1.6 Cell Division

WJEC A-Level Biology

Structure of a chromosome

How many pairs of homologous chromosomes do humans have?

23

Main features of mitosis

• produces two identical daughter cells

• maintains chromosome number throughout

• allows growth and repair

• asexual reproduction

What can uncontrolled mitosis lead to?

Growth of cancerous cells

What are the 6 main stages of mitosis?

1. Interphase

2. Prophase

3. Metaphase

4. Anaphase

5. Telophase

6. Cytokinesis

What happens when a cell is in interphase?

• forms new organelles e.g. centrioles

• DNA replication

• builds an energy store

What happens in prophase? - mitosis

1. chromosomes condense and become visible

2. centrioles move to opposite poles of the cell

3. spindle fibres develop from centrioles

4. nuclear membrane breaks down and nucleolus disappears

What happens in metaphase? - mitosis

1. chromosomes line up at centre of cell

2. spindle fibres attach to the individual chromtids by the centromere

What happens in anaphase? - mitosis

1. spindle fibres contract and pull chromatids apart to opposite poles of the cell

2. mitrochondria gather to provide energy

What happens in telophase? - mitosis

1. spindle fibres break down

2. chromatids become indistinct

3. nuclear membrane surrounds both sets of chromatids

4. nucleolus reforms

What happens in cytokinesis? - mitosis

• forms the two daughter cells

• organelles and cytoplasm equally divide

Main features of meiosis

• sexual reproduction

• genetic variation

• 2 divisions

• chromosomes half in each daughter cell

What are the main stages of meiosis?

1. interphase

2. prophase 1

3. metaphase 1

4. anaphase 1

5. telophase 1

6. cytokinesis

7. prophase 2

8. metaphase 2

9. anaphase 2

10. telophase 2

11. cytokinesis

What happens in prophase 1? - meiosis

1. chromosomes condense and become visible

2. centrioles move to opposite poles of cell and spindle fibres develop

3. paternal and maternal chromosomes associate in their homelogous pairs (bivalents)

4. chromosomes wrap around each other and join at the chiasmata in a process called synapsis

5. nuclear membrane breaks down and nucleolus disappears

What happens in metaphase 1? - meiosis

1. bivalents arrange themselves at centre of cell

2. the maternal and paternal chromosomes are arranged randomly allowing new genetic combinations (random arrangement)

What happens in anaphase 1? - meiosis

1. chromosomes in each bivalent separate and spindle fibres pull them to opposite poles of cell

2. each pole receive one chromosome from a homologous pair

3. it will be a random mixture of maternal and paternal chromosomes (independent assortment)

What happens in telophase 1? - meiosis

1. nuclear membrane surrounds both sets of chromosomes

What happens in cytokinesis 1? - meiosis

1. 2 haploid cells formed

What happens in prophase 2? - meiosis

• new spindle fibres develop at right angles to old

What happens in metaphase 2? - meiosis

• chromosomes line up separately in middle and spindle fibres attach

What happens in anaphase 2? - meiosis

• chromatids pulled to opposite poles of cell

What happens in telophase 2? - meiosis

• chromatids become indistinct

• spindle fibres disappears

• nuclear membrane and nucleolus reform

What happens in cytokinesis 2? - meiosis

• 4 haploid cells formed