Mutation & Redundancy
Mutation & Redundancy HOME
2) Redundancy of the Code
Success Criteria & Vocabulary
Click this drop-down menu to see the Success Criteria.
Describe what mutations and mutagens are.
Describe what a point mutation is.
Describe the processes of base substitution, insertion, and deletion.
Discuss the effect of each type of gene mutation will have on the structure and function of the protein produced.
Click this drop-down menu to see the list of Vocabulary.
mutation: Permanent changes to the DNA sequence, which results in a new allele.
mutation: Substance or environmental factor that changes the genotype/base sequence.
insertion mutation: When a base is added into the DNA sequence increasing the number of bases in a gene.
deletion mutation: When a base is taken out/removed from a DNA sequence, decreasing the number of bases in a gene.
substitution mutation: When a base is changed/swapped, which may or may not result in a codon that codes for a different amino acid.
frameshift : When bases downstream an insertion or deletion mutation shift positions.
missense: Any mutation that causes a change in the sequence of amino acids.
nonsense: Any mutation that results in the production of a stop codon.
same sense: Any mutation that does not result in a change in amino acid. Also called silent mutations.
Self-Directed Learning Tasks
Task 3: Complete Education Perfect:
Task called 'Types of Mutations'.
Task 4: Complete sciPad:
Mark your own work using the sciPad online answers.
What is a Mutation and What Causes It?
Mutations and the Genotype
Remember from 2.5 Genetic Variation and Change that 'genotype 'is the combination of alleles in our DNA, which is responsible for a particular trait an individual has.
A mutation is a permanent change in the base sequence of a gene, altering the genotype, which results in a new allele. This could result in different amino acids being coded for, resulting in the shape of the protein being altered.
Mutagens are environmental factors that, when exposed to cells, can alter the genetic material of an organism. In other words, mutagens increase the rate of mutations in an organism. If any mutagen results in the formation of cancerous cells, it is known as a carcinogen.
Long term exposure to sunlight (UV radiation) can cause a variety of skin cancers such as melanoma. UV radiation is absorbed by our skin cells and therefore, our DNA. Here it acts as a mutagen by causing lesions on the DNA strand, which can result in bases being replaced mutated (substitution, deletion, addition).
Many chemical compounds can also act as mutagens. Examples of mutagenic chemical compounds are benzene, formaldehyde, nitrous oxide, and asbestos. (You do not need to know examples, they will be given to you).
Examples (INTEREST ONLY)
Asbestos is a naturally occurring heat-resistant group of minerals, that can be manufactured into fluffy fibres and used for many different kinds of construction materials. Asbestos began to be used commercially in the late 1880’s where it was used as an insulator and to fire-proof buildings.
Unfortunately, the tiny fibres that make up asbestos are highly dangerous. These fibres cause large gene mutations and the deletion of bases from the genome. Once these fibres are inhaled, they become stuck in the lungs, and overtime they case various diseases such as lung cancer and asbestosis - long term inflammation and scarring of the lungs.
In New Zealand, around 170 people die each year as a result of these diseases. The use of asbestos in most building materials New Zealand was phased out after the mid 1980’s, once the health risks were realised.
Viruses can also act as mutagens. For example, the hepatitis B virus invades healthy cells and integrates its genome with the DNA of the host cell.
What are the Different Types of Point/Gene Mutations?
The mutations you need to know about are point mutations.
Point mutations are permanent changes to the base sequence of a gene (and therefore the genotype). Point mutations occur within a single gene, and result in the alteration of the base sequence of that particular gene. They can be advantageous to an organism, harmful, or neutral (have no effect).
Point mutations can be passed onto an organism’s offspring, but only if they are present in gametes. These mutations are known as gametic mutations. Whereas mutations that occur in general body cells (somatic mutation) cannot be inherited by an organism’s offspring.
(In the next topic you will learn about how the ‘redundancy of the genetic code’ can potentially lessen the effect of a genetic mutation).
There are 3 Types of Point/Gene Mutations
Point mutations can be one of three types:
A base substitution mutation is when one base on the DNA is removed and replaced by another base. A base substitution mutation will result in one of three possible outcomes:
Silent/same sense mutation
Outcome #1: Silent/Same Sense Mutation
While a base substitution mutation sounds like it would be harmful to the individual involved, many substitution mutations have no noticeable effects on an organism’s phenotype or outward characteristics. This is due to the redundancy of the genetic code (more on this in the topic).
If a base substitution does not result in a change in amino acid, it is called a silent mutation, or same sense mutation. This is because even though the mutation has caused a change in the DNA sequence, it has had no effect on the structure/shape of the protein, and no visible effect on the individual. The protein will still carry out the same function.
Outcome #2: Missense Mutation
If the base substitution mutation creates a codon that codes for a different amino acid, it is called a missense mutation.
The consequence of the missense mutation on protein structure and function will depend on what amino acid is put in place of the original one.
