Year 13 - Gene expression

Question 1

Describe the following biological terms and when they are found as per the spec:​
- Stem cells ​
- Totipotent​
- Pluripotent ​
- Multipotent ​
- Unipotent

Answer 1

Stem cells- cells that can divide by mitosis into two genetically identical cells. One remains as stem cell and the other differentiates into a specialised cells by translating part of their DNA. ​

Totipotent- divide and differentiate into any type of cell and can produce a whole new organism. Found up to 8 cell stage in early mammalian embryos for a limited time. ​

Pluripotent- divide unlimited times and differentiate into any type of cell. Found in an embryo. ​

Multipotent- divide a limited number of times and differentiate into a limited number of different cell types. Found in mature mammal tissues eg bone marrow. ​

Unipotent- divide a limited number of times and differentiate into one cell type. Found in the heart as cardiomyocytes.

Question 2

Suggest how ’molecule’ increasing gene expression works

Answer 2

->molecule is a Transcription factor ​
->binds to promotor region in DNA as complementary ​
-> stimulates RNA polymerase ​
-> transcription.

Question 3

Describe how oestrogen acts as a transcription factor

Answer 3

• Oestrogen is lipid soluble so diffuses through phospholipid cell surface membrane and nuclear envelope. ​
• Oestrogen is complementary to oestrogen receptor so binds. ​
• This changes tertiary structure of PROTEIN RECEPTOR (NOT OESTROGEN AS THIS IS NOT A PROTEIN) ​
• Releasing the transcription factor ​
• Transcription factor is complementary to a specific sequence of DNA known as the promotor region ​so binds
• Stimulates RNA polymerase to transcribe the gene, increasing transcription so mature mRNA is produced.

Question 4

How do interfering RNA (RNAi eg siRNA and miRNA) work?

Answer 4

• siRNA/miRNA is specifically Complementary to named genes/protein’s mRNA​
• So binds by H bonds ​
• Results in destruction of mRNA OR stops ribosome binding to mRNA so stops translation initiating.​
• So reduces translation of named protein ​
• So reduces named protein in cell so less named protein function

Question 5

Define epigenetics

Answer 5

Heritable changes in gene function​ without changes to the base sequence of DNA

Question 6

Describe and explain the epigenetic modifications that keep genes turned off.

Answer 6

• Increased methylation of DNA means named transcription factor can’t bind to promotor region of DNA
• Decreased acetylation of histone protein tails (more tightly packed)
• less expression of gene
• so less RNA polymerase binds to DNA next to promotor
• less mRNA of named gene produced
• less named protein
• less named proteins function

Question 7

Describe and explain the epigenetic modifications that keep genes turned on.

Answer 7

• Decreased methylation of DNA means named transcription factor binds to promotor region of DNA
• Increased acetylation of histone protein tails (loosely packed)
• more expression of gene
• so RNA polymerase binds to DNA next to promotor region
• transcribes *named gene8 producing more mRNA
• so more named protein
• so more named protein function

Question 8

Describe the purpose of tumour suppressor and proto-oncogenes

Answer 8

Tumour suppressor genes code for polypeptides that stop cell division

Proto-oncogenes code for polypeptides that start cell division

Question 9

What is a tumour?

Describe the difference between malignant and benign tumours

Answer 9

A mass of cells from uncontrolled mitosis

Malignant = Cells have metastasised and can spread around the body​

Benign = cells can’t metastasise and will not spread to other parts of the body.

Question 10

Describe how tumours could form because of epigenetic modifications to tumour suppressor genes

Answer 10

More methylation of tumour suppressor gene DNA (promotor region/ both copies of gene), ​

less acetylation of histone proteins around tumour suppressor gene​

Less/no expression (inactivation) of named tumour suppressor gene​

Less/no transcription of gene ​

Less/no mRNA formed​

Less/no translation of it’s mRNA ​

Less/no named protein that stops cell cycle​

leads to uncontrolled cell division.

Question 11

Describe how tumours could form because of epigenetic modifications to proto-oncogenes

Answer 11

less methylation of named proto-oncogene DNA (promotor region/ both copies of gene), ​

More acetylation of histone proteins around named proto-oncogene​

More expression (inactivation) of named proto-oncogene​

More transcription of gene ​

More mRNA formed​

More translation of it’s mRNA ​

More named protein that starts cell cycle​

leads to uncontrolled cell division.

Question 12

Describe the result of mutations to promotor regions. ​

1. Tumour suppressor genes​
2. proto-oncogenes.

Answer 12

Tumour suppressor genes:​

Mutation in promotor regions for both tumour suppressor genes ​
Transcription factor cannot bind ​
So RNA polymerase is not stimulated ​
So no transcription of gene ​
no mRNA formed​
no translation -> less protein -> uncontrolled cell division

Proto-oncogenes:

Mutation in promotor regions for proto-oncogenes​
Transcription factor cannot bind ​
So RNA polymerase is not stimulated ​
So no transcription of gene ​
no mRNA formed​
no translation -> less protein -> can’t start the cell cycle.