Question
Asked Mar 9, 2020
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Answer the questions that follow for each stage of the
cell cycle (G1, S, G2, prophase, metaphase, anaphase,
telophase). If necessary, use an arrow to indicate a
change that occurs during a particular cell cycle stage
(for example, 1 → 2 or yes → no).
a. How many chromatids make up each chromosome
during this stage?
b. Is the nucleolus present?
c. Is the mitotic spindle organized?
d. Is the nuclear membrane present?

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Expert Answer

Step 1

The cell cycle is a process through which the DNA of a cell gets replicated and division of the cell takes place. I could be either through mitosis or meiosis. There are certain phases of a cell cycle as follows: -

1. I-phase :- in this phase cell prepares itself for replication and division.

  • G1 (gap1) phase: - The cell prepares itself for DNA duplication by increasing its size.
  • S (synthesis) phase: - in this phase genetic material gets doubles but the chromosome number remains the same. Chromosome acquires an X shape.
  • G2 (gap2) phase: - in this phase cell prepares itself for M-phase by doubling its content

2. M phase

  • Mitosis: - in this type of cell division the number of chromosomes remains the same in daughter cells as of the parents.
  • Meiosis: - in this type of cell division number of chromosomes gets reduced by half in the daughter cell as of the parent cells.
Step 2

a) Let the number of chromosomes in a diploid cell be 2n. Hence the number of chromatids in:-

  • G1 phase – chromosomes – 2n, chromatids - 2n
  • S phase – chromosomes – 2n, chromatids - 2n
  • S phase → G2 phase – chromosomes - 2n, chromatids – 4n

mitosis

  • Prophase – chromosomes - 2n, chromatids – 4n
  • Metaphase – chromosomes - 2n, chromatids – 4n
  • Anaphase – chromosomes – 4n, chromatids – 4n (each chromatid now considered as a chromosome)
  • Telophase - chromosomes – 4n, chromatids – 4n (each chromatid now considered as a chromosome)
  • Telophase → cytokinesis - chromosomes – 2n chromatids – 2n

meiosis I

  • prophase I - chromosomes - 2n, chromatids - 4n
  • metaphase I - chromosomes - 2n, chromatids - 4n
  • anaphase I - chromosomes - 2n, chromatids - 4n
  • telophase I - chromosomes - 2n, chromatids - 4n

meiosis II

meiosis I → meiosis II - chromosomes - n, chromatids - 2n

  • prophase II - chromosomes - n, chromatids - 2n
  • metaphase II - chromosomes - n, chromatids - 2n
  • anphase II - chromosomes - 2n. chromatids - 2n (each sister chromatid considered as a chromosome)
  • telophase II - chromosomes - 2n, chromatids - 2n
  • Telophase → cytokinesis - chromosomes – n, chromatids – n
Step 3

 b) Nucleolus starts disappearing during the initiation of cell division and reappears at the end of cell division.

  • G1 phase – yes
  • S phase – yes
  • G2 phase – yes
  • G2 phase → prophase &...

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