Mutations And Control Of Gene Expression

Question 1

Define a mutation.

Answer 1

A change in the amount, arrangement or structure in the DNA or in some cases RNA of an organism.

Question 2

What are mutations described as?

Answer 2

Spontaneous or random. Spontaneous = no apparent cause, random = equal probability anywhere in the genome of diploid organisms.

Question 3

What are beneficial mutations?

Answer 3

Ones that are rare but give a selective advantage, contributing to the variation between individuals so the basis of natural selection and evolution.

Question 4

When do mutations most occur?

Answer 4

During DNA replication, prior to cell division. In crossing over of prophase 1 or non-disjunction in anaphase 1/2.

Question 5

Which organisms are more likely to suffer from mutations?

Answer 5

Organisms with short life cycles and frequent meiosis/cell division show a greater rate of mutation.

Question 6

What 2 ways are the frequency of mutations increased?

Answer 6

Ionising radiation - gamma rays, x-rays, UV light.

- mutagenic chemical - policyclic hydrocarbons in cigarette smoke, methanal, mustard gas.

Question 7

How is UV light mutagenic?

Answer 7

Is the wavelength DNA absorbs most efficiently. Radiation joins adjacent pyrimidine bases in a DNA strand so DNA polymerase may insert and incorrect nucleotide.

Question 8

How are mutagenic chemicals like acridine mutagenic?

Answer 8

Flat molecules that slide in between base pairs and prevent DNA polymerase inserting the correct nucleotide.

Question 9

Define gene (point) mutation.

Answer 9

DNA not copied accurately in S phase before cell division, involving one or a small number of bases.

Question 10

Define chromosome mutation.

Answer 10

Broken chromosomes may not repair themselves correctly, altering their structure and a large number of genes.

Question 11

Define aneuploidy and polyploidy.

Answer 11

Aneuploidy is when a whole chromosome may be lost or added in non-disjunction.
Polyploidy is where the number of chromosomes may double if cells fail to divide following the first nuclear division after fertilisation.

Question 12

Define non-disjunction.

Answer 12

When chromosomes fail to separate to the poles of dividing cells at anaphase 1 or chromatids at anaphase 2.

Question 13

What are the 5 different types of gene (point) mutations?

Answer 13

1. Addition - base added.
2. Duplication - same base twice.
3. Subtraction - base deleted.
4. Substitution - different bases incorporated.
5. Inversion - adjacent bases switch.

These change MRNA so change polypeptide.

Question 14

What are the potential effects of gene (point) mutations?

Answer 14

New codon may code for the same amino acid so no change
New codon may code for a similar chemical nature so the effect may be small
If the mutation is at a significant site it may make a significant difference to the activity eg active site of an enzyme.

Question 15

What is an example of a substitution point mutation?

Answer 15

Sickle cell anaemia. A DNA triplet coding strand codes for glutamate, but a substitution at the second position causes valine instead.

Question 16

How does sickle cell anaemia occur and why is it bad?

Answer 16

Glutamate is coded for and is large and hydrophilic whereas valine is small and hydrophobic. This leads to a red blood cell becoming sickle shaped, and they’re more fragile and may break in capillaries.

Question 17

What are the symbols for normal haemoglobin/sickle cell anaemia?

Answer 17

HbA / HbS. Individuals with HBSHBS have sickle cell and individuals with HBSHBA have sickle cell trait.

Question 18

What are the symptoms of sickle cell anaemia?

Answer 18

Joint pain and organ damage. Ability of red blood cells to carry oxygen is reduced resulting in anaemia and possibly death. People with sickle trait have less severe symptoms.

Question 19

How can changes in the structure of chromosomes occur?

Answer 19

During prophase 1, crossing over occurs. Mutation arises when the chromosome doesn’t rejoin accurately at the corresponding point on the homologous partner. Therefore they end up with different genes. Further meiosis won’t be possible as homologous pairs won’t be able to pair up.

Question 20

How can changes in chromosome number occur?

Answer 20

At anaphase 1 + 2, a faulty spindle may result in chromosomes not being shared equally between daughter cells. This is Non-disjunction. One daughter cell gets 2 copies and the other gets none.

Question 21

How does Down syndrome occur?

Answer 21

Non-disjunction occurs in oogenesis so a secondary oocyte has no chromosome 21 or 2 chromosome 21s. If it had no C21, the embryo is not viable. If it has 2, it fuses with a normal sperm and an embryo with 3 C21s forms. This condition is trisomy 21.

Question 22

How does translocation downs occur?

Answer 22

A fragment of chromosome 21 attaches to the chromosome 14 in a gamete. So it fuses with a normal sperm producing an embryo with 2 normal chromosome 21s and an extra one on chromosome 14.

Question 23

Define euploid and aneuploid.

Answer 23

Euploid = complete set of chromosomes
Aneuploid = not enough chromosomes due to something like nondisjunction.

Question 24

How can polyploidy arise?

Answer 24

A defect in the spindle at meiosis results in all chromosomes at anaphase 1 or chromatids at anaphase 2 moving to the same pile of the cell making gametes with 2 sets of each chromosomes, a diploid gamete. When fertilised by a haploid gamete a triploid zygote forms. The zygote will be infertile because it can’t make homologous pairs.

Question 25

What is endomitosis.

Answer 25

Replication of chromosomes not followed by cytokinesis, so in an early embryo four lots of chromosomes are made, followed by mitosis making tetraploid cells.

Question 26

Why is polyploidy more common in plants than animals?

Answer 26

Can reproduce asexually

Hermaphrodite so don’t use chromosomes to determine sex.

Question 27

What is a carcinogen?

Answer 27

An agent that causes cancer.

Question 28

What are tumour suppressor genes?

Answer 28

Genes regulating mitosis and prevent cells dividing too quickly are tumour suppressor genes.

Question 29

How can tumour suppressor gene mutations cause cancer?

Answer 29

It may lose its regulatory function. The cell could go through continual, repeated mitosis which characterises cancer. If the cell isn’t attacked by the immune system it could cause a collection of cells called a tumour, which can be benign or malignant.

Question 30

What is a proto oncogene?

Answer 30

Codes for a protein that contributed to cell division.

Question 31

How can a mutation in a proto oncogene cause cancer?

Answer 31

Mutation may switch it on permanently so excessive amounts of protein are made causing repeated, rapid mitosis ie cancer.

Question 32

How can a proto oncogene be turned on?

Answer 32

Mutation causes chromosomes to rearrange it, and places the protooncogene next to a DNA sequence that turns it on permanently.
Extra copy so extra protein made.

Question 33

How can polycyclic hydrocarbons from smoking cause cancer?

Answer 33

Can enter the nuclei of alveolar cells and slide between base pairs causing mutations by preventing accurate replication.

Question 34

What is epigenetics?

Answer 34

The study of the control of gene expression by factors other than changes in DNA.

Question 35

Define DNA methylation.

Answer 35

Addition of methyl groups preventing bases being recognised, possibly leading to genes not being expressed.

Question 36

Define histone acetylation.

Answer 36

Modification of the histone proteins which can cause the histone to coil more tightly, preventing gene expression. The histone may also coil more loosely increasing the expression of the gene.