mitosis and meiosis Flashcards
what are the phases of mitosis
there are 3 main phases: S phase, M phase and cytokinesis
there are 2 gap phases: gap phase 1 (G1) and gap phase 2 (G2)
what happens during S phase (interphase)
DNA is synthesised and chromosomes duplicate
what are the 5 stages of M phase
prophase, prometaphase, metaphase, anaphase and telophase
what happens during prophase
centrioles move to opposite poles of the nucleus
chromosomes condense so they can easily separate
what happens during prometaphase
spindle fibres elongate away from the spindle poles
what happens during metaphase
pairs of chromosomes are aligned on the metaphase plate
what happens during anaphase
pairs of sister chromatids are separated to opposite poles of the cell
what happens during telophase
the chromosomes de-condense
nuclear membrane reforms
what happens during G1 phase and when does it happen
happens between the end of M phase and the start of S phase
cell increases in size
RNA is produced
DNA prepares for synthesis
what is the restriction point in G1 phase
this is where cells stay for around 2 hours before entering S phase
allows cells to check that signalling proteins that tell the cell to proliferate are still being received
if the signals have stopped the cell will exit the cycle into G0 where they will stop dividing until they can re-enter the cycle
what happens during G2 phase and when does it happen
between the end of S phase and the start of M phase
the cell checks DNA
cell prepares for division
what causes the transition from G2 to M phase
a factor called maturation promoting factor (MPF) forms complexes with cyclin dependent kinases (CDKs) which phosphorylate target proteins to turn signalling cascades on
cyclins activate CDKs to ensure mitosis proceeds in one direction
what are the 3 types of cyclins and how do their levels change throughout mitosis
cyclin E: low levels during G1, increases after the restriction point
cyclin A: levels increase through the S phase
cyclin B: levels increase through the M phase
how does a cell enter mitosis
- cyclin binds to CDK1 forming inactive M-CDK
- Wee1 phosphorylates M-CDK but it is still inactive
- inactive phosphatase (cdc25) is phosphorylated and becomes active
- active phosphatase (cdc25) acts on M-CDK to remove the inhibitory phosphate
- cdk-activating kinase phosphorylates M-CDK activating it
- active M-CDK acts on Wee1 so more active M-CDK is formed (positive feedback loop)
- this causes chromosomes to condense, nuclear envelope to break down and spindle fibres to assemble
how does the cell transition from metaphase to anaphase
- inactive anaphase-promoting complex (APC) is activated by cdc20
- the active APC/C attacks the cyclin-cdc complex so it can’t function
- APC/c also destroys securin which holds chromatids together
what are the 3 types of microtubule that make up spindle fibres
astral: link spindle poles to the outside of the cell
interpolar: overlap with each other in the middle of the cell and slide over each other
kinetochore: structure that links spindle pole to chromatids
what is loss of heterozygosity
loss of an allele from a cell
what is hemizygosity
loss of one allele leaving one mutant chromosome
this is rare
how can non-disjunction cause loss of heterozygosity
the cell tries to lose the extra chromosome but could lose the normal chromosome instead of the mutant one
how can mitotic recombination cause loss of heterozygosity
chromosomes can break and when they are being repaired the genetic material can be exchange leading to one chromosome having both mutant alleles
how can gene conversion cause loss of heterozygosity
if DNA polymerase begins replication on one chromosome and then switched to another the mutant allele could be copied across
what happens in prophase 1 in meiosis
the synaptonemal complex forms between homologues causing them to pair up through base pairing
this forms bivalent chromosomes
there is genetic recombination between the bivalent chromosomes
what is the structure of the synaptonemal complex
it is made up of cohesin which interacts with DNA
axial cores within homologues bind with chromatin and transverse filaments to join them together
transverse filaments overlap slightly to bring chromosomes close together
what happens during anaphase 1 in meiosis
- homologues separate because both kinetochores on one chromosome attach to the same spindle pole
- cohesion is removed so the chromosomes can separate
what is aneuploidy in meiosis
abnormalities in the number of chromosome
can be monosomy (missing a chromosome) or trisomy (an extra chromosome)
this is caused by chromosome non-disjunction
what is chromosome non-disjunction
where homologous chromosomes fail to separate in meiosis
or sister chromatids fail to separate in mitosis or meiosis
how does non-disjunction affect cells in meiosis 1 and 2
meiosis 1: one pair of gametes has less genetic material and the other has more, all gametes produced are affected
meiosis: less severe as one pair of gametes is normal and the other has a different number of chromosomes
what is necrosis and what causes it
a whole-scale form of cell death where one cell dies which causes surrounding cells to die
caused by external messages like physical damage to tissues
what is apoptosis and what causes it
a form of programmed cell death where a small number of cells die for a specific reason
in response to an external or internal signal like physiological issues or pathological situations
what is reversible and irreversible necrosis
reversible: the cell swells and organelles get bigger, membrane integrity is compromised, this leads to irreversible necrosis
irreversible: intracellular calcium levels increase, autolysis occurs (cell is destroyed by its own enzymes) and there is cell lysis (cell bursts)
what happens to a cell during apoptosis
the cell shrinks
nuclear membrane breaks down
apoptotic bodies form
phagocytosis occurs
what are 2 examples of developmental apoptosis
when tadpoles undergo metamorphosis they lose their tail though apoptosis
in mice some of the cells in their paws die by apoptosis to form webbing
what are CED genes
genes involved in the recognition of apoptotic signals which causes apoptotic vesicles to be engulfed by phagocytes
what are the roles of the different CED genes
CED 3 and 4 stimulate apoptosis
CED 9 inhibits apoptosis
ELG-1 can inhibit CED 9 to activate apoptosis
what are caspases
enzymes that trigger apoptosis
what are the two types of caspases and what do they do
initiator: activated to apoptotic signals and activate executioner caspases
executor: cause apoptosis
what do caspases target
executioner caspases cleave nuclear laminas to break down the nuclear membrane
they cleave the DNA repair enzyme PARP
cause cytoskeleton changes by attacking actin
what is the extrinsic pathway of apoptosis activation
- killer lymphocytes have a Fas receptor ligand on their surface that will bind to the Fas death receptor on cells
- this causes cell-induced signalling complexes (DISCs) to form
- DISCs activate initiator caspases which activate executioner caspases to stimulate apoptosis
what is the intrinsic pathway of apoptosis activation
- triggered by stress signals such as DNA damage
- mitochondria release the protein cytochrome C which activates apoptotic protease activating factor (Apaf-1)
- Apaf-1 activates initiator caspases which activate executioner caspases to stimulate apoptosis
