What is the primary difference between the $G_1$ and $G_2$ phases of interphase?

In $G_1$, the cell prepares for DNA replication by synthesizing enzymes and growing. In $G_2$, the DNA has already been replicated, and the cell focuses on error-checking that DNA and producing proteins like tubulin for the mitotic spindle.

How does cytokinesis differ between animal and plant cells?

Animal cells undergo cytokinesis via the constriction of the cytoplasm (cleavage furrow). Plant cells cannot constrict due to their rigid cell wall, so they form a cell plate at the equator using Golgi vesicles to build a new wall from within.

What is the difference between mitosis and the M phase of the cell cycle?

Mitosis refers specifically to the division of the nucleus into two identical nuclei. The M phase is a broader term that encompasses both mitosis (nuclear division) and cytokinesis (the physical division of the entire cell).

Why is it a mistake to say the chromosome number doubles during the S phase?

While the DNA mass doubles during the S phase, the chromosome number remains the same. This is because the replicated DNA strands (sister chromatids) stay attached at the centromere, and a 'chromosome' is defined by the presence of a single centromere.

What happens if a cell fails the $G_1$ checkpoint?

If DNA damage is detected at the $G_1$ checkpoint, the cell will not advance to the S phase. It will either enter a non-dividing state to attempt repairs or undergo apoptosis (cell suicide) to prevent the replication of damaged genetic code.

Why is interphase often incorrectly described as a 'resting phase'?

Interphase is highly active metabolically; the cell is growing, synthesizing proteins, and replicating its entire genome. It only appears 'resting' because the dramatic movements of chromosomes seen in mitosis are not yet visible.

Define 'Cyclins' and their role in the cell cycle.

Cyclins are a family of proteins that act as chemical signals to trigger the transition from one phase of the cell cycle to the next. Their concentrations fluctuate to ensure the cycle proceeds in the correct order and only when the cell is ready.

What is the 'S phase' of the cell cycle?

The S phase, or Synthesis phase, is the specific period during interphase when DNA replication occurs. This ensures that each daughter cell will receive a complete and identical set of genetic instructions.

What is a 'Checkpoint' in the context of cell biology?

A checkpoint is a critical control point in the cell cycle where stop and go-ahead signals regulate the cycle. They ensure that essential processes, such as DNA replication or spindle attachment, are completed accurately before the cell proceeds.

Why must the cell cycle be strictly regulated?

Strict regulation ensures that DNA is copied without errors and distributed equally. Without this control, cells could divide with mutations or incorrect chromosome numbers, leading to dysfunctional tissues or cancerous growths.

The Cell Cycle | OCR A-Level Biology

2. Foundations in Biology

The Cell Cycle

Summary

The cell cycle is a highly regulated, continuous sequence of events that a cell undergoes between one division and the next. It ensures that a cell grows sufficiently, replicates its genetic material accurately, and divides its contents into two genetically identical daughter cells.

1. Definition & Core Concepts

The cell cycle is the regulated sequence of growth and division stages that occurs in eukaryotic cells. It is not merely a division process but a comprehensive life cycle that prepares the cell for successful reproduction.
The cycle is divided into three primary phases: interphase, nuclear division (mitosis), and cell division (cytokinesis). While mitosis is the most visible part under a microscope, it represents only a small fraction of the total cycle time.
The duration of the cycle varies significantly between organisms and cell types. For instance, rapidly dividing epithelial cells may complete a cycle in half the time required by specialized plant tissue cells, depending on environmental conditions and internal signals.

A circular diagram of the cell cycle showing G1, S, G2 (Interphase) and the M phase (Mitosis and Cytokinesis).

2. The Phases of Interphase

G1 Phase (First Gap): This is the initial growth phase where the cell increases in mass and size. During this time, the cell actively synthesizes RNA, enzymes, and proteins required for subsequent DNA replication.
S Phase (Synthesis): This is a critical, relatively short period where DNA replication occurs. Every chromosome is copied, resulting in two identical sister chromatids joined at a centromere, effectively doubling the DNA content of the nucleus.
G2 Phase (Second Gap): Following DNA synthesis, the cell continues to grow and prepares for division. It synthesizes tubulin proteins for microtubule formation and performs a vital check of the newly replicated DNA for errors.

3. Regulation & Checkpoints

4. Key Distinctions

Animal vs. Plant Cytokinesis

Animal Cells: Cytokinesis occurs through the constriction of the cytoplasm. A cleavage furrow forms as the cell membrane pinches inward between the two new nuclei until the cell is split in two.
Plant Cells: Because of the rigid cell wall, constriction is impossible. Instead, vesicles from the Golgi apparatus align at the equator to form a cell plate, which eventually develops into a new primary cell wall.

DNA Mass vs. Chromosome Number

During the $S$ phase, the mass of DNA doubles because every molecule is replicated. However, the chromosome number remains the same because the identical copies (chromatids) remain attached at the centromere and are counted as a single unit.

5. Exam Strategy & Tips