3 Cell Division

what is the difference between mitosis & meiosis?

• mitosis: division results in 2 identical daughter cells

• meiosis: division results in 4 unidentical gametes (sex cells) with half the chromosome no.

define chromosome

threadlike structure of DNA & histones found in the nucleus

define chromatid

2 threadlike structures (held together by a centromere) that make up the chromosome

define centromere

a specialised sequence of DNA that links the sister chromatids together

define telomere

a cap at the end of each chromosome arm which maintains stability

what are the stages of interphase?

G1, S, G2

what occurs in G1 phase?

• prepares for DNA replication by synthesising mRNAs & proteins required

• cell grows larger & organelles (needed for DNA synthesis) replicate

what occurs in S (synthesis) phase?

DNA replication occurs:

• sister chromatids replicate

• note: these sister chromatids are identical pairs of chromosomes

what occurs in G2 phase?

• cell continues growing & repleneshies energy stores

cell prepares & reoranises cytoplasmic components for division:

• organelles such as mitochondria duplicate

• dismantling of the cytoskeleton

outline the 3 checkpoints in the cell cycle.

1. cell growth checkpoint

2. DNA synthesis checkpoint

3. mitosis checkpoint

what occurs in the cell growth checkpoint?

at the end of G1 phase, checks whether the cell is big enough & has right proteins for S phase

what occurs if the cell isnt big enough or doesnt have the right proteins for S phase?

then it will go through a resting period (G0) until it is ready

what occurs in the DNA synthesis checkpoint?

during S phase, checks whether DNA has been replicated properly

what occurs in the mitosis checkpoint?

during mitosis, checks whether mitosis is complete

define cyclins

a family of proteins that controls the progression of a cell though the cell cycle by activating cyclin-dependent kinase (CDK) enzymes needed for synthesis of cell cycle. 

which cyclins regulate transition from G1 to S phase?

• cyclin D which binds to CDK4

• cyclin E which binds to CDK2

which cyclin promotes progression of S phase?

cycin A which binds to CDK2

Cyclins accumulate steadily during __ phase & are abruptly destroyed as cells exit from _____.

Cyclins accumulate steadily during G2 phase & are abruptly destroyed as cells exit from mitosis

which cyclin regulates progression from G2 to M (mitosis) phase

cyclin B which binds to CDK1

Define chromatin

DNA coild around histone proteins

whats the diff between chromosomes & chromatin?

• chromatin is lower order of DNA organization whereas chromosomes are higher order of DNA organization

• before cell division, chromatin forms itself into chromosomes

what are the 4 stages of mitosis?

1. prophase

2. metaphase

3. anaphase

4. telophase

what occurs in prophase?

• chromosomes condense & become visible inside the nuclear membrane

• they become shorter & thicker & seen as separate structures

• nuclear envelope begins to breakdown

• centrioles migrate to oppsoite ends of the cell

• spindle of microtubules begin to form

what 2 proteins catalyse the condensation process of chromosomes?

cohesin & condensin

outline the function of cohesin & condensin.

• cohesin forms rings that holds sister chromatids together

• condensin forms rings that coil the chromsomes compactly

define pro-metaphase

when the nuclear envelope breaks down allowing chromosomes to attach to the spindle microtubules.

what occurs in pro-metaphase?

• phosphphorylation of nuclear lamins by M-CDK causes the nuclear membrane to break down into small vesicles

• spindle microtubules connect to each chromosome at its kinetochore

define kinetochore

a complex of proteins positioned at the centromere

what are polar & astral microtubules?

• polar microtubules extend from the spindle pole across the equator

• astral microtubules extend from the spindle pole to the cell membrane

what occurs in metaphase?

• chromosomes attach to spindle fibres by their centromere

• they are pulled by the microtubules so they line up along the equator of the spindle

what occurs in anaphase?

• centromeres split

• spindle microtubules attached to centromers shorten & drag chromatids to oppsosite ends of the cell

what causes the centromeres to split in anaphase?

• enzymatic breakdown of cohesin

• cohesin linked sister chromatids together during prophase, so its breakdown causes the centromeres to separate

whats the diff between the 2 parts of anaphase?

Anaphase A - kinetochore microtubules shorten & draw the chromosomes toward the spindle poles

Anaphase B - astral microtubules anchored to the cell membrane pull the poles further apart and the interpolar microtubules slide past each other, exerting additional pull on the chromosomes

what occurs in telophase?

• chromosomes assemble at opposite poles & uncoil (making them invisible again)

• nuclear membrane forms around each set of chromosomes

• nuecloli reform

what occurs in cytokinesis?

the 2 nuclei separate to form 2 genetically identical daughter cells

in meiosis what occurs?

PMAT twice

whic stage is different in mitosis & meiosis & why?

• prophase 1

• crossing over occurs

note: only in prophase 1

how does crossing over occur?

• during synapsis - the pairing of homologus chromosomes

• tetrads/bivalents are created when spindle fibres pull homologus chromosomes together

• this = crossing over = of genetic material between sister chromatids

what does meiosis produce?

4 genetically non-identical haploid daughter cells