Week 8 the eukaryotic cell cycle, mitosis and apoptosis

Question 1

Cell division

Answer 1

Allows the continuity of life

Question 2

Cell cycle

Answer 2

Series of cyclic events for cell duplication and division, essential for all living things to reproduce and grow

Question 3

In unicellular organisms

Answer 3

Each cell division produces a new organism

Question 4

Cell cycle in Eukaryotic cells

Answer 4

4 Phases:
1)G1 phase
2)S phase (DNA replication)
3)G2 phase
G1 -> S -> G2 -> interphase
4)M phase, comprising of;
-mitosis (nucleus division)
-cytokinesis (cytoplasm division)

Question 5

Cell division rates

Answer 5

Not all cell types divide and cell division rates are different between cells
Non-dividing cells are in a quiescent state, called G0 phase

Question 6

Cell cycle control system

Answer 6

Uncontrolled cell division can result in cancer
The cell-cycle control system regulates the normal progression of the cell cycle

Question 7

It occurs at 3 cell cycles transition checkpoints…

Answer 7

It occurs at 3 cell cycles transition checkpoints
(G1/S, G2/M & metaphase/anaphase checkpoints)

Question 8

Cell cycle control system roles

Answer 8

It ensures that all the events required in each phase are completed before the next one begins
If not;
-It halts the cell cycle
-Or enters the G0 resting phase (outside the cell)
-Or activates apoptosis

Question 9

Cdks

Answer 9

Progression at checkpoints depends on the cyclical activation of distinct proteins,
Cyclin-dependent kinases (Cdks), when associated to regulatory proteins, cyclins

Question 10

Cdk activation

Answer 10

Cdks require interaction with specific cyclins to become active
->Cyclins regulate the activation of CDKs
Cdks are also phosphorylated to be active
Activated Cdks phosphorylate target proteins to drive cell cycle progression

Question 10

Cdk activation

Answer 10

Cdks require interaction with specific cyclins to become active
->Cyclins regulate the activation of CDKs
Cdks are also phosphorylated to be active
Activated Cdks phosphorylate target proteins to drive cell cycle progression

Question 11

Different Cyclin–Cdk complex levels

Answer 11

Distinct cyclin–Cdk complexes control various cell cycle phases (G1, S, G2, M)
->Transition through checkpoints is irreversible
->4 different cyclins are synthesised in response of specific signals and degraded after the cell progresses past the corresponding checkpoint
->CDKs are not destroyed, but cyclically activated/deactivated by phosphorylation / dephosphorylation

Question 11

Different cyclin–Cdk complex levels

Answer 11

Distinct cyclin–Cdk complexes control various cell cycle phases (G1, S, G2, M)
->Transition through checkpoints is irreversible
->4 different cyclins are synthesised in response of specific signals and degraded after the cell progresses past the corresponding checkpoint
->CDKs are not destroyed, but cyclically activated/deactivated byphosphorylation /
dephosphorylation

Question 12

G1 phase

Answer 12

Period of metabolic activity, cell growth, and general repair to prepare the cell for division

Question 12

G1 phase

Answer 12

Period of metabolic activity, cell growth, and general repair to prepare the cell for division

Question 13

G1/S checkpoint

Answer 13

The cell assesses DNA integrity and determines if conditions are favourable to progress into cell cycle

Question 13

G1/S checkpoint

Answer 13

The cell assesses DNA integrity and determines if conditions are favourable to progress into cell cycle

Question 14

checkpoint G1-to-S transition - key decisions

Answer 14

->Proceed to S Phase: Triggered by extracellular signals (mitogens) → good conditions
Delay S Phase Entry: Allows for further growth or DNA repair
->Exit the cell cycle by entering the G0 Phase: Can occur temporarily or permanently
->Initiate Apoptosis (programmed cell death): A response to severe DNA damage

Question 14

Checkpoint G1-to-S transition - key decisions

Answer 14

->Proceed to S Phase: Triggered by extracellular signals (mitogens) → good conditions
->Delay S Phase Entry: Allows for further growth or DNA repair
->Exit the cell cycle by entering the G0 Phase: Can occur temporarily or permanently
->Initiate Apoptosis (programmed cell death): A response to severe DNA damage

Question 15

S phase

Answer 15

In the S phase, each chromosome (nuclear
DNA) is replicated - DNA synthesis
->S-Cdk activates helicases and other enzymes to initiate DNA replication
->Chromosomes are not yet visible in their X-shape; they exist as chromatin
In S phase, centrosome is also duplicated

Question 15

S phase

Answer 15

In the S phase, each chromosome (nuclear DNA) is replicated - DNA synthesis
->S-Cdk activates helicases and other enzymes to initiate DNA replication
Chromosomes are not yet visible in their X-shape; they exist as chromatin
In S phase, centrosome is also duplicated

Question 16

Centrosome

Answer 16

Is a cellular structure that controls the microtubules organisation within the cell
Composed of 2 centrioles
(9 triplets of microtubules)

Question 16

Centrosome

Answer 16

Is a cellular structure that controls the microtubules organisation within the cell
-Composed of two centrioles
(9 triplets of microtubules)

Question 17

Centrosome main functions;

Answer 17

->Facilitates the assembly / disassembly of microtubules to arrange cytoskeleton
->It organises microtubules during cell division to form the mitotic spindle
-The mitotic spindle ensures accurate chromosome segregation in mitosis

Question 17

Centrosome main functions;

Answer 17

->Facilitates the assembly / disassembly of microtubules to arrange cytoskeleton ->It organises microtubules during cell division to form the mitotic spindle -The mitotic spindle ensures accurate chromosome segregation in mitosis

