The cell cycle

Question 1

What is the purpos of the cell cycle?

Answer 1

duplication of cell contents (DNA, organelles and cytoplasm)

division into new daughter cells

Question 2

how many organisms does the cell cycle give rise to in unicellular organsims?

Answer 2

2

Question 3

purpose of the cell cycle in multicellular organisms

Answer 3

A zygote must undergo many rounds of the cell cycle to make a new fully grown organism

Must also constantly replace any cells that die during the lifetime of the organism

Question 4

cells which are in G0 and cannot re enter the cell cycle

Answer 4

Nerve cells cell cycle re-entry is not possible

Hepatocytes are maintained in G0 unless stimulated to divide 

Epithelial cells and haematopoietic cells in the bone marrow are constantly in the cell cycle

Question 5

DNA contents during cell cycle

Answer 5

FACS analysis allows us to see that DNA content is greatest during G1 phase and the lowest during the S phase

Question 6

stages of interphase and mitosis

Answer 6

G1 = growth and preparation for S phase

S = chromosome duplication

G2 = Growth and preparation for M phase

Interphase = G1+S+G2

M = mitosis+cytokinesis

Question 7

mitosis

Answer 7

Prophase:
Chromosomes condense
Centrosomes move to opposite poles
Mitotic spindles form

Prometaphase: 
Nuclear envelope breakdown 
Chromosomes attach to the mitotic spindle 

Metaphase: 
Centrosomes are at opposite poles 
Chromosomes are at there most condensed and line up at the equator of mitotic spindle 

Anaphase: 
Sister chromatids separate 
Each new daughter chromosome move to opposite spindle pole 

Telophase: 
Chromosomes arrive at the spindle fibres 
These chromosomes expand 
Nuclear envelope forms 

Cytokinesis 
Cytoplasm divides

Question 8

mitotic spindle

Answer 8

Bipolar array of microtubules

Start assembling during prophase from the centrosomes at each pole 

Attach to the chromosmes via the kinetochore 

Pull apart the sister chromatids 

3 types of spindle fibres: 

Astral microtubules 

Kinetochore microtubules 

interpolar microtubules

Question 9

kinetochore

Answer 9

a protein structure formed on a chromatid, where the spindle fibres attach and pull the chromatids apart during cell division

Question 10

centromere

Answer 10

a part of the chromosome connected to the spindle fibre

Question 11

chromatids

Answer 11

the two chromosomes that have been replicated and linked through the centromere

Question 12

Centrosome cycle

Answer 12

Microtubule organizing centre in somatic animal cells

Centrosomes consists of a pair of centrioles surrounded by pericentriolar matrix 

Duplicated during interphase  

Migrate to opposite poles in preparation for M phase

Question 13

Cytokinesis

Answer 13

Final step of the cell cycle

Divides the cytoplasm into two daughter cells  

Contractile ring: 

Cytoskeletal structure composed of actin and myosin bundles 

Accumulate between the poles of the mitotic spindle beneath the plasma membrane 

Ring contracts and forms an identation or cleavage furrow, dividing the cell in two

Question 14

Cell organelle division

Answer 14

The cell organelles must be redistributed between the 2 new daughter cells

Cell organelles spontaneously regenerate so must be already present in the new daughter cells

Question 15

Mitosis vs meiosis

Answer 15

Mitosis:

Two cells 

Diploid 

Cell divides once 

No recombination between homologous chromosomes 

Meiosis: 

Four cells 

Haploid 

Cell divide twice 

Homologous recombination occurs

Question 16

Meiosis

Answer 16

Division which starts with one diploid cell and ends with 4 haploid cells

Produces sperm and egg cells  

First there’s a round of DNA replication during the S phase to double the genetic content 

Meiosis one: 

Homologous chromosomes line up on the spindle and then separate to opposite spindle poles 

Meiosis two: 

Sister chromatids line up on the spindle and separate to opposite spindle pole 

Recombination occurs between homologous chromosomes

Question 17

Nondisjunction

Answer 17

Failure of homologous chromosomes to separate from one another either at meiotic division one or meiotic division two

If in autosomes, usually fatal however there are a few exceptions: 

Trisomy 21 (downs syndrome) 

Sex chromosomes: 

XO (turners syndrome) 

Regulation of the cell cycle:

Entry into the cell cycle must be strictly controlled  

Each phase must only occur only once per cycle 

Phases must be in the correct order and non overlapping

Question 18

Checkpoints in the cell cycle (G1 checkpoint)

Answer 18

Check extracellular environment, growth factors, mitogenic signals and check for DNA for damage

Induction/ expression of cyclin D 

Binding/ activation of Cdk4 

Phosphorylation of pRB 

Release and activation of E2F 

S phase gene transcription 

DNA Damage repair: 

Normal p53 degrades very quickly, is unstable and maintained at very low levels 

Phosphorylated p53 is not degraded 

Active p53 promotes the transcription of the p21 gene that induce cell cycle arrest which gives more time to repair damaged DNA

Question 19

Checkpoints in the cell cycle (G2 checkpoint)

Answer 19

Check DNA replicated properly, check for DNA damage

Question 20

Checkpoints in the cell cycle (metaphase checkpoint)

Answer 20

Are all chromosomes aligned on the mitotic spindle

Question 21

Cell cycle regulators

Answer 21

Cdks
Enzymes which phosphorylate the target proteins
Become active when bound to a corresponding cyclin

Cyclins 
Regulators of Cdks 
Different cyclins are produced at each phase of the cell cycle

Question 22

Cell cycle control

Answer 22

Cdk levels fairly stable throughout the cell cycle

Cyclin levels vary as part of the cell cycle 

Cdk bound to cyclin is active and can phosphorylate target protein 

Cdk activation triggers the next step in the cell cycle such as entry into S phase or M phase 

Cyclin degradation terminates Cdk activity

Question 23

Cyclin-dependant kinase inhibitors

Answer 23

INK4

Inhibit G1 CDK’s 

CDK inhibitory protein 

Inhibit all other CDK-cyclin complexes gradually sequestered by G1 CDKs thus allowing activation of later CDKs