Differentiation and Gene Expression

Question 1

Define cell differentiation

Answer 1

the process through which a cell undergoes changes in gene expression and gene activity to specialise and take on specific roles in an organism

Question 2

Define stem cells

Answer 2

undifferentiated and unspecialized cells of the human body

Question 3

Recall the different types of stem cells

Answer 3

• Totipotent: Can differentiate into every type of cell in the body. Eg. Zygote
• Pluripotent: Can differentiate into most, but not all type of cells like extraembryonic cells. Eg embryonic- cells present within the blastocyst that forms after fertilisation in humans
• Multipotent: Can differentiate into a limited number of cell types. Eg. Hematopoietic and mesenchymal stem cells

• Unipotent: Can form one type of differentiated cell only eg. Spermatogonial stem cells and dermatocytes
“

Question 4

Describe the difference between asymmetric and symmetric stem cells

Answer 4

Asymmetric: A stem cell produces one differentiated cell and one stem cell
Symmetric: A stem cell produces two differentiated cells or two stem cells

Question 5

How does asymmetric division arise?

Answer 5

· A fate regulator (e.g. polarity protein) distributes unequally in the daughter cells and makes the different daughter cells

Question 6

What is significant about pluripotent embryonic stem cells?

Answer 6

Have been shown to be transiently existing cells, not stem cells.

They differentiate so quickly that sometimes they are not considered stem cells.

Question 7

Describe adult stem cells (4)

Answer 7

· Multipotent

· Typically give rise to various types of differentiated cells within the tissue they reside in. Tissue-specific stem cells.

· Their function: dead cells replacement (cell turnover).

· Low number in tissues where low rate of cell turnover (adult brain) and abundant in tissues such as intestine epithelium and blood cells because the number of cells dying is a lot.
“

Question 8

What are hemaopoietic stem cells?

Answer 8

· multipotent stem cells anchored to fibroblast-like osteoblasts of the marrow of long bones.

Produce all blood cells and some immune system cells. Regular self-renewal.

Question 9

What are mesenchymal stem cells?

Answer 9

· stromal cells found in bone marrow and other organs. Poorly defined and heterogeneous.

Question 10

What type of cells can mesencgymal cells give rise to?

Answer 10

cartilage (chondrocytes), bone (osteoblasts) and muscle cells (myocytes) and adipocytes

Question 11

Describe the difference between hematopoietic and oligopotent cells(2)

Answer 11

1. Oligopotent because their potency to differentiate is lower than the hematopoietic stem cells.
2. Hematopoietic can replicate indefinitely but oligopotent have finite replication.”

Question 12

Describe general transcription factors using TFII and sigma factor.

Answer 12

· Most basic set of factors and bind to five prime regions

· The ability of sigma factor and TFII to recruit RNA polymerases to promoters are generic because they happen at every promotor.

This does not account for the ability to vary the level of transcription from a promotor

Question 13

Describe specific transcription factors

Answer 13

· differentially regulate expression by binding to enhancing region or the promotor regions.
Critical to ensure right genes are expressed in right cell of the right amount.

Question 14

What are housekeeping genes?

Answer 14

Some genes are transcribed in all or nearly all cell types

Question 15

What are pioneer factors?

Answer 15

Transcription factors that can bind condensed/close chromatin, remodel it (open it and close it) and initiate cell-fate and differentiation (master regulators)

Question 16

Give examples of pioneer factors

Answer 16

OCT4, BMP4, SOX2 and NANOG

Question 17

What are some process that pioneer factors do?(3)

Answer 17

· Can activate or inhibit gene expression

· Histone modification

· DNA methylation blockage- they can increase or decrease accessibility

Question 18

Why are pioneer factors highly expressed in embryonic stem cells?

Answer 18

· needed to maintain their pluripotency

Question 19

Describe DNA methylation(5)

Answer 19

1. Covalent addition of the methyl group at the 5-carbon of the cytosine ring resulting in 5-methylcytosine (5-mC)
2. In somatic cells, 5-mC occurs on CpG sites- located upstream before transcription site
3. Exception: embryonic stem cells, where is also present in non-CpG regions.
4. Methylation pattern is “remembered” in daughter cells.
5. During differentiation unmethylated CpG pairs can become methylated by a de novo methyltransferase.
6. Copied to opposite strand (also CpG) by a maintenance methyltransferase

Question 20

What are CpG islands?

Answer 20

1. CpG islands are associated with genes, particularly housekeeping genes, in vertebrates.
2. CpG islands are typically common near transcription start sites and may be associated with promoter regions.

Question 21

What are the roles of CpG methylation?(3)

Answer 21

· Methylation of a gene (especially its promoter or control sequence)

2. increase folding (heterochromatin) 3.silencing transcription.
3. When transcription is needed the CpG islands are demethylated”

Question 22

Are the majority of CpG islands methylated in embryonic stem cells?

Answer 22

· In embryonic stem cells the majority of CpG sites are unmethylated

Question 23

Describe the process of Somatic Cell Nuclear Transfer(3)

Answer 23

1· The zygote the methylation is removed so silenced genes are then switched on

2· Introduced nucleus is reprogrammed by factors in the egg cytoplasm.

3.The new egg behaves like a zygote (totipotent)Dolly the SheepIt is time consuming”

Question 24

Describe induced pluripotent stem cells(4)

Answer 24

1· Pluripotent stem cells artificially produced from somatic cells.

2· Although differentiation is not reversible, this is an exception

3· Can potentially produce almost all cells of the organisms
Cells are exposed to only four transcription factors.
4. Depending on the type of cells add different factors

Question 25

Recall the 4 transcription factors used in iPSCs

Answer 25

“OCT4, SOX2, KLF4 and c-MYC(Yamanaka’s cocktail)”

Question 26

Why are iPSCs significant?(3)

Answer 26

· gene therapy

· regenerative medicine

· model disease and drug screening

Question 27

How do master gene regulators work?(4)

Answer 27

→Transcription of the gene for the master gene regulator occurs

→ creating the regulatory proteins

→These proteins then bind to and activate the promoters of different genes coding for proteins that will differentiate the cell.

→these specialised proteins are made, and then proceed to work on the cell.