For example if the structure and properties of the substituted amino acid are very similar to the original amino acid, the mutation is said to be conservative and will have little effect on the resultant protein's structure and function. But if the substitution leads to an amino acid with very different structure and properties the mutation is non-conservative, meaning it will totally change the resulting protein's structure and function (i.e. the sickle cell point mutation).
Above example:If the second triplet on the DNA molecule above changes from GTA to GTC as a result of a substitution mutation, the codon on the mRNA will also change from CAU to CAG. In turn, the amino acid coded for will change from HIS to GLU.
When an amino acid is produced that is different from the one that was originally coded for, the structure and therefore the function of the final protein will most likely be altered. Mutations that cause a change in the sequence of amino acids that are coded for are called missense mutations.
If a missense mutation causes a severe change to a protein’s structure, it will be unable to function as normal. This will be harmful to an individual, especially if the non-functional protein produced is involved in vital cellular processes.
For example, enzymes are important proteins that are responsible for catalysing different reactions in our cells. To do this, enzymes have a very specific active site, which is where the chemicals involved in a reaction bind. If the substitution mutation results in the production of an enzyme with a differently shaped active site, the enzyme would not be able to function and vital cellular reactions would not be carried out! (More on the active site in later blocks of lessons).
Outcome #3: Nonsense Mutations
If the base substitution mutation creates a codon that codes for a STOP codon, it is called a nonsense mutation. A nonsense mutation will truncate or stop translation altogether, and result in a shorter polypeptide chain.
The consequence of the nonsense mutation on protein structure and function will depend on how far down the DNA sequence this mutation happens in. If it happens early in the DNA sequence, the resultant protein will almost certainly be non-functional.
It is important to remember that there are 3 stop codons that can be found on mRNA: UAA, UAG, and UGA. These codons will signal the end point of protein synthesis.
Above Example: A nonsense mutation occurred, changing TYR to a STOP codon. If this stop codon is produced too early, then protein synthesis (specifically, translation) will end before all of the amino acids have been added to the growing polypeptide chain. The protein that forms from this polypeptide chain will be shorter and incomplete and will likely have no biological function.
Summary of Substitution Mutation Consequences
Base Insertion and Base Deletion Mutations
Base insertions and deletions are mutations that, unlike base substitutions, are almost always harmful to an individual. Base insertions result in the addition of a new base or sequence of bases to a DNA strand, while base deletions result in the removal of one or more bases from a DNA strand.
Adding or removing a single base affects every codon from the point mutation onwards. These mutations both result in “reading frameshifts”, creating different codons on the mRNA. Once a base in a triplet is inserted or deleted from the genetic code, it causes all of the bases downstream of the mutation to shift positions.
One result could be that a completely different sequence of amino acids will be coded form, and a non-functional protein will most likely be produced. This could be very harmful to the organism.
Let’s look at the sentence:
“THE BIG FAT CAT ATE THE SLY RAT”
Here, each word represents a triplet in a sequence of DNA. If we insert an “A” before the word “FAT”, the letters in the sentence shift position and read:
“THE BIG AFA TCA TAT ETH ESL YRA T”...... which no longer makes sense!
Another result could be that the frameshift could cause a stop codon to occur much earlier, which would affect the length of the polypeptide chain produced. This will likely produce a non-functional protein.
Let’s look at the example above of a reading frameshift caused by a base insertion. The normal sequence of bases in the DNA strand above is: GAG TCA CGG ACA TGC.
A mutation in the DNA causes the base G to be inserted into the second codon in the sequence. This shifts along all of the bases that follow one step to the right, so the DNA sequence now reads: GAG TGC ACG GAC ATG.
The insertion of a new base has altered all of the triplets downstream from the mutation. This also alters the arrangement of bases on the mRNA, and as a result the sequence of amino acids changes from LEU, SER, ALA, CYS, and THR to LEU THR CYS LEU and TYR. Only the position of the first amino acid stayed the same. This is likely to change the way the polypeptide chain folds, and therefore the 3-dimensional structure of the protein. Therefore this insertion mutation will probably result in the creation of a protein that is unable to carry out its normal function.
Now, let’s look at the above example of a reading frameshift caused by a base deletion. The normal sequence of the bases in the DNA sequence above is: GAG TGC ACG GAC ATG. This time, a mutation causes the second base in the second triplet, G, to be deleted. This shifts all of the bases that follow, one position to the left, so the DNA sequence now reads GAG TCA CGG ACA TGC.
The deletion of this base has altered all of the triplets downstream of the mutation. The sequences of bases on the mRNA will also be altered, as a result of the sequence of amino acids has changed drastically from LEU, THR, CYS, LEU, and TYR to LEU, SER, ALA, CYS, and THR. Only the position of the first amino acid stayed the same.
This is likely to change the way the polypeptide chain folds, and therefore the 3-dimensional structure of the protein. So as with the insertion mutation, this deletion of a single base will result in the creation of a protein that is unable to carry out its normal function.