Hand-typed summary notes of Cell Division, diversity and differentiation OCR-A Level Biology Unit 2. Very clear and efficient, providing you with all the key points you need to ace your exam. I achieved an A* in Biology, using these notes!
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Cell division, diversity and differentiation
The Cell Cycle
There are 2 stages. Mitosis is when the cell divides and is shorter than
Interphase, which is when the cell prepares for division.
CDK enzymes push the cell through the cycle.
Apoptosis = cells purposely die
Senescence = old cells won’t replicate (gone past programmed times of
division)
Stages of the cell cycle:
G1: Cell grows - protein synthesis occurs, e.g. enzyme needed for DNA
replication in S phase. Organelles duplicate.
G0: Resting phase triggered during early G1. Cells may undergo apoptosis
(deliberate cell death), differentiation or senescence. Some cells, e.g.
neurones, remain here indefinitely. Others go here temporarily, e.g. lack
of resources.
S: DNA replicates, each chromosome contains a pair of identical
chromatids. Once a cell enters S phase, it’s committed to completing the
cycle. Cell is vulnerable as the exposed bases on DNA are susceptible to
mutations. This stage is rapid.
G2: Cell grows further to prepare for mitosis. Chemicals check DNA was
replicated correctly by stimulating proteins that will be involved in making
chromosomes condense and in formation of the spindle.
M: Mitosis occurs and then cells divide by cytokinesis.
Cell cycle checkpoints:
G1 Checkpoint: check cell is growing well enough or if DNA is damaged.
Ensures cell is ready to enter S phase.
Metaphase checkpoint: checks chromosomes are lined up in middle and
attached to the spindle correctly.
Purpose of checkpoints:
Prevent uncontrolled division that would lead to tumours (cancers)
Detect and repair damage to DNA, e.g. caused by UV light.
,Mitosis
Mitosis produces 2 genetically identical daughter cells.
Mitosis is required for:
Asexual reproduction: some plants and single-cells organisms
reproduce this way
Growth: all multicellular organisms produce identical cells
Tissue repair: cells divide to repair damaged areas
Stages of Mitosis (PMAT)
Prophase
Chromatin condenses to form chromosomes.
Nuclear envelope breaks down
Centrioles divide and migrate to opposite poles
Spindle fibres begin to form
Metaphase
Chromatids attach to spindle along the equator of the cell
Attached by the centromere
Anaphase
Centromere splits by motor proteins pulling chromatids to opposite poles.
, Telophase
Chromosomes are at either pole. Nuclear envelope forms. 2 identical
nuclei.
Cytokinesis
Once mitosis is complete, cell splits into two identical daughter cells.
In animal cells: plasma membrane folds inwards
In plant cells: end plate forms where the equator of the spindle was. New
cell membrane and wall material are laid down either side along the end
plate.
Meiosis
Results in 4 genetically different daughter cells. Involves combining of genetic
material from 2 unrelated individuals of the same species.
Haploid = having one set of chromosomes (‘n’)
Diploid = having two sets of chromosomes (‘2n’)
Diploid germ cells that undergo meiosis are located in organs called gonads –
ovaries or testes.
Homologous = carry same genes, same loci (position), same size, but different
alleles.
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