Cell Cycle

Question 1

What happens during G1-phase?(3)

Answer 1

1. the cell increases in size and prepares to copy its DNA2.
2. genes are expressed into the proteins that are needed by the cell
3. the organelles are replicated

Question 2

What happens during S-phase?

Answer 2

DNA and histone syntheses, all DNA is copied.

Question 3

What happens during G2-phase?

Answer 3

the proteins required for M phase are synthesized such as microtubules and membrane proteins for the daughter cells.

Question 4

What is the G0 phase?

Answer 4

· Many cells in our body are in G0 and not all can re-enter the cell cycle
· Cell cycle re-entry not possible e.g. nerve cells
Maintained in G0 unless stimulated to divide”

Question 5

Which cells are constantly in the cell cycle?

Answer 5

hematopoietic e.g. epithelial cells of the gut

Question 6

What happens during prophase?(4)

Answer 6

o chromosomes condense-tightly packaged

o centrosomes move to opposite poles,

o mitotic spindle forms

o nucleolus disappears

Question 7

What happens during prometaphase?(3)

Answer 7

o breakdown of nuclear envelope

o chromosomes attach to mitotic spindle

o Long chromosomes as they are not condensed

Question 8

What happens during metaphase?(3)

Answer 8

o centrosome are at opposite poles

o chromosome are at their most condensed

o chromosome line up at the equator of the mitotic spindle (metaphase plate)

Question 9

What happens during anaphase?

Answer 9

o sister chromatids separate synchronously,

o each new daughter chromosome moving to the opposite spindle pole

Question 10

What happens during telophase?(3)

Answer 10

o chromosome arrives at the spindle poles

o chromosomes decondense
nuclear envelope reforms around each nuclei

Question 11

What happens during cytokenesis?

Answer 11

cytoplasm divides by a contractile ring of actin and myosin II constrict the cell into 2 daughter cells by forming an indentation which divides the cell into 2.

Question 12

Describe the structure of the mitotic spindle?

Answer 12

bipolar array of microtubules with the +ve and -ve ends.

Question 13

What are the three types of microtubules?

Answer 13

– astral microtubules- projecting from the poles- orientation and position of the spindle within the cell because it is a symmetrical structure. Site of kinesis is influenced by astral
– kinetochore microtubules- contacts kinetochore
– interpolar microtubules- projected from both poles
-motor proteins associated with the interpolar microtubules drive the assembly of the spindle

Question 14

How do chromosomes attach to the spindle?

Answer 14

Chromosomes attach to the spindle via the kinetochore

Question 15

Describe the process of chromosome attachment to the microtubles(4)

Answer 15

1. Microtubules contacts the kinetochore- which is made from two plate
2. Fibrous corona- assembly of proteins involve in microtubules capture and spindle assembly.
3. Need to be dissembled before anaphase to avoid mistakes in chromosome segregation.
4. Kinetochores shorten toward poles and drag chromatids toward the poles

Question 16

Describe the structure of kinetochore(4)

Answer 16

1. made from two plates: the outer plate and the inner plate.
2. It is a big micromolecular complex made of proteins different. Cent A, B, C, D.
3. Outer plate has proteins that interact with the microtubules
4. the inner plate has proteins next to the centromere with specialized histones

Question 17

What is the centrosome?(3)

Answer 17

• microtubule-organizing centre in animal cells
• centrosome consists of a pair of centrioles surrounded by pericentriolar material(protein)
• migrate to opposite poles in preparation for M phase

Question 18

What is the state of parent and daughter centriole and how they disengage?

Answer 18

• The daughter centriole and the mother centriole are bound by a cohesion ring and disengage in G1 by the enzyme separase.

Question 19

Describe the structure of the contractile in cytokinesis (4)

Answer 19

1. Contractile ring divides the cell in two
2. Cytoskeletal structure composed of actin and myosin
3. Accumulates between the poles of the mitotic spindle beneath the plasma membrane
4. Ring contracts and forms an indentation or cleavage furrow, dividing the cell into two new cells

Question 20

What happens in meiosis I?

Answer 20

o chromosomes line up on the spindle and separate to opposite spindle poles.

Question 21

What happens in meiosis II?

