Cell Cycle

Question 1

Why do Prokaryotic and Eukaryotic Cells Divide?

Answer 1

 The life cycle of an organism is linked to cell division.
 Unicellular organisms use cell division primarily for
reproduction.
 In multicellular organisms, cell division is important for
growth and repair of tissues.

Question 2

consequences of cell division

Answer 2

 Growing up!
 Hair growth
 Wound healing
 Root growth
 Gecko tail regeneration
 Yeast (fermented products)
 Bacteria (yogurt

Question 3

How do Prokaryotic and Eukaryotic Cells Divide?

Answer 3

Four events must occur for cell division.
1. Reproductive signal: To initiate cell division.
2. Replication: of DNA.
3. Segregation: Distribution of the DNA into the two new
cells.
4. Cytokinesis: Separation of the two new cells.
Note1: Protecting DNA is a cell’s top priority.
Note2: Cell division is a tightly regulated process

Question 4

What happens if cells start dividing (proliferating)
in an uncontrolled way?

Answer 4

Diseases such as cancer.
Inhibition of cell proliferation and killing the cancer cells is the top priority of many cancer treatments

Question 5

Prokaryote Cell Division

Answer 5

 The process of cell division in prokaryotes is called ‘binary fission’
 Binary fission results in two new cells.
 External factors such as
nutrient concentration and
environmental conditions are
the reproductive signals that
initiate cell division.
 For many bacteria, abundant
food supplies speed up the
division cycle.

Question 6

Chromosomes and Regions

Answer 6

Most prokaryotes have one chromosome, a single molecule
of double-stranded DNA, usually circular.
 This chromosome contains 0.6 – 4.6
million base pairs (bp) = ~4000 protein
coding genes.
 and contains two important regions:
 ori – (origin of replication) where
replication starts
 ter – (termination of replication)
where replication ends

Question 7

Binary Fission Process

Answer 7

1. DNA replication begins at centre of cell
   2.Chromosomal DNA replicates as the cell grows
   3.The daughter DNAs separate, lead by the region including ori. Cells begin to divide
   4.Cytokinesis is complete. 2 new cells are formed

Question 8

Cytokinesis

Answer 8

 Cytokinesis after DNA replication, begins by a pinching in of
the plasma membrane; protein fibres form a ring.
 As the membrane pinches in, new cell wall materials are
synthesized resulting in separation of the two cells.

Question 9

Eukaryotic cell division is tightly controlled

Answer 9

 Different cells have different rates of cell division.
 Some cells, such as those in an early embryo, divide
rapidly and continuously.
 Others, such as neurons in the brain, don’t divide at all.
 This suggests that the signalling pathways for cells to
divide are highly controlled.

Question 10

What’s the cell cycle?

Answer 10

The phases through which a cell
passes to produce daughter cells
by cell division is called the cell
cycle.

 The cell cycle has four phases:
G1, S, G2, and M.
 G1, S and G2 phase make up the
phase named INTERPHASE.
 M phase includes MITOSIS and
CYTOKINESIS.

Question 11

Durations of CC

Answer 11

 The duration of the cell cycle
varies considerably in different
cell types.
 In the early embryo the cell cycle
may be as short as 30 minutes,
 A rapidly dividing cell in an adult
human typically complete the cycle
in about 24 hours.

Question 12

What happens in the interphase? G1

Answer 12

 Cells prepare for DNA synthesis
 check the quality of DNA,
 produce necessary proteins
 and if everything is okay, cells commit
to replicate DNA and proceed with cell
cycle.
 Duration of G1 determines the cell cycle length in different cell types.
 Some rapidly dividing embryonic cells get rid
of it entirely, whereas other cells may remain in G1 for weeks or years.

Question 13

What happens in the interphase? S

Answer 13

 DNA replication takes place.
 Each chromosome is duplicated – consists oftwo sister chromatids.
 Sister chromatids remain joined until the M phase (mitosis).

Question 14

What happens in the interphase? G2

Answer 14

 Cell prepares for mitosis, by
synthesizing and assembling the
structures that move the chromatids
to opposite ends of the dividing cell.

Question 15

What happens if the cell does not need to divide?

Answer 15

 Some cells in G1 enter an inactive
resting phase of the cell cycle called
G0.
 These cells can enter cell cycle when
required or stay at G0 (for example:
cardiac muscle cells and neurons).
 There are checkpoints through the
cell cycle.
 If, for example, there is DNA
damage, cell either undergo cell cycle
arrest to repair the damage or
undergo programmed cell death
(apoptosis). Restriction point (R)

Question 16

Regulation of cell cycle

Answer 16

Regulated expression or activation of phase specific proteins.
Cyclins and Cyclin Dependent Kinases (CDKs)

Question 17

What is a kinase?

