L7 - Mitosis and Meiosis Flashcards
Why must the rates of cell division be carefully controlled
To regulate the cell numbers in a given tissue
How many division every 24 hours - typically
1
What cells don’t divide any more
Terminally differentiated cells
What are the two phases of M phase
Divison of the nucleus
Division of the cytoplasm
What happens in S phase
DNA synthesis
What occurs at prophase
Condensation of the sister chromatids
What occurs at metaphase
Attachement of the mitotic spindle to the kinetochore
What occurs at anaphase
Separation of the sister chromatids
What are the two types of yeast lifecycle
Budding and fission
What is significant about yeast undergoing a budding lifecycle
No G2 phase
What is significant about yeast undergoing a fission lifecycle
Short G1 phase
What forms as the result of a budding lifecycle
Forms separate daughter cells one of which is smaller and buds off
What forms as a result of the fission lifecycle
Divides an stays together forming filaments
What are the advantages of using Yeast as a model organism to study the cell cycle
Rapid division rate
Cell cycle control genes are highly conserved
Yeast can be grown as haploid or diploid
What is a genetic trick to do with the fact that yeast can be grown as haploid and as diploid
Diploids are used to maintain lethal muations that are studied in haploids
Knock out one copy of the gene - then turn into a haploid and study
What are temperature sensitive mutations
At a low tempature the protein is functional - when the temperature is increased there is no further function of the protein
What can temperature sensitive mutations be used for
To arrest cells at a particular stage of the cycle and then to synchronise these cells at a stage
What are the genes controlling cell division known as
Cell cycle control genes - Cdc genes
Advantages of using the Xenopus to study the cell cycle
Well known // Well studied lifecycle
Rapid division rate (every 30 mins)
Large size - easier to purify proteins
Manipulation by injection of RNAs/chemical into the oocyte
Describe the three requirements for cell free mitosis
Cyctoplasm from a frogs egg
Nucleus from the sperm of a frog
ATP
What are two uses of the cell free mitosis technique
Deplete the cytoplasm of proteins using antibodies
Remove cytoplasm at any stage to study changes e.g to protein phosphorylation over time
What are the three checkpoints of the cell cycle
G1/S (start)
G2/M
Metaphase-anaphase trnasition
G1/S transition
Is the environment favourable
G2/M transition
Is all of the DNA replicated
Is the environment favourable
What is checked at the metaphase - anaphase transition
What type of signalling is used?
Check if all of the chromosomes are attached to the spindle
Negative signalling
When does G1/S cyclin peak
Once cell pases G1/S (start) and commits to division
When does G1/S cyclin levels fall to the min level
At the end of G1 phase
When do s-cyclin levels peak
During S and G2 phase
When do s-cyclin levels fall
At the beginnig of M phase
When do m-cyclin levels rise
During G2 and M phases
Cyclins are proteins which
Are expressed at different levels during different stages of the cell cycle
What do cyclins bind
Cdk
What do cdks do
Phosphorylation of target proteins which are specific to certain stages of the cell cycle
How many phosphate groups req for activation
1
When a second phosphate group is added onto a cdk what occurs
Inhibition and inactivation
What enzyme removes a phosphate group from CDK
Cdc25 phosphatase
What enzyme adds a phosphate group from CDk
Wee1 kinase
What is the effect of p27
Binds to CDK-cyclin forming p27-cdk-cyclin complex which causes inactivation
What are the two sub units which must combine for the active APC/C complex to form
Inactive APC/C and Activating subunit cdc25
What is the anaphase promoting complex
A ubiquitin ligase
What does the APC/C do
Adds a polyubiquitin chain to M-cyclin ==> leads to degredation of m-cyclin by the proteasome
Also ubiquitinates S-cyclin and securin
What is the role of securin
Holds the sister chromatids together `
What is also required by the APC/C for the ubiquitination of M-cyclin
Ubiquitination enzymes E1 and E2 and ubiquitin
A favourable EC environment activates
G1-CDK
G1-CDK activates … which then activates
G1/S cyclin synthesis
S Cyclin synthesis
S cyclin activates …
s-cdk
What does DNA damage inhibit
G1/S CDK
S-CDK
M-CDK
Unreplicated DNA inhibits
M-CDK
Unattached chromsomes inhibit
APC/C
What does diploid mean
Have two copies of each chromosomes which are known as homologous chromosomes either maternal or paternal
Meiosis is similiar to mitosis except for …
There must be extra steps to separate homologous chromosomes
What does the pairing of homolgous chromosomes before segregation allow
Crossing over - homologous recombination
Do the X and Y chr behave as homologous chr
Behave like homologous due to small regions of homology
During meiosis prophase I what is crossing over aided by
Synaptoneal complex as well as DNA base pairing between homologes
What does homologous recombination between non sister chromatids serve
Aligns chromosomes ready for anaphase and aids in the formation of the synaptoneal complex
Allows for genetic recombination between paternal and maternal chromosomes
Where does crossing over occur
Chiasma
What occur where chr missing/added to a cell
Called non disjunction
Any gametes that arise are aneuploid
How many mammalian sperm aneupoloid
4%
How many mamalian eggs are aneuploid
20%
What is the purpose of regulating the rate of division in cells and tissue
Enables you to maintain cell numbers in each tissue
How often to mammalian cells divide on average
Every 24 hours
During DNA duplication, each chromosome pair (maternal and paternal) is duplicated to give rise to sister chromatids, T or F
T
Sister chromatids are then segregated between the two daughter cells, T or F
T
What is meant by G1 phase and what is going on the cell during this phase
G1 or gap 1 phase is the phase in which most cells are in. The cell is growing and constantly checking its environment. This growth is required to maintain cell size with subsequent divisions.
