Unit 1 - Control of Cell Division

Question 1

G0 phase

Answer 1

Occurs if the go-ahead signal is not reached at the G1 checkpoint.

No cyclin proteins are produced and the cell is in a ‘resting state’.

The cell is active and fully functioning - just not dividing.

Most cells are in G0 and may return to the cell cycle if conditions change.

Question 2

G1 checkpoint

Answer 2

Occurs near the end of G1 and monitors cell size. There must be sufficient mass to form two daughter cells.

Controls entry to S phase.

Question 3

Cell cycle check points

Answer 3

Regulate progression through the cell cycle.

They assess the condition of the cell and halt progression to the next phase if certain requirements are not met.

Question 4

Proto-oncogene

Answer 4

A normal gene used to control cell growth and division.

May mutate to form a tumour-promoting oncogene.

Question 5

Tumour formation

Answer 5

Caused by an uncontrolled increase in the rate of the cell cycle.

May be benign or result in a malignant cancer.

Question 6

Degenerative disease

Answer 6

Caused by an uncontrolled reduction in the rate of the cell cycle.

Insufficient replacement cells are formed for normal tissue function. eg. Alzheimer’s

Question 7

Cytokinesis

Answer 7

Division of the cytoplasm to form two daughter cells.

Animal cells - the membrane is pinched by a circle of actin and myosin fibres.

Plant cells - a middle lamella and new cell wall forms between daughter cells before a new membrane is formed.

Question 8

Telophase

Answer 8

Separated chromosomes form daughter nuclei at opposite poles.

The chromosomes de-condense and nuclear membranes form.

Question 9

Anaphase

Answer 9

Depolymerisation at the kinetochore end of the spindle fibres separates sister chromatids.

Once separated, they are known as chromosomes and are rapidly pulled to opposite poles.

Question 10

Metaphase

Answer 10

Chromosomes line up on the metaphase plate at the equator of the spindle.

They move due to polymerisation/ depolymerisation of tubulin.

Question 11

Cell cycle

Answer 11

A sequence of events in which cell contents are duplicated and divided into two, allowing cells to reproduce themselves.

The cell cycle regulates the growth and replacement of genetically identical cells throughout the life of the organism.

There are 2 phases - interphase and mitosis.

Question 12

Polar microtubules

Answer 12

Made from spindle fibres.

Help to push the spindle apart during mitosis and meiosis.

Question 13

Kinetochore

Answer 13

Proteins found in the centromere of each chromatid, where spindle fibre microtubules attach.

Question 14

Aster

Answer 14

A star shaped tuft of microtubules at each centrosome (MTOC) which ensures that the cell division apparatus is correctly located.

Question 15

Spindle fibres

Answer 15

Control the movement of chromosomes during mitosis and meiosis.

Formed by the disassembly of cytoplasmic microtubules.

Question 16

Cytoskeleton

Answer 16

A network of protein fibres extending through the cytoplasm of all eukaryotic cells.

It is attached to membrane proteins and gives mechanical support and shape to the cell.

Organelles are attached to the cytoskeleton, and it is involved in the movement of cellular components and whole cells (using pseudopodia, flagella and cilia).

Question 17

Microtubules

Answer 17

Hollow cylinders which are polymers of the soluble globular proteins alpha and beta tubulin (which bond together to form a dimer).

Microtubules are 25 nm in diameter, but vary in length due to polymerisation (assembly) or depolymerisation (disassembly) of tubulin at the ends.

They control the location and movement of membrane bound organelles and other cell components.

Question 18

MTOC (centrosome)

Answer 18

Microtubule organising centre.

Microtubules radiate out from it.

Centrioles form part of the MTOC and are involved in the organisation of spindle fibres during cell division.

Question 19

Interphase

Answer 19

An active period of growth and metabolism. There are 3 phases :

G1 - 1st growth stage. New proteins and organelles made

S - DNA replication

G2 - 2nd growth stage. More proteins and copies of organelles are made.

Question 20

Mitosis

Answer 20

Chromosomal material is separated by the spindle microtubules.

