L10 - Cell Division Flashcards
Cell destinies
1) live and function without dividing
2) grow and divide
3) die
Various signals tell a cell which path to take
Homeostasis maintenance
Balance between cell proliferation and death
Cell division frequency by cell type
Skin, blood, liver cells divide frequently
Brain, nerve and muscle cells don’t divide after maturing
G1
normal cell processes + some prep for division
- most cellular activities (metabolically active cell)
- duration variable (cell type specific)
- organelle/cytoplasm component duplication
- Centrosome replication begins
S
DNA replication
- Strands separate at H bonds holding nucleotides together
- New strand of DNA synthesised opposite each old strand
- Catalysed by DNA polymerase
- Proofreading (careful process)
S onwards is mostly prep for cell division
G2
- cell growth continues
- checks for correct DNA synthesis
- prepares for mitotic phase (synthesis of required proteins, enzymes, gather reactants)
- Centrosome replication complete
G0
- exit from cell cycle
- become non-dividing cells (e.g most nerve cells)
- cells held so they don’t proceed through rest of cycle
Mitosis prophase
- DNA condenses further
- mitotic spindles (microtubules) form
- two identical chromatids (sister chromatids) per chromosome
Mitosis prometaphase/late prophase
- nuclear membrane fragments
- kinetochore microtubules attach to kinetochore of chromosome
- chromosomes condensed
Mitosis metaphase
- individual chromosomes line up at centre (metaphase plate)
- centrosomes are at each cell pole
Mitosis anaphase
- sister chromatids separated and pulled to opposite cell poles by spindle fibres
Mitosis telophase
- nuclear envelope reforms
- spindle fibres disappear
Chromosome numbering
Longest is 1
Homologous chromosomes
One maternal one paternal, same genes
Sister chromatids
Identical, result of DNA replication
Non-sister chromatids
Sister chromatids of each homologous chromosome
Centromere
One on each chromatid
Sister chromatid cohesion
Several attachment points that keep sister chromatids together
Mitotic cell cycle checkpoints
G1 checkpoint, G2 checkpoint, M checkpoint
G1/S transition
Checks cell conditions for S phase - DNA undamaged? - health/quality of cells (size/nutrition) - sufficient organelles - appropriate signals present? If standards not met, cell enters G0
M checkpoint (prior to completion of mitosis)
- are all chromosomes attached to spindles
- inappropriate complement of chromosomes = apoptosis (cell dies instead of contributing to associated tissue/organ incorrectly - better for the organism)
Key proteins involved in checkpoints
- cyclin and CDK (cyclic changes work as mitosis clock, controls by phosphorylating other proteins)
- MPK
- external signals (e.g growth factors): control via signalling transduction pathways
Meiosis prophase 1
- synapsis to form tetrads, crossing over
Synapsis
Two sister chromatids of each pair of homologous chromosomes (non-sister chromatids) pair up
Tetrad
4 chromatids paired from synapsis
Crossing over
Non-sister chromatids within tetrads cross over at chiasmata
- results in genetic recombination
Meiosis metaphase 1
Homologous chromosomes pairs line up in centre
- independent assortment
Meiosis anaphase 1
Homologous chromosomes separated to each cell pole
- Sister chromatids remain attached
Meiosis 2
Similar to mitosis
Sources of variation in meiosis
Independent assortment, crossing over, gamete fusion
no. chromosomes and chromatids after meiosis 1 and 2
both result in haploid meiosis 1: - 23 chromosomes - 46 chromatids meiosis 2: - 23 chromosomes - 23 chromatids 1 chromosome refers to both when there is 1 chromatid only and when there is a pair of sister chromatids
