Mutations: a source of variation

1. Mutations:a source of variation

2. Mutations are the source of variation • A mutation is a heritable change in genetic material. • Different forms of the same gene resulting from mutations are known as alleles. • Alleles are the forms of a gene located at the site or locus on the chromosome. • Alleles are the basis of heritable variation. • Over the course of evolution, many mutations have occurred giving rise to considerable genetic allelic variation within species, and the process of meiosis ensures random distribution of particular parental alleles to offspring.

3. Point Mutations • Mutations in genes involving the alteration of a single base in DNA are referred to as point mutations. • These include: • substitution mutations, which occur as a result of a substitution of one nucleotide for another, or • frameshift mutations, resulting from the addition or deletion of a single nucleotide.

4. Substitution Mutations There are three types of substitution mutations: • Missense mutations • involve a substitution that results in the replacement of the original amino acid with a different amino acid. • Silent mutation • involve substitution that does not affect the amino acid coded for. An example would be a change from CAC to CAT, both codons are for the amino acid histidine. • Nonsense mutation • involve a nucleotide substitution that results in the replacement of the original amino acid with a stop codon.

5. Original Sequence Silent Mutation Missense Mutation Nonsense Mutation Examples of substitution mutations

6. Frameshift Mutations • If a single base pair is inserted in or deleted from the DNA, the reading frame (codon sequence) of the mRNA is altered and the wrong amino acids may be incorporated for the remainder of the sequence. • Original Sequence THE FAT CAT SAT • Insertion ATH EFA TCA TSA T • Deletion THE FAC ATS AT

7. Block Mutations • Block mutations are different types of chromosome mutations that can only occur during meiosis. • They are called block mutations as they involve the rearrangement of whole blocks of genes rather than individual bases within a gene. • Block mutations include: • deletions • inversions • duplications • translocations

8. Deletions • A break may occur at two points in the chromosome, and the middle piece falls out. • The two ends then rejoin to form a chromosome missing some genes. Alternatively the end of a chromosome may break off.

9. Inversions • The middle piece of the chromosome falls out and rotates through 180 degrees and then rejoins. • There is no loss of genetic material but the genes will be in a reverse order for this segment of the chromosome.

10. Translocations • Involves the movement of a group of genes between different (non homologous) chromosomes. • A piece of one chromosome breaks off and joins onto another chromosome. • This will cause major problems when the chromosomes are passed to gametes as some will receive extra genes, while others will miss out.

11. Duplications • A segment is lost from one chromosome and added to another. • The chromosome with the missing segment removed is deficient in genes. • Some gametes will receive double the genes will others will have no gene for the affected segment.

12. Somatic and Germ Line Mutations • Germ line mutations occur in germ line cells and will be inherited by offspring formed by fertilization of these germ cells. • Examples include PKU and albinism. • Somatic mutations only affect the individuals in which they arise; they cannot be inherited by future generations. • Example – person with one blue eye and one blue and brown eye. The size of the sport reflects the number of cell divisions that occurred after the somatic mutations. • Somatic mutations may be important in cancer as many cancers are due to a genetic change in a single cell resulting in uncontrolled growth and division.

13. Detecting Mutations • Mutations are usually because of the effect they have on the individual carrying them. • A large range of molecular techniques allows mutations in genes to be analysed. • DNA sequencing can provide a complete DNA sequence for the gene under investigation. Alternatively, if the gene is large, rapid PCR-based procedures have been developed. • Example: Finding the common A → T missense mutation associated with sickle cell anaemia: • The relevant region of the b-globin gene is amplified by PCR and then digested with the restriction enzyme MstII. • The restriction enzyme sequence is present in the normal sequence but not in the sickle cell sequence, therefore gel electrophoresis will show different sized bands depending on the alleles carried by the person being tested.

14. Finding the common A → T missense mutation associated with sickle cell anaemia (a) A portion of the β-globin gene is shown. The MstII site is present in the normal allele, but missing in the sickle-cell allele. The position of the PCR primers used is shown. (b) A diagram of an electrophoretic gel showing the DNA fragments produced when the DNA of an individual carrying two copies of the normal allele (Track 1) or an individual carrying two copies of the sickle-cell allele is PCR-amplified and exposed to the restriction enzyme MstII. The DNA size standards in Track 3 allow the size of the bands in Tracks 1 and 2 to be estimated with accuracy.

