Genetic Mutations

Jamie Z 2024-11-07

Learning Goals

• Recall that a 3-base codon codes for one amino acid in a protein molecule.
• Define Mutation
• Recall factors which may contribute to causing mutations
• Describe the main categories of mutations: chromosomal mutations, point mutations, frameshift mutations.
• Classify examples of mutations as chromosomal, point or frameshift
• Deduce how mutations can impact on both an organism's phenotype & genotype and that of its offspring.
• Compare the likely severity of the consequences of the three categories of mutation to the organism’s genotype and phenotype.
• Identify a chromosomal mutation shown in a karyotype

Mutations

• Genetic material can change
• This is called a mutation - Likely occurs in parental gametes
• Mutagenic agents can cause mutations, which could be causative or non-causative agents

Causative Agents

• Induced mutation
• X-rays
• UV radiation
• Nuclear radiation
• Chemical substances

Non Causative Agents

• Spontaneous mutation (occurs more frequently with age)

How does DNA code for proteins?

• The DNA code is responsible for the construction of PROTEINS – structural, or enzymes.
• Proteins are made of amino acids. There are 20 different amino acids in humans
• The bases of the nucleotides are read in groups of 3 called codons
• Each codon codes for one amino acid

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• Example: GGG AAC CCT CTC TTT TCG TGA

What is a Mutation

• A change in the DNA code which alter the instructions in the DNA and alter the protein product

Main Kinds

1. Chromosomal - Individual inherits too many or too little chromosomes or large parts of chromosomes are altered
2. Point substitution - Individual has the wrong base at a particular point in the gene
3. Frame Shift - an insertion or deletion of one or more bases has occurred, resulting in the disruption of subsequent codons
4. Inversion - Chromosome rearrangement in which a segment of a chromosome is reversed end to end.

Chromosomal Mutations

• During meiosis, whole sections of homologous chromosomes can be swapped over (crossing over)
• This increases diversity among offspring
• If rearrangement of whole blocks of genes occurs (eg: deletion, inversion, duplication or movement to the wrong chromosome), were use the term Chromosomal Mutation
• Most of these types of mutations result in spontaneous termination of a pregnancy – the consequences are too severe for the embryo/ foetus to survive. Some are not so catastrophic, however.

Point Shift Mutations

• The replacement of one nucleotide in DNA
• Example: AAT CTC GGA ——> AAT  GTC  GGA

Because one 3-base codon codes for one amino acid, the consequence of substitution can be

• None - If the codon is the same amino acid Eg: TAT and TAC both code for Tyrosine.
• Incorrect amino acid Eg: TAT = tyrosine, but CAT = histidine.
• Premature STOP Eg: TAT = tyrosine, but TAA = STOP

Frame Shift Mutations

Deletion

• Removal of one or more nucleotides from the DNA strand
• Example : AAT ~~C~~TC GGA —> AAT TCG GA

Insertion

• Insertion of one ore more nucleotides into the DNA strand
• Example: AAT CTC GGA —> AAT ACT CGG A

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Reptation Mutations

• The addition or deletion of large number of repeating bases
• Eg: Addition of many copies of AGG AGG AGG AGG AGG AGG AGG AGG etc

Do mutations affect the next generation?

• Mutation that occurs in a body cell of an organism is a somatic mutation and only daughter cells by mitosis will have the mutation
• A mutation that is occurring in a cell that produces gametes is a germ-line mutation - Definitely Heritable