Problem 4 - Campbell Biology

Question

If the DNA content of a diploid cell in the \(\mathrm{G}_{1}\) phase of the cell cycle is \(x,\) then the DNA content of the same cell at metaphase of meiosis I will be (A) 0.25\(x\) (B) 0.5\(x\) \((\mathrm{C}) x\) \((\mathrm{D}) 2 x\)

Step-by-Step Solution

Answer

The DNA content at metaphase of meiosis I is \(2x\).

Step 1 - Understand the cell cycle phases

A diploid cell in the \(\mathrm{G}_{1}\) phase has a DNA content of \(x\). In this phase, the cell has two sets of chromosomes (one set from each parent).

Step 2 - Identify DNA replication phase

In the \(\mathrm{S}\) phase (Synthesis phase) of the cell cycle, the DNA content duplicates. Therefore, the DNA content becomes \(2x\).

Step 3 - Check metaphase of meiosis I

During metaphase of meiosis I, cells have already replicated their DNA in the \(\mathrm{S}\) phase. The DNA content is still \(2x\) because the chromosomes are aligned but not yet separated.

Step 4 - Conclusion

The DNA content of a diploid cell at metaphase of meiosis I is \(2x\). The correct choice is (D).

Key Concepts

Cell Cycle

The cell cycle is a series of phases that cells go through to grow and divide. It consists of several stages:

• G1 Phase: This is the first gap phase where the cell grows and carries out normal functions.
• S Phase: The synthesis phase where DNA is replicated, doubling its content.
• G2 Phase: The second gap phase where the cell prepares for mitosis or meiosis.
• M Phase: The mitosis or meiosis phase where cell division occurs and creates two daughter cells.

Understanding these phases is crucial as they explain the changes that happen to the DNA content during the cell's life cycle.

Diploid Cells

Diploid cells contain two sets of chromosomes, one from each parent. In humans, diploid cells have 46 chromosomes in total. These cells go through the cell cycle to grow and divide.
Before a diploid cell enters the S phase of the cycle, its DNA content is denoted as 'x' during the G1 phase. This is an important baseline for understanding changes during meiosis and mitosis.

Metaphase of Meiosis I

Metaphase of meiosis I is a specific stage during cell division where chromosomes align at the cell's equatorial plane. Here, homologous chromosomes (pairs of similar chromosomes, one from each parent) are lined up next to each other.
The key point is that the cell has already replicated its DNA during the S phase. As a result, each homologous chromosome pair forms a tetrad with duplicated DNA, keeping the content at 2x until they are separated.

DNA Replication

DNA replication is a critical process that occurs during the S phase of the cell cycle. During this time:

• Enzymes unwind the DNA double helix.
• Each strand serves as a template for the formation of a new complementary strand.
• This results in two identical daughter DNA molecules, effectively doubling the DNA content to 2x in a diploid cell.

This doubling is essential for the cell to later divide and pass on the correct amount of genetic material to its daughter cells.

S Phase

The S Phase, or synthesis phase, is the part of the cell cycle where DNA replication occurs. In this phase:

• The cell's DNA content increases from 'x' to '2x' as each chromosome is duplicated.
• Once DNA replication is complete, the cell enters the G2 phase, preparing for the division processes of mitosis or meiosis.
• This duplication is crucial for ensuring that each daughter cell receives a complete set of chromosomes during cell division.