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Cell division - Binary fission

- carried out by prokaryotic cells


Cell division - Mitosis

- carried out by most eukaryotic cells, resulting in the formation of identical daughter cells


Cell division - Meiosis

- special type of division which produces non-identical daughter cells w/ half the DNA of parent cell


Cell division - Bacterial

- no sexual life cycle, reproduction is simple
- single, circular bacterial chromosome is rep.
- replication begins @ origin of replication & proceeds bidirectionally to site of termination
- new chromosomes are partition to opposite ends of cells
- septum forms to divide the cell into 2 cells


human genome

- CHROMOSOMES carry genes in linear order
- 23 chromosome pairs: 22 pairs of AUTOSOMES, 2 sex chromosomes
- SOMATIC CELLS have all 23 pairs of chromosomes


Chromosomes - eukaryotic

Composed of chromatin
- a long thin mixture of DNA & protein
- chromatin can be folded & coiled to form chomosomes
- nucleosome = histone proteins + DNA



- special region of the chromosome where the two copies of DNA become attached using cohesins
- location where the chromatids are attached to the mitotic spindle via KINETOCHORES



- region of highly repetitive DNA sequence at the end of telomeres which 'seals' the end of chromosomes & prevents it being joined to other chromosomes


Cell cycle

1. G1 (gap phase 1)
- primary growth, phase, longest phase
2. S (synthesis)
- replication DNA
3. G2 (gap phase 2)
- growth & prep. organelles replicate microtubules organise DNA copies are checked

4. M (mitosis)
5. C (cytokenisis)
- separation to form 2new cells


Cell cycle - duration

- fruit fly embryo = 8min

- mature cells take longer
→ typical mammalian cell = 24hrs
→ liver cells take more than a year

- growth occurs during G1, G2, S phases
→ M phase takes 1hr

- most variation in length of G
→ resting phase G0 - cells spend more/less time here



G1, S & G2 phases

G1: cells undergo major portion of growth

S: replicate DNA

G2: chromosomes coil more tightly using motor proteins; centrioles replicate; tubulin synthesis



- attachment site of microtubules
- consists of specialised proteins that bind tightly & specifically to the chromosome centromeres



- microtubules are filamentous structures within cells
- aid movement, organisation of cellular structure & intracellular transport, cilia and flagella and CELL DIVISION
- made of protein called TUBULINS (alpha and beta)



Individual condensed chromosomes first become visible w/ light microscope
- condensation continues throughout prophase

Spindle apparatus assembles
- 2 centrioles move to opposite poles forming spindle apparatus (no centrioles in plants)
- asters- radial array of microtubules in animals

nuclear envelope breaks down



transition occurs after disassemble of nuclear envelope

microtubule attachment
- 2nd group grows poles & attaches to kinetochores
- each sister chromatid connected opposite poles

Chromosomes begin to move to centre of cell-congestion
- assembly & disassembly of microtubules
- motor proteins at kinetochores



Alignment of chromosomes along metaphase plate
- not an actual structire
- future axis of cell division
- chromosomes are attached to opposite poles & under tension



- begins when centromeres split
- key event: removal of cohesion proteins from al chromosomes
- sister chromatids pulled to opposite poles
- 2 forms of movement
→ anaphase A: kinetochores pulled towards poles
→ anaphase B: poles move apart



- spindle apparatus disassembles
- nuclear envelopes forms around each set of sister chromatids → now called chromosomes
- chromosomes begin to uncoil
- nucleolus reappears in each new nucleus


G0 & G2

- most cells of the human body are actually in G0 phase (non-dividing)
- some cells in humans may never divide while others can be "triggered" to go from G0 back to G1
- if a cell receives a 'go' signal to divide it will usually complete G1, S G2 & M phase



- cleavageof the cell into equal halves
- animal cells: constriction of actin filaments produces a cleavage furrow
- plant cells: cell plate forms between nuclei
- fungi: nuclear membrane does not dissolve; mitosis occurs within the nucleus ; air of nucleous occurs w/ cytokenesis


Cell cycle - checkpoints

1 G1/S checkpoint :
- decides to divide
- primary point for external sig. influence

2. G2/M checkpoint:
- commited to mitosis
- DNA replication complete
- Assess success of DNA replication

3. Late metaphase (spindle) checkpoint
- cells ensure that all chromosomes are attached to the spindle



- unrestrained, uncontrol growth of cells failure of cell cycle control