Cell Cycle Flashcards
Events occurring in interphase
G1-cell decides whether conditions are favourable for division
S-DNA synthesis, DNA polymerase duplicates genome so cells can divide, sister chromatids are held together by cohesion complexes that physically link around DNA to hold it together
Importance of interphase
Allows cells to grow and prepare for division
If conditions not favourable then cell doesn’t divide
Events occurring in prophase
Chromosome condensation
Spindle begins to form, co-ordinated by centromeres
Chromatin wraps around histones a few times to reduce length
Histones wrap around each other and associate with a scaffold protein
Spindle catches chromosomes
Some spindle grows to edge of cell, some grows to centre
Importance of prophase
Condensing stops the chromosomes tangling irreversibly with each other and makes moving them around more manageable
Spindle pushes and pulls chromosomes to the right place
Events occurring in prometaphase
Nuclear envelope breaks down
Importance of prometaphase
Maximises space in the cell to move chromatids around
Also allows spindle to reach chromosomes
Events occurring in metaphase
Chromosomes align at equator
Kinetochores attach to microtubules
Kinetochores are protein complexes assembled at centromere that allow the DNA to attach to the microtubules, contain motor proteins that can move along the microtubule
Importance of metaphase
Kinetochores allow DNA to bind to microtubules
Events occurring in anaphase
Sister chromatids separate -
Cohesin complex is cleaved by a separase
Securin is bound to separase, inactivating it, anaphase promoting complex breaks down securin so separase is no longer bound and it active
Daughter chromosomes migrate to opposite poles as spindles move apart
GTP cap on microtubule is stable form, GDP tubulin dimers are unstable
Grow by adding dimers
When GTP cap is removed the microtubule shrinks as dimers are lost
Moves microtubules to poles
Sliding force is generated between overlapping microtubules from opposite poles to push the poles apart and a pulling force acts directly on the poles to move them apart
Anaphase A and B
Anaphase A- segregation of sister chromatids by the shortening of microtubules
Anaphase B- the separation of the spindle poles themselves, motor proteins that cross link overlapping inter polar microtubules push them apart
Importance of anaphase
Shrinking of microtubules pulls daughter chromosomes to opposite poles, separating them
Cleaving of cohesin complexes allows daughter cells to be separated
Events occurring in telophase
Daughter chromosomes arrive at poles and decondense
Nuclear envelope reforms
Importance of telophase
Reforming nuclear envelope separates genetic material
Events occurring in cytokinesis
Cytoplasmic separation
Formation of actin ring around the equator that contracts to separate the 2 cells
Organelle distribution
Mitochondria and chloroplasts by stochastic mechanism- random
Golgi and ER by active distribution- they divide as the cell divides
Importance of cytokinesis
Organelle distribution makes sure daughter cells have the correct organelles
Actin ring allows the cytoplasm to cleave to form 2 cells
Nurse experiments for discovery of cdc2
Mutagenise proteins in genome of yeast, saucing either long yeast or short yeast (divided too early)
Short yeast had one mutant, long had 60 mutants so 61 genes involved in regulation
Complementation- take mutant yeast, put wild type genes in one at a time, the one that reverses the mutation is the one causing it, select yeast that divide normally, isolate plasmid and sequence
Led to discovery of cdc2 which is a protein kinase, phosphorylates other proteins to activate them
Hunt experiments for cyclin
Sea urchin oocytes (egg cells)
Large and all at same stage of cell cycle waiting to be fertilised
Injected with radioactively labelled methionine and injected to stimulate fertilisation
Found that levels of cyclin increase just before mitosis, maximum abundance at G2
Properties of cdks and cyclin
Cdc2 levels are always constant but activity correlates with different stages of cell cycle
Cyclin activity maximal at G2
Cyclin is regulatory subunit of cdc2, activates it
In humans multiple cycling bind to cyclin dependent kinase (cdk)
Multiple versions in humans, different ones expressed in different stages of the cell cycle to regulate different stages independently
Gives cell cycle directionality as cyclin cause cdks to phosphorylate the preceding phase cyclin which leads to its degradation, ensures cell cycle only goes forward
What does each cyclin and cdk do
G1 cyclin/cdk -pass through restriction point
S phase cyclin/cdk- initiation of DNA replication
G2 cyclin/cdk- verify DNA replication, if not complete and accurate then causes cell death
M phase cyclin/cdk- promotes nuclear envelope breakdown and activates APC
Role of Rb and E2F
In G1 Rb is hypophosphorylated, in S/G2/M Rb is hyperphosphorylated
Rb can be phosphorylated by G1 cyclin/cdks
In G1 Rb in hypophosphorylated state is bound to E2F
When Rb is hyperphosphorylated by cdks, changes conformation so dissociates from E2F
E2F able to bind as a transcription factor to trigger S phase as it carries out trans activation of genes for S phase cyclin, DNA polymerase and DHFR (a rate limiting enzyme for nucleotide biosynthesis)
Restriction point and control of S phase entry
Restriction point is Rb being phosphorylated, after this mitosis has to occur and the cell is committed to cycle as S phase is entered
If cell doesnt pass restriction point it enters G0 phase or quiescence which is a waiting phase when not point through cycle
