Cell Determination and Cellular Senescence

Question 1

Define cell determination

Answer 1

cell fate becomes stable. It is followed by cell differentiation. When a cell chooses a particular “fate”, it is said to be determined. Implies a stable change.

Question 2

What is asymmetric cell division?

Answer 2

produces two daughtercellswith differentcellularfates.

The two daughter cells = different fates= different gene expression profile

Question 3

What are the causes of asymmetric cell division?(2)

Answer 3

1. Inductive signals from neighbouring cells is the most common cause-One group of cells influences the development of another group of cells
2. TFs that access silent chromatin, remodel it and initiate cell-fate. e.g. OCT4, SOX2 and NANOG

Question 4

What is cell senescence and why does it occur?

Answer 4

1. irreversible cell-cycle arrest mechanism in which cells cease to divide
2. Occurs as response to excessive extracellular or intracellular stress

Question 5

What is apoptosis?

Answer 5

cell death that plays an essential role during the individual’s life.
Many stimuli Can lead to apoptosis

Question 6

What is the Hayflick limit?

Answer 6

Hayflick found out that normal human cells divided 50 times and became senescence

Question 7

Define cell lifespan

Answer 7

the total number of doublings that a cell population goes through before senescence. The length of time for which a cell exists. Eg 53 doublings

Question 8

What are the morphological changes during senescence?(4)

Answer 8

1. Prominent nucleoli
2. Nuclear lamina degradation so edge of nucleus is hard
3. Vacuolised
4. Chromatin reorganisation

Question 9

Recall the best-known molecular markers in senescence

Answer 9

1. lysosomal β-galactosidase

2. protein p16- a cell cycle inhibitor.

Question 10

What is SASP?

Answer 10

Senescence Associated Secretory Phenotype consists of inflammatory cytokines, growth factors and proteases.

Question 11

What are telomeres?

Answer 11

regions at the end of the chromosomes composed of TTAGGGn DNA sequences whose function is to preserve chromosome integrity during each DNA replication thus preventing from DNA damage (constitutive heterochromatin).

Question 12

What is the result of telomere loss?

Answer 12

1. results in unstable chromosome end that can fuse with other broken chromosomes
2. be involved in recombination events or be degraded.
3. It can also involve the damage of telomere-flanking genes or important gene cells.”

Question 13

How are chromosome caps/shield made?

Answer 13

Telomeric DNA is associated with a six-member protein shelterin complex that facilitates the formation of loops which “cap/shield” the chromosome end.

Question 14

How is the telomere problem overcome?

Answer 14

ribonucleoprotein enzyme which replicates telomeric DNA by reverse transcribing DNA hexamers (TTAGGG) from RNA using its RNA subunit -TERC(Telomerase RNA Component) and its protein component -TERT

Question 15

Describe the steps of telomerase action(4)

Answer 15

1. Telomerase elongates telomeric DNA by repetition of two-steps cycle: synthesis and translocation.
2. In the lagging strand, telomerase can bind with the first few nucleotides of the template to the last telomere sequence on the chromosome
3. Adds complementary repeat sequence to the telomere sequence -GGTTAG
4. Realigns 3’ to the template.

Question 16

How are germline cells able to maintain full length telomeres?

Answer 16

They express TERT

Telomerase Reverse Transcriptase

Question 17

Give examples of germline cells

Answer 17

oocytes, sperm and their diploid progenitors

Question 18

What molecules are expressed in cancer cells?

Answer 18

TERT, p53 defects and p16 defects

Question 19

What molecule is associated with human senile defects such as CVD, and Diabetes?

Answer 19

p16 (CDKN2A)

Question 20

Recall some senescence and ageing symptoms(3)

Answer 20

1. Bone marrow
2. Hair greying linked to decreased melanocyte stem cell maintenance in hair follicles
3. Reduced healing ability of skin with age, increased risk of skin ulcers.
   Proposed to be due to senescence in dermal fibroblasts.

Question 21

What are symptoms of waardenburg syndrome?

Answer 21

white patches on skin and hair and loss of pigment in the iris (melanocytes)

Question 22

What are two examples of positive feedback in master gene regulators in differentiation?(4)

Answer 22

→Melanocytes
→MITF (MIcrophthalmia-associated Transcription Factor) -
→Skeletal Muscle
→MYOD1 family

Question 23

How does the positive feedback of melanocytes happen?(6)

Answer 23

→ MITF gene gets translated into the MITF protein

→which produces specialized proteins one of which is MC1R (melanocortin 1 receptor)

→ MSH (melanocyte-stimulating-hormone) binds to MC1R

→ activates cAMP

→activates PKA

→PKA then phosphorylates CREB and activated it to transcribe more MITF genes.

Question 24

How do the MYOD regulators work in differentiation in the embryo?(7)

Answer 24

→In migrating myoblasts MYOD1 and MYF5 are present but they are not able to work because of the IDI which blocks the MYOD from binding to the E protein.

→When the myoblasts get to their designated place, there are less growth factors such as FGF and IGF(insulin-like growth factor)

→this destabilizes the ID 1 so the MYOD1 and the MYF5 are able to work because the ID1 has degraded.

→They promote each other’s transcription and bind with the E proteins and stimulate transcription.

→the change in environment destabilizes the inhibitor

→ myogenic factors are now free to bind and make dimers (active complexes) with the E-proteins.

→these go on to activate different muscle genes

Question 25

What are the mechanisms of cell senescence?(9)

Answer 25

The telomeres shorten
→The DNA damage signal phosphorylates p53 (a tumour suppressor), activating it
→ The p53 stimulates the expression of p21 (a growth inhibitor)
→P21 inhibits CDK 1/2/4/6
→That means that there is no phosphorylation of pRB, meaning that it remains bound to E2F, blocking transcription, thus effectively arresting cell division.

→ Radiation, oxidative stress or DNA damage occurs.
→ This activates p53 (and that continues down the same path as before) and p16
→P16inhibits CDK 4/6
→ That also continues down the same path as before.

Question 26

What is the situation in normal skeletal muscle and in myoblasts expressing ID-1?

Answer 26

NORMAL MUSCLE
→MYOD1, MYF5, etc. bind and activate muscle gene promoters, working as dimers with E-proteins.

MYOBLASTS
→ID1 bind strongly to E-proteins, and prevents activation.
→ID1 has no DNA-binding domain. So ID1 inhibits differentiation.

Question 27

What are the myogenic factors?(4)

Answer 27

→MYOD1
→MYF5
→MYOG
→MRF4

Question 28

What are myogenic factors?

Answer 28

master gene regulators in skeletal differentiation.

Question 29

What are E-Proteins?

Answer 29

widely expressed as transcription factors and the myogenic factors work as dimers with E proteins.

Question 30

what is ID-1?

Answer 30

(inhibitor of differentiation) it is an inhibitory protein found in myoblasts which are migrating cells in the embryo