C2.1 Cell and nuclear division

Cytokinesis

splitting of cytoplasm of parent cell to create daughter cells

Cytokineses in animals

1. Actin and myosin form a contractile ring pinching cell membrane together

2. this creates a clevage furrow which deepens and seprates the two daughter cells

Cytokineses in plant cells

1. Vesicles fuse to create a cell plate

2. The cell plate expands until it has reached the cell wall

3. Fuses with cell wall spliting parent cell into two

Examples of unequal cytokinesis

• oogenesis: produces 4 haploid cells but only one can form a zygote due to unequal sharing

• budding in yeast: yeast grow smaller cells (buds) which when grow are then separted from parent but are smaller

Why is nuclear division important

it ensures that cells are not anuceleate after cell division

Mitosis aim

maintains chromosome number and genome of cell

Meiosis aim

contirbutes to genetic diversity

priori to mitosis or meiosis

DNA replication needs to occur

• two sister chromatids joinmed at centromere

Role of histones

• coiling and supercoling → forms chromatin

Role of microtubules

• kinetochores → attach to the centromere

• Over-lapping microtubules → motor proteins direct their movement

• microtubules have a positive and negative end

Phases of mitosis

• interphase - prior to mitosis - cell just exists

• prophase

• metphase

• anaphase

• telophase

prophase in mitosis

• nuclear membrane disintegrates

• centromose move to oppsoite sides of the cell

• microtubules form

• chromatin forms chromosomes

metaphase of mitosis

• spindle fibres attach to kinetochores

• they create a metaphase plate

• centromeres align on the plate

Anaphase mitosis

• microtubules seprate at cenrtomere

• spindle fibres shorten

• chromosomes pulled to opposite poles

Telophase

• nuclear membrane reforms

• spindle fibres break down

• chromosomes begin to decondnese

• cell ready for cytokineses

Meiosis I prophase

• nuclear membrane disinetgartes

• spindle fibres form

• homologous chromosomes form bivalents at chiasmata crossing over occurs - exchange of gentic information between non sister chromatids

meiosis 1 metaphase

• homologous chromosmes form the metaphase plate → random orientation

• spindle fibres attach to cenromeres

meiosis 1 anaphase

• microtubules shorten seperating homologous chromsomes pulling sister chromatids to opposite poles

meiosis 1 telophase

• spindle fibres break down

• nuclear membrane forms

• chromosomes decondense

• cell ready for cytokinesies

→ haploid cells formed

Meisois 2 prophase

• nuclear membrane disintegartes

• centrosomes move to oposite sites of the poles

• spindle fibres begin to form

• dna condenses into chromosomes

Meisois 2 metaphase

• spindle fibres attach

• chromosomes form metaphase plate

• microtubules attach to kinetochores of each sister chromatids

meiosis 2 anaphase

• spindle fibres shorten

• split centromeres

• pulling sister chromatids towards opposite poles

meiosis 2 telophase

• nuclear membrane reforms

• chromosomes decondense

• spindle fibres break down

• cell ready for cytokineses

Non disjunction

• Down syndorme

• Trisomy 21

• 47 chromosomes instead of 46

• failure of chromosomes to seperate in anaphase 1

Meiosis and increased genetic diveristy

• crossing over → forms recombinants

• random orientation leads to independent assortemnt

Role of mitosis

• growth - in meristems mitosis needs to occur

• embryo developemnt - embryonic stem cells needs to undergo mitosis to form an embryo

• replace and repair - dead skin needs to be replaced

Interphase

• cell growth

1. S1 → cell grows

2. G → Dna replication

3. S2 → growth, prep for mitosis and some organelles increase in number

Role of cyclins

• cyclins are proetins

• they bind to CDKs enzymes

• thos ohosporylate proetins

• if conc of cyclins is high

• a chekcpoint is resched which moves the cell into next cell phase

Oncogenes

• created by proto oncogenes

• proto oncogenes regulate division

• if they turn uncontrolled cell division

tumour suppresor genes

control cell proliferation, cell cycle and apoptosis

if inactiavted uncontrolled cell growth and accumulation of mutations

primary tumour

grows in area of orgin

secondary tumour

spread from origin

Beging tumour

not cancerous

doesnt spread and cause damage

Malignant tumour

cancerous

spread and causes damage to orgnas