Answer 21

“in meiosis II sister chromatids line up on the spindle and separate to opposite spindle pole”

Question 22

How does nondisjunction occur?

Answer 22

• failure of homologues to separate from one another, either at meiotic division I or meiotic division II

Question 23

Name some autosome mutations(3)

Answer 23

• trisomy 21 Down’s Syndrome
• trisomy 18 Edward’s syndrome
• trisomy 13 Patau Syndrome

Question 24

Why isn’t there a viable autosomal monosomies?

Answer 24

The genetic imbalance resulting from the loss of an entire chromosome in individuals is generallylethal, except for the full X chromosomemonosomy.

Question 25

Name some sex chromosome mutations(3)

Answer 25

*         XO Turner's syndrome *         XXX Triple X *         XXY Klinefelter's syndrome

Question 26

What are cyclin dependent kinases? (3)

Answer 26

o        active when bound to a cyclin o        phosphorylates target proteins Cdk levels are stable in the cell cycle. CDK activation triggers the next step in the cell cycle"

Question 27

What are cyclins?(2)

Answer 27

o       different cyclins produced at each phase of the cell cycle o       Regulators of CDKs and different ones are produced at each phase of the cell cycle. Cyclin levels vary as part of the cycle.

Question 28

Recall the name of cyclins and their Cdk(4)

Answer 28

o       Cyclin D-CDK4 o       Cyclin E-CDK2 o       Cyclin A-CDK2 o       Cyclin B(Cdc2)-CDK1

Question 29

How is Cdk4 and pRB involved in transcription?(5)

Answer 29

1) Extracellular signals (mitogens) lead to the expression of cyclin D which associates with CDK4 2. this phosphorylates pRB (target protein of CDK) pRB  3. this association prevents the E2F from binding to DNA sequences (E2F sites) that promote cell division.  4 ) when pRB becomes phosphorylated it changes the 3D structure and dissociates from E2F, E2F then binds to the genes that produce cyclin E and cyclin A. 5. At this stage the G1 checkpoint occurs, ATM is the proof reader gene which codes for proteins that proofread the DNA,

Question 30

What is the role of p53 and p21 genes?(3)

Answer 30

1.  It promotes transcription of genes that induce cell cycle arrest via the use of proteins.  2. It binds to the promoter regions of the p21 gene and stimulate the expression of p21.  3. P21 binds and inhibits G1-S Cdk complexes

Question 31

What must happen to the cylcins after they trigger phase?

Answer 31

"Each is degraded after triggering the phase to stop the CDK from being active •        Cyclin degradation terminates Cdk activity

Question 32

When can Cdks phosphorylate target protein?

Answer 32

When bound to cyclins

Question 33

Describe the levels of Cdks and cyclins in the cell cycle(2)

Answer 33

*        Cdk levels stable throughout the cell cycle | *         Cyclin levels vary as part of the cell cycle

Question 34

What is neoplasia, and how does it arise?

Answer 34

1. uncoordinated cell growth | 2.  Cells escape normal cell cycle regulation - no barriers to prevent uncontrolled progression through the cell cycle

Question 35

What happens at the G1 checkpoint if there is damage to the DNA?(6)

Answer 35

1. p53 is degraded quickly as it is unstable, and maintained at very low levels → phosphorylated (active) p53 is not degraded → active p53 promotes transcription of genes that induce cell cycle arrest, → it binds to the promoter region of the p21 gene and stimulates p21 expression →p21 binds and inhibits G1/S-Cdk/S-Cdk complexes →cell arrests in G1 (allowing time to repair the damaged DNA)

Question 36

What normally happens at the G1 checkpoint to allow the cell to enter the S phase?(5)

Answer 36

→ induction/expression of Cyclin D which associates with CDK 4 →binding/ activation of Cdk4 →phosphorylation of pRB by CDK4 →release and activation of E2F →S phase gene transcription

Question 37

What is ATM?

Answer 37

ataxia-telangiectasia mutated ATM is the proof reader gene which codes for proteins that proofread the DNA, if there is a mistake the ATM proteins will ‘tell’ the p53 gene that there is a mistake and p53 will produce DNA repair enzymes.