MCM 2-12 Cell Division Flashcards Preview

MSI Unit II > MCM 2-12 Cell Division > Flashcards

Flashcards in MCM 2-12 Cell Division Deck (60)
Loading flashcards...

what are the four phases of the cell cycle and brief description

G1 phase - Gap before DNA synthesis

S Phase - DNA synthesis phase

G2 Phase - Gap after DNA synthesis

M Phase - Mitosis


Describe G1

The gap before DNA synthesis. Cells in this phase have two copies of each chromosome (one copy of each homologue). It is the typical “resting” phase wherein general biosynthesis occurs. External signals (e.g. receptor activation-MAP kinase receptors) and internal state (e.g. cell size, DNA integrity) jointly influence whether a cell proceeds to division (S phase) or remain in G1 (G0 phase).


S Phase

The DNA synthesis phase. Cells in this phase have an amount of DNA that gradually increases from that found in G1. During this phase histones and other proteins needed exclusively for DNA replication are produced. DNA is replicated from multiple replication origins (bidirectionally). After this is accomplished, the replication licensing factors are removed from the origin recognition complex to prevent re-replication. Centrioles duplicate but remain as a single centrosome (for now).

afterwards, replication "licensing factors" are removed from original recognition complex to prevent re-replication
-centrioles duplicate


G2 phase

The gap after DNA synthesis. Cells in this phase have four copies of each chromosome (two copies of each homologue). Proteins required for mitosis and cytokinesis are continued to be produced. Addition of cohesins links sister chromatids together, and addition of condensins combined with phosphorylation of histone H1 allows for chromosomes to condense at the beginning of M phase.


M Phase

Mitosis. The onset of mitosis is controlled by kinase activity…the nuclear and cytoplasmic events are coordinated during the stages of mitosis largely through the actions of kinases and phosphatases that are active throughout the cytoplasm. Cells undergoing mitosis have four copies of each chromosome (two copies of each homologue) until cytokinesis. This phase is triggered by the activity of a cell-cycle regulatory kinase. Mitosis is further broken down into the following phases:


what are the major checkpoints controlling progression from one stage to the next?

G1/S border (start)
G2/M border (enter M)
Metaphase/anaphase transition (exit M) - tension on all kinetochores activates APC (an E3 ubiquitin kinase) targeting the mitotic cyclin and other proteins for destruction.
-Securin destruction releases separase to separate sister chromatids.
-Destruction of mitotic cyclin turns off Motitoc CDK.


describe mechanism of cyclin dependent kinase (CDK) Regulation

CDK's regulate cell cycle
complete CDK's (cyclin-CDK complexes) are composed of catlytic CDK subunit and regulatory (cyclin) subunit
-changes in cyclin concentration change CDK activity- the cell cycle is dependant upon this activity

-in order for an active cyclin-CDK complex to be formed, cyclin subunit must be synthesized and CDK (catalytic subunit) must be phosphorylated at one of two sites (one is inhibitory, one is activating).

when catalytic subunit is phosphorylated- both sites receive phosphase, keeping the subunit inactive. A phosphorylase then removes the inhibitory phosphate, activating the cyclin CDK complex


what are the kinases that phosphorylate CDK?

two different kinases are involved in phosphorylating the CDK's two sites
wee1 adds the inhibitory phosphate
cak (CDK activating kinase) adds the activating phosphate.

cdc25 removes the inhibitory phosphate to activate the complex


How are CDK's turned off?

by ubiquination. An E3 ubiquitin ligase tags the cyclin with polyunbiquitin chain, causing its degredation by the proteosome.

a phsophatase removes the activating phosphate.


describe the DNA damage repair pathway

p53 is major protein.

in response to DNA damage, protein kinases are activated that phosphorylate P53, stabilizing/activating it

active p53 binds to regulatory regions of small inhibitory protein genes leading to the synthesis of the small inhibitory proteins that inhibit active CDK's. This arrests the cell cycle in whichever phase it is in.

protein produced depends on phase of the cell cycole which the damage is detected.

