mitosis and meiosis

Question 1

what are the phases of mitosis

Answer 1

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)

Question 2

what happens during S phase (interphase)

Answer 2

DNA is synthesised and chromosomes duplicate

Question 3

what are the 5 stages of M phase

Answer 3

prophase, prometaphase, metaphase, anaphase and telophase

Question 4

what happens during prophase

Answer 4

centrioles move to opposite poles of the nucleus
chromosomes condense so they can easily separate

Question 5

what happens during prometaphase

Answer 5

spindle fibres elongate away from the spindle poles

Question 6

what happens during metaphase

Answer 6

pairs of chromosomes are aligned on the metaphase plate

Question 7

what happens during anaphase

Answer 7

pairs of sister chromatids are separated to opposite poles of the cell

Question 8

what happens during telophase

Answer 8

the chromosomes de-condense
nuclear membrane reforms

Question 9

what happens during G1 phase and when does it happen

Answer 9

happens between the end of M phase and the start of S phase
cell increases in size
RNA is produced
DNA prepares for synthesis

Question 10

what is the restriction point in G1 phase

Answer 10

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

Question 11

what happens during G2 phase and when does it happen

Answer 11

between the end of S phase and the start of M phase
the cell checks DNA
cell prepares for division

Question 12

what causes the transition from G2 to M phase

Answer 12

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

Question 13

what are the 3 types of cyclins and how do their levels change throughout mitosis

Answer 13

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

Question 14

how does a cell enter mitosis

Answer 14

1. cyclin binds to CDK1 forming inactive M-CDK
2. Wee1 phosphorylates M-CDK but it is still inactive
3. inactive phosphatase (cdc25) is phosphorylated and becomes active
4. active phosphatase (cdc25) acts on M-CDK to remove the inhibitory phosphate
5. cdk-activating kinase phosphorylates M-CDK activating it
6. active M-CDK acts on Wee1 so more active M-CDK is formed (positive feedback loop)
7. this causes chromosomes to condense, nuclear envelope to break down and spindle fibres to assemble

Question 15

how does the cell transition from metaphase to anaphase

Answer 15

1. inactive anaphase-promoting complex (APC) is activated by cdc20
2. the active APC/C attacks the cyclin-cdc complex so it can’t function
3. APC/c also destroys securin which holds chromatids together

Question 16

what are the 3 types of microtubule that make up spindle fibres

Answer 16

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

Question 17

what is loss of heterozygosity

Answer 17

loss of an allele from a cell

Question 18

what is hemizygosity

Answer 18

loss of one allele leaving one mutant chromosome
this is rare

Question 19

how can non-disjunction cause loss of heterozygosity

Answer 19

the cell tries to lose the extra chromosome but could lose the normal chromosome instead of the mutant one

Question 20

how can mitotic recombination cause loss of heterozygosity

Answer 20

chromosomes can break and when they are being repaired the genetic material can be exchange leading to one chromosome having both mutant alleles

Question 21

how can gene conversion cause loss of heterozygosity

Answer 21

if DNA polymerase begins replication on one chromosome and then switched to another the mutant allele could be copied across

Question 22

what happens in prophase 1 in meiosis

Answer 22

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

Question 23

what is the structure of the synaptonemal complex

Answer 23

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

Question 24

what happens during anaphase 1 in meiosis

Answer 24

1. homologues separate because both kinetochores on one chromosome attach to the same spindle pole
2. cohesion is removed so the chromosomes can separate

Question 25

what is aneuploidy in meiosis

Answer 25

abnormalities in the number of chromosome can be monosomy (missing a chromosome) or trisomy (an extra chromosome) this is caused by chromosome non-disjunction

Question 26

what is chromosome non-disjunction

Answer 26

where homologous chromosomes fail to separate in meiosis or sister chromatids fail to separate in mitosis or meiosis

Question 27

how does non-disjunction affect cells in meiosis 1 and 2

Answer 27

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

Question 28

what is necrosis and what causes it

Answer 28

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

Question 29

what is apoptosis and what causes it

Answer 29

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

Question 30

what is reversible and irreversible necrosis

Answer 30

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)

Question 31

what happens to a cell during apoptosis

Answer 31

the cell shrinks nuclear membrane breaks down apoptotic bodies form phagocytosis occurs

Question 32

what are 2 examples of developmental apoptosis

Answer 32

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

Question 33

what are CED genes

Answer 33

genes involved in the recognition of apoptotic signals which causes apoptotic vesicles to be engulfed by phagocytes

Question 34

what are the roles of the different CED genes

Answer 34

CED 3 and 4 stimulate apoptosis CED 9 inhibits apoptosis ELG-1 can inhibit CED 9 to activate apoptosis

Question 35

what are caspases

Answer 35

enzymes that trigger apoptosis

Question 36

what are the two types of caspases and what do they do

Answer 36

initiator: activated to apoptotic signals and activate executioner caspases executor: cause apoptosis

Question 37

what do caspases target

Answer 37

executioner caspases cleave nuclear laminas to break down the nuclear membrane they cleave the DNA repair enzyme PARP cause cytoskeleton changes by attacking actin

Question 38

what is the extrinsic pathway of apoptosis activation

Answer 38

1. killer lymphocytes have a Fas receptor ligand on their surface that will bind to the Fas death receptor on cells 2. this causes cell-induced signalling complexes (DISCs) to form 3. DISCs activate initiator caspases which activate executioner caspases to stimulate apoptosis

Question 39

what is the intrinsic pathway of apoptosis activation

Answer 39

1. triggered by stress signals such as DNA damage 2. mitochondria release the protein cytochrome C which activates apoptotic protease activating factor (Apaf-1) 3. Apaf-1 activates initiator caspases which activate executioner caspases to stimulate apoptosis