Sign up to access the complete lesson and track your progress!
Unlock This CourseMutations are changes in the genetic material (DNA) of an organism. They are the raw material for evolution and can lead to new traits in a population. However, most mutations are either harmful or have no effect at all. In this session, we'll explore the different types of mutations and understand how they influence living organisms.
Key Definitions:
Mutations are crucial for evolution as they create genetic diversity. Without mutations, all organisms would be genetically identical and species wouldn't be able to adapt to changing environments. While many mutations are harmful, some can provide advantages that help organisms survive and reproduce more successfully.
Mutations occur naturally all the time in all living things. Most cells have mechanisms to repair DNA damage, but sometimes these mechanisms fail, resulting in permanent changes. These changes can be passed down to offspring if they occur in reproductive cells (eggs or sperm).
Gene mutations are changes that affect one or a few nucleotides in a gene. These small changes can have significant effects on the proteins produced and, consequently, on the organism itself.
A substitution mutation occurs when one nucleotide base is replaced with another. For example, changing an A to a G in the DNA sequence. This can lead to a different amino acid being added to a protein, potentially changing its function.
An insertion mutation happens when extra nucleotides are added to the DNA sequence. This can shift the reading frame of the gene, causing all the subsequent amino acids to be incorrect. This is called a frameshift mutation.
A deletion mutation occurs when one or more nucleotides are removed from the DNA sequence. Like insertions, deletions can cause frameshift mutations if the number of nucleotides removed is not a multiple of three.
The genetic code is read in groups of three nucleotides (codons), each specifying a particular amino acid. Mutations can affect protein synthesis in several ways:
Sometimes, a mutation changes a codon but still codes for the same amino acid (due to the redundancy of the genetic code). These mutations have no effect on the protein produced and are called silent mutations.
A missense mutation changes a codon so that it codes for a different amino acid. This can alter the protein's structure and function, sometimes causing disease. For example, sickle cell anaemia is caused by a single nucleotide change that results in a different amino acid in haemoglobin.
A nonsense mutation changes a codon to a stop codon, causing protein synthesis to end prematurely. This usually results in a shortened, non-functional protein that can cause serious health problems.
When nucleotides are inserted or deleted (and the number isn't a multiple of three), the reading frame shifts. This changes all the codons that follow the mutation, usually resulting in a completely different and non-functional protein.
Chromosome mutations involve larger changes that affect the structure or number of chromosomes. These can have more dramatic effects than gene mutations because they often involve many genes.
An inversion occurs when a segment of a chromosome breaks off, flips around and reattaches. The genetic material is still present but in reverse order, which can disrupt gene function or regulation.
A translocation happens when a piece of one chromosome breaks off and attaches to another chromosome. This can lead to genetic disorders if important genes are disrupted at the breakpoints.
Duplication occurs when a section of a chromosome is copied, resulting in extra copies of the genes in that section. This can lead to overproduction of certain proteins.
Sometimes, errors during cell division can lead to changes in the number of chromosomes:
Aneuploidy is when there are extra or missing individual chromosomes. Down syndrome is a well-known example, caused by an extra copy of chromosome 21 (trisomy 21). Most aneuploidies are harmful and many result in miscarriage.
Polyploidy occurs when an organism has extra sets of chromosomes. While this is often lethal in animals, it's common in plants and has played a significant role in plant evolution. Many crop plants, like wheat and cotton, are polyploid.
Sickle cell anaemia is caused by a single nucleotide substitution in the gene for haemoglobin. The mutation changes the amino acid glutamic acid to valine, resulting in abnormal haemoglobin that causes red blood cells to become sickle-shaped when oxygen levels are low. These sickle cells can block small blood vessels, causing pain and organ damage.
Interestingly, people with just one copy of the sickle cell gene (carriers) have some protection against malaria. This is why the mutation is more common in areas where malaria is prevalent – it's an example of how a harmful mutation can provide an advantage in certain environments.
Mutations can occur spontaneously or be induced by external factors:
Mutagens are agents that increase the rate of mutations. These include:
During DNA replication, enzymes occasionally make mistakes, inserting the wrong nucleotide. While cells have mechanisms to correct these errors, some slip through, resulting in mutations. These spontaneous mutations occur naturally at a low rate in all organisms.
Mutations provide the genetic variation that drives evolution. Natural selection acts on this variation, favouring mutations that increase an organism's fitness in its environment.
Though rare, beneficial mutations can give organisms an advantage. For example, a mutation that allows bacteria to break down a new food source or resist antibiotics can help them survive and reproduce more successfully.
Over time, beneficial mutations can spread through a population through natural selection, leading to adaptation. This process has allowed species to adapt to diverse environments, from the deepest oceans to the highest mountains.
Antibiotic resistance in bacteria is a perfect example of mutation and natural selection in action. When bacteria are exposed to antibiotics, most die. However, if a random mutation gives some bacteria resistance to the antibiotic, these bacteria survive and reproduce, passing the resistance gene to their offspring.
Over time, with continued antibiotic use, the resistant bacteria become more common in the population. This is why antibiotic resistance is a growing problem in medicine – bacteria are evolving faster than we can develop new antibiotics.
Mutations are changes in DNA that can affect genes or chromosomes. They can be caused by environmental factors or occur spontaneously during DNA replication. While many mutations are harmful or neutral, some can be beneficial and contribute to evolution through natural selection.
Understanding mutation types is crucial for studying genetics, evolution and many genetic diseases. It also helps us understand how species adapt to changing environments and how new species can emerge over time.