2.1.6 Cell Division, Cell Diversity and Cellular Organisation

What is the cell cycle?

A regular sequence of events between one cell division & the next; 3 phases:

1. Interphase (G₁+S+G₂)

2. Mitosis (nuclear division)

3. Cytokinesis (cell division)

What happens during the G₁ phase of interphase?

G₁ phase (GROWTH):

• Cells produce RNA, enzymes & other proteins required for cell growth

• Cells receive signal to divide controlled by cyclins

What happens during the S phase of interphase?

S phase:

• DNA in nucleus replicates → each chromosome contains 2 identical sister chromatids

What happens during the G₂ phase of interphase?

G₂ phase (FURTHER CELL GROWTH):

• Newly synthesised DNA is checked for any errors

• Tubulin is made - protein needed for mitosis

Why is mitosis important?

Mitosis is essential for:

• Growth of multicellular organisms

• Replacement of cells & repair of tissues

• Asexual reproduction

What are the 4 stages of mitosis?

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

What happens during prophase?

• Chromatin condenses, forming chromosomes that are joined together at the centromere

• Centrosomes move to opposite sides → spindle fibres emerge

• Nuclear envelope breaks down into vesicles & nucleolus dissapears

What happens during metaphase?

• Chromosomes line up at the equator of the spindle (AKA metaphase plate)

• Spindle fibres attach to centromeres

What happens during anaphase?

• Sister chromatids separate at centromere

• Spindle fibres shorten & pull the separated sister chromatids to opposite poles

What happens during telophase?

• Chromosomes arrived at opposite poles & decondense, reforming chromatin

• Nuclear envelope reforms around each set of chromosomes

• Spindle fibres break down

What is cytokinesis?

The physical separation of the parent cell into two genetically identical daughter cells

How does cytokinesis occur in animals?

The cytoskeleton pulls the cell surface membrane inwards, causing it to invaginate (pinch in)

→ Causes cleavage furrow to forms, until cell membrane is pinched off to form 2 new cells

How does cytokinesis occur in plants?

Vesicles from the Golgi apparatus line up along the metaphase plate, then fuse with each other & the cell surface membrane, to form a new cell surface membrane in the middle of the cell (cell plate)

→ New sections of the cell wall are produced, separating the new daughter cells

How can the cell cycle be regulated?

Includes checkpoints where proofreading & repair enzymes check & correct any errors, to avoid any mutations

What are the 4 types of checkpoints in the cell cycle?

• G₁ checkpoint

• S phase checkpoint

• G₂ checkpoint

• Metaphase checkpoint

What happens at the G₁ checkpoint?

Checks chromosomes for damage before entering S phase

What happens at the S phase checkpoint?

Ensures all DNA has been successfully replicated. If not, cell cycle stops

What happens at the G₂ checkpoint?

Checks replicated DNA for any damage; cycle pauses until repairs occur

What happens at the metaphase checkpoint?

Confirms chromosomes are correctly attched to spindle fibres before anaphase