7: Cell Division & Death

Question 1

What is meiosis & why is it needed?

Answer 1

Specialised nuclear division process producing haploid cells carrying 1 copy of each chromosome. The haploid cells can differentiate into gametes. Genetically distinct offspring to the parents are produced = diversity

Question 2

How is the process of meiosis different to mitosis (generally)?

Answer 2

There are 2 rounds of chromosome segregation: Meiosis I and Meiosis II

Question 3

What happens in meiosis I?

Answer 3

Duplicated paternal & maternal homologs pair up and become physically linked (by genetic recombination).
In 1st meiotic anaphase, duplicated homologs are pulled apart & segregated into 2 daughter nuclei

Question 4

What happens in meiosis II?

Answer 4

Sister chromatids are pulled apart and segregated to produce haploid daughter nuclei

Question 5

How many haploid nuclei are produced from each diploid nucleus that enters meiosis?

Answer 5

4 haploid nuclei produced

Question 6

What is a haploid/diploid?

Answer 6

Haploid: one copy of each chromosomes
Diploid: 2 copies of each chromosome

Question 7

What are bivalent chromosomes and how are they involved in DNA crossing over?

Answer 7

They are chromosomes from male & female parents which line up to form exchange points (chiasmata) in prophase 1, as a result of recombination. This leads to genetic variability

Question 8

What happens in late interphase (meiosis)?

Answer 8

Synapsis and crossing over begin
DNA replicated = pair of chromatids

Question 9

Outline what happens in Prophase I (meiosis I)?

Answer 9

*Crossing over continues. Paired chromosomes condense & become visible
*Homologous chromosomes pair to form bivalent chromosomes
*DNA cross over occurs
*Members of each chromosomal pair repel each other but are still held at crossing-over point (chiasmata)
*5 stages

Question 10

What are the 5 sequential stages of meiotic prophase I?

Answer 10

1. Leptotene- homologs condense & pair and genetic recombination begins
2. Zygotene- synaptonemal complex assembles at sites where the homologs are closely associated, recombination events are occurring
3. Pachytene- the assembly process is complete, & the homologs are synapsed along their entire lengths. Crossing over & recombination.
4. Diplotene-disassembly of synaptonemal complex & condensation and shortening of chromosomes. Pairs of chromosomes linked at crossover points, chiasmata, now play crucial role in holding compact homologs together. Homologs now ready for segregation
5. Diakinesis I = chromosomal condensation reaches max

Question 11

What happens in metaphase I, anaphase I & telophase I (meiosis I)?

Answer 11

In metaphase I, bivalents line up across equator of spindle, attached by centromeres. Spindle formed
1. Chromatids separate
2. Reformation of nuclear envelope
3. The cell begins to divide (cytokinesis)

Question 12

What happens in meiosis II?

Answer 12

Starting to form haploid cells. In anaphase II centromeres divide, microtubules pull chromatids to opposite poles. In telophase II (same as in mitosis, but produces 4 haploid daughter cells) & cytokinesis occurs for the second time

Question 13

What is the major difference between mitosis and meiosis?

Answer 13

In mitosis, cells divide producing 2 genetically identical daughter cells. In meiosis, genetically different/unique cells, containing half as much DNA.

Question 14

What happens in the S phase of the cell cycle?

Answer 14

-‘Synthesis’ of DNA
- During replication, DNA & chromatin proteins must be reproduced accurately
- Its important that replication only happens once per cycle
- DNA replication in eukaryotes begins at origins of replication & leads to elongation phase
- Once replication origin is activated, it cannot be reused until new Mcm (inactive DNA helicases) loaded at late mitosis or G1
- DNA given permission to start division at replication origin due to Mcm helicases, during G1 phase. Then unwinding of DNA and ignition of synthesis occurs in S phase.

Question 15

Explain the duplication of chromatin structure in interphase (mitosis)

Answer 15

-Chromatin production increases in S phase. DNA is replicated and doubled, chromatin is remodelled
-Increase in histone subunits - form histone octamers (DNA is wrapped around these, forming chromatin)
-Subunits assembled into nucleosomes by nucleosome assembly factors
-Chromatin packaging helps control gene expression (is highly condensed making replication more difficult)
-Sister-chromatid cohesion depends on cohesion protein complex

Question 16

Explain the role of cohesin in the duplication of chromatin structure

Answer 16

(Cohesin is a protein complex with 4 subunits; 2 being SMC proteins = large family of proteins)
- First loaded around unduplicated chromosomes before S phase
- During S phase, the cohesin ring is held in place during passage of the replication fork and encircles the pair of sister chromatids during synthesis
- ATP binding promotes the interaction of the ATPase domains while ATP hydrolysis dissociates the ATPase domains
During S phase, acetylation of the ATPase domain of Smc3 ring

Question 17

What are the causes of cell death and why is it important?

Answer 17

caused by infection or trauma
- is a critical process for maintaining tissue homeostasis and eliminating harmful cells. Important for maintenance of tissue size; normal cell death & turnover; homeostasis (balance between cell growth, division & death)

Question 18

What are the different types of cell death and how do they vary?

Answer 18

*Necrosis (accidental health e.g. due to trauma or lack of blood supply, radiation or extreme temps. cells swell and burst, contents released = inflammation)
*Apoptosis/programmed cell death (cells die in an organised manner : engulfed and digested. ATP dependent, no inflammation)
*Autophagy (degradation of dysfunctional and unnecessary cellular components / when something is not working)

Question 19

What can happen when cell death goes wrong?

Answer 19

killing cells that are not meant to die (neurodegeneration e.g. Parkinson’s disease)
failing to kill cells that are generated in excess (cancer)

excessive general apoptosis = atrophy of tissues

Question 20

What are the roles of apoptosis?

Answer 20

• for tissue structure (e.g. development of hands & feet)
• when structure is no longer needed e.g. tadpoles
• quality control : eliminating cells that are abnormal, misplaces, nonfunctional or dangerous e.g. T and B cells that don’t produce antigen receptors, or produce self reactive receptors)

Question 21

Describe the process of apoptosis

Answer 21

Cell shrinks and chromatin condenses (cell also loses water).
Membrane starts blebbing & organelles disintegrate
Nucleus and organelles collapse & membrane continues to bleb
Apoptotic bodies form (chunks of membrane)
Macrophages phagocytose apoptotic bodies
= no inflammation

Question 22

What are caspases and what do they do?

Answer 22

proteases that mediate Apoptosis by cleaving specific intracellular proteins (at specific amino acid sequences)

Question 23

Describe the mechanism of apoptotic caspases

Answer 23

• these are inactive precursors in most nucleated animal cells
• Initiator & executioner caspases
  ^ initiator caspases activated by apoptotic stimulus which activates adaptor proteins
• activated initiator cascade dimers then cleave themselves and activate downstream executioner cascade dimers by cleaving them

Question 24

How are initiator caspases activated?

Answer 24

extrinsic pathway and intrinsic pathway
EXTRINSIC: death receptors recruit caspase-8 via adaptor proteins
INTRINSIC: intracellular signals induce mitochondrial outer membrane permeabilisation, which releases soluble proteins from the mitochondrial intermembrane space into the cytosol; released cytochrome c activates the adaptor protein Apaf1 which recruits cascade-9 monomers to form a large activation complex called an apoptosome