Question

If a diploid cell has 4 chromosomes at interphase (before S phase), how many chromosomes AND sister chromatids will it have at each of the following phases?

Prophase 1

Metaphase 1

Anaphase 1

Telephase 1

Prophase 2

Metaphase 2

Anaphase 2

Telephase 2

Answer 1

The cell cycle is divided into two basic phases

• interphase
• M phase (meiosis/mitosis phase)

interphase is further divided into the following phases:

• G₁ phase (gap 1)
• S phase (synthesis)
• G_{2 phase} (gap 2)

G₁ phase corresponds to the interval between meiosis and initation of DNA replication. During this phase the cell is metabolically active but does not replicate its DNA.

DNA is replicated during the S phase in which the amount of DNA per cell doubles but the chromosome number remains same.

Chromosomes are single stranded structures formed by the condensation of chromatin(complexes of small positively charged histone proteins and negatively charged DNA wrapped around it).

During the S phase, the single stranded chromosomes duplicate to form an X-shaped structure, known as the sister chromatids.

During the G₂ phase, proteins are synthesised in preparation of meiosis while cell growth continues.

Meiosis involves two sequential cycles of nuclear and cell division, known as Meiosis I and Meiosis II. Meiosis I is only initiated after the replication of parental chromosomes(homologous chromosomes). The homologous chromosomes pair and recombination occurs between them.

Number of chromosomes and sister chromatids during Meiosis I

MEIOSIS I	NO. OF CHROMOSOMES	NO. OF SISTER CHROMATIDS
PROPHASE I	4	8
METAPHASE I	4	8
ANAPHASE I	4	8
TELOPHASE I	4	8

No. of chromosomes and sister chromatids during meiosis II

MEIOSIS II	NO. OF CHROMOSOMES	NO. OF SISTER CHROMATIDS
PROPHASE II	2	2
METAPHASE II	2	2
ANAPHASE II	2	2
TELOPHASE II	2	2

Since meiosis is also known as reductional division, the number of cells formed at the end of meiosis II are 4 which are haploid (n) i.e., chromosome number becomes half in each cell.