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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