49 The Eukaryotic Cell Cycle

Key Idea: The eukaryotic cell cycle can be divided into phases, although the process is continuous. Specific cellular events occur in each phase.

The life cycle of a eukaryotic cell is called the cell cycle. The cell cycle can be divided into two broad phases; interphase and M phase. Specific activities occur in each phase.

Interphase

Cells spend most of their time in interphase. Interphase is divided into three stages: ` The first gap phase (G1). ` The S-phase (S). ` The second gap phase (G2). During interphase the cell increases in size, carries out its normal activities, and replicates its DNA in preparation for cell division. Interphase is not a stage in mitosis.

First gap phase (G1): Cell increases in size and makes the mRNA and proteins needed for DNA replication.

Cytokinesis: Cytoplasm divides and the two cells separate. It is distinct from mitosis. S (synthesis) phase: DNA replication, the chromosomes are duplicated.

G1

grow Interphase

S replicate DNA

G2 grow and prepare for division

Mitosis

Mitosis and cytokinesis (M-phase)

Mitosis and cytokinesis occur during M-phase. During mitosis, the cell nucleus (containing the replicated DNA) divides in two equal parts. Cytokinesis occurs at the end of M-phase. During cytokinesis the cell cytoplasm divides, and two new daughter cells are produced.

Mitosis: Nuclear division Second gap phase (G2): Rapid cell growth and protein synthesis. Cell prepares for mitosis.

During interphase, the cell grows and acquires the materials needed to undergo mitosis. It also prepares the nuclear material for separation by replicating it. During interphase the nuclear material is unwound. As mitosis approaches, the nuclear material begins to reorganise in readiness for nuclear division. During mitosis the chromosomes are separated. Mitosis is a highly organised process and the cell must pass checkpoints before it proceeds to the next phase.

1. Briefly outline what occurs during the following phases of the cell cycle:

(a) Interphase:

(b) Mitosis:

(c) Cytokinesis: