What changes in a gene?

A gene mutation is a structural change in the base sequence of a gene that can alter the genetic information copied into RNA. Keep the scale clear here: this is not about whole chromosomes moving around. It is about nucleotide changes within a gene.

Genes are made of DNA, and DNA is chemically stable enough to be copied accurately. Still, replication and repair are not perfect. If an altered sequence is copied into a DNA molecule and stays in the cell lineage, it counts as a mutation, not just a temporary pairing error.

Three main molecular types

A substitution is a gene mutation in which one nucleotide base at a particular position is replaced by a different base. For example, a position that previously held adenine may now hold guanine, cytosine or thymine.

An insertion is a gene mutation in which one or more additional nucleotides are added into the base sequence of a gene. The sequence becomes longer.

A deletion is a gene mutation in which one or more nucleotides are removed from the base sequence of a gene. The sequence becomes shorter.

Use these three words precisely. In a substitution, the length of the gene is unchanged; in an insertion or deletion, the length changes. That difference matters enormously when the sequence is later read in codons during translation.

Why one changed base may or may not matter

A base substitution changes one base pair. Its effect depends on where the change happens and which codon it produces. In non-coding DNA, many substitutions have no obvious effect on a polypeptide. In a coding sequence, though, a substitution may change a codon in mRNA, so it may change the amino acid added during translation.

A single-nucleotide polymorphism is a DNA sequence variant at one nucleotide position that occurs among individuals in a population and originated by base substitution. SNPs are common in genomes. Some occur in non-coding regions; others lie in coding or regulatory sequences and can be associated with differences in disease risk or drug response.

The degeneracy of the genetic code is the property of the code in which more than one codon can specify the same amino acid. For that reason, a base substitution may be silent at the level of the polypeptide: the codon changes, but the amino acid does not.

Possible outcomes in a coding sequence

A silent mutation is a base substitution in a coding sequence that changes a codon without changing the amino acid specified. The polypeptide sequence stays the same, so the protein often functions normally.

A missense mutation is a base substitution in a coding sequence that changes a codon so that a different amino acid is inserted into the polypeptide. The effect can be tiny or severe. If the new amino acid has similar chemical properties, or lies in a less important region of the protein, the protein may still work. If the change affects an active site, binding site, folding pattern or structural region, function may be reduced or lost.

A nonsense mutation is a base substitution in a coding sequence that changes a codon for an amino acid into a stop codon. Translation stops too early, producing a shortened polypeptide that usually fails to function.

This is where the link to protein diversity fits neatly. A protein is a polymer of amino acid subunits folded into a specific three-dimensional structure that gives it a function. Change the subunit sequence, and folding, binding and activity can change too. So the diversity of proteins made by a cell depends strongly on the order of amino acid subunits, and mutation can alter the outcome of gene expression by changing that order.

Genetic tests and interpretation

Commercial genetic tests may report SNPs linked with future disease risk. That information is not the same as a diagnosis. Risk depends on the allele tested, other genes, environment, lifestyle and the quality of the evidence connecting the SNP with the disease. Without expert interpretation, a result can sound more alarming than it should, or more reassuring than the evidence allows.

Reading frames make small changes large

A reading frame is the way nucleotide bases are grouped into consecutive triplets during translation. Ribosomes read mRNA three bases at a time, so if one or two nucleotides are inserted or deleted, every codon after the mutation is shifted.

A frameshift mutation is an insertion or deletion mutation that changes the reading frame of codons from the point of mutation onwards. After that point, many amino acids usually change, and a stop codon may appear soon after. The resulting polypeptide is very likely to be non-functional.

Major insertions and deletions

Large insertions or deletions in a gene are also likely to stop a polypeptide functioning. A functional domain might be removed, an inappropriate stretch of amino acids might be added, or folding may be disrupted so badly that the protein cannot take up its working shape.

Insertions or deletions of three nucleotides, or multiples of three, do not shift the reading frame. Don’t over-celebrate that. They still add or remove one or more amino acids, which can be enough to alter the tertiary structure and function of a protein.

The big idea is simple: substitutions often affect one codon; insertions and deletions often affect many codons. That is why insertions and deletions are, on average, more likely to destroy protein function.

Errors during replication and repair

Gene mutation can happen when DNA is copied incorrectly. DNA polymerases usually choose the right complementary nucleotides, and proofreading plus repair systems catch many of the mistakes. A few still get through, especially when a wrongly paired base is copied again in the next round of replication.

