The Cell Cycle

Question 1

What are the two outcomes of the (mitotic) cell cycle?

Answer 1

Duplication of cell contents- DNA, organelles, cytoplasm

Division into two new daughter cells

Question 2

What is the purpose of the cell cycle in unicellular and multicellular organisms?

Answer 2

Unicellular organisms, such as bacteria or yeast:
Each cell cycle gives rise to two new organisms
Multicellular organisms, such as humans:
Single-celled 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 3

What are the three categories of cells in G0?

Answer 3

Many cells in our body are in G0 and not all can re-enter the cell cycle
Cell cycle re-entry is not possible in e.g. nerve cells
Maintained in G0 unless stimulated to divide e.g. hepatocytes
Constantly in the cell cycle e.g. epithelial cells of the gut

Question 4

What happens in each stage of the cell cycle?

Answer 4

G1- Gap 1 phase; growth and preparation for S phase
S- Synthesis phase; chromosome duplication
G2- Gap 2 phase; growth and preparation for the M phase
M- Mitotic phase; mitosis and cytokinesis
Interphase incorporates G1, S and G2

Question 5

What is mitosis? What happens in each of the stages of mitosis?

Answer 5

Mitosis is nuclear division
Prophase- chromosomes condense, centrosomes move to opposite poles, mitotic spindle forms
Prometaphase- breakdown of the nuclear envelope, chromosomes attach to mitotic spindle
Metaphase- centrosome are at opposite poles, chromosomes are at their most condensed and line up at the equator of the mitotic spindle
Anaphase- sister chromatids separate synchronously, each new daughter chromosome moving to the opposite spindle pole
Telophase- chromosome arrives at the spindle poles, chromosomes decondense, nuclear envelope reforms

Question 6

What is cytokinesis and how does it occur?

Answer 6

Cytokinesis is a cytoplasmic division
At the position of the metaphase plate
Contractile ring of actin and myosin II constricts the cell into two new daughter cells
Contractile ring divides the cell in two
Cytoskeletal structure composed of actin and myosin
Accumulates between the poles of the mitotic spindle beneath the plasma membrane
Ring contracts and forms an indentation or cleavage furrow, dividing the cell into two new cells

Question 7

What is the mitotic spindle and what are the three types of spindles?

Answer 7

Starts to assemble during prophase from the centrosomes
Function is to separate the duplicated chromosomes
Constructed from microtubules
3 types of spindle microtubules:
Astral microtubules- role in orientation and positioning of the spindle; cytokinesis is influenced by the positioning of these astral MT
Kinetochore microtubules- attaches to the kinetochore
Interpolar microtubules- interacting in the middle

Question 8

What is the kinetochore?

Answer 8

It is a protein complex assembled on the centromere

Chromosomes attach to the spindle via the kinetochore

Question 9

How do kinetochore and the respective microtubules attach?

Answer 9

There are kinetochore proteins that work together, where the outer plate is, that are kinetochore microtubule interacting
And the inner plate has got proteins that are next to the centromere
They are next tot he centromere and are organised and interact with the specialised histones within this region
The fibrous corona is an assembly of dozens of proteins that have functions around microtubule capture and spindle assembly- its mechanisms for how its assembled are yet to be discovered
Needs to be disassembled before anaphase to avoid mistakes

Question 10

What is the centrosome cycle?

Answer 10

Microtubule-organising centre in animal cells
Centrosome consists of a pair of centrioles surrounded by pericentriolar material
Duplicated during interphase
Migrate to opposite poles in preparation for Mphase

Question 11

How are the cell organelles divided evenly?

Answer 11

Cell organelles cannot spontaneously regenerate so must be present in both daughter cells
In animal cells, the organelles are scattered randomly so that after division the amount of organelles will be roughly equal

Question 12

What is meiosis?

Answer 12

Meiosis is a specialised cell division that starts with one diploid cell and end with 4 haploid cells
The purpose is to produce gametes:
Sperm
Egg
One round os DNA replication during S phase and two rounds of cell division
In meiosis I, homologous chromosomes line up on the spindle and separate to opposite spindle poles
In meiosis II sister chromatids line up on the spindle and separate to opposite spindle poles
Recombination occurs between homologous chromosomes

Question 13

What happens nondisjunction happens during meiosis?

Answer 13

Nondisjunction- failure of homologues to separate from one another, either at mitotic division I or meiotic division II
For autosomes, it is usually fatal, exceptions being:
Trisomy 21 Down's syndrome
Trisomy 18 Edward's syndrome
Trisomy 13 Patau syndrome
For sex chromosomes;
XO Turner's syndrome
XXX Triple X
XXY Klinefelter's syndrome

Question 14

What are the criteria for a cell cycle?

Answer 14

Each phase must occur only once per cell cycle
Phases must be in the correct order- G1-S-G2-M
Phases must be non-overlapping
There are checkpoint at G1, G2 and M

Question 15

How do cell cycle regulators work here?

Answer 15

Cyclin dependent kinases (Cdks):
Active when bound to a cyclin
When active, phosphorylates target proteins
Cyclins:
Different cyclins produced at each phase of the cell cycle

Question 16

How is the yeast cell cycle controlled

Answer 16

A single Cdk binds successively to different cyclins
In G1, the Cdk becomes bound by S-cyclin, which triggers the S phase
S-cyclin is degraded allowing Cdk to go through the cell cycle, allowing it to bind to M-cyclin and trigger M phase
After that the M-cyclin is degraded

Question 17

How is the human cell cycle controlled?

Answer 17

Different classes of Cdk and cyclins act at each stage of the cell cycle
Cyclin D+ CDK4 at G1
Cyclin E+ CDK2 at G1/S
Cyclin A+ CDK2 at S
Cyclin B+ Cdc2 (CDK1) at G2 phase

Question 18

What is neoplasia?

Answer 18

Uncoordinated cell growth
Cells escape normal cell cycle regulation- no barriers to prevent uncontrolled progression through the cell cycle
Many genes mutated in human cancers are regulators of the cell cycle such as p53 and pRB, known as tumour suppressor genes which lose their function after mutation