Cell Cycle Flashcards
What is the cell cycle?
period of time between the birth of a cell and its own division to produce 2 daughter cells
How long does one full cycle take?
at least 12 hours but takes much longer in majority of adult tissues
What are the 2 basic phases of the cell cycle?
Mitosis (M)
Interphase (G1,S,G2)
What are cyclins?
- family of proteins that modify cells in interphase
- activate CDKs (enzyme)
- variable concentration
What occurs during S phase?
- DNA replication
- promoted by cyclin A and CDKs
- begins 8 hours after the end of mitosis
- takes ~8 hours to finish
Describe the structure of DNA.
- 2 chains coiled around common helical axis
- hydrophilic deoxyribose-phosphate backbone on outside
- hydrophobic bases stacked on inside
What does the spatial relationship between the 2 strands in the helix create
- major groove (wide)
- minor groove (narrow)
What can cause damage to DNA during replication?
- deoxynucleotide incorporation error during replication
- spontaneous deamination of bases during normal genetic functions
- x-radiation causing nicks in the DNA
- UV radiation
- various chemicals that interact with DNA
List 3 disorders caused by defective DNA repair.
- xeroderma pigmentosum
- homologous recombination gene mutation
- hereditary nonpolyposis colon cancer
Describe the cause of xeroderma pigmentosum.
- when DNA repair mechanisms are defective
- DNA damaged by UV
- results from inborn deficiency of the enzyme nicking endonuclease
Describe the symptoms of xeroderma pigmentosum.
- dry keratosis
- hyperpigmentation
- skin atrophy
- increased cutaneous sensitivity to UV light
- potential corneal ulcer formation
- squamous cell carcinoma of the skin
Describe homologous recombination.
- major pathway for the repair of DNA
- defining step is strand exchange directed by RAD51
- breast cancer suppressors BRAC1 and 2 are required
What occurs if there is a mutation in one of the genes involved in homologous recombination?
mutations have been linked to tumour predisposition
What causes hereditary nonpolyposis colon cancer
- mismatch repair of DNA
- chromosome 2 has been shown to contain genetic errors in the effected gene (hMSH2)
- mutations of hMSH2 account for up to 60% of cases
When does G1 occur?
gap in the cycle between the end of mitosis and beginning of S phase
What occurs during G1?
- cells respond to growth factors directing the cells to initiate
- irreversible
- most molecular machinery required to complete another cell cycle is generated
What controls G1?
cyclin E and CDKs
How long does G1 take?
- shows considerable variation
- less than 2 hours in rapidly dividing cells
- up to 100 hours in the same tissue
When does G0 occur?
after G1
What occurs during G0?
- undifferentiated cells go back into the cells cycle
- no S phase
- cells termed quiescent
What stimulates cells to leave G0 and enter the cell cycle?
growth factors
What blocks the cell cycle in G1?
proteins encoded by certain tumour suppressor genes
When does G2 occur?
between the end of S and beginning of mitosis
What occurs during G2?
- cell prepares for division
- ends with the breakdown of the nuclear membrane and onset of chromosome condensation
What controls G2 phase?
cyclin B and CDKs
How long does G2 last?
2 - 4 hours
When does interphase occur?
just before mitosis
Are chromosomes visible during interphase?
no
- uncoiled into very long narrow structures
What occurs during interphase?
- replication of DNA takes place in chromosomes
- centrioles are located in cytoplasm
What does the nucleus contain during interphase?
- nuclear envelope
- network of chromatin threads or granules
- nucleolus
What are the 4 phases of mitosis?
- prophase
- metaphase
- anaphase
- telophase
What is the result of mitosis?
distribution of identical copies of the parent cell genome to the 2 daughter cells
What occurs in prophase?
- individual chromosomes are visualised as thin threads
- threads tightly coiled along the entire length of cell
- chromosomes spilt into 2 chromatids
- 2 centrioles are separated from each other by elongation of continuous microtubules
When does prometaphase occur?
between prophase and metaphase
What happens during prometaphase?
- nuclear membrane and nucleolus disappear
- chromosomes now entangled in a meshwork of continuous microtubules
What occurs during metaphase?
- centromere region of each chromosome presents kinetochores
- set of microtubules are organised and extend bilaterally towards opposite centrioles
What does the growth of microtubules during metaphase allow?
permits the centromere of individual chromosomes with their paired chromatids to occupy equatorial plane of the spindle
What is the role of colchicine in the cell cycle?
- arrests cell division at metaphase
- prevents formation of microtubules of the spindle
- chromosomes don’t align properly during metaphase
What occurs during anaphase?
- separation of genetic material
- centromeres split longitudinally
- chromatids form 2 new chromosomes
- spindel fibres contract
- new chromosomes separated and migrate to opposite poles
What is non-disjunction?
when one daughter cell receives extra chromosomes and the other daughter cell is deficient in those chromosomes
How does non-disjunction occur?
- abnormal function of spindle apparatus
- one or more chromosomes fail to migrate properly in anaphase
What is isochromosome?
formation of 2 daughter chromosomes of unequal length
How does isochromosome occur?
when the centromere splits transversely instead of longitudinally
What occurs during telophase?
- reorganisation period
- daughter chromosomes enveloped by new nuclear membrane and nucleolus appears
- chromosomes become uncoiled and individual identity is lost
What occurs by the end of telophase?
- cytoplasm divides
- 2 complete cells formed