Question 18

G2 phase

Answer 18

Rapid cell growth and protein synthesis (enzymes) in preparation for mitosis Thorough check for unreplicated/damaged DNA

Question 18

G2 phase

Answer 18

Rapid cell growth and protein synthesis (enzymes) in preparation for mitosis Thorough check for unreplicated / damaged DNA

Question 19

Checkpoint end of G2/entry into mitosis

Answer 19

->If DNA is fully replicated and not damaged, proteins involved in early mitosis are activated, allowing the cell to enter mitosis ->Incomplete replication can arrest the cell cycle

Question 19

Checkpoint end of G2/entry into mitosis

Answer 19

->If DNA is fully replicated and not damaged, proteins involved in early mitosis are activated, allowing the cell to enter mitosis ->Incomplete replication can arrest the cell cycle

Question 20

M Phase Overview

Answer 20

Cell division is a continuous sequence of events (5 stages), usually symmetric; 1)Prophase 2)Prometaphase 3)Metaphase 4)Anaphase 5)Telophase Comprised of mitosis and cytokinesis

Question 20

M Phase Overview

Answer 20

Cell division is a continuous sequence of events (5 stages), usually symmetric

Question 21

Stage 1 - Prophase

Answer 21

Chromatin is condensed into visible chromosomes (by condensins) All the nuclear activities are ceased Outside the nucleus, the mitotic spindle assembles between the two duplicated centrosomes, which have begun to move apart

Question 21

Stage 1 : Prophase

Answer 21

->Chromatin is condensed into visible chromosomes (by condensins) ->All the nuclear activities are ceased ->Outside the nucleus, the mitotic spindle assembles between the two duplicated centrosomes, which have begun to move apart

Question 22

Stage 2 : Prometaphase

Answer 22

->Nuclear envelope disintegrates ->The 2 centrosomes are at the spindle poles (opposite ends) of the cell ->Chromosomes are attached to spindle microtubules of one pole via their kinetochores (protein complexes of the chromosomes’ centromere on both chromatids)

Question 23

Stage 3 : Metaphase

Answer 23

->Mitotic spindle is fully developed ->Kinetochores on each sister chromatid attach to opposite poles of the spindle ->The chromosomes are aligned at the spindle equator (midway between poles) ->M checkpoint controls the proper chromosomes alignment and attachment

Question 24

Stage 4 : Anaphase

Answer 24

->Cohesins, holding sister chromatids together, break down → X-shaped chromosomes ->Sister chromatids of each chromosome segregate (now individual chromosomes) and are pulled toward the opposite spindle poles -Because kinetochore microtubules shorten and the spindle poles also move apart

Question 25

Stage 5 : Telophase

Answer 25

->The two sets of chromosomes arrive at the spindle poles and decondense ->Nuclear envelopes and lamina reassembles around each set of chromosomes, formation of two nuclei → final event of mitosis It starts the cytoplasm division → formation of the contractile ring, between poles

Question 26

Stage 6 : Cytokinesis

Answer 26

The cytoplasm is divided in two by the contraction of contractile ring, which pinches the cell into two daughters → each with one nucleus (same DNA)

Question 27

M phase overview

Answer 27

Prophase: condensation of chromosomes; starting formation of two poles Prometaphase: nuclear envelope breakdown; chromosomes attachment to spindle microtubules Metaphase: chromosomes aligned at the equator (midway) of the spindle Anaphase: sister chromatids separated; moving to the two poles Telophase: reformation of nuclear envelopes (nuclear separation), formation of contractile ring Cytokinesis: separation of two daughter cells

Question 28

Apoptosis

Answer 28

Programmed cell death induced by external and internal stimuli e.g., -Removing cells during embryonic development -Cells no longer needed -Cells with severe DNA damage -Cells infected by viruses (preventing damage to neighbouring cells) Different from other cell death processes, like Necrosis (death due to acute injury) -> non controlled event, causing cell rupture and leakage outside → inflammation

Question 29

Apoptosis activation

Answer 29

Internal and external signals activate a proteolytic cascade mediated by proteases (cleaving peptide bonds of proteins) switching from procaspases (inactive) into caspases (active)

Question 30

Intrinsic pathway

Answer 30

An intracellular signal in response to stress -> radiations, heat, viral infection ->Altering the permeability of mitochondrial outer membrane that release cytochrome c from intermembrane space into the cytosol ->Cytosolic cytochrome c induces procaspases activation into caspases

Question 31

External pathway

Answer 31

Exposure of a extracellular death signal (Fas ligand) from a different cell Each cell has ‘death receptors’ on its surface Ligand-receptor binding recruits and activate procaspases into active caspases

Question 32

Apoptosis & caspases

Answer 32

Both pathways activate initiator caspases Initiator caspases cleave, and activate, downstream caspases, which break down key proteins (e.g. enzymes, structural proteins) to kill the cell quickly and neatly (no leakage outside) ->Cytoskeleton collapses ->The nuclear envelope disassembles ->The nuclear DNA breaks up into fragments ->Cell shrinkage Survival factors promoting cell survival and suppress apoptosis

Question 33

Apoptosis and health

Answer 33

The rate of cells dying should balance the rate of cells produced by mitosis Apoptosis plays a role in maintaining the correct balance

Question 34

Insufficient apoptosis

Answer 34

Leads to the formation of tumours Cancer cells acquire mutations allow them to escape apoptosis (cancer hallmark)

Question 35

Excessive apoptosis

Answer 35

Leads to cell loss and degeneration