Topic 8A: Mutations and Gene Expression

Question 1

Definition: Mutation

Answer 1

A change in the base sequence of DNA.

Question 2

What are the different types of mutation?

Answer 2

• Substitution
• Deletion
• Addition
• Duplication
• Inversion - a sequence of bases is reversed.
• Translocation - a sequence of bases is moved from one location in the genome to another

Question 3

What are the affects of mutations?

Answer 3

1) Base sequence of DNA is changed.
2) The sequence of amino acids in the polypeptide could be changed.
3) The tertiary structure of the protein is changed.
4) Unable to function properly.

Question 4

What are hereditary mutations?

Answer 4

When a gamete containing a mutation is fertilised, meaning the mutation is present when the foetus forms. Therefore, they are passed on to the offspring.

Question 5

Why are not all mutations harmful?

Answer 5

1) The degenerate nature of the genetic code means some amino acids are coded for by more than one DNA triplet.
2) Therefore, not all types of mutation will cause a change in the amino acid sequence of a polypeptide.

Question 6

What are frameshift mutations?

Answer 6

1) Addition, duplication or deletion occurs meaning there is a change in the number of bases in the DNA code.
2) Therefore, there is a shift in the base triplets that follow, so the triplet code is read in a different way.
3) This means you get a completely amino acid sequence.

Question 7

Definition: Mutagenic agents

Answer 7

Something which increases the rate of mutations by:
• Acting as a base.
• Altering bases.
• Changing the structure of DNA.

Question 8

Definition: Acquired mutations

Answer 8

Mutations that occur after fertilisation.

Question 9

Formation of tumours: Mutations in tumour suppressor genes

Answer 9

1) When functioning normally, tumour suppressor genes slow down cell division by producing proteins which stop cells dividing and cause them to self-destruct in a process called apoptosis.
2) If a mutation occurs, the gene will becomes inactive, meaning the protein it codes for isn’t produced.
3) Therefore cells divide uncontrollably, resulting in a tumour.

Question 10

Formation of tumours: Mutations in proto-oncogenes

Answer 10

1) When functioning normally proto-oncogenes stimulate cell division by producing proteins which make cells divide.
2) If a mutation occurs, the gene becomes overactive, stimulating the cells to divide uncontrollably, resulting in a tumour.

Question 11

What is an oncogene?

Answer 11

A mutated proto-oncogene.

Question 12

Tumours: Malignant tumours

Answer 12

• Cancerous tumours which grow rapidly and invade and destroy surrounding tissues.
• Cells can break off the tumour and spread to other parts of the body in the bloodstream or lymphatic system.

Question 13

Tumours: Benign tumours

Answer 13

• Non-cancerous tumours which grow slower and contain fibrous tissue preventing them from invading other tissues.
• Often harmless, but can cause blockages or put pressure on organs.

Question 14

Features of tumour cells

Answer 14

• Large, dark nucleus.
• Irregular shape.
• Different surface antigens.
• Don’t respond to growth regulating processes.
• Divide by mitosis more frequently.

Question 15

Growth of tumours: Abnormal methylation of tumour suppressor genes

Answer 15

1) If tumour suppressor genes are hyper-methylated the proteins they produce to slow cell division aren’t made.
2) Therefore, the cells divide uncontrollably by mitosis and a tumour develops.

Question 16

Growth of tumours: Abnormal methylation of proto-oncogenes

Answer 16

1) If proto-oncogenes are hyper-methylated they act as oncogenes, causing increased production of the protein that causes cell division.
2) Therefore, the cells divide uncontrollably by mitosis and a tumour develops.

Question 17

Growth of tumours: Oestrogen

Answer 17

1) Oestrogen stimulates breast cells to divide and replicate, meaning the more oestrogen you have, the higher the rate of cell division and so the chances of a mutation occurring are increased.
2) Also, if cells do become cancerous, their rapid replication could be further assisted by oestrogen, helping tumours to grow quickly.

Question 18

Definition: Stem cell

Answer 18

An unspecialised cell which has the ability to develop into other types of cell.

Question 19

Stem cells: Totipotent

Answer 19

Totipotent stem cells are stem cells which are able to develop into any type of body cell.

Question 20

Stem cells: Pluripotent

Answer 20

Pluripotent stem cells are stem cells which are able to develop into any type of body cell, apart from placental cells.

Question 21

Stem cells: Multipotent

Answer 21

Multipotent stem cells are only able to divide into a few different types of cell.

Question 22

Stem cells: Unipotent

Answer 22

Unipotent stem cells are only able to develop into one type of cell.

Question 23

How do stem cells become specialised?

Answer 23

1) Stem cells become specialised because during their development they only transcribe and translate part of their DNA.
2) This means only certain genes are expressed and the others are switched off.
3) Expressed genes get transcribed into mRNA which is then translated into proteins which modify the cell.
4) This causes the cell to become specialised.

