Mitosis and Meiosis

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Mitosis, Prophase

chromosomes condense, they become shorter and thicken =pair of chromatids

the pairs separate and move to the opposite poles organising themselves as they move

microtubules make the spindle, which extend from the poles to each centromere

nuclear envelope disintergrates and the nucleolus disappears

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Mitosis, Metaphase

chromosomes attach to the spindle fibres at their centromere and align at the equator

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Mitosis, Anaphase

centromere separates

spindle fibres shorten pulling the chromatids to the poles, centromere first

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Mitosis, Telophase

chromosomes uncoil and lengthen

spindle fibres break down

nuclear envelope reforms and nucleolus reappears

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Mitosis, Cytokinesis

division of the cytoplasm

animals- constriction of the parent cell around the equator from outside inwards

plants- cell wall material (cell plate) forms across the equator of the parent cell from the centre outwards to form a new cell wall

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Significance of Mitosis

genetically identical to the parent cell = genetic stability

repairs tissues and dead cells, continuous and identical

asexual reproduction = genetically identical to the parent = no genetic variation

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Damage and Disease

cells may fail to divide/divide too frequently/wrong time

genes control the division so if they become mutated the brake may become too damaged and replicate too fast

if in solid tissue, a tumour may form

'tumour supressor genes' prevent rapid replication

if a gene is un altered/doesn't create tumours = proto-oncogene

if a gene is altered/creates tumours = oncogene

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Meiosis

in reproductive organs prior to sexual reproduction

diploid is halved to haploid

when 2 haploid gamees fuse at fertilisation the zygote formed is now diploid

if it didnt half during gamete formation the number of chromosomes would double very time

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Meiosis 1, Prophase 1

each homologus chromosome pair is a bivalent

chromosomes condense

centrioles move to the poles and organise the polymerisation of the microtubules and the spindle forms

chromatids undergo crossing over at the chiasma = genetic variation

nuclear envelope disintergrate and nucleolus disappears

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Meiosis 1, Metaphase 1

Pairs of homologus chromosomes arrange themselves at the equator

= independent assortment = genes from both parents go into both daughter cells

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Meiosis 1, Anaphase 1

chromosomes separate

spindle fibres shorten pulling a pair to each side

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Meiosis 1, Telophase 1/Cytokinesis

in some species the nuclear envelpo reforms and the chromosomes decondense

in many species they do not decondense

Cytokinesis - cytoplasm divides and makes 2 haploid cells

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Meiosis 2, Prophase 2

centrioles organise a new spindle at right angles to the old one

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Meiosis 2, Metaphase 2

chromosomes align at the equator, attached to the spindle at the centromere

independent assortment

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Meiosis 2, Anaphase 2

centromeres divide and the spindle fibres shorten pulling the chromatids to the oppposite poles by their centromeres

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Meiosis 2, Telophase 2/Cytokinesis

chromatids lengthen, spindle fibre disintergrates

nuclear envelope reforms and nucleoli reappear

Cytokinesis - produces 4 haploid cells

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Significance of Meiosis

keeps the chromosome number constant

genetic variation due to =

= crossing over at the chiasma at P1

= independent assortment at M1 M2

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