T8 The Control Of Gene Expression

Question 1

What is a mutation and when does it occur?

Answer 1

Change in base sequence of DNA
Arises spontaneously during cell replication (in interphase)

Question 2

What is a mutagenic agent? Give some examples

Answer 2

Something that increases the rate of genetic mutations occurring e:g ionising radiation (x-rays), carcinogens (cigarettes), some viruses

Question 3

What are the 6 types of gene mutation?

Answer 3

Addition, Substitution, Deletion, Duplication, Inversion, Translocation

Question 4

Explain addition

Answer 4

1 or more bases added to base sequence
RESULTS IN frameshift

Question 5

Explain substitution

Answer 5

1 base replaces another
RESULTS IN no change due to degenerate nature of genetic code OR 1 triplet/codon change > 1 amino acid change

Question 6

Explain duplication

Answer 6

A sequence of bases is inserted twice or multiple times
RESULTS IN frameshift

Question 7

Explain deletion

Answer 7

1 base removed from the base sequence
RESULTS IN frameshift

Question 8

Explain inversion

Answer 8

A sequence of bases is separated
from DNA and inserted at the same position, backwards
RESULTS IN no frameshift (no of bases = same) but triplets/codons in inverted region change > different sequence of amino acids

Question 9

Explain translocation (mutation)

Answer 9

Sequence of bases taken out and inserted at a different position on the same or diff chromosome
RESULTS IN diff amino acid sequence at old and new therfore diff gene expression

Question 10

What is frameshift?

Answer 10

Frameshift; triplets / codons change
downstream of mutation → amino acid sequence changes
OR
• If multiple of 3 bases lost – no frameshift, but missing triplets / codons → missing amino acids

Question 11

How can a mutation create a non functional polypeptide?

Answer 11

Stop codons terminate translation, Substitution or frameshift can produce premature stop codons shortening the length of the polypeptide producing a shorter non functional polypeptide

Question 12

How do mutations produce a non functional protein or enzyme? (6)

Answer 12

1. Change in base / triplet sequence of DNA / gene
2. Changes sequence of codons on mRNA
3. Changes sequence of amino acids in primary structure of polypeptide
4. Changes position of hydrogen / ionic / disulphide bonds in protein tertiary structure
5. Changes tertiary structure / shape of protein and in the case of enzymes, the active site
   will change shape
6. In the case of enzymes, the substrate will be unable to bind to active site and form an enzyme-substrate complex

Question 13

What is a stem cell? Give its two characteristics

Answer 13

Unspecialised cell
self renewal (can divide to replace itself)
potency (have the capacity to differentiate into different cell types)

Question 14

Cell differentiation is

Answer 14

The process by which cells become specialised for different functions

Question 15

4 types of stem cell are

Answer 15

Totipotent
Pluripotent
Multipotent
Unipotent

Question 16

Totipotent stem cell

Answer 16

Can differentiate into any type of cell & form an entire organism. Present in a zygote & the early
embryo

Question 17

Pluripotent cell

Answer 17

Can form most cell types. Usually derived from the inner cell mass of the blastocyst (Embryonic
& fetal stem cells).

Question 18

Multipotent stem cell

Answer 18

Can differentiate into a number of closely related cell types (e.g. haemopoietic adult stem cells in
bone marrow, umbilical cord blood)

Question 19

Unipotent stem cell

Answer 19

Differentiate into a single cell type Maintain and repair adult tissues

Question 20

How are induced pluripotent stem cells (iPS cells) made?

Answer 20

Unipotent cells that have been converted back to pluripotent stem cells

1) Isolate somatic cells and grow
2) Treat cells with reprogramming viruses to turn genes back on
3) Change culture conditions to stimulate cells to differentiate into variety of cell types

Question 21

Evaluate the use of stem cells

Answer 21

For:
• Use of embryonic stem cells
• Tiny ball of cells, incapable of feeling pain, not equivalent to a human
• Would otherwise be destroyed (if from infertility treatment which creates more
than needed)
• Duty to apply knowledge to relieve human suffering
Against:
• Use of embryonic stem cells
• Embryo is a potential human; should be given rights
• Scientific
• Induced pluripotent stem cells – cannot yet reliably reprogramme stem cells
• Could begin to multiply out of control, and cause tumours

Question 22

What are transcription factors?
explain what they do and how they work

Answer 22

Transcription factors are proteins Move from cytoplasm → nucleus and bind to DNA at a specific DNA base
sequence on a promotor region

Stimulate or inhibit transcription (production of mRNA) of target
gene(s) by helping/preventing RNA polymerase binding

Question 23

Explain the role of oestrogen in initiating transcription (5)

Answer 23

1. Oestrogen (steroid hormone) diffuses across phospholipid bilayer because lipid soluble
2. In cytoplasm it binds to an inactive transcription factor forming a hormone receptor complex > active
3. Diffuses from cytoplasm into nucleus + binds to promoter region on DNA base sequence
4. Stimulates transcription by helping RNA polymerase bind

Question 24

Nucleosome

Answer 24

DNA wrapped around histone proteins
how closely the DNA and histone are packed together

Question 25

Epigenetics

Answer 25

heritable changes in gene function without changes to the base sequence of DNA

caused by changes in the environment

Question 26

What are the two epigenetic changes?

Answer 26

Methylation and acetylation

Question 27

What is the effect of increased methylation of DNA?

Answer 27

Methyl groups added to cytosine bases in DNA
Nucleosomes pack more tightly together → prevents transcription
factors binding; genes not transcribed (RNA polymerase can’t bind)
• Irreversible

Question 28

Explain acetylation