1. Higher Biology
Unit 1:
1.6 Mutations

2. Learning Intentions
To be able to describe where mutations occur and what changes this causes
To be able to describe different types of mutations and their impact

3. Mutations
Mutations are changes in the structure or amount of an organism’s DNA.
These changes can be tiny changes in DNA structure to large scale alterations in chromosome structure or number
Mutations can result in no protein or an altered protein being formed

4. When a mutation occurs and produces a change in the phenotype of an organism we refer to this organism as a mutant.

5. Mutation Frequency Mutations occur spontaneously and at random
They occur rarely with mutation frequency varying from one gene to another

6. Complete the table Organism Mutant Characteristic
Mutation Frequency (mutations at gene site/ million gametes)
Chance of new mutation occurring
Fruit Fly
Ebony body
White eye 20
1 in
Mouse
Albino coat 10
Human
Haemophilia
Muscular
dystrophy 80
1 in

7. Types of mutation
A point mutation (or single gene mutation) involves a change in one of the base pairs in the DNA sequence of a single gene
This can occur in the protein coding region of a gene or in a DNA sequence that regulates the gene
Point mutations can alter the expression of the gene and therefore the protein synthesised

8. Substitution
In a substitution mutation one of the base pairs has been changed to a different base. Substitutions bring about a minor change (normally a change in one amino acid). This may affect the organism only slightly or not at all. In some cases if the substituted amino acid occurs at a critical position then a major defect may arise.

9. Insertion
Insertion mutations result in a new base pair being added into the original sequence. Insertion mutations lead to a major change as it causes a large portion of the gene’s DNA to be misread. The protein produced differs from normal by many amino acids and is usually non functional.

10. Deletion
A deletion mutation occurs when a base is lost from the original sequence. Deletions bring about major changes as they cause a large portion of the gene’s DNA to be misread. The protein produced differs by many amino acids and is normally non functional.

11. Point mutations
Point mutations can be classified further depending on the impact they have on the sequence of amino acids produced.
They fall into five categories:

12. Silent
As a result of a substitution one base pair is altered. The resulting codon is found to code for the same amino acid as before so the protein synthesised is unaffected.

13. Neutral
As a result of a substitution one of the amino acids formed as part of the protein is different. The new amino acid has similar chemical properties so there is little or no effect on the protein formed.

14. Missense
As a result of a substitution the amino acid formed as part of the protein is different. A protein is formed but as this amino acid has different chemical properties it causes the protein to function in a different way.

15. Nonsense
As a result of a substitution the amino acid formed as part of the protein is different. The new amino acid formed acts as a stop codon and causes protein synthesis to be halted prematurely. The polypeptide chain formed is shorter than normal.

16. Frameshift
As a result of either an insertion or a deletion all of the codons along the gene are affected. The protein formed is almost certain to be non functional.

17. Sickle cell anaemia
Sickle cell anaemia is an example of a point mutation
It is caused by the presence of abnormal haemoglobin known as haemoglobin s
The allele for normal haemoglobin is represented by H and the allele for haemoglobin s is represented by S

18. People who are homozygous for haemoglobin s (SS) produce blood cells which are sickle shaped and stick together interfering with blood circulation
Alleles H and S are co-dominant meaning they are both expressed in a heterozygous person.

19. People who are heterozygous for haemoglobin type (HS) express mild anaemia
Allele S is rare as it usually causes death, but in some areas of Africa up to 40% of the population are heterozygous
This is because allele S offers a resistance to malaria
Natural selection favours heterozygous people in malarial regions as they are more likely to survive an outbreak of malaria than those who are homozygous for normal haemoglobin (HH)

20. Chromosome structure mutations
These types of mutations involve the breakage of one or more chromosomes
These broken ends can then join to one another
This can bring about a change in the number or sequence of genes in a chromsome

21. Deletion
In a deletion mutation the chromosome breaks in 2 places and the segment in between becomes detached
The two ends then join up creating a shorter chromosome that lacks some genes
This normally has drastic effects on the organism

22. Cri du Chat
Cri du chat is a condition that occurs when part of chromosome 5 in humans is deleted
The sufferer has severe learning difficulties

23. Duplication
Duplication occurs when a segment of genes attaches to the end of a chromosome or becomes inserted somewhere along its length
This results in a set of genes being repeated
This can be detrimental as in the case of certain genes that when duplicated cause cancer

24. Duplication In some circumstances duplication of genes can be useful
A second copy of a gene is produced that is free from selection pressure which allows changes to occur without affecting the original gene’s function
This gene can undergo mutations and produce new DNA sequences that may offer an advantage to the organism

26. Inversion
Inversion occurs when a chromosome breaks in 2 places and the segment between turns around before joining up again
This brings about a reversal of the normal sequence of genes
When this chromosome meets its normal non mutated partner at gamete formation they have to form a complicated loop to pair up
This can result in non viable gametes

28. Translocation
Translocation mutations occur when a section of a chromosome breaks off and attaches to another chromosome which is not its matching partner
This normally leads to problems during pairing of chromosomes at gamete formation and results in non viable gametes

29. Non reciprocal translocation
A section of one chromosome breaks off and attaches to the other. One chromosome has increased in size and the other has decreased.

30. Reciprocal Translocation
Sections have broken off from each chromosome and attached to one another. Each chromosome now contains a different sequence of genes.