Mutations and Gene expression

Question 1

what is a gene mutation

Answer 1

● A change in the base sequence of DNA
● Can arise spontaneously during DNA replication (interphase)

Question 2

What is a mutagenic agent?

Answer 2

A factor that increases rate of mutation, eg. ultraviolet (UV) light or alpha particles

Question 3

Explain how a gene mutation can lead to the production of a non-functional protein or enzyme

Answer 3

1. Changes sequence of base triplets in DNA so changes sequence of codons on mRNA
2. So changes sequence of amino acids in the encoded polypeptide
3. So changes position of hydrogen / ionic / disulphide bonds (between amino acids)
4. So changes tertiary structure (shape) of protein
5. Enzymes - active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form

Question 4

what is a substitution gene mutation

Answer 4

A base / nucleotide is replaced by a different base / nucleotide in DNA

Question 5

what is an addition gene mutation

Answer 5

1 or more bases / nucleotides are added to the DNA base sequence

Question 6

what is a deletion gene mutation

Answer 6

1 or more bases / nucleotides are lost from the DNA base sequence

Question 7

what is a duplication gene mutation

Answer 7

A sequence of DNA bases / nucleotides is repeated / copied

Question 8

what is an inversion gene mutation

Answer 8

A sequence of bases / nucleotides detaches from the DNA sequence, then rejoins at the same position in the reverse order

Question 9

what is a translocation gene mutation

Answer 9

A sequence of DNA bases / nucleotides detaches and is inserted at a different location within the same or a different chromosome

Question 10

Explain why not all gene mutations affect the order of amino acids

Answer 10

● Some substitutions change only 1 triplet code / codon which could still code for the same amino acid
○ As the genetic code is degenerate (an amino acid can be coded for by more than one triplet)
● Some occur in introns which do not code for amino acids

Question 11

explain why a change in amino acid sequence is not always harmful

Answer 11

● May not change tertiary structure of protein (if position of ionic / disulphide / H bonds don’t change)
● May positively change the properties of the protein, giving the organism a selective advantage

Question 12

What are stem cells?

Answer 12

Undifferentiated / unspecialised cells capable of:
1. Dividing (by mitosis) to replace themselves indefinitely
2. Differentiating into other types of (specialised) cells

Question 13

Explain what is meant by a frameshift

Answer 13

● occurs when gene mutations change the number of nucleotides / bases by any number not divisible by 3
● This shifts the way the genetic code is read, so all the DNA triplets/ mRNA codons downstream from the mutation change
● The sequence of amino acids encoded changes accordingly and the effects on the encoded polypeptide are significant

Question 14

Describe how stem cells become specialised during development

Answer 14

● Stimuli lead to activation of some genes (due to transcription factors )
● So mRNA is transcribed only from these genes and then translated to form proteins
● These proteins modify cells permanently and determine cell structure / function

Question 15

Describe totipotent cells

Answer 15

● Occur for a limited time in early mammalian embryos
● Can divide AND differentiate into any type of body cell (including extra-embryonic cells eg. placenta)

Question 16

Describe pluripotent cells

Answer 16

● Found in mammalian embryos (after first few cell divisions)
● Can divide AND differentiate into most cell types (every cell type in the body but not placental cells)

Question 17

Describe multipotent cells

Answer 17

● Found in mature mammals
● Can divide AND differentiate into a limited number of cell types

Question 18

Describe unipotent cells, using an example

Answer 18

● Found in mature mammals
● Can divide AND differentiate into just one cell type

Question 19

Explain how stem cells can be used in the treatment of human disorders

Answer 19

● Transplanted into patients to divide in unlimited numbers
● Then differentiate into required healthy cells (to replace faulty / damaged cells)

Question 20

give examples of how stem cells are used in the treatment of human disorders

Answer 20

● Potential treatment of Type 1 diabetes by creating healthy islet cells that produce insulin
● Bone marrow stem cell transplant for SCD / blood cancers
1. Destroy patient’s bone marrow before treatment → so no faulty cells are produced
2. Transplant stem cells from healthy person → divide and differentiate into healthy cells

Question 21

Explain how induced pluripotent stem (iPS) cells are produced

Answer 21

1. Obtain adult somatic (body) cells (non-pluripotent cells or fibroblasts) from patient
2. Add specific protein transcription factors associated with pluripotency to cells so they express
   genes associated with pluripotency (reprogramming)
   ○ Transcription factors attach to promoter regions of DNA, stimulating or inhibiting transcription
3. Culture cells to allow them to divide by mitosis

Question 22

What are transcription factors?

Answer 22

● Proteins which regulate (stimulate or inhibit) transcription of specific target genes in eukaryotes
● By binding to a specific DNA base sequence on a promoter region

Question 23

Describe how transcription can be regulated using transcription factors

Answer 23

1. Transcription factors move from cytoplasm to nucleus
2. Bind to DNA at a specific DNA base sequence on a promoter region (before / upstream of target gene)
3. This stimulates or inhibits transcription (production of mRNA) of target gene(s) by helping or
   preventing RNA polymerase binding

Question 24

Explain how oestrogen affects transcription

Answer 24

1. Oestrogen is a lipid-soluble steroid hormone so diffuses into cell across the phospholipid bilayer
2. In cytoplasm, oestrogen binds to its receptor, an inactive transcription factor, forming an oestrogen-receptor complex
3. This changes the shape of the inactive transcription factor, forming an active transcription factor
4. The complex diffuses from cytoplasm into the nucleus
5. Then binds to a specific DNA base sequence on the promoter region of a target gene
6. Stimulating transcription of target genes forming mRNA by helping RNA polymerase to bind

Question 25

Explain why oestrogen only affects target cells

Answer 25

Other cells do not have oestrogen receptors.

