Mutations and Their Causes
- Junessa Masaya
- Apr 15
- 4 min read
HSC Biology | Free Study Notes
In this lesson
what mutations are
what point mutations are
what chromosomal mutations are
how mutagens cause mutations
why coding and non-coding DNA both matter
What is a mutation?
A mutation is a change in the DNA sequence.
Mutations are important because they can introduce new alleles into a population. This is one reason Module 6 begins with mutation as the inquiry question, How does mutation introduce new alleles into a population?
Point mutations
A point mutation is a small-scale mutation involving a change in a single nucleotide or base pair.
What this means
A point mutation may involve:
substitution of one base for another
insertion of a base
deletion of a base
At this level, the key idea is that point mutations affect a very small part of the DNA sequence. HSC materials and marking guidelines explicitly identify point mutation as a distinct mutation type.
Possible effects of point mutations
A point mutation may:
have no noticeable effect
change one amino acid in a protein
affect gene expression
create a new allele
The effect depends on where the mutation happens and what kind of change it causes.
Chromosomal mutations
A chromosomal mutation is a large-scale change involving whole chromosomes or sections of chromosomes.
Types of chromosomal mutation
Examples include:
deletions
duplications
inversions
translocations
changes in chromosome number
HSC marking guidance specifically lists deletions, duplications, inversions and translocations as chromosomal alterations, and also describes numerical abnormalities caused by problems such as non-disjunction.
Why chromosomal mutations matter
Because chromosomal mutations affect much larger sections of genetic material, they can have bigger effects than point mutations.
Point mutations and chromosomal mutations compared
Type of mutation | Scale of change | Example idea |
Point mutation | Small, one nucleotide or base pair | Single base substitution |
Chromosomal mutation | Large, chromosome section or number | Deletion, duplication, non-disjunction |
Mutagens
A mutagen is something that increases the chance of mutation.
The Module 6 syllabus specifically requires students to explain how mutagens operate, including:
electromagnetic radiation sources
chemicals
naturally occurring mutagens
Electromagnetic radiation
Examples include:
UV radiation
X-rays
How they cause mutation
These forms of radiation can damage DNA structure.
HSC materials describe X-rays as mutagenic electromagnetic radiation that damages DNA, and another HSC question models UV causing abnormal bonding between thymine bases.
Chemicals
Some chemicals can act as mutagens by:
changing DNA bases
interfering with DNA replication
damaging chromosome structure
At this level, the key idea is that chemicals can alter DNA and increase mutation risk.
Naturally occurring mutagens
Not all mutagens are human-made.
Some mutagens occur naturally in the environment and can still cause changes to DNA.
Coding vs non-coding DNA
The Module 6 syllabus specifically requires students to assess the significance of coding and non-coding DNA segments in the process of mutation.
Coding DNA
Coding DNA contains the information used to make proteins.
Why mutations in coding DNA matter
A mutation in coding DNA may:
change a codon
change an amino acid in a polypeptide
affect the structure and function of a protein
Because proteins are important in cell function, mutations in coding DNA can sometimes have a direct effect on phenotype.
Non-coding DNA
Non-coding DNA does not directly code for amino acid sequences in proteins.
Why non-coding DNA still matters
Mutations in non-coding DNA are not always harmless.
A 2022 HSC question identified the correct significance of mutations in non-coding DNA as that they may affect gene expression, and a 2025 HSC question used a cancer example where non-coding DNA affected whether an oncogene was switched on.
Key idea
Non-coding DNA can still be important because it may regulate when, where, or how strongly genes are expressed.
Why mutation effects differ
Not all mutations have the same outcome.
The effect of a mutation depends on:
whether it is a point or chromosomal mutation
whether it occurs in coding or non-coding DNA
whether it affects a somatic or germ-line cell
whether it changes gene expression or protein structure
This is why some mutations have no noticeable effect, while others can cause major genetic change.
Worked example
Exam-style question
Explain why a mutation in non-coding DNA can still be significant.
Worked answer
A mutation in non-coding DNA can still be significant because non-coding regions may affect gene expression. Even if the mutation does not directly change an amino acid sequence, it may influence whether a gene is switched on or off, which can change the phenotype.
Why this works
This answer:
shows that non-coding DNA still has a function
links mutation to gene expression
explains why the mutation may still matter
Common mistakes
Saying all mutations are harmful.
Mixing up point mutations and chromosomal mutations.
Forgetting that mutagens increase mutation risk rather than guaranteeing mutation.
Saying non-coding DNA has no significance.
Assuming every mutation in coding DNA changes the phenotype.
Quick quiz
What is a mutation?
What is a point mutation?
What is a chromosomal mutation?
Name two types of mutagens.
Why can a mutation in non-coding DNA still matter?
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