cell division

what is the cell cycle ?

series of events in a cell leading to the duplication of its DNA and division of cytoplasm and organelles to produce two daughter cells

what type of cells go through the cell cycle ?

only stem cells as specialised cells dont divide

mitosis

division of the nucleus (prophase metaphase anaphase telophase )

cytokinesis

division of the cell (cytoplasm)

interphase

G1

S

G2

G1

growth phase 1 (growth of cell / synthesis of proteins / inc in mass / vol)

S

DNA synthesis (chromosome replication)

G2

growth phase 2 - replication of organelles e.g. mitochondria

what happens to cells that enter G0

they differentiate into specialised cells or die by apoptosis

what happens to cells w/ DNA damage that fail to pass checkpoints ?

apoptosis - controlled cell death

whats the purpose of the G1 checkpoint ?

check for DNA damage - stops proliferation of cells w/ damaged DNA

restriction point where cell becomes commited to entering the cell cycle

whats the purpose of the G2 checkpoint ?

checks for DNA damage
enables some repair of DNA
stops proliferation of damaged cells

whats the purpose of the M (metaphase) checkpoint ?

checks all centrioles are attached to spindle fibres before separation of chromatids

describe the process of DNA replication ?

the 2 strands unzip and unwind as H bonds are broken by DNA helicase

free DNA nucleotides form H bonds w/ the exposed complementary bases on both DNA strands

DNA polymerase forms phosphodiester bonds between DNA nucleotides by condensation reactions synthesising new DNA strands

each strand of the DNA is a template for the synthesis of a new complementary strand

why is DNA replication described as semi conservative

after replication the new DNA molecule formed contains :
one strand of the OG parent DNA

one newly synthesised DNA strand

DNA helicase

breaks hydrogen bonds between complementary bases to help unzip and unwind DNA

DNA polymerase

catalyses the formation of phosphodiester bonds between DNA nucleotides

what direction do all enzymes work in ?

5 to 3 direction

what needs to happen before mitosis ?

DNA must be copied so each cell gets 2 copies of each chromosomes

organelles must be replicated so each cell gets equal amnts

state 3 roles of mitosis

growth - allows organisms to get larger

repair - allows damaged / destroyed cells to be replaced

renewal of stem cells

what happens after DNA replication to chromosomes ?

the 2 sister chromatids remain attached at the centromere

prophase

2 pairs of replicated centrioles move to opposite poles of the cell

the spindle forms between them

chromosomes condense

nuclear envelope breaks down

nucleoli disappear

identifiable features of the prophase

condensed chromosomes

inside circular shape - nuclear envelope still present

metaphase

chromosomes become attached to spindle fibres at the centromeres and are lined up across the equator of the spindle

anaphase

centromeres divide and sister chromatids are pulled apart towards opposite poles of the cell as the spindle fibres shorten

telophase

the 2 sets of chromosomes have reached the poles of the cell

chromosomes lengthen and uncoil

nuclear envelopes reform around the chromosomes

nucleoli reform

cytokinesis in animal cells

a cleavage furrow develops in the cell membrane and deepens unti the 2 cells separate

cytokinesis in plant cells

remnants of spindle are used as a scaffold to form a new cell plate which eventually forms a new cell wall

what stage of the cell cycle is being showed here ?

anaphase

what stage of the cell cycle is being showed here ?

prophase

what stage of the cell cycle is being showed here ?

metaphase

what stage of the cell cycle is being showed here ?

telophase

what stage of the cell cycle is being showed here ?

interphase

what happens to cells that leave the cell cycle ?

they enter G0 and differentiate into specialised cells or die via apoptosis

chromosome

double stranded DNA molecule containing thousands of genes

centromere

part of chromosome which attaches to spindle fibres

sister chromatid

replicated chromosome - remains attached at centromere until anaphase

centriole role

forms spindles but not in plants

spindle fibre

microtubule fibre that attaches to centriole and shortens to separate sister chromatids

define the role of meiosis

to produce haploid gametes and introduce genetic variation

how is genetic variation introduced in meiosis ?

in prophase 1

where homologous chromosomes condense to form bivalents attached at the chiasma where crossing over occurs as the chromatids break and the detached parts swap over and recombine

at metaphase 1

through the independent assortment of chromosomes where the bivalents line up on the spindle in a random orientation

what does independent assortment do ?

introduces genetic variation because it produces 4 gametes that each carry a different combo of maternal & paternal chromosomes

Prophase 1

centriole pairs move to opposite poles of the cell

nucleoli & nuclear envelope disappear

spindle forms

Metaphase 1

bivalents line up along the equator of the spindle

centromeres attach to spindle fibres

Anaphase 1

homologous chromosomes separated (spindle fibres shorten pulling centromeres to opposite poles of the cell)

Telophase 1

does not always occur

chromosomes begin to decondense

nuclear envelopes form

Prophase 2

chromosomes condense

nucleoli & nuclear envelope disappear

new spindle forms at 90o to the first

Metaphase 2

chromosomes line up along equator of the spindle

centromeres attach to spindle fibres

Anaphase 2

centromeres divide

sister chromatids separated (spindle fibres shorten & pull each chromatid towards the opposite pole of the cell)

Telophase 2

does not always occur

chromosomes decondense

nucleoli & nuclear envelopes form

cytoplasm divides (cytokinesis)