An introduction to genetic variation and the formation of mutations and some of the consequences of mutations

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Genetic variants are caused by alterations in the common nucleotide sequences in the DNA of genes. The term variant can be used to describe an alteration that may be benign (harmless), pathogenic (harmful), or of unknown significance. The term variant is increasingly being used in place of the term mutation. Variants are key to successful evolution because genotype changes (usually of the smaller type) can lead to changes in phenotype. Human genetic variation is the genetic differences both within and among populations. There may be multiple variants of any given gene in the human population. A mutation may defined as any change in a DNA compared to normal that results in a rare and abnormal variant.



(a) Changes can happen to the DNA of the genome - mutations and variants

Sometimes DNA may mutate, meaning a random change occurs in the DNA sequence.

This automatically changes the sequence of bases in the DNA molecule.

All the different versions of genes are called genetic variants or alleles and are formed by mutations (but do not assume they are all harmful to the functioning of an organism!).

Genetic variants can be inherited from one generation to another e.g. mother to child.

There are various different ways that mutations can occur and change the base sequence in DNA e.g.

Deletion mutation:

A base might be deleted at random from the DNA base sequence. Usually just one base is deleted.

This will change the way in the nucleotide base sequence is read and will affect other bases further down the DNA strand. In other words an abnormal amino acid sequence is produced.

The wrong triplets are recognised so the wrong amino acids will be coded for!

In the diagram ATC GTT AGC CGA etc. becomes ATC TTA GCC GA... etc.

Insertion mutation:

A new base may be inserted into the DNA base sequence into a position it should NOT occupy in a gene.

This will change the way the triplet codes are read i.e. it changes the amino acid code and code for the wrong amino acids.

In the diagram the original triplet codons are ATC GTT AGC CGA etc. but after the insertion of base T after the first triplet, the triplet codons now read quite differently:  ATC TGT TAG CCG A... etc.

Also, as a consequence, more than one amino acid triplet is changed because a whole sequence of bases can be affected. The wrong amino acids will be coded for.

One or bases may be inserted in a single mutation. If one or two bases are inserted the above applies.

BUT, if three bases are inserted, the original sequence before and beyond the insertion remains intact! Is the consequence an extra amino acid in the polypeptide-protein? Can the same functioning protein still be made?

Substitution mutation:

Another base in the DNA is substituted at random with a different base changing the base sequence.

Here there are two possible outcomes:

(i) there might not be any overall effect because some amino acids are coded for by more than one triplet and the substitution might make one of those other triplet codes.

e.g. in the diagram the 2nd triplet GTT mutated to ATT, but may still code for the same amino acid.

(ii) the sequence can't be read correctly because the code doesn't match the particular amino acid required. The wrong amino acid will be coded for.

 


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(a) What are the consequences of mutations?

Important reminders:

(i) Enzymes are proteins. They catalyse most reactions in organisms. They have a specific shape and molecular structure that enables them to catalyse specific reactions.

If the enzyme protein molecule is not correctly synthesised, then it cannot perform its catalytic role in biochemistry.

(ii) If a mutation produces a change in the triplet codes for amino acids then the final protein formed may have a different structure and function than the one that was supposed to have been formed (this was explained in section (a) above.

The protein produced is unlikely to be able to perform the function that was intended from the DNA code.

The protein might do something different or may be incapable of doing anything.

A single mutation changing the function of a single protein molecule can have a significant effect on the phenotype.

Examples:

The genetic disorder cystic fibrosis is caused by the deletion of three bases with a massive detrimental effect on the phenotype. The 'damaged' gene codes for a protein that controls the movement of salt and water in and out of cells -semi-permeable membrane control. Unfortunately, the protein produced by the cystic fibrosis variant doesn't work correctly. The result in the individual is excess mucous production in the lungs and digestive systems and this causes difficulty in breathing and digesting food.

Some mutations have a slight effect on protein function and have a relatively small effect on the phenotype - I presume the protein molecule is sufficiently well formed enough to do its function, but perhaps not perfectly.

Mutations of coding DNA do not necessarily change the amino acid sequence of a protein. Here, such mutations have no effect on the phenotype i.e. no effect on the characteristics of an organism. This is in sharp contrast to the sufferers of cystic fibrosis.

It appears that many sections of DNA are described as 'non-coding', meaning they do not code for any proteins. However these non-coding sections of DNA perform other essential functions including switching genes 'on and off'. This means whether or not a gene is expressed. The word 'expression' in genetics means that that gene is used to make a protein. Therefore, any mutations in this non-coding DNA may prevent the synthesis of protein and the lack of this protein may adversely affect the organism's phenotype.

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notes to myself!

link to genome 3 page genetic diagrams - in progress

repeat stuff from top of page and expand with more examples or leave until a later page?

more links to examples of mutations see ...


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