.

18