Gene Mutation

Gene Expression and Genetics

Define mutation.

A permanent change in the DNA of an organism.

When can mutations occur?

They can occur when DNA is copied during cell division.

Can mutations occur in any cell?

Yes. Mutations can occur in both body cells and cells that form gametes.

Why are mutations in gametes particularly important?

They can be passed on to offspring and may cause inherited genetic disorders.

What are gametes?

Haploid sex cells that fuse during sexual reproduction to form a diploid zygote.

What is a gene mutation?

A change in the sequence of bases within a gene.

What is another name for a gene mutation affecting a single base?

A point mutation.

Define point mutation.

A change involving a single base in the DNA sequence.

What are the three main types of point/gene mutation?

Substitution, deletion and insertion.

Define substitution mutation.

A mutation in which one DNA base is replaced by another base.

Define deletion mutation.

A mutation in which a DNA base is completely removed from the sequence.

Define insertion mutation.

A mutation in which an extra DNA base is added to the sequence.

What can the inserted base in an insertion mutation be?

It may be a repeat of an existing base or a completely different base.

What happens to DNA during a substitution mutation?

One base is exchanged for another.

What happens to DNA during a deletion mutation?

One base is lost from the DNA sequence.

What happens to DNA during an insertion mutation?

An additional base is added to the DNA sequence.

Why do cells have DNA repair systems?

Errors can occur during DNA replication, so repair systems detect and repair damaged or incorrectly copied DNA.

Why do some mutations remain despite DNA repair systems?

Some replication errors escape repair and are copied into new DNA molecules.

What is a chromosomal mutation?

A mutation involving a change in the position or arrangement of whole genes within a chromosome.

How are chromosomal mutations different from point mutations?

Point mutations affect individual bases, whereas chromosomal mutations affect larger sections containing whole genes.

What is a whole-chromosome mutation?

The loss or duplication of an entire chromosome.

How can whole-chromosome mutations occur?

Errors during meiosis can cause chromosomes to be lost or duplicated in the resulting cells.

What chromosome abnormality causes Down syndrome?

An individual has three copies of chromosome 21 instead of the usual two.

What is the relationship between mutation and variation?

Mutations create new genetic variation by producing new DNA base sequences.

Define variation.

Differences between organisms resulting from genetic differences, environmental factors or both.

Can mutations be beneficial?

Yes. A mutation may produce a new or improved protein that gives an organism a reproductive or survival advantage.

Why can beneficial mutations become more common in a population?

Individuals with the advantageous mutation may reproduce more successfully and pass the mutation to more offspring.

Can mutations be neutral?

Yes. Many mutations neither improve nor reduce an organism's chance of survival.

Can mutations be harmful?

Yes. A mutation may alter an important protein and disrupt normal cell or body function.

How can a mutation affect a protein?

It can change the DNA base sequence, which may change the mRNA codon sequence and therefore the amino acid sequence of a polypeptide.

What is the sequence showing how a gene mutation can affect phenotype?

DNA base sequence changes → mRNA codon changes → amino acid sequence may change → protein structure may change → protein function may change → phenotype may change.

Why can changing one amino acid affect a protein?

The change can alter interactions within the polypeptide, changing its folding, three-dimensional shape and function.

How could a mutation affect an enzyme?

A changed amino acid sequence may alter the enzyme's tertiary structure and active site, preventing the substrate from binding properly.

What could happen if the shape of an enzyme's active site changes?

The substrate may no longer be complementary to the active site, so fewer or no enzyme-substrate complexes form.

Why do many mutations have no observable effect?

They may occur in non-coding DNA or the genetic code may be degenerate so the mutation does not change the amino acid produced.

Why might a mutation in non-coding DNA have no effect?

The altered sequence may not code for a protein and may not affect how the genetic code is read or regulated.

How can the degenerate genetic code reduce the effects of mutations?

More than one codon can code for the same amino acid, so a base substitution may still produce the same amino acid.

Does every substitution mutation change the amino acid sequence?

No. Because the genetic code is degenerate, the new codon may still code for the same amino acid.

Why can insertion and deletion mutations have particularly large effects?

They can alter how subsequent bases are grouped into triplet codons, potentially changing many amino acids in the polypeptide.

What genetic diseases are given as examples of mutations in gametes?

Thalassaemia and cystic fibrosis.

How are mutations involved in thalassaemia?

Mutations affect the production of blood proteins so they are not manufactured correctly.

How are mutations involved in cystic fibrosis?

A mutation causes a membrane protein to function incorrectly.

What can mutations in somatic cells cause?

They can contribute to the development of cancers.

What are somatic cells?

Body cells other than gametes.

Are somatic-cell mutations normally inherited by offspring?

No. They are generally not passed to offspring because they do not occur in gametes.

What is sickle cell disease?

A genetic disease caused by a mutation affecting the haemoglobin protein in red blood cells.

What type of mutation causes sickle cell disease?

A point mutation involving the substitution of a single DNA base.

What happens to the DNA code in sickle cell disease?

One base in a codon is changed, causing the codon to specify a different amino acid.

Which amino acid change is shown for sickle cell haemoglobin?

Glutamic acid is replaced by valine.

What is the healthy haemoglobin DNA coding-strand codon shown for glutamic acid?

GAG.

What is the corresponding sickle cell DNA coding-strand codon shown for valine?

GTG.

What single-base substitution occurs in the sickle cell example?

GAG changes to GTG.

How can a single-base mutation cause sickle cell disease?

The base substitution changes a codon → a different amino acid is incorporated → haemoglobin's structure changes → haemoglobin molecules stick together → red blood cells become sickle-shaped.

What happens to haemoglobin molecules in sickle cell disease?

The altered haemoglobin molecules can stick together and form rigid rods.

How do altered haemoglobin molecules affect red blood cells?

They cause the cells to become rigid and sickle-shaped.

Why are sickle-shaped red blood cells harmful?

They transport oxygen less efficiently and can obstruct small blood vessels such as capillaries.

What can blockage of capillaries by sickle-shaped cells cause?

Reduced blood flow, severe pain and potentially serious tissue damage.

How does the sickle cell example demonstrate the importance of DNA sequence?

A change in just one nucleotide can change one amino acid, altering protein structure and producing major effects on phenotype.

What does phenotype mean in relation to mutations?

The observable characteristics of an organism that may change as a result of altered protein structure or function.

What is a mutagen?

Anything that increases the rate at which mutations occur.

Give examples of mutagens.

X-rays, ionising radiation and certain chemicals.

How does exposure to mutagens affect mutation rate?

It increases the likelihood of DNA mutations occurring.

Why should exposure to mutagens be minimised?

They increase mutation rates and therefore increase the risk of harmful mutations, including those associated with cancer.

Are mutations always caused by mutagens?

No. Mutations can occur spontaneously, particularly as errors during DNA replication.

What is the difference between spontaneous mutations and mutations caused by mutagens?

Spontaneous mutations arise naturally, whereas mutagens increase the rate at which mutations occur.

Why can the same type of mutation have very different effects?

Its effect depends on where in the DNA it occurs and whether it changes the structure or function of an important protein.