A hydrogen bond is an intermolecular attraction between a partially positive hydrogen atom and an electronegative atom such as oxygen or nitrogen. DNA uses hydrogen bonding for complementary base pairing, so replication, repair, transcription and CRISPR guide-RNA targeting all depend on the same basic chemical principle: bases recognise their partners through specific hydrogen-bond patterns.

Mutagens

A mutagen is an agent that increases the rate at which mutations occur by damaging DNA or increasing errors during DNA replication or repair. Mutagens don't usually select useful changes; they just make molecular damage more likely.

Chemical mutagens include substances in tobacco smoke such as polycyclic aromatic hydrocarbons and nitrosamines. Some chemicals change bases so that they pair incorrectly; others damage the DNA backbone or disrupt repair. A carcinogen is a mutagenic or non-mutagenic agent that increases the risk of cancer, often by increasing mutation in genes controlling cell division.

Mutagenic radiation includes ultraviolet radiation, X-rays, gamma rays and alpha particles from radioactive materials. Ultraviolet radiation can form abnormal bonds between adjacent bases in DNA. Higher-energy ionising radiation, such as X-rays and gamma rays, can break DNA strands or produce reactive particles that damage DNA.

Mutation is not just an internal copying problem. Environment matters: exposure to mutagens can increase mutation rate, and therefore can increase the probability of harmful genetic change.

Random does not mean equally likely everywhere

A random mutation is a mutation whose occurrence is not directed by the needs of the organism or by the usefulness of the trait it might produce. For example, a bacterium exposed to an antibiotic does not deliberately change the exact base that would make it resistant.

Mutations can occur anywhere in the base sequence of a genome, but they do not occur at the same rate in every base or every region. Some bases are chemically more vulnerable than others. Some regions get repaired more efficiently. Some stretches of DNA are simply more prone to copying errors. So mutation is random with respect to benefit, but it is not perfectly uniform across the genome.

No natural search-and-replace system for useful traits

No natural mechanism is known that deliberately changes a particular base with the purpose of changing a trait. That point matters in evolution. Variation appears first; selection acts afterwards. Organisms don’t produce the exact mutation they require just because the environment demands it.

Most new mutations are neutral or harmful because existing genes have been shaped by long periods of selection. If a functioning sequence changes at random, it is more likely to have no effect or to make things worse than to improve it. Rare beneficial mutations still matter, because natural selection can increase their frequency in later generations.

Germ cells: mutations that can be inherited

A germ cell is a cell in a reproductive lineage that can give rise to gametes. When a mutation happens in a germ cell, the changed allele may end up in a sperm or egg, so offspring can inherit it.

An inherited mutated gene may do nothing, cause a genetic disease, or, in rare cases, give an advantage. Helpfulness isn't the deciding factor. What matters is that it can move from one generation to the next.

Somatic cells: mutations within one body

A somatic cell is a body cell that is not part of the reproductive lineage. Mutations in somatic cells are usually not passed to offspring. They affect only the individual, and often only one tissue or clone of cells within that individual.

Most somatic mutations have limited consequences: the cell may keep working normally, malfunction, or die and be replaced. The serious exception is a mutation in genes controlling the cell cycle, DNA repair or cell death. If these controls fail, cells may divide when they should not.

Cancer is a disease in which cells divide uncontrollably because genetic and cellular controls over the cell cycle have been disrupted. Somatic cell mutations can therefore cause tumours, but those tumour mutations are not normally inherited by the next generation.

New alleles begin with mutation

An allele is a version of a gene that differs from other versions of the same gene by one or more differences in DNA base sequence. When mutation changes a base sequence, it can create a new allele.

Genetic variation is the presence of differences in DNA sequence or inherited traits among individuals in a population. Meiosis and sexual reproduction shuffle existing alleles into new combinations. They don’t create the original differences. Gene mutation is the original source of all genetic variation.

Why mostly harmful or neutral mutations are still essential

Most mutations are harmful or neutral for an individual organism. That sounds like a poor bargain, but populations need to be viewed over long time scales. Without mutation, selection would gradually use up variation, leaving populations without the raw material needed for adaptation.

Natural selection is a process in which heritable variants that improve survival or reproduction become more common in a population over generations. Selection can reduce variation within one population by increasing the frequency of favourable alleles and removing less favourable ones. In different environments, though, different alleles can be favoured in different populations; over long periods this can increase biological diversity, especially when populations diverge into separate species.

This connects back to polymers too. DNA and proteins are polymers whose subunit sequences carry information or determine structure. Variation in the order of DNA nucleotides can alter codons, while variation in the order of amino acids can alter protein shape and function. A tiny molecular change can therefore matter later at the level of phenotype, population and evolution.