What is G0 phase
A quiescent, non-dividing phase
What is the restriction or start point
A point in the cell cycle after G1 phase that determines the commitment of the cell to S phase and the completion of the rest of the cycle to G1 again
What happens during G2 phase of the cell cycle
More growth of the cell and environmental checking. It has a shorter duration than G1 phase
What are the constituent parts of M phase
Nuclear division – mitosis and cytoplasmic division (cytokinesis)
What are the stage of mitosis
Prophase, prometaphase, metaphase, anaphase and telophase
During which stage of mitosis do the chromosomes attach to the mitotic spindle via their kinetochores and the microtubules
Metaphase
During which phase of mitosis do the sister chromatids condense
Prophase
What happens during anaphase
Sister chromatids are separated
Telophase corresponds to cytokinesis, T or F
T
What can be said about the genes that control the cell cycle
They are extremely conserved and almost identical across the species
What are the two different types of yeast models
Fission yeast and budding yeast
What are the advantages of using yeast to study the cell cycle
Rapid division rate <1hr, cell cycle control genes almost identical to human, can be grown as haploids or diploids
How can the ability of yeast to be haploid or diploid be harnessed to study the cell cycle
If you want to study a gene involved in cell cycle, manipulation will have a very deleterious effect on that organism. If you have carriers of the deleterious gene in diploid state and then switch the yeast to haploid when you want. Thus, diploids can be used to maintain lethal mutations that are then studied as haploids
How can you use temperature sensitive mutations in yeast to study the cell cycle
You can introduce mutations that only disrupt gene functions at particular temperatures into the yeast. Keeping the yeast at a low permissive temperature maintains gene function but switching to a restrictive (high) temperature enables you to study the genes function
What are the advantages using Xenopus laevis as a biochemical model when looking at the cell cycle
Easy to collects its eggs, rapid division rate, large sized eggs makes protein purification easier and they can be manipulated by injection of RNA or chemicals into the oocytes
What is cell-free mitosis
Technique that allows you to observe mitosis/nuclear division in a test tube outside of the cell membrane. This can be used to study changes i.e. in protein phosphorylation over time after cytoplasmic depletion using antibodies
What is the role of the start checkpoint
Checkpoint at the end of G1 that checks to see if the environment is favourable before triggering the DNA replication machinery to replicate the DNA. It ensures that the cells have enough resources to go through the cell cycle to G1 again
What is the purpose of the G2/M checkpoint
Checks to see is all DNA has replicated and if the environment is favourable before assembling the mitotic machinery
What happens at the metaphase to anaphase transition
Cell checks that all the chromosomes are attached to the mitotic spindle before triggering anaphase and completing cell division
Cyclins are a class of protein involved in the regulation of the cell cycle, explain how they interacts with their targets
Cyclins bind to cyclin-dependant kinases (Cdks) to activate them
What are Cdks and how do they act
Cyclin dependant kinases (Cdks) are kinases that phosphorylate proteins involved in specific stages of the cell cycle
Give examples of different cyclins and how they act with Cdks at different times in the cell cycle
M-cyclin is high during mitosis and the Cdk-M-cyclin complex leads to phosphorylation of protein involved in the assembly of the mitosis machinery. Whereas S-cyclin is high during S phase and the Cdk-S-cyclin complex phosphorylates proteins involved in the assembly of DNA replication machinery
Cyclins are expressed at constant levels during the cell cycle, T or F
F – cyclins are transiently expressed and are synthesised and degraded throughout the cell cycle
Levels of Cdk remain constant throughout the cell cycle, T or F
T
Cdks require cyclin to become active, T or F
T
How are Cdks further regulated other than the action of cyclins
Cdks are also regulated by their phosphorylation state. Active Cdk contains 1 phosphorylated serine residue whereas inactive Cdk has 2 phosphorylated serine residues. Unusually phosphorylation is an inhibitory signal in the case of Cds.
Explain the enzymes involved in the regulation of Cdk activity
Wee1 kinases is responsible for the phosphorylation and inactivation of Cdk. Whereas Cdc25 phosphatase removes a phosphate group from the inactive Cdk hence activating it. The interplay between Wee1 kinase and Cdc25 phosphatase determines that activity of Cdks.
What is the name of the protein inhibits the activity of the cyclin-cdk complex
p27
What is the name of the complex that marks cyclins for degradation and how is this achieved
Anaphase promoting complex (APC) marks cyclins for degradation by ubiquitination
What is the main difference between meiosis and mitosis
Meiosis resembles mitosis except there are extra steps that segregate homologous chromosomes
Pairing of homologues before segregation allows for crossing-over via homologous recombination, T or F
T
In meiosis I sister chromatids aren’t separated, what does happen during this stage
Homologous chromosomes pair up and crossing over takes place. These cells will be haploid with each homologue represented by two sister chromatids
Sex chromosomes don’t cross over, T or F
F – they behave like homologues during sperm formation due to small regions of homology
What happens during meiosis II
Chromosomes line up on the metaphase plate and sister chromatids are separated
Explain how pairing of chromosomes is facilitated in meiotic prophase I
Pairing is facilitated by the synaptonemal complex as well as DNA base pairing between homologues
What are the two purposes of homologous recombination between non-sister chromatids
Aligns the chromosomes ready for anaphase and facilitates formation of the synaptonemal complex as well as allowing for genetic recombination between paternal and maternal DNA on the same chromosome
Mistakes during meiosis I result in gametes with an extra chromosome or lacking a homologue, what is the name given to this event
Nondisjunction
How are the cells that arise from gametes that contain an extra chromosome or missing homologue referred to
Aneuploid
What percentage of mammalian sperm and eggs contain extra chromosomes or missing homologues
4% of sperm, 20% of eggs
