6.1.1 Gene mutations Flashcards
What is a gene mutation?
A change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide
When do mutations occur?
Continuously, and can occur spontaneously during DNA replication
How can the probability of a mutation occurring be altered?
It can increase with the presence of certain factors, known as mutagens:
- Ionising radiation (x rays) can break DNA strands = can be altered during repair process
- Deaminating chemicals can alter chemical structure of bases
- Methyl or ethyl groups can be added to bases, leading to incorrect pairing
- Viruses - insert sections of viral DNA into DNA
Why do mutations usually have no effect on us?
Most mutations - dont ater polypeptide or only alter slightly so function/structure isnt affected (as genetic code is degenerate)
Many mutations occur in non-coding sections of DNA
What are the three main ways in which a mutation in the DNA base sequence can occur?
- Substitution
- Insertion
- Deletion
SID the sloth
Define the insertion mutation
Mutation that occurs when a nucleotide is randomly inserted into the DNA sequence.
What is the effect of an insertion mutation?
Changes the amino acid that would have been coded for by the original base triplet, as it creates new, different triplets of bases.
It can also have a knock-on effect further on in the DNA sequence by changing the triplets, known as a frameshift mutation.
May dramatically change amino acid sequence produced and therefore the polypeptides function
Define the deletion mutation
A mutation that occurs when a nucleotide is randomly deleted from the DNA sequence
What does a deletion mutation change?
The amino acid that would have been coded for
Changes the groups of three bases further on in the DNA sequence (frameshift mutation)
What is the effect of a deletion mutation
May dramatically change the amino acid sequence produced from the gene and therefore the ability of the polypeptide to function.
Define the substitution mutation
A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base
What is the effect of a substitution mutation
It will only change the amino acid for the triplet in which the mutation occurs; it will not have a knock-on effect
What are the three forms of substitution mutations
- Silent = mutation does not alter the amino acid sequence
- Missense = mutation alters a single amino acid
- Nonsense = mutation creates a premature stop codon
What is a silent mutation
Where the mutation does not alter the amino acid sequence of the polypeptide (may be because certain codons may code for same amino acids - genetic code is degenerate)
What is a missense mutation
The mutation alters a single amino acid in the polypeptide chain (sickle cell anaemia is an example of a disease caused by a single substitution mutation)
What is a nonsense mutation
When the mutation creates a premature stop codon, causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function (eg cystic fibrosis)
What are the three main categories od the effects mutations have on an organism
- Beneficial
- Harmful
- Neutral
What is a beneficial mutation
The alteration to a polypeptide that results in an altered characteristic in an organism that causes beneficial effects for the organism.
Give an example of a beneficial mutation
Production of the pigment melanin:
- Early humans of Africa had dark skin as they produced lots of melanin
- Provided protection from harmful UV radiation whilst allowing vitamin D to be synthesised
- However, at low sunlight intensities, pale skin = faster at synthesising vit D than dark skin
- Humans moved to cooler areas = mutations occurred which decreased the production of melanin
- Paler skinned individuals would have a selective advantage as they could synthesise more vitamin D (reduced chance of rickets, and protection against heart disease/cancers)
What is a harmful mutation
By altering a polypeptide, some mutations can lead to an altered characteristic in an organism that causes harmful effects for the organism
What are two examples of genetic diseases caused by harmful mutations
Haemophilia and sickle cell anaemia
Describe the harmful mutation involved in cystic fibrosis
Deletion mutation of three nucleotides in the gene coding for the protein CFTR
Loss of function of CFTR protein caused by this deletion mutation results in a number of symptoms, including lung and pancreatic problems as a result of extremely thickened mucus
What is a neutral mutation?
They offer no selective advantage or disadvantage to the individual organism
Why are some mutations neutral?
- Mutation does not alter the polypeptide
- Mutation only alters polypeptide slightly so that its structure or function is not changed
- Mutation does alter structure/function but this difference provides no particular advantage or disadvantage to the organism