Inferring function by removing function

A gene knockout is a research technique where a gene is deliberately changed so it no longer works. If a trait changes after the gene is removed, the normal gene is likely to play a role in that trait.

A model organism is a species used in research because it is practical to study, and findings from it can help explain biology in other organisms. Mice, yeast and some plants are common examples. You don’t need the laboratory details of how knockouts are produced, but you should know the logic: disable one gene, observe what changes, then infer the gene’s function.

Knockouts are particularly useful when genome sequencing finds a gene-like sequence with an unknown function. Researchers compare knockout organisms with normal organisms, then look for differences in development, physiology, behaviour or cell function.

For some model species, researchers have built large libraries of knockout organisms. A knockout library is a collection of strains in which each strain carries a different inactivated gene. This saves time because scientists can select a strain with a particular gene already disabled instead of making it from scratch.

CRISPR sequences and Cas9

A CRISPR sequence is a region of DNA with short repeated sequences separated by spacer sequences, which can be used to make guide RNA. In gene editing, scientists use the guide part to send the editing machinery to a chosen DNA sequence.

Cas9 is an enzyme that binds guide RNA and cuts DNA at a sequence complementary to that guide RNA. A guide RNA is an RNA molecule whose sequence binds to a target DNA sequence by complementary base pairing, directing Cas9 to that site.

Gene editing means deliberately altering a specific DNA sequence in a genome. In CRISPR-Cas9 editing, the guide RNA brings Cas9 to the target, Cas9 cuts the DNA, and the cell’s repair mechanisms disrupt the gene or introduce a changed sequence.

Base pairing between guide RNA and target DNA relies on hydrogen bonding. It’s the same chemical theme again: hydrogen bonds are weak enough to be reversible, but specific enough for molecular recognition.

A successful use of CRISPR-Cas9

One successful medical use is CRISPR-Cas9 editing of a patient’s blood-forming stem cells to treat sickle-cell disease and transfusion-dependent beta-thalassaemia. Here, cells are taken from the patient, edited outside the body to increase production of fetal haemoglobin, and then returned. The edited cells can produce red blood cells with improved haemoglobin function.

This is a useful example because it isn’t vague “designer baby” talk. It’s a targeted treatment in somatic cells. The edit is intended to help the treated person, not to be inherited by future generations.

Ethics and regulation

Some possible uses of CRISPR raise serious ethical issues before they are used widely. Somatic editing for severe disease, editing embryos, ecological gene drives and enhancement of traits do not raise the same risk-benefit questions.

Scientists work under different regulatory systems in different countries. That becomes a problem if one country allows work that another prohibits, especially for technologies with possible global effects. For this reason, international efforts aim to harmonize regulation of genome editing technologies such as CRISPR, so that safety, consent, equity and environmental risk are considered consistently.

Conserved and highly conserved sequences

A conserved sequence is a DNA base sequence that is identical or very similar across individuals of a species or across a group of species. A highly conserved sequence is a DNA base sequence that remains identical or very similar over long periods of evolutionary time, often across distantly related groups.

Sequence conservation gives biologists a clue. If a sequence has stayed similar while many other parts of the genome have changed, there is probably a reason — sometimes more than one.

Hypothesis 1: functional requirement

One hypothesis is that conserved sequences stay similar because their products or roles matter for function. If a gene product needs a precise amino acid sequence to fold correctly or bind another molecule, many mutations in that sequence will be harmful and removed by natural selection.

The same idea can apply outside protein-coding sequences. Conserved sequences may also occur in genes for functional RNAs, or in regulatory regions that control when and where genes are expressed. If a sequence change disrupts development or cell function, individuals carrying harmful variants leave fewer offspring, so the original sequence is retained in the population.

Hypothesis 2: slower mutation rate

A second hypothesis is that some conserved sequences lie in regions of the genome with lower mutation rates. They may be less exposed to damage, copied more accurately, or repaired more efficiently. In this explanation, the sequence is conserved partly because fewer mutations arise there in the first place.

The two hypotheses can both be true. A sequence could mutate slowly and also be under strong selection because it has an important function. In practice, biologists compare sequence data, gene expression, protein function and mutation rates to decide which explanation is better supported.

Conserved sequences also help us understand change. Natural selection can preserve sequences whose functions are essential, while allowing other sequences to diverge. Related species can therefore share deeply conserved molecular machinery and still evolve different forms, behaviours and ecological roles.