Cell Division (Mitosis and Meiosis)

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what are the phases of the cell cycle?


what phases does Interphase consist of?


what phases does mitosis consist of?

Prophase, Prometaphase, Metaphase, Anaphase, Telophase, Karyokinesis, Cytokinesis

when does G1 start?

end of mitosis

why is G1 the longest phase?

cell growth: gather nutrients, synthesize RNA and proteins

what are the two checkpoints in G1?

G1 DNA-damage checkpoint
-monitor integrity of newly replicated DNA

G1 Restriction checkpoint
-the point of no return
-mediated by Rb
-evaluate replication potential for S phase

what are the two phases of S?

synthesis and checkpoint

what happens during synthesis of S phase?

DNA replication

what happens during checkpoint at end of S phase?

S DNA-damage checkpoint
-check quality of newly replicated DNA

what are the three phases of G2?

G2 phase
-cell prepares for division

2 checkpoints

what are the two checkpoints of G2 phase?

1. unreplicated DNA checkpoint
-prevents progression into mitosis before completion of DNA synthesis

2. DNA-damage checkpoint
-monitor integrity of newly replicated DNA

what are the two checkpoints of the M phase?

1. spindle-assembly checkpoint
-make sure all chromosomes, spindles, kinetochores are attached
-prevents premature entry into anaphase

2. chromosome segregation checkpoing
-prevents cytokinesis until full separation of chromsomes

what is the kinetochore?

-large protein complex
-forms near centromere
-attaches chromosome to mitotic spindle

what are centrosomes?

-hollow, rigid cytoskeleton structure with alpha and beta subunits

what is the microtubule organizing center?

contains a pair of centrioles at the poles

what does the mitotic spindle consist of?

centrosomes + microtubules + motor proteins

which way does dynein move?

towards negative end

which way does kinesin move?

towards positive end

what happens during Interphase?

G2 -> M transition

-centrosomes separate
-centrosomes move to opposite poles
-orientation of spindles determines cell's plane of division

what happens during Prophase?

-condensation of replicated chromosomes
-chromatids connected at centromere
-kinetochores form near centrosomes
-mitotic spindle assembly
-nucleolus disassembles

what happens during Prometaphase?

-breakdown of nuclear envelope (dissolution of nuclear lamina)
-spindle microtubules bind to kinetochores

what happens during Metaphase?

-chromosomes align at equatorial, metaphase plate
-kinetochore microtubules attach sister chromatids to opposite poles
-motor proteins move chromatids into position

what happens during Anaphase?

-anaphase promoting complex (APC)
--> cleavage of chromatids
-sister chromatids separate
--> each pulled to opposite spindle poles by dyneins

what is anaphase A?

kinetochore microtubules get shorter

what is anaphase B?

-overlap microtubules lengthen
-spindle poles move apart

what happens during Telophase?

-sister chromatids arrive at opposite poles and decondense
-nuclear envelope reassembly
-assembly of contractile ring

in what phase is karyokinesis? how does the cell look?


one cell with two distinct nuclei

what happens during Cytokinesis?

-cleavage furrow
-separation due to contractile ring
-creation of 2, genetically identical, diploid 2n daughter cells
-completion of cell cycle

what is the contractile ring?

separates cytoplasm during cytokinesis; actin and myosin II

what are the regulators during G1?

cyclin D, Cdk 4/6
phosphorylation of Rb

what are the regulators during S?

cyclins E and A, Cdk 2

what are the regulators during G2?

cyclin A, Cdk 1

what are the regulators during M?

cyclin B, Cdk 1

explain the regulation of retinoblastoma protein Rb?

in resting cells of G1, Rb is active (hypophosphorylated)
--> binds E2F and is an inhibitor of cell cycle progression

in G1 -> S, Rb is inactive (hyperphosphorylation)
--> due to increased cyclin D, Cdk 4/6 activity, Rb dissociates from E2F (which is now free to activate gene expression)

what happens during G2 to M progression?

CdC 25 phosphatase removes inhibitory phosphorylatin of Cdk1 so Cdk1 can bind to cyclin B; this complex activates mitosis

what are two roles of p53?

1. apoptosis
2. cell cycle regulation during G1 phase

what does p53 do during cell cycle regulation?

stimulates transcription of Cdk inhibitory proteins

what is the phase of anaphase promoting complex?

to separate sister chromatids
-degradation of securing
-activation of separate
-cleavage of cohesin complexes

degradation of M phase cyclin B

what are the proteins involved in G1 DNA Damage checkpoint?

p53 activation
p21 activation (Cdk inhibitor)

what are the proteins involved in S DNA Damage checkpoint?

BRCA1 activation (repairs double stranded DNA breaks)

what are the proteins involved in G2 DNA Damage checkpoint?

Cdc25 inactivation

what are tumor suppressor genes?

-genes that maintain normal cell growth
-prevent unregulated cell cycle progression
-loss of function mutations enhance susceptibility to cancer

how are cancer cells regulated?

-no cell cycle arrest (lack of p53)
-cell division with damaged DNA
-continued mutation, selection, and tumor progression
-increased genetic instability

what are protooncogenes?

genes whose protein products control cell growth, proliferation, and differentiation

what are oncogenes?

mutated protooncogenes

what are the 4 stages of Prophase I in Meiosis?

1. leptotene
2. zygotene
3. pachytene
4. diplotene

what happens during leptotene?

-chromatin condensation
-sister chromatids condense and connect to each other
-pairing of homologous maternal/paternal chromosomes beings

what happens during zygotene?

-synapsis (synaptonemal complex binds chromosomes together)

what happens during pachytene?

synaptonemal complex complete and over

what happens during diplotene?

-synaptonemal complex begins to break down
-homologous chromosomes begin to separate
-chiasmata (newly formed junctions between chromosomes)

what happens during Metaphase I in Meiosis?

1. homologous chromosomes randomly align on equatorial plate at chiasmata
2. spindle microtubules attach to kinetochores

what happens during Anaphase I in Meiosis?

homologous maternal/paternal chromosomes pulled to opposite sides

what are the differences between Meiosis II and Mitosis?

-no DNA replication
-no crossover
-2nd meiotic division separates sister chromatids
-number of chromosomes in haploid gametes half the number of diploid parent

why don't we want errors in cell division?

increase in genetic instability

what does non-disjunction cause?

aneuploidy (improper separation during anaphase)

what are 4 types of chromosome damage or fragmentation?

-inversion (fragment reattaches to same chromosome but reversed)
-duplication (fragment treated as a separate chromosome)

what is polyploidy?

more than 2 sets of chromosomes (i.e. 3n, 4n etc)

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