Question

1. In which phase of the cell cycle is DNA replicated? In which phase (of the cell cycle) does the nucleus divide?

2. The toes cell of an organism that belongs to an unidentified sexually reproducing animal contains 42 chromosomes; based on this information

A. A cell of this organism formed upon completion of meiosis I contains _______ chromosomes.

B. Each toe cell of the unidentified animal contains _____ sister chromatids.

C. In Anaphase I, _____ pairs of homologous chromosomes will line up in the middle of the sex cell of the organism described above.

3. List two ways in which mitosis differs from meiosis.

4. Explain the process of ‘crossing over,’ and include the essence of the process.

5. Explain the importance of checkpoints in the cell cycle, and their relevance.

Answer 1

Answer 1:

DNA is replicated in S phase of cell cycle resulting in the duplication of each chromosome.

Nucleus division takes place in second growth phase thai is M phase where it divides through mitosis.

Answer 2:

A. Meiosis I halves the number of chromosomes, So the number of chromosomes will be 21 ( n=21)

B. Each toe cell of the unidentified animal will contain 42 chromatids. This is because each chromosome contains 2 chromatids.

C:

In Anaphase I, 21 pairs of homologous chromosomes will line up in the middle of the sex cell of the organism.

Entry into Anaphase I is related with two events which together results in the separation of the homologous chromosome from one another.

The sister chromatid cohesion is lost along the arms of the chromosome and the chiasma between the homologs are resolved. Together, these events result in the separation of homologs chromosome from one another.

Answer 3:

Meiosis	Mitosis
This occurs in se* cells. In this division number of chromosome are reduced by half producing two haploid cells	It happens in all cells except gametes ( Se*) cells. Number of chromosome remains same at the end of the mitosis.
Meiosis produces different cells or organism.	Mitosis produces identical cells or organisms.

Answer 4:

Crossing over happens between prophase I and metaphase I of meiosis. Genetic exchange works constantly to blend and rearrange DNA most obviously during meiosis when homologous chromosome pair prior to the first nuclear division. During this pairing, genetic exchange between the chromosome occurs. This exchange is termed as crossing over.

It involves the physical exchange of DNA sequences between the chromosome.

Crossing over is vital to the genetic diversity of a species. This provides a buffer against genetic defects, susceptibility to disease and survival of possible extinction events, as there will always be certain individuals in a population better able to survive changes in environmental condition.