Cell replication

Types of cell replication

Mitosis Meiosis

Making clone cells For growth, repair

and reproduction Occurs in somatic

cells Output: 2 identical

(diploid) cells One cycle

Making gametes For sexual

reproduction Occurs in the ovaries

and testes Output: 4 daughter

(haploid) cells Two cycles

Mitosis

Making clones

The cell cycle

The stages of mitosis

Interphase The DNA is relaxed

and in long threads The cell replicates

the organelles, including the centrioles

The cell replicates the DNA

2 checkpoints occur to ensure the correct number of organelles and chromosomes

Prophase The DNA condenses

into chromosomes The threads of DNA

wrap around proteins to make a tightly wound structure

This ensures no DNA is lost in the split

The nuclear membrane dissolves

The centrioles migrate

Metaphase The chromosomes

pair up along the equator of the cells

The centrioles are at opposite poles and begin to extend the microtubules (spindle fibres)

The spindle fibres attach to the centromere of each chromosome

Anaphase The centrioles begin

to contract the spindle fibres

The retracting fibres pull each chromosome to the opposing poles of the cell

The spindle fibres detach

Telophase A new nuclear

membrane forms around the chromosomes

The cell begins to cleave in two

Cytokinesis The cell completely

cleaves in two In plants, a cell plate

also forms to create a dividing cell wall

Each new cell moves into interphase, where the DNA will unwind again

Meiosis

Making gametes

The stages of meiosis

Meiosis Prophase 1

the replicated DNA condenses into chromosomes Metaphase 1

the homologous (matching) chromosomes pair up along the equator

This is where crossing over can occur, when homologous chromosomes exchange corresponding DNA. Mixing up the allele combination on each chromosome.

The spindle fibres attach

Meiosis (cont.) Anaphase 1

The homologous chromosomes are separated by the retracting spindle fibres

This is where non-disjunction can occur, resulting in abnormal chromosome numbers

The chromosomes are pulled to opposing sides of the cell and the spindle fibres detach

Telophase 1 The cell begins to cleave No nuclear membrane forms

Meiosis (cont.) Prophase 2

The chromosomes begin to align Metaphase 2

The chromosomes line up across the equator The spindle fibres attach

Anaphase 2 The centromere splits The chromatids are pulled to opposing sides of the

cells The spindle fibres detach

Telophase 2 The nuclear membrane begins to form The cells begin to cleave

The result Four daughter cells

Sperm in men, eggs in women Each haploid – with half of the chromosome

number of a somatic cell This means that when two gametes meet, they

will form a diploid cell that can undergo mitosis to make a new individual

Crossing over creates variation amongst the daughter cells, resulting in new allele combinations