Topic 7.2 Factors Affecting Gene Expression

Question 1

RNA splicing

Answer 1

• Modifications of pre-mRNA involve the removal of introns and exons.
• The exons are joined together with splicosomes to produce mature, functional mRNA.
• The splicosomes may join the same exons in a variety of ways; producing different phenotypes.

Question 2

Epigenetics

Answer 2

• Changes that affect gene activation.
  –> Involved in differentiation.
  –> Involved in changes in proteins synthesised.
• Factors that affect gene control are:
  -DNA methylation
  -histone modification
  -non-coding RNA

Question 3

Regulation of gene expression

Answer 3

• Genes can be switched off when they are not needed.
• This prevents cellular resources being wasted.
• Typical human cells express 3-5% of their genes at any given time.
• Cancer results when genes do not turn off properly.

Question 4

DNA methylation

Answer 4

• Addition of a methyl group to a base.
• Added by DNA methyltransferase enzyme.
• Can also modify the structure of the histones.
• Changes arrangement of DNA and prevents transcription.
• Switch off genes which cannot be switched on so proteins cannot be transcribed.

Question 5

DNA demethylation

Answer 5

• Removal of methyl group.
• Genes become active so can be transcribed.

Question 6

Histone modification

Answer 6

• Positively charged proteins.
• DNA helices wind around histones to form chromatin.
• Makes up chromosomes
• When densely supercoiled and condensed genes are not available to be copied (heterochromatin).
• Active chromatin is loose with uncoiled regions opening up more genes for transcription.

Question 7

Histone acetylation

Answer 7

• Acetyl group is added to one of the lysines in histone.
• Opens up structure and activates chromatin allowing transcription
• Removing group produces heterochromatin again.

Question 8

Histone methylation

Answer 8

• Methyl group is added to a lysine in histone.
• Depending on the position of lysine methylation may cause inactivation of the DNA or activation of a region.
• Methylation is often linked to silencing of a gene.

Question 9

Transcription factors

Answer 9

• A protein that controls the transcription of genes so that only certain parts of the DNA are expressed.
  -These proteins are which move in from the cytoplasm into the nucleus and bind to specific DNA sites called promoters.
  -Promoters are found at the start of their target gene.

Question 10

The transcription factors can be:

Answer 10

• Activators: stimulate or increase the rate of the transcription
  -Help RNA polymerase to bind to the start of the target gene and activate transcription.
• Repressor: inhibit or decrease the rate
  -Inhibit or decrease the rate by preventing RNA polymerase from binding to the target gene, therefore stopping transcription.
  -They create a loop in the DNA molecule that activates the gene. This allows RNA polymerase to attach to the DNA chain and transcription will start.

Question 11

Gene probes

Answer 11

Gene probes identify a particular section of DNA and mRNA.

Question 12

Gene expression in action

Answer 12

• Each cell type produces specific proteins that relate to the particular function of the cell.
• This means that different genes must be expressed in different types of cells.
• By comparing the proteins found in different types of cells, scientists can measure the level of differentiation that has taken place and work out which genes have been expressed and which had been suppressed.

Question 13

Controlling gene expression

Answer 13

• The Human genome Project found that the human genome has 20,000-25,000 individual genes
• In a differentiated cell, between 10,000 and 20,000 of those genes are actively expressed
• Different combinations are expressed in different cells, creating the variety of structure and function seen in cells of different tissues.

Question 14

Control of gene expression

Answer 14

• Gene expression is controlled by switching on and off the transcription of genes.
• Several different transcription factors will be involved in the expression of a gene giving many levels of control.

Question 15

Promoter sequences

Answer 15

Are usually found just above the starting point for transcription upstream of the gene

Question 16

Enhancer sequences

Answer 16

Regulate the activity of DNA by changing the structure of chromatin, making it open to RNA polymerase.

Question 17

Transcription

Answer 17

The genetic code of the DNA is copied to a complimentary strand of RNA before protein synthesis can take place.

Question 18

Transcription factors

Answer 18

Protein that bind to the DNA in the nucleus and affect the process of transcribing the genetic material.

Question 19

Stem cells

Answer 19

Stem cells are unspecialised cells that divide and develop into other types of cell.

Question 20

Becoming specialised

Answer 20

Genes that are expressed get transcribed into mRNA which is then translated. These proteins will then modify the cell (eg. determine structure, cell processes).

Question 21

Totipotent

Answer 21

• Can differentiate into any cell type.
• Occur for only a short length of time in mammalian embryos.
  –> Zygote.

Question 22

Pluripotent

Answer 22

• Can create most cell types.
• Occur in embryo can create most cell types.
  –> Blastocyst.

Question 23

Multipotent

Answer 23

• Can only produce a limited number of cell types.
• Found in mature mammals.
  –> Cells in embryo.

Question 24

Unipotent

Answer 24

Found in mature mammals, can only differentiate into one cell type

Question 25

Epigenetic control of stem cells

Answer 25

• The zygote starts as a totipotent and then cells become pluripotent a they divide.
• This occurs due to transcription factors and epigenetic controls switching on and off the transcription of certain genes.
• As development progresses more genes become silenced, and thus become more specialised.

Question 26

Stem cell therapy

Answer 26

Scientists are trying to find ways of using stem cells to treat medical conditions where the patient’s own cells are damaged or faulty.

Question 27

Therapeutic cloning

Answer 27

Experimental technique to produce large quantities of healthy tissue (eg. cells to replace damaged pancreas in someone with type 1 diabetes).
1) Remove the nucleus from a patients normal body cell
2) Transfer it to human ovum which has had its nucleus removed, fuse them with an electric shock which further causes development to take place
3) Pre embryo starts to develop and divide, producing embryonic stem cells
4) The genetics perfectly match the patient. The stem cells can be collected and cultured in a suitable environment to differentiate into the tissue needed.
5) They can then be transferred back into the patient.

Question 28

Induced pluripotent stem cells (iPS cells)

Answer 28

• Adult mouse cells and reprogrammed to become pluripotent again.
• They have introduced certain transcription factors back into the cells which allows them to become pluripotent.
  (Removes the ethical issues of stem cells and also removes the risk of rejection).
  –> Adult cells made into pluripotent cells.

Question 29

Pros and cons of stem cell therapy

Answer 29

+Revolutionise medicine
+Save lives
+Cure diseases
+Avoid risk of rejection
-Not enough research
-Could cause the development of cancers in the body
(This is evidence that people who have had bone marrow transplants are at higher risk of developing other cancers later)

Question 30

How can histone modification affect gene expression?

Answer 30

• Addition of an acetyl group activates chromatin, allowing transcription.
• Addition of a methyl group can either activate or inactivate chromatin depending on the position of the lysine.

Question 31

How can DNA methylation affect gene expression?

Answer 31

• Involves addition of CH3 group to cytosine bases, which prevents transcription factors from binding.
• Therefore gene expression is suppressed.

Question 32

How are epigenetic involved in cell differentiation?

Answer 32

• Epigenetics control the amount of a cells DNA that is transcribed.
• So ensures only the proteins needed for that specific cells function are produced.

Question 33

How do totipotent cells develop into other types of stem cell?

Answer 33

• Epigenetic modifications result in selective translation of the relevant parts of DNA.
• Totipotent cells develop into pluripotent cells in the early stages of embryonic development.
• Then later into fully differentiated somatic cells.

Question 34

Similarities between iPS and embryonic

Answer 34

• Potential to divide indefinitely.
• Potential to differentiate into a number of different cell types.