There are 4 continuous phases which flow into each other - prophase, metaphase, anaphase and telophase.

Question 21

Prophase

Answer 21

DNA condenses into chromosomes in preparation for being moved, and become visible.

Each chromosome is made of 2 sister chromatids (formed by DNA replication).

The cell’s microtubules disassemble and reassemble into spindle fibres and asters from the MTOC by polymerisation of tubulin.

Spindle fibres attach to the kinetochore proteins in the centromere.

The nuclear membrane breaks down.

Question 22

G2 checkpoint

Answer 22

Occurs at the end of G2.

Assesses the success of DNA replication and any DNA damage to make sure that each daughter cell can receive a complete copy of the DNA.

Controls entry to mitosis.

Question 23

Metaphase checkpoint

Answer 23

Occurs during metaphase.

Monitors chromosome alignment to ensure that each daughter cell receives one chromatid from each chromosome.

Controls progression from metaphase to anaphase.

Progression is halted until the chromosomes are correctly aligned on the metaphase plate and are attached to the spindle microtubules.

Question 24

Cyclins

Answer 24

Proteins that accumulate during cell growth, and are involved in regulating the cell cycle. There are different cyclins produced in each stage eg. G1 - cyclin D, E and A.

They combine with and activate regulatory proteins called cyclin-dependent kinases (CDKs).

Question 25

Active CDKs (cyclin dependent kinases)

Answer 25

Result in the phosphorylation of target proteins that regulate progression through the cell cycle. No active CDKs results in the cell entering G0. (Resting state) Sufficient phosphorylation results in a checkpoint being passed and the cell moves to the next stage. If there is insufficient phosphorylation the cell is held at the checkpoint.

Question 26

Retinoblastoma protein

Answer 26

Acts as a tumour suppressor by inhibiting the transcription of genes that code for proteins required for DNA replication. It inhibits the transcription factor E2F by binding to it and preventing essential proteins required in the S phase from being made. It prevents the cell from entering S phase when CDK levels are too low. When G1 cyclin-CDK reaches a sufficient threshold, Rb is prevented from binding to E2F due to repeated phosphorylation of the Rb. This allows the cell to progress to the S phase.

Question 27

p53 protein

Answer 27

A transcription factor that can stimulate DNA repair, arrest the cell cycle or trigger cell death at the G1 checkpoint. DNA damage results in p53 causing the expression of genes that stimulate DNA repair and arrest the cell cycle. If the DNA repair is successful, the cell cycle continues. If unsuccessful, p53 instructs the cell to kill itself through apoptosis.

Question 28

Apoptosis

Answer 28

Programmed cell death - the deliberate destruction of cells. Essential during the development of tetrapod limbs and metamorphosis. Triggered by cell death signals.

Question 29

Caspases

Answer 29

Degradation enzymes that achieve apoptosis. Overactive caspases result in degenerative diseases. Under active caspases result in tumour development.

Question 30

Extrinsic pathway (external death signal)

Answer 30

Cell death signals that originate outwith a a cell. eg. from a lymphocyte that has detected the abnormal behaviour of a cell. Ligands bind to a surface receptor protein, the receptor changes conformation leading to signal transduction and a protein cascade within the cytoplasm that produces caspases.

Question 31

Intrinsic pathway (internal cell death signal)

Answer 31

Death signals that originate within a cell, eg. as a result of DNA damage. p53 activates a caspase cascade. p53 detects internal damage and disrupts mitochondria, resulting in a cascade.

Question 32

Caspase cascade

Answer 32

A series of post-translational modifications to proteins that are already present in the cytoplasm (allowing a rapid response), Inactive caspases undergo cleavage, which removes a subunit to form the active caspase. Each initiator caspase at the beginning of the cascade activates several executioner caspases which act as DNAases and proteases to destroy the cell.

Question 33

Cell fragmentation

Answer 33

Once the cellular components are destroyed, the cell can no longer function as it has no cytoskeleton. The cell breaks into small vesicle fragments which are engulfed and destroyed by phagocytic white blood cells.