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) \(0.5 x\). (C) \(x\). (D) \(2 x .\)

Step-by-Step Solution

Answer

The DNA content at metaphase of meiosis I is \(2x\). Thus, the correct answer is (D).

Step 1: Understand the \(\text{G}_{1}\) Phase

In the \(\text{G}_{1}\) phase of the cell cycle, the cell is diploid (2n) and the DNA content is denoted as \(x\). This phase is before DNA replication.

Step 2: DNA Replication

During the S phase, which follows the \(\text{G}_{1}\) phase, DNA replication occurs. The DNA content of the cell doubles from \(x\) to \(2x\). The cell, however, remains diploid with duplicated chromosomes.

Step 3: Entering Meiosis I

After DNA replication, the cell enters meiosis I. However, unlike mitosis, meiosis involves two consecutive divisions. The first division is meiosis I, where homologous chromosomes pair and then segregate.

Step 4: Metaphase of Meiosis I

During metaphase of meiosis I, chromosomes align at the metaphase plate. Each chromosome consists of two sister chromatids, meaning the DNA content is still \(2x\) since replication has already doubled it during the S phase.

Step 5: Determine DNA Content

Given that the DNA content was \(x\) during \(\text{G}_{1}\), after replication and during metaphase of meiosis I, the DNA content remains \(2x\).

Key Concepts

G1 phase

The G1 phase, also known as the Gap 1 phase, is a part of the cell cycle where the cell grows and prepares for DNA replication. In this phase, the cell is diploid (containing two sets of chromosomes) and has a DNA content designated as x. This phase occurs just after a cell has divided and before it begins to replicate its DNA. It's a crucial stage where the cell checks for any damage to the DNA and ensures that it has the necessary resources to proceed with replication. Importantly, during the G1 phase, no DNA replication occurs yet, and the cell's primary aim is growth and preparation.

DNA replication

DNA replication is a fundamental biological process occurring during the S phase (Synthesis phase) of the cell cycle. Following the G1 phase, the cell enters the S phase where its entire DNA content is copied. This replication doubles the amount of DNA, changing the DNA content from x to 2x. However, even though the DNA amount doubles, the cell remains diploid. Each chromosome now consists of two identical sister chromatids joined at a point called the centromere. This process ensures that when the cell eventually divides, each daughter cell receives an accurate copy of the original genome.

Metaphase of meiosis I

During meiosis I, the cell undergoes a special type of cell division to produce gametes (sperm or eggs) with half the number of chromosomes of the original cell. In metaphase of meiosis I, homologous chromosomes (each consisting of two sister chromatids) align at the metaphase plate. It's important to understand that each chromosome is still duplicated, meaning the total DNA content is 2x. This alignment is crucial for the accurate separation of homologous chromosomes into two new cells. This stage ensures genetic diversity through the distribution of maternal and paternal chromosomes to each gamete.

Cell cycle phases

The cell cycle is a series of phases that a cell goes through as it grows and divides. It consists of four primary phases:

• G1 phase: Growth and preparation for DNA replication.
• S phase: DNA replication, doubling the DNA content.
• G2 phase: Further growth and preparation for mitosis or meiosis.
• M phase: Mitosis or meiosis, where the cell divides.

Additionally, there is a resting phase known as the G0 phase, where the cell is not actively preparing to divide. In the context of meiosis, these phases help prepare and successfully carry out the division that leads to the formation of haploid gametes. Understanding these phases is key to comprehending how cells manage growth, replication, and division accurately.