Question

Number of chromatids in g1 and g2 respectively in a somatic cell with 2n = 20

Answer 1

Answer:

The number of chromatids in G$1$ and G$2$ respectively in a somatic cell with $2n = 20$ is $20$ and $40$.

Explanation:

A typical eukaryotic cell cycle includes Mitosis (M), G$1$, S, G$2$ phases. Karyokinesis and cytokinesis occur in the M phase. In the G$1$ phase, the cell grows continuously but no DNA replication will happen, so the chromosome number and amount of DNA will be the same. In the S phase, DNA synthesis or replication occurs, so the amount of DNA per cell doubles. But the chromosome remains the same as in G$1$. In G$2$, the cell checks the duplicated chromosome for error.

The number of chromatids doubles in DNA replication. So, in the S phase, there is an increase in the number of chromatids but not in the chromosomes. Therefore, the number of chromatids in G$2$ will be $40$ but the chromosome will be the same $2n$. That is $2n = 40$. While, In the G$1$, $2n = 20.$

Answer 2

Given:

A cell with 2n = 20

To Find:

The number of chromatids in the cell after G₁ and G₂ phase of the cell cycle

Solution:

What does '2n' signify?

• 2n refers to the number of chromosomes in a cell. It signifies that the cell is diploid, i.e, the chromosomes occur in pairs.
• Here 2n = 20 means that in a genome there are 10 chromosomes and there are 2 genomes.

What are chromatids?

• Each half of a chromosome is called a chromatid.
• In a chromosome, two sister chromatids are joined at the centromere.

The Cell Cycle

• The human cell completes a cycle in almost 24 hours.
• The cell cycle can be divided into phases:

1. Interphase - Preparatory Phase
2. M Phase - most active phase

• During the interphase, the cell goes through the following stages:

                                   G₁ ⇒ S ⇒ G₂

• The G₁ phase is characterized by the replication of some organelles.
• In the S phase, DNA synthesis/replication occurs but the number of chromosomes remains the same.
• During the G₂ phase, the remaining organelles replicate. Proteins are also synthesized as the cell prepares to enter the M phase.

Thus, the number of chromosomes during interphase remains unchanged.

∴ The number of chromosomes in G₁ = the number of chromosomes in G₂

Number of chromosomes ⇒ 2n = 20

Each chromosome is made of 2 chromatids

Hence, the number of chromatids in G₁ and G₂ = 20 × 2 = 40

Thus, in a somatic cell with 2n = 20, the number of chromatids in G₁ and G₂ is 40.