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 Prophase MetaphaseAnaphase Telophase Cytokinesis

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 Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 Prophase 2 Metaphase 2 Anaphase 2 Telophase 2

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