Cell Cycle

Question 1

What is the most basic function of the cell cycle ?

Answer 1

to duplicate accurately the DNA in chromosomes and segregate the DNA into genetically identical daughter cells

Question 2

What are the 4 phases of the cell cycle ?

Answer 2

M phase
G1 phase
S phase
G2 phase

Question 3

Describe G1 phase

Answer 3

• growth phase
• increases metabolic/biosynthetic activity
• in g1 phase a cell has 3 options

Question 4

What are the 3 options for cells in the G1 phase ?

Answer 4

1. to continue the cell cycle and enter the S phase
2. stop the cell cycle and enter G0 phase for undergoing differentiation
   - become arrested in G1 then enter G0 or re-enter the cycle

Question 5

Describe G2 phase

Answer 5

• occurs after DNA replication and is a period of protein synthesis
• rapid cell growth to prepare the cell for mitosis
• before proceeding to mitotic phase, cells must be checked at the G2 checkpoint for any DNA damage

Question 6

Describe S phase

Answer 6

starts when DNA synthesis commences

Question 7

Describe M phase

Answer 7

• consists of Nuclear division
• divided into 6 phases
  1. prophase
  2. pro metaphase
  3. metaphase
  4. anaphase
  5. telophase
  6. cytokinesis

Question 8

Describe the control of the cell cycle

Answer 8

1. the restriction point in late G1, where the cell commits to cell-cycle entry and chromosome duplication
2. G2/M transition –> where control system triggers early mitotic events the lead to chromosome alignment in metaphase

Question 9

What are the central components of the cell-cycle control system ?

Answer 9

Cyclin- dependent kinases

Question 10

Describe Cyclin-Dependent Kinases

Answer 10

• activity of these kinases rise & fall as the cell progresses during the cycle
• leads to cyclical changes in phosphorylation of intracellular proteins that initial/regulate the major event of the cell cycle
• Cdk activity is controlled by a complex array of enzymes & other proteins - example = cyclin

Question 11

Describe Cdk Activation

Answer 11

• in the inactive state, without cyclin bound, the active site is blocked by a region called the T-loop
• binding of cyclin causes the T-loop to move, resulting in the partial activation of the Cdk2
• Phosphorylation of Cdk2 at a threonine residue in the T-loop further activates the enzyme by changing to shape of the T-loop

Question 12

What are the different classes of cyclins ?

Answer 12

1. G1/S-cyclins
2. S-cyclins
3. M-cyclins
   - they’re defined by the stage of the cell cycle

Question 13

Describe the role of G1/S cyclins

Answer 13

• G1/S-cyclins activate Cdks in late G1 and thereby help trigger progression through start
• results in commitment to cell-cycle entry
• level falls in the S phase

Question 14

Describe the role of S-cyclins

Answer 14

• bind Cdks soon after progression through start and help stimulate chromosome duplication
• S-cyclin levels remain elevated until mitosis
• these cyclins also contribute to control some early mitotic events

Question 15

Describe the role of M-cyclins

Answer 15

• activate Cdks that stimulate entry into mitosis at the G2/M transition
• M-cyclin levels fall in mid-mitosis

Question 16

Describe BrdU Analysis

Answer 16

• S-phase cells can be identified in the microscope by supplying molecules that are incorporate into newly synthesised DNA
• Artifialy thymidine analog bromodeoxyuridine (BrdU)
• cell nuclei that have incorporated BrdU are then revealed

Question 17

Define Prophase (1)

Answer 17

• condensation of chromosomes
• formation of 2 poles

Question 18

Define pro metaphase (2)

Answer 18

• nuclear envelope breakdown
• bipolar attachment at kinetochores

Question 19

Define metaphase (3)

Answer 19

chromosomes aligned in the mid zone

Question 20

Define anaphase (4)

Answer 20

• sister chromatids separated
• moving to the 2 poles

Question 21

Define telophase (5)

Answer 21

• reformation of nuclear envelopes

Question 22

Define cytokinesis (6)

Answer 22

• formation of contractile ring
• separation of 2 daughter cells