Answer 17

An enzyme that phosphorylates a target protein

Question 18

What is phosphorylation?

Answer 18

Transfer of a phosphate group from ATP to a target protein.

Question 19

Overall Function of Kinases

Answer 19

So, kinases catalyse transfer of a phosphate group from ATP to a target protein

Question 20

Cyclin dependent kinases are not active when they are on their own

Answer 20

They depend on a cyclin partner to get activated.

Question 21

CDK target at G1/S transition

Answer 21

A protein called ‘Retinoblastoma (RB) acts as an inhibitor of the cell cycle at the restriction point (R).
To begin S phase, a cell must
get by the RB block.
 G1/S cyclin–CDK complex
catalyses the
phosphorylation of RB.
 This causes a change in the
three-dimensional structure
of RB, thereby inactivating
it.
 Cell cycle now can proceed
into S phase

Question 22

Cyclins

Answer 22

Different cyclins are expressed at different phases of the cell cycle.

Question 23

As CDKs can only be activated by their specific cyclin partners,
cyclic production and degradation of cyclins regulate cell cycle
progression.

Answer 23

Cyclin is degraded

Question 24

Important cyclin-CDK complexes

Answer 24

 Cyclin D-Cdk4 promotes progression through G1
 Cyclin E-Cdk2 commits cell to S-phase
 Cyclin A-Cdk2 promotes DNA replication
 Cyclin B-Cdk1 initiates mitosis
 In order of cell cycle phases: DEAB

Question 25

Distinct cyclin-CDK complexes

Answer 25

control progression through
cell cycle checkpoints.

Question 26

If the cell should not proceed with cell cycle

Answer 26

cells either try
and repair the damage or undergo apoptosis (programmed cell
death)

Question 27

How do cells stop cell cycle?

Answer 27

Proteins called CDK inhibitors are synthesised, which inactivate CDKs.
Cell cycle will stop if the CDK is inactivated.

Question 28

If there was DNA damage, at G1/S checkpoint (R),
cell needs to stop cell cycle and try to repair the damage

Answer 28

DNA damage is detected by specialised proteins.
 These proteins induce expression of a CDK inhibitor
named p21 (p=protein, 21= mass of the protein: 21
kilodaltons).
 p21 binds to G1/S CDK
 When p21 is bound to CDK, cyclin cannot bind to CDK!
 CDK cannot be activated!

Question 29

p53

Answer 29

More than half of all
human cancers contain
defective p53,
resulting in the absence
of cell cycle controls
Cell Cycle arrest
due to inhibition of CDKs

Question 30

Intrinsic control:

Answer 30

cyclin/CDK/CDKi expression/interaction

Question 31

Extrinsic Control- Growth Factors

Answer 31

Example:
 If you cut yourself and bleed, specialized cell fragments
called platelets gather at the wound to initiate blood
clotting.
 The platelets produce and release a protein called
platelet-derived growth factor which diffuses to the
adjacent cells in the skin
 And stimulates them to divide and heal the wound.
Some white blood cells produce
interleukins that promote cell
division in other white blood cells.
Erythropoietin (Epo) produced in
the kidneys stimulates division of
bone marrow cells and production
of red blood cells.

Question 32

Gap 1 (Growth 1 – First gap)

Answer 32

 The cell cycle begins with G1.
 The cell grows to prepare for DNA replication.
 At this stage, the cell is diploid (also known as 2n) i.e., it has 2
copies of each chromosome.
 In humans, a normal diploid cell has 46 chromosomes (23 pairs).
 Contains the ‘Restriction point’ (G1-checkpoint).

Question 33

S (DNA Synthesis)

Answer 33

 DNA replication occurs in S phase.
 Quality control checks are made in S phase to ensure DNA
replication is accurate.
 The cell DNA content transitions from 2n to 4n i.e., it has 4
copies of each chromosome.
 A human cell contains around 3,000 bp of DNA, which encode
~21,000 protein coding genes

Question 34

Gap 2 (Growth 2 – Second gap)

Answer 34

 The G2 checkpoint ensures any new DNA damage is repaired.
 The cell DNA content is now 4n i.e., it has 4 copies of each
chromosome, 2 for each daughter cell.
 Cyclin B/Cdk1 complexes begin to accumulate as the cell
prepares for mitosis.