The cell cycle

Question 1

what are the 4 phases of the cell cycle? what do each of these entail?

Answer 1

M- consists of mitosis followed by cytokinesis, G1- gap phase, S- period of DNA replication, G2- gap phase

Question 2

what regulates progression thru transitions (control points) in the cell cycle?

Answer 2

Extracellular signals and cell size

Question 3

what are the 3 major transitions in the cell cycle? what happens in each one?

Answer 3

Start - late G1cell commits to cell-cycle entry and chromosome duplication, G2/M - early mitotic events with chromosome alignment on the spindle occur; DNA repair too, Metaphase to anaphase - sister chromatids separate, anaphase triggered

Question 4

can the cell cycle be stopped? why would this happen?

Answer 4

cell cycle progression can be arrested if DNA is irreparably damaged OR if environments are not favorable

Question 5

what are cyclins? how are these activated?

Answer 5

proteins that activate the Cdks (cyclin dependent kinases); cyclins are activated by specific cues such as cell size or intra/extracellular signals such as growth factors, hormones, etc.

Question 6

what are cyclin-dependent kinases (Cdks)?

Answer 6

protein enzymes that add phosphates to various proteins involved in cell cycling

Question 7

what is the role of cyclin proteins and Cdk enzymes in the cell cycle? how is this activity regulated?

Answer 7

Distinct pairs of cyclins and Cdks regulate progression through different stages of the cell cycle; Activity of Cdks is regulated by association with cyclins, activating and inhibitory phosphorylations, and the binding of Cdk inhibitors.

Question 7

what is the cyclosome (APC/C)? how is it structured? why is it used?

Answer 7

APC/C is the cyclosome or anaphase promoting complex; a ubiquitin ligase that catalyzes the destruction of major proteins like S- and M-cyclins and securin (protects protein links between sister chromatids and unleashes anaphase) required for completion of mitosis and the beginning of cytokinesis

Question 8

what are the 4 classes of cyclins?

Answer 8

G1/S cyclins, S-cyclins, M-cyclins, G1 cyclins

Question 9

what does G1/S cyclin do?

Answer 9

activate Cdks in late G1 to trigger progression through the restriction point (commits to entry into cell cycle)

Question 10

what does S cyclin do?

Answer 10

bind Cdks to help stimulate chromosome replication and control early mitosis

Question 11

what do M-cyclins do?

Answer 11

activate Cdks that stimulate entry into mitosis

Question 12

what do G1 cyclins do?

Answer 12

govern activities of G1/S cyclins

Question 13

what happens in S phase? what is important here? what is found along the chromosome that are important in this phase?

Answer 13

central event of chromosome duplication; must be copied accurately: genome must be copied only once during the cycle; origins of replication- DNA replication initiated at these sites during S phase (ORCs are initiator proteins)

Question 14

when is the first step of initiation of replication? what happens here? which enzymes are present here?

Answer 14

During late mitosis and early G1; prereplication complex (pre-RC) assembles by ORCs (called “licensing”)
* M-Cdk inhibits this step, so it happens when Cdk (cyclin M and CdK 1) is low
* APC/C stimulates this step, so it happens when APC is high; G1/S complex with high cyclin G1

Question 15

when is the second step of initiation of replication? what happens here? which enzymes are present here?

Answer 15

At onset of S phase, pre-RCs induce formation of preinitiation complex which unwinds the DNA helix and loads DNA polymerases;
* S-CdK is activated and triggers preinitiation complex
* Elevated S-Cdk keeps #1 from happening until early G1

Question 16

what is the pre-replication complex made of? why does it work? what happens when this is deconstructed?

Answer 16

the ORC + the origin sequences on dsDNA + helicase + helicase loaders; works because of low S-Cdk; S-Cdk presence and deconstruction of pre-RC bring on the pre-initiation complexes and phosphorylation of ORC

Question 17

how and when are cohesins involved in the cell cycle? what happens to them?

Answer 17

involved at end of S phase to hold sister chromatids together; these proteins must be destroyed during metaphase to anaphase transition

Question 18

what are the 5 stages of M phase (mitosis)? (+ the last one)

Answer 18

prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis

Question 19

what is M-Cdk? what is it made up of?

Answer 19

One single protein kinase complex, controls all of the occurrences in early stages of mitosis
Cdk 1 and cyclin M = M-Cdk

Question 20

what role does M-Cdk play in mitosis? (5 roles)

Answer 20

M-Cdk phosphorylates key proteins:
– induces assembly of mitotic spindle in prophase
– ensures sister chromatids are attached to opposite poles
– triggers chromosome condensation (reorganization of intertwined sisters)
– promotes breakdown of nuclear envelope
– rearranges actin cytoskeleton

Question 21

what is condensin’s role in cell cycle?

Answer 21

To avoid sister chromatid breakage, condensation (compaction of chromatids) and resolution (resolved into separable units) must occur

Question 22

where is the mitotic spindle formed and what is it made of? what are 4 important parts of the spindle?

Answer 22

made of microtubules is formed in the centrosome;
– minus end of tubule is in the centrosome, plus end extends away
– centrosome contains a bunch of proteins, motor proteins, etc.
– poles of spindles within centrosome
– motor proteins (kinesins and dynein) important

Question 23

what can cause an abnormal mitotic spindle?

Answer 23

defective cell division control by faulty M-Cdk action (loss of phosphorylation)

Question 24

why is nuclear envelope breakdown necessary? what are some general steps of this process and what initiates it?

Answer 24

must break down for sister chromatids to attach to spindle; M-Cdk phosphorylates several subunits of nuclear pore complexes
* initiation of pore complex disassembly
* initiation of nuclear lamina dissociation
* envelope broken into small vesicles

Question 25

what are kinetochores? what are their roles in the cell cycle?

Answer 25

large multilayered proteins built on heterochromatin at the centromere of a chromosome; kinetochore microtubules binding kinetochores and motor proteins pushing and pulling

Question 26

what are the 3 steps take for chromatid separation?

Answer 26

– Metaphase-to-anaphase transition initiated by anaphase-promoting complex (APC/C)
– Cohesins disrupted by APC/C via securin and activated separase, allowing chromatids to separate
– S- and M-cyclins are destroyed and Cdks are inactivated

Question 27

how does APC/C become activated?

Answer 27

unclear; process involves increased transcription of certain proteins (promoted by M-Cdk)

Question 28

what happens during anaphase? what are the two processes?

Answer 28

segregation of chromosome; – anaphase A is initial poleward movement of chromosomes
– anaphase B is separation of spindle poles

Question 29

what happens during telophase? what 4 steps are taken to accomplish this?

Answer 29

final stage; 2 sets of chromosomes are packaged into daughter nuclei;
* Disassembly of the mitotic spindle
* Re-formation of nuclear envelope by coalescing around chromosome clusters and re-formation of pore complexes; help from ER
* Contractile ring formation
* Lots of dephosphorylation moves process backward

Question 30

what is the difference between meiosis and mitosis?

Answer 30

In meiosis, chromosome duplication is followed by two rounds of chromosome segregation; DNA breaks are formed in each sister so they hold on to each other (crossover) called homologous recombination; in mitosis homologous chromosomes behave independently of each other (they don’t line up together)

Question 31

what is formed when crossing over occurs?

Answer 31

when homologs are tightly connected chiasma crosses form

Question 32

what is the basis of homologous recombination?

Answer 32

crossing over