Gene Mutation (AQA A-Level Biology): Revision Notes
Gene Mutation
What are mutations?
A mutation refers to any change in the quantity or base sequence of DNA within an organism. These changes can occur spontaneously during DNA replication or may be induced by external factors. When mutations occur during gamete formation, they can be inherited and passed to offspring, creating genetic differences between individuals.
When mutations occur in gamete cells (sex cells), they become heritable and can be passed to future generations. This is how genetic diversity is maintained in populations and how evolution occurs over time.
A gene mutation specifically involves changes to one or more nucleotide bases within a gene, or alterations in the sequence of these bases. Since DNA sequences are transcribed into mRNA and then translated into amino acid sequences that form polypeptides, any changes to the DNA base sequence can potentially affect the final protein product.
Types of gene mutations
Substitution of bases
Substitution mutations occur when one nucleotide in a DNA molecule is replaced by another nucleotide containing a different base. The impact of these mutations depends on several factors.
When a substitution occurs, the affected DNA triplet codes for a different amino acid in the resulting polypeptide. For example, if the DNA triplet GTC (which normally codes for glutamine) has its final cytosine base replaced by guanine, it becomes GTG. This new triplet still codes for glutamine, so the polypeptide remains unchanged.
The effect varies significantly depending on the specific amino acid change. If the original amino acid plays a crucial role in protein structure - such as forming important bonds that determine protein shape - then the replacement amino acid may not form the same bonds. This can alter the protein's three-dimensional structure and affect its function. In enzymes, this might mean the active site no longer fits the substrate properly, preventing catalysis.
The degenerate nature of the genetic code means that most amino acids are coded for by more than one DNA triplet. This redundancy acts as a protective mechanism - some base changes do not result in amino acid changes in the final protein, making the mutation essentially "silent."
Deletion of bases
Deletion mutations arise when a nucleotide is lost from the normal DNA sequence. Although losing a single nucleotide from thousands in a typical gene might seem minor, the consequences are usually severe.
DNA is read in groups of three bases (triplets) during protein synthesis. When one nucleotide is deleted, all subsequent triplets in the sequence are shifted by one position. This creates a reading frame shift that completely alters the amino acid sequence of the polypeptide from that point onwards.
Why deletion mutations are more severe: Deletion mutations generally cause more damage than substitution mutations because they affect the entire protein structure downstream from the mutation site. The resulting protein typically has an entirely different amino acid sequence and is unlikely to function correctly.
Chromosome mutations
Chromosome mutations involve changes to the structure or number of whole chromosomes, rather than individual bases. These mutations can arise spontaneously and take two main forms:
- Changes in whole sets of chromosomes
- Changes in the number of individual chromosomes
Polyploidy
Polyploidy occurs when organisms possess three or more complete sets of chromosomes instead of the usual two. This condition is particularly common in plants and can arise through several mechanisms.
One pathway involves chromosomes failing to separate into two distinct sets during meiosis. Gametes produced through this process contain both chromosome sets rather than just one. When such gametes fuse during fertilisation, the resulting offspring are polyploid.
Non-disjunction
Non-disjunction refers to the failure of homologous chromosome pairs to separate properly during meiosis. This results in gametes having either one more or one fewer chromosomes than normal. After fertilisation, the offspring will have abnormal chromosome numbers in all their body cells.
An example in humans is Down's syndrome, where individuals possess an additional chromosome 21, giving them 47 chromosomes instead of the typical 46.
Real-world example: Wheat evolution through polyploidy
Real-World Application: How Modern Wheat Evolved Through Chromosome Mutations
Modern wheat varieties demonstrate how chromosome mutations, particularly polyploidy, can be beneficial. The development of bread wheat involved multiple hybridisation events over thousands of years.
Step 1: Original wild wheat Einkorn wheat (Triticum urartu) represents the original wild form with 14 chromosomes.
Step 2: First hybridisation Around 500,000 years ago, this hybridised with a wild goat grass species (Aegilops speltoides), also containing 14 chromosomes. The hybrid offspring, emmer wheat (Triticum turgidum), possessed 28 chromosomes - making it tetraploid.
Step 3: Second hybridisation Later hybridisation occurred between tetraploid emmer wheat and another goat grass species (Aegilops tauschii) with 14 chromosomes. This cross produced bread wheat (Triticum aestivum) with 42 chromosomes - a hexaploid containing six complete chromosome sets.
Result: This polyploid condition allows chromosomes to find homologous partners during meiosis, enabling successful reproduction. Modern wheat plants have thus arisen through combining chromosome sets from different ancestral species, creating fertile organisms with enhanced characteristics.
Key Points to Remember:
- Gene mutations involve changes to individual DNA bases, while chromosome mutations affect whole chromosomes
- Substitution mutations may have no effect due to the degenerate genetic code, but can be serious if they affect critical protein regions
- Deletion mutations cause reading frame shifts that typically produce non-functional proteins
- Polyploidy (multiple chromosome sets) is common in plants and can arise from hybridisation between different species
- Modern wheat varieties demonstrate how chromosome mutations can be beneficial, creating new species through polyploidy