Cell Cycle Flashcards
Cell Cycle
Period of time between birth of a cell and its own division to produce 2 daughter cells
At least 12hrs but can last longer
Cycling Cells
Continue to divide regularly in definite intervals
Cyclins & CDKs
Protein family
Concentration increases and decreases throughout cell cycle
Cyclin A
Cyclin B
Cyclin E
Cyclin Dependent Kinases (CDKs) - act with cyclins
Cell cycle completion - transcription and degradation (ubiquitin)
Interphase
Interval between end of mitosis and beginning of the next cycle
Comprises –> G1, S, G2 phases
Also G0 phase in some cases
Individual chromosomes not visible - uncoiled
Nucleus posses a nuclear envelope and chromatin thread/granule network and a nucleolus
DNA replication takes place in chromosome
Two paired centrioles in cytoplasm
S Phase
DNA Synthesis - not in mitosis
Cyclin A and CDK
8hrs after mitosis
7-8hrs to complete
Cell Cycle Arrest
When crucial steps in the cell cycle are not completed
DNA Damage
Can arise during DNA replication
Deoxynucleotide incorperation error
Spontaneous deamination of bases
X-Radiation - nicks produced
UV radiation
CHemical agents
Disorders from defective DNA repairing
Xeroderma Pigmentosum - UV radiation, deficiency of endonuclease
Homologous Recombination - repairing of dsDNA breaks, RAD51 protein defining step, BRCA1/2 breast cancer, mutations of HR genes
Hereditary Nonpolyposis Colon Cancer - Mismatch repair, Chr2 contains genetic error in hMSH 2 gene
G1 Phase
Between end of mitosis and beginning of S phase
Cells respond to GFs to initiate another cycle (irreversible)
Most cell machinery required to complete another cycle
Cyclin E and CDKs
Less than 2hrs or more than 100hrs
G0 Phase
Cells retain capacity for proliferation and differentiation, but are not dividing
Quiescent
GF stimulate quiescent cells to leave GO and re-enter cycle
Proteins encoded ny tumour suppressor genes block cycle in G1
G2 Phase
Between S phase and beginning of mitosis
Cell prepares for division
Ends in nuclear membrane breakdown and chromosome condensation onset
Cyclin B and CDKs
2-4hrs
Mitosis (M)
Nuclear changes that achieve this distriubtion - 4 stages
Prophase
Metaphase
Anaphase
Telophase
Occurs in most somatic cells
Distribution of identical copies of parent cell gneome
Prophase
1st Stage
Chromosomes are visualised as thin threads and are tighlty coiled
Each chromosome splits longitudinally into two chromatids at centromere
Two paired centrioles are separated by microtubule elongation (achromatic spindle) and are at opposite poles
Prometaphase
Nuclear membrane and nucleolus degraded
Chromosomes entangled in microtubule network
1.5hrs
Metaphase
Centromere region from each chromosome present a bilateral disc Kinetochore
From kinetochores, a set of chromosomal microtubules are organised and extend bilaterally toward opposite centrioles
Growth of chromosomal microtubules allows centromere of individual chromosomes with their paired chromatids to occupy the equator
20mins
Anaphase
Centromeres split longitudinally and chromatids separate to form two new chromosomes
Spindle fibres contract and the pair of newly formed chromosomes separates and migrates to opposite poles
Seperation of chromatids to form new chromosomes occurs by contraction of chromosomal microtubules which exert a centrifugal force in centromere region
Telophase
Reorganisation period
Daughter chromosomes are enveloped by new nuclear membrane and nucleolus appears
Chromosomes become uncoiled and individual identity lost
Cytoplasm divides and two complete cells are formed
Cytokinesis
Cleavage furrow develops around equator region of achromatic spindle
Contraction of fibrillar component of cytoplasm help in hourglass constriction of cleavage
Colchicine
Arrests cell division at metaphase
Prevents microtubule formation
Chromosomes found to assemble around centriole
Important tool in karyotyping
Non-disjunction
Abnormal function of spindle apparatus
One or more chromosomes fail to migrate properly in anaphase
Isochromosome
Centromere splits transversely
Two daughter chromosome of unequal length
