Question

Chromosomes are duplicated during what portion of the cell cycle? a. \(\mathrm{G}_{1}\) phase b. S phase c. prophase d. prometaphase

Short answer

b. S phase

Step-by-step solution

Overview of the Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide. It is divided into two main parts: interphase and the mitotic phase. Interphase consists of three sub-phases: the G1 phase (Gap 1), the S phase (Synthesis), and the G2 phase (Gap 2). The mitotic phase involves the actual division of the cell and consists of prophase, prometaphase, metaphase, anaphase, and telophase, followed by cytokinesis.

G1 Phase

During the G1 phase, the cell grows and prepares for DNA replication by synthesizing necessary proteins and organelles. Chromosome duplication does not occur in this phase.

S Phase

The S phase is the part of the cell cycle when DNA replication occurs. Each single-stranded DNA molecule is copied to produce two identical double-stranded DNA molecules, which will become the two chromatids of a duplicated chromosome.

G2 Phase

During the G2 phase, the cell continues to grow, produces necessary proteins, and prepares for mitosis. Chromosome duplication is already completed at this point.

Prophase

Prophase is the first stage of mitosis, during which the chromatin in the nucleus condenses to form visible chromosomes. Chromosome duplication has already occurred before entering this phase.

Prometaphase

Prometaphase is the second stage of mitosis, during which the nuclear envelope breaks down and spindle fibers begin to attach to chromosomes. At this stage, chromosomes have already been duplicated.

Identifying the Correct Answer

Now that we've reviewed each phase, it is clear that chromosomes are duplicated during the S phase of the cell cycle. Therefore, the correct answer is: b. S phase

Key concepts

DNA replication

DNA replication is a fundamental biological process occurring during the S phase of the cell cycle. The purpose of DNA replication is to ensure that each new cell receives an exact copy of the parent cell's genetic material.

During replication, the double helix structure of DNA unwinds, and enzymes like DNA helicase break the hydrogen bonds between the base pairs. This creates two single strands that serve as templates. Another enzyme, DNA polymerase, then adds nucleotides to each template strand according to the base pairing rules—adenine with thymine and cytosine with guanine—resulting in two identical DNA molecules.

Significance of Accuracy in DNA Replication

DNA replication must be highly accurate to prevent mutations that could lead to diseases or cellular dysfunction. The proofreading abilities of DNA polymerase play a crucial role in ensuring the fidelity of this process.

Replication Forks and Replication Bubbles

Replication occurs at several points in the DNA molecule simultaneously, creating replication forks that extend outward and form 'replication bubbles'. These allow replication to proceed quickly and efficiently within the nucleus.

S phase

The S phase, or synthesis phase, is the period of the cell cycle during which DNA replication occurs. It is characterized by several critical events that ensure the doubling of the cell's genetic material before cell division.

In addition to DNA replication, the S phase involves repair and synthesis of histone proteins, which help in packaging the newly replicated DNA into chromatin. As the phase progresses, each chromosome now possesses two sister chromatids, connected at a region known as the centromere.

Preparation for Cell Division

Completing the S phase is very important as it prepares the cell for the complex process of mitosis. Without successful DNA replication, cell division would result in daughter cells with incomplete or damaged DNA, which can lead to serious consequences for organismal growth and development.

Chromosome duplication

Chromosome duplication refers to the process by which a cell copies its entire set of chromosomes during the S phase, producing sister chromatids for each original chromosome. This is essential because, during cell division, each new cell needs a complete set of chromosomes to function properly.

After replication, the sister chromatids are held together by cohesin proteins at the centromere until they are separated into two new cells during mitosis. The mechanism ensures that each daughter cell inherits the same genetic information as the parent cell.

Role in Genetic Stability

Accurate chromosome duplication and segregation are critical for maintaining genetic stability. Errors in these processes can result in aneuploidy, where cells have an abnormal number of chromosomes, leading to developmental disorders or diseases like cancer.