During G1/S checkpoint, if damage is detected, p53 upregulated the gene p21, which leads to synthesis of the protein p21; it inhibits G1/S- and S-CDK's


4 basic goals of cell division

approximate doubling in volume and content

exact duplication of genome

exact segregation of duplicated genomes

division into 2 new cells,
each toerh approximate equal volumes/content and 1 genome per copy


describe G1, S, G2, and M in terms of chromosome number

G1 - cell growing but dna not replicating, 2n
S - dna syntehsis. from 2 to 4n
G2 - 4n cells
mitosis - reduction of 4n into 2 2n cells


what phase is this?
typical resting phase
general biosynthesis occurs (growth/maintenance)



what influences the movement from G1 to S

external - receptor mediated pathways (growth factors, hormones, MAP kinase)

Internal - look at internal state, must have accumulated enough "stuff", enough nutrition, decides its big enough to divide


What events occur during S phase

Duplicating DNA, means you must also duplicate histones

-synthesizing nezymes required for DNA replication

-DNA replication occurs at multiple origines

-removal of replication licensing factors (via the phosphorylation of cdc6 by s-CDK)

centrioles duplicate, but do not separate until mitosis


why is S-phase unidirectional?

because you destroy the replication licensing factors (cdc6) that recongie origin. must wait until cell cycle ends to make more.


G2 events

continued syntehsis of proteins required for mitosis and cytokinesis

addition of cohesins links sister chromatids

once condensins and histones are phosphorylation, enters into Mitosis prophase


Mitosis - Prophase

defined by chromosomes condensing and 2 centrosomes separating.

KEY - still has nuclear membrane. This defines prophase.

1. chromosomes condense
2. spindle forms (duplicated chromosomes separate)
3. kinetochores (proteins) assemble on the centromeric portion of DNA


what markes transition from Mitosis prophase to prometaphase?

breakdown of nuclear envelope by the phorphorylation of lamins which is a function of CDK (cyclin dependant kinase)


what triggers mitosis?

activity of a cell-cycle regulatory kinase



after nuclear envelope breakdown
-allows microtubules radiating from centrosomes to be captured by the kinetochores, forcing chromosome pair to middle of cell



defining feature - all chromosomes attatched to both sides and lined up on metaphase plate.

Once lined up, kinetochore cascade gets turned off (if cascade on, wont proceed).


describe the balanced forces during metaphase

tension on kinetochore fibers pull inward, balanced by outward forces of overlapping polar microtubules being pulled in opposite directions by kinesin


metaphase/anaphase checkpoint

tension on kinetochores activates the Anaphase Promoting Complex (APC) that triggers destruction of coheins (which keep sister chromtids togetner).

Chromatids separate and move towards opposite poles (anaphase)


anaphase can be broken down into two types of movement

1. A - microtubules get shorter, the kinetochores chew the microtubules, puling the chromatids towards the poles

2. B - kinesins continue to actively push the poles apart on the bipolar microtubule arrays, furhter separating and elongating the cell.



finish dividing cell by forming ring of actin filaments (non-muscle myosin 2)
-spindle disassembles, ring assembles via signals sent at middle of spindle.

chromosomes decondense via phosphatase activity reversing effects of mitotic kinase on the chromosomes (causing condensation)

prepares for cytokinesis



contractile ring of antiparellel microfilaments associated with cell membrane and non-muscle myosin II. Similar to belt junction, constricts the cell

orientation of mitosis apparatus determines position of contractile ring


what does cell lose during mitosis and cytokinesis?

cells lose adhesion and round up


cells that fail to complete cytokinesis...

cells that fail to complete mitosis...


polyploid - multiple copies of genome in one nucleus


Describe CDK's

what do they do?

2 subunit kinase. the cyclin dependant kinase subunit (catalytic) and the cyclin subunit (Regulatory)

regulate the cell cycle

Decks in MSI Unit II Class (46):