Question 26

Describe what is meant by epigenetics

Answer 26

● Heritable changes in gene function / expression without changes to the base sequence of DNA ● Caused by changes in the environment (eg. diet, stress, toxins)

Question 27

Describe what is meant by epigenome

Answer 27

All chemical modification of DNA and histone proteins - methyl groups on DNA and acetyl groups on histones.

Question 28

explain how methylation can inhibit transcription

Answer 28

1. Increased methylation of DNA - methyl groups added to cytosine bases in DNA 2. So nucleosomes (DNA wrapped around histone) pack more tightly together 3. Preventing transcription factors and RNA polymerase binding to promoter

Question 29

explain how acetylation can inhibit transcription

Answer 29

1. Decreased acetylation of histones increases positive charge of histones 2. So histones bind DNA (negatively charged) more tightly 3. Preventing transcription factors and RNA polymerase binding to promoter

Question 30

what factors can lead to epigenetic changes

Answer 30

Environmental factors (eg. diet, stress, toxins)

Question 31

What is RNA interference (RNAi)?

Answer 31

● Inhibition of translation of mRNA produced from target genes, by RNA molecules eg. siRNA, miRNA ● This inhibits expression of (silencing) a target gene

Question 32

Describe the regulation of translation by RNA interference

Answer 32

1. Small interfering RNA (siRNA) or micro-RNA (miRNA) binds to a protein, forming an RNA-induced silencing complex (RISC) ○ siRNA synthesised as double-stranded RNA → 1 strand incorporated ○ miRNA synthesised as a double-stranded hairpin bend of RNA → both strands incorporated 2. Single-stranded miRNA / siRNA within RISC binds to target mRNA with a complementary base sequence 3. This leads to hydrolysis of mRNA into fragments which are then degraded OR prevents ribosomes binding 4. Reducing / preventing translation of target mRNA into protein

Question 33

Describe how tumours and cancers form

Answer 33

● Mutations in DNA / genes controlling mitosis can lead to uncontrolled cell division ● Tumour formed if this results in mass of abnormal cells ○ Malignant tumour = cancerous, can spread by metastasis ○ Benign tumour = non-cancerous

Question 34

main characteristics of benign tumours

Answer 34

-Usually grow slowly (cells divide less often) -Cells are well differentiated / specialised -Cells have normal, regular nuclei -Well defined borders and often surrounded by a capsule so do not invade surrounding tissue -Do not spread by metastasis (as cell adhesion molecules stick cells together -Can normally be removed by surgery and they rarely return

Question 35

main characteristics of malignant tumours

Answer 35

-Usually grow faster (cells divide more often) -Cells become poorly differentiated / unspecialised -Cells have irregular, larger / darker nuclei -Poorly defined borders and not encapsulated so can invade surrounding tissues -Spread by metastasis - cells break off and spread to other parts of the body, forming secondary tumours (due to lack of adhesion molecules) -Can normally be removed by surgery combined with radiotherapy / chemotherapy but they often return

Question 36

Describe the function of tumour suppressor genes

Answer 36

Code for proteins that: ● Inhibit / slow cell cycle (eg. if DNA damage detected) ● OR cause self-destruction (apoptosis) of potential tumour cells (eg. if damaged DNA can’t be repaired)

Question 37

Explain the role of tumour suppressor genes in the development of tumours

Answer 37

● Mutation in DNA base sequence → production of non functional protein ○ By leading to change in amino acid sequence which changes protein tertiary structure ● Decreased histone acetylation OR increased DNA methylation → prevents production of protein ○ By preventing binding of RNA polymerase to promoter region, inhibiting transcription ● Both lead to uncontrolled cell division (cell division cannot be slowed)

Question 38

Describe the function of proto-oncogenes

Answer 38

Code for proteins that stimulate cell division

Question 39

Explain the role of oncogenes in the development of tumours

Answer 39

● Mutation in DNA base sequence → overproduction of protein OR permanently activated protein ○ By leading to change in amino acid sequence which changes protein tertiary structure ● Decreased DNA methylation OR increased histone acetylation → increases production of protein ○ By stimulating binding of RNA polymerase to promoter region, stimulating transcription ● Both lead to uncontrolled cell division (cell division is permanently stimulated)

Question 40

Suggest why tumours require mutations in both alleles of a tumour suppressor gene but only one allele of an oncogene

Answer 40

● One functional allele of a tumour suppressor gene can produce enough protein to slow the cell cycle ● One mutated oncogene allele can produce enough protein to lead to rapid / uncontrolled cell division

Question 41

Explain the relevance of epigenetics in cancer treatment

Answer 41

Drugs could reverse epigenetic changes that caused cancer, preventing uncontrolled cell division. For example: ● Increasing DNA methylation OR decreasing histone acetylation of oncogene ○ To inhibit transcription / expression ● Decreasing DNA methylation OR increasing histone acetylation of tumour suppressor gene ○ To stimulate transcription / expression

Question 42

Explain the role of increased oestrogen concentrations in the development of some (oestrogen receptor-positive) breast cancers

Answer 42

1. Some breast cancers cells have oestrogen receptors, which are inactive transcription factors 2. If oestrogen concentration is increased, more oestrogen binds to oestrogen receptors, forming more oestrogen-receptor complexes which are active transcription factors 3. These bind to promoter regions of genes that code for proteins stimulating cell division 4. This increases transcription / expression of these genes, increasing the rate of cell division

Question 43

Suggest how drugs that have a similar structure to oestrogen help treat oestrogen receptor-positive breast cancers

Answer 43

● Drugs bind to oestrogen receptors (inactive transcription factors), preventing binding of oestrogen ● So no / fewer transcription factors bind to promoter regions of genes that stimulate the cell cycle