Question 24

What are cardiomyocytes?

Answer 24

Heart muscle cells which can only be replaced (if damaged) by new cardiomyocytes derived from a small store of unipotent stem cells in the heart.

Question 25

Existing stem cell therapies: Bone marrow transplants

Answer 25

• Bone marrow contains stem cells that can become specialised to form any type of blood cell, meaning bone marrow transplants can be used to replace faulty bone marrow in patients that produce abnormal blood cells.
• The stem cells in the transplanted bone marrow divide and specialise to produce healthy blood cells.

Question 26

Future stem cell therapies

Answer 26

• Stem cells could be used to replace damaged nerve tissue.
• Stem cells could be used to replace damaged heart tissue.
• Stem cells could be used to grow whole bladders which are used to replace diseased ones.
• Stem cells can be used to grow whole organs.

Question 27

Sources of stem cells: Adult stem cells

Answer 27

• Obtained from the body tissue of an adult.

✔️ Relatively easy to obtain with little risk, only discomfort.

✖️ Only able to specialise into a limited range of cells.

Question 28

Sources of stem cells: Embryonic stem cells

Answer 28

• Obtained from embryos at an early stage of development.

✔️ Can divide an unlimited number of times and develop into all types of body cell.

✖️ Embryos are destroyed - right to life?

Question 29

Sources of stem cells: Induced pluripotent stem cells

Answer 29

• Created by scientists in a lab by reprogramming adult body cells to become pluripotent.

✔️ Can divide an unlimited number of times and develop into all types of body cell.
✖️ More research needed before they are properly used.

Question 30

Why is transcription controlled?

Answer 30

To determine which genes are expressed, and which are not, to allow proteins to carry out their specific functions.

Question 31

Function: Transcription factors

Answer 31

They bind to promoter regions near the start of target genes, and control expression by controlling the rate of transcription.

Question 32

Transcription factors: Activators

Answer 32

Stimulate or increase the rate of transcription by helping RNA polymerase to bind to the start of gene and activate transcription.

Question 33

Transcription factors: Repressors

Answer 33

Inhibit or decrease the rate of transcription by binding to the start of the target gene, preventing RNA polymerase from binding, stopping transcription.

Question 34

Factors affecting gene expression (transcription): Oestrogen

Answer 34

1) Oestrogen binds to an oestrogen receptor to form an oestrogen-oestrogen receptor complex.
2) This moves from the cytoplasm into the nucleus, where it binds to specific DNA sites near the start of the target gene.
3) This helps RNA polymerase to bind to the start of the target gene, causing the gene to be expressed.

Question 35

Factors affecting gene expression (translation): siRNA

Answer 35

1) Once mRNA has been transcribed it leaves the nucleus and enters the cytoplasm.
2) Here, double-stranded siRNA associates with several proteins and unwinds.
3) One of the resulting single strands of siRNA is broken down and the other is selected.
4) The single strand of siRNA then binds to the target mRNA, through complementary base pairing.
5) The proteins associated with the siRNA cut the mRNA into fragments, so that it can no longer be translated, and is instead degraded.
6) Therefore, the gene isn’t expressed.

Question 36

Definition: Gene expression

Answer 36

When a gene is transcribed and used to make a protein.

Question 37

Factors affecting gene expression (translation): miRNA

Answer 37

1) Once miRNA has been transcribed it leaves the nucleus and enters the cytoplasm, as a long folded double strand.
2) Here, it associates with several proteins and unwinds.
3) One of the resulting single strands of miRNA is broken down and the other is selected.
4) The single strand of miRNA then binds to the target mRNA, meaning the target mRNA isn’t translated.
5) The mRNA is then degraded and the gene isn’t expressed.

Question 38

What is epigenetic control?

Answer 38

The attachment or removal of chemical groups (called epigenetic marks) to or from DNA/Histone proteins to determine whether or not a gene is expressed.

Question 39

How do epigenetic marks work?

Answer 39

They alter how easy it is for the enzymes and other proteins needed for transcription to interact with and transcribe DNA.

Question 40

Controlling gene expression: Increased methylation of DNA

Answer 40

1) Methyl group is attached to the DNA.
2) This changes the DNA structure so that transcriptional machinery (such as enzymes) cannot interact with the gene meaning it is not expressed.

Question 41

Controlling gene expression: Decreased acetylation of histones

Answer 41

1) By adding an acetyl group, the chromatin is less condensed, allowing the transcriptional machinery to access the DNA, allowing genes to be transcribed.
2) By removing an acetyl group, the chromatin becomes more condensed, meaning the transcriptional machinery is unable to access the DNA, and therefore the genes cannot be transcribed.