15. How do mutations arise? • Mutations may either be spontaneous or induced. • Spontaneous mutations arise naturally as random changes in the base sequence of DNA. • Induced mutations occur following deliberate or accidental exposure to radiation or other agents.

16. Spontaneous Mutations • Arise naturally as random changes in the base sequence of DNA. • Occur at an average rate of approximately one in a million • Are the result of rare, undetected and unrepaired errors of DNA synthesis. • Types of Spontaneous mutations include: • Tautomerism - A base is changed by the repositioning of a hydrogen atom. • Depurination - Loss of a purine base (A or G). • Deamination - Changes a normal base to an atypical base; C → U, (which can be corrected by DNA repair mechanisms), or spontaneous deamination of 5-methycytosine (irreparable), or A → HX (hypoxanthine). • Transition - A purine changes to another purine, or a pyrimidine to a pyrimidine. • Transversion - A purine becomes a pyrimidine, or vice versa..

17. Tautomeric Shifts and Deamination

18. Induced Mutations • Mutations that occur following deliberate or accidental exposure to radiation or other agents are called induced mutations. • The agents that induce mutations are called mutagens. • Induced mutations can occur more frequently than spontaneous mutations • Exposure to naturally occurring mutagens during the course of evolutionary history has contributed to the spontaneous mutation rate, however, in recent history, the number of chemicals added to the environment as a result of agricultural and industrial practices and warfare has increased. • Chemicals that cause mutations are called mutagens. Chemicals that cause an increase in cell division without direct changes to genetic material are caused carcinogens. A chemical can be both a mutagen and a carcinogen. • Mutagens have a cumulative effect on an individual – i.e. repeated small doses over a long period of time may be just as harmful as a single, larger dose.

19. Types of Mutagens • Radiation • Ionising radiation e.g. nuclear fallout, UV light, X rays, gamma rays. • Overexposure to UV rays (sunlight) can result in mutagenesis and skin cancer. • X rays have higher energy than UV radiation and can break DNA and even cause chromosome breakage, resulting in cell death or mutational change. • Radiation from radioactive elements can have similar effects. • Viruses and microorganisms • Viruses such as hepatitis B, HIV, and Epstein-Barr virus can upset genes and potentially trigger cancer when they integrate into the genome. • Alcohol and dietary components • Diets high in fat, especially those containing burned and/or fatty highly preserved meat, slow the passage of food through the gut giving time for mutagenic irritants to form in the bowel. • Environmental poisons and irritants • Many chemicals are mutagenic. • Synthetic and natural examples include organic solvents such as benzene, asbestos, formaldehyde, tobacco tar, vinyl chlorides, coal tars, some dyes and nitrites.

20. DNA Repair Systems • DNA repair is the process by which a cell identifies and corrects damage to DNA. • In human cells, both normal metabolic activities and environmental factors such as UV light can cause DNA damage, resulting in as many as 1 million individual molecular lesions per cell per day. • Many of these lesions cause structural damage to the DNA molecule and can alter or eliminate the cell's ability to transcribe the gene that the affected DNA encodes. • Other lesions induce potentially harmful mutations in the cell's genome, which will affect the survival of its daughter cells after it undergoes mitosis. • Consequently, the DNA repair process must be constantly active so it can respond rapidly to any damage in the DNA structure.

21. DNA Repair Systems • DNA repair is mediated by enzymes. • In many cases DNA repair occurs during replication by randomly incorporating bases into the gap. If the base incorporated is not the same as the ‘missing base’, this results in a missense mutation in the synthesized strand. • The rate of DNA repair is dependent on many factors, including the cell type, the age of the cell, and the extracellular environment. • A cell that has accumulated a large amount of DNA damage, or one that no longer effectively repairs damage incurred by its DNA, can enter one of three possible states: • an irreversible state of dormancy, known as senescence • cell suicide, also known as apoptosis • unregulated cell division, which can lead to the formation of a tumor

22. DNA Repair