PLEASE HELP Which of the following statements are true? a. In meiosis, the chrom

Question

PLEASE HELP

Which of the following statements are true?

a. In meiosis, the chromosomal material doubles during the last premeiotic interphase.

b. The greater the number of chromosome pairs, the greater is the effect of meiosis in producing genetically distinct gametes.

c. The diploid number of chromosomes is half the haploid number.

d. A cell that will not divide again remains in the G­2 stage of interphase.

e. The correct sequence of events during meiosis is: synapsis dyad formation chiasma formation

f. In an organism with n = 3, there is a 1 in 8 probability of a gamete with all maternally derived chromosomes contributing to the production of a zygote.

g. An organism with n = 3 can produce 8 different kinds of gametes in relation to combinations of maternal and paternal chromosomes.

h. Separation of sister chromatids occurs during anaphase I of meiosis and is the physical basis for the law of independent assortment.

i. A single diploid cell that undergoes meiosis in the male testis will produce 4 genetically different gametes if crossing-over is ignored.

j. The independent alignment of different tetrads at metaphase I of meiosis is the physical basis of the law of independent assortment.

k. If a diploid cell has 20 pairs of chromosomes, there will be 10 tetrads during prophase I of meiosis.

l. If a diploid cell has 20 pairs of chromosomes, there will be 20 bivalents during prophase I of meiosis.

m. If a diploid cell has 20 pairs of chromosomes, there will be 80 chromatids present during metaphase of mitosis.

n. If a diploid cell has 20 pairs of chromosmes, the organism can make 202 types of gametes if crossing-over is ignored.

o. Segregation of sister chromatids occurs during anaphase I of meiosis and is the physical basis of the law of segregation.

p. For an individual with a diploid number of 30 chromosomes, there will be 60 chromatids at metaphase I of meiosis.

q. For an individual with a diploid number of 30 chromosomes, there will be 15 tetrads at metaphase II of meiosis.

Explanation / Answer

a.   In meiosis, the chromosomal material doubles during the last premeiotic interphase.
Ans:True
Explanation:
Interphase is divided into three phases:
Growth 1 (G1) phase: In this very active phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In G1, each of the chromosomes consists of a single linear molecule of DNA.
Synthesis (S) phase: The genetic material is replicated; each of the cell's chromosomes duplicates to become two identical sister chromatids attached at a centromere. This replication does not change the ploidy of the cell since the centromere number remains the same. The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope. This will take place during prophase I in meiosis.
Growth 2 (G2) phase: G2 phase as seen before mitosis is not present in meiosis. The first four stages of prophase I in many respects correspond to the G2 phase of mitotic cell cycle.

b.   The greater the number of chromosome pairs, the greater is the effect of meiosis in producing genetically distinct gametes.
Ans:False
c.   The diploid number of chromosomes is half the haploid number.
Ans:True
d.   A cell that will not divide again remains in the G¬2 stage of interphase.
Ans:False
e.   The correct sequence of events during meiosis is: synapsis dyad formation chiasma formation
Ans:False

Explanation:
Prophase I is the longest phase of meiosis. During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination. This often results in chromosomal crossover. This process is critical for pairing between homologous chromosomes and hence for accurate segregation of the chromosomes at the first meiosis division. The new combinations of DNA created during crossover are a significant source of genetic variation, and result in new combinations of alleles, which may be beneficial. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. The process of pairing the homologous chromosomes is called synapsis. At this stage, non-sister chromatids may cross-over at points called chiasmata.

•When the chromosomes first become visible they are already doubled, each homologue having been duplicated during the preceding S phase.
•Result: pairs of homologous dyads each dyad consisting of two sister chromatids held together by a protein complex called cohesin.
•Pairing: Each pair of homologous dyads align lengthwise with each other.
•Result: a tetrad. (These structures are sometimes referred to as bivalents because at this stage you cannot distinguish the individual sister chromatids under the microscope.)
•The two homologous dyads are held together by
oone or more chiasmata (sing. = chiasma) which form between two nonsister chromatids at points where they have crossed over.
othe synaptonemal complex (SC), a complex assembly of proteins (including cohesin)

f.In an organism with n = 3, there is a 1 in 8 probability of a gamete with all maternally derived chromosomes contributing to the production of a zygote.
Ans:False
g.An organism with n = 3 can produce 8 different kinds of gametes in relation to combinations of maternal and paternal chromosomes.
Ans:True

h.Separation of sister chromatids occurs during anaphase I of meiosis and is the physical basis for the law of independent assortment.
Ans:false
Explanation:
Kinetochore microtubules shorten, pulling homologous chromosomes (which consist of a pair of sister chromatids) to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates in preparation for division down the center. Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected. This allows the sister chromatids to remain together while homologs are segregated.

i.A single diploid cell that undergoes meiosis in the male testis will produce 4 genetically different gametes if crossing-over is ignored.

Ans:false

j.The independent alignment of different tetrads at metaphase I of meiosis is the physical basis of the law of independent assortment.
Ans:True
Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both centrosomes attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line.The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. In mitosis, the force of kinetochore microtubules pulling in opposite directions creates tension. The cell senses this tension and does not progress with anaphase until all the chromosomes are properly bi-oriented. In meiosis, establishing tension requires at least one crossover per chromosome pair in addition to cohesin between sister chromatids.

k. If a diploid cell has 20 pairs of chromosomes, there will be 10 tetrads during prophase I of meiosis.
Ans:True
Prophase of meiosis I (prophase I) is a more elaborate process than prophase of mitosis .
When the chromosomes first become visible they are already doubled, each homologue having been duplicated during the preceding S phase.
•Result: pairs of homologous dyads each dyad consisting of two sister chromatids held together by a protein complex called cohesin.
•Pairing: Each pair of homologous dyads align lengthwise with each other.
•Result: a tetrad. (These structures are sometimes referred to as bivalents because at this stage you cannot distinguish the individual sister chromatids under the microscope.)
•The two homologous dyads are held together by
oone or more chiasmata (sing. = chiasma) which form between two nonsister chromatids at points where they have crossed over.
othe synaptonemal complex (SC), a complex assembly of proteins (including cohesin)
At metaphase I, microtubules of the spindle fibers attach to the
•sister kinetochores of one homologue, pulling both sister chromatids toward one pole of the cell;
•sister kinetochores of the other homologue pulling those sisters toward the opposite pole.

l.If a diploid cell has 20 pairs of chromosomes, there will be 20 bivalents during prophase I of meiosis.
Ans:True

m. If a diploid cell has 20 pairs of chromosomes, there will be 80 chromatids present during metaphase of mitosis.
Ans:Flase
Explanation: pairs of homologous dyads each dyad consisting of two sister chromatids held together by a protein complex called cohesin.
n. If a diploid cell has 20 pairs of chromosmes, the organism can make 202 types of gametes if crossing-over is ignored.
Ans:false
o. Segregation of sister chromatids occurs during anaphase I of meiosis and is the physical basis of the law of segregation.
Ans:false

p. For an individual with a diploid number of 30 chromosomes, there will be 60 chromatids at metaphase I of meiosis.
Ans:True
q. For an individual with a diploid number of 30 chromosomes, there will be 15 tetrads at metaphase II of meiosis.
Ans:True

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