Problem 7 - Campbell Biology

Question

Cell A has half as much DNA as cells B, C, and D in a mitotically active tissue. Cell \(\mathrm{A}\) is most likely in (A) \(\mathrm{G}_{1}\) (C) prophase. (B) \(\mathrm{G}_{2}\) (D) metaphase.

Step-by-Step Solution

Answer

G1

Step 1: Understand the cell cycle phases

The cell cycle consists of the following phases: G1 (first gap phase), S (synthesis phase), G2 (second gap phase), and M (mitosis phase). DNA is replicated during the S phase.

Step 2: Analyze DNA content

G1 phase is before DNA synthesis, meaning the cell has a single set of chromosomes. After synthesis in S phase, the DNA content is doubled in G2, prophase, and metaphase phases.

Step 3: Compare DNA content with given information

Cells B, C, and D have double the DNA of Cell A. This implies that A is before the DNA replication phase.

Step 4: Determine the phase with given options

If Cell A has half the DNA, it must be in the G1 phase. Cells in G2, prophase, and metaphase have already completed DNA synthesis and have double the DNA.

Step 5: Verify the correct answer

Among the given options: (A) G1, (B) G2, (C) prophase, and (D) metaphase, the phase where the cell has half the DNA is G1.

Key Concepts

G1 phase

The G1 phase, also known as the first gap phase, is the initial phase of the cell cycle. During G1, the cell grows in size, producing RNA and synthesizing proteins necessary for DNA replication. It is a critical checkpoint where the cell decides whether to divide, delay division, or enter a non-dividing state, known as G0. Cells in G1 have a single set of chromosomes, meaning they have not yet undergone DNA replication. This is why, in the given exercise, Cell A, which has half the DNA content compared to other cells, is identified to be in the G1 phase.
Key characteristics of the G1 phase include:

• Cell growth and maturation
• RNA and protein synthesis
• Pre-replication checkpoint to ensure conditions are favorable for DNA synthesis

This phase is crucial for the cell to gather resources and energy needed for the upcoming DNA replication in the S phase.

DNA replication

DNA replication occurs during the S phase (synthesis phase) of the cell cycle. This is when the cell synthesizes a complete copy of the DNA, doubling its content. By the end of S phase, each chromosome consists of two sister chromatids connected by a centromere.
Key aspects of DNA replication include:

• Origin recognition and unwinding of the DNA helix
• Formation of a replication fork
• Synthesis of new DNA strands by DNA polymerase
• Proofreading and error correction

Accurate DNA replication is crucial as it ensures that each daughter cell receives an identical copy of the genetic material. In the context of the exercise, cells B, C, and D, which have double the DNA content, must have completed the S phase, indicating they are either in G2, prophase, or metaphase.

Mitosis

Mitosis is the process by which a eukaryotic cell divides its nucleus and chromatids to produce two genetically identical daughter cells. It follows the G2 phase and is divided into several stages, including prophase, metaphase, anaphase, and telophase.
During mitosis:

• Prophase: Chromosomes condense, and the mitotic spindle begins to form.
• Metaphase: Chromosomes line up at the cell's equatorial plane called the metaphase plate.
• Anaphase: Sister chromatids separate and move toward opposite poles of the cell.
• Telophase: Nuclear membranes re-form around the separated chromatids, now called chromosomes.

Finally, cytokinesis occurs, splitting the cytoplasm to create two daughter cells.
In the given exercise, if the cells are in mitotically active tissue, Cells B, C, and D with doubled DNA content are likely in the later stages of the cell cycle such as G2, prophase, or metaphase. This reinforces why Cell A, which has half the DNA content, is identified as being in the G1 phase.