(2) Changes can happen to the DNA of the genome - mutations - genetic variants

Sometimes DNA may mutate, meaning a random change occurs in a DNA sequence of an organism.

It is possible for the mutation to be inherited.

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

Therefore the gene expression may be altered or inhibited.

Here we are dealing with a different version of the gene - a genetic variant (also called an allele).

Any mutation changes the sequence of bases in a strand of DNA which produces a different form of the gene (allele), and is called a genetic variant.

In the course of evolution advantageous mutations are more likely to be inherited through successive generations.

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!).

Despite the frequency of mutations, most have no or very little effect on the protein synthesised in the ribosomes.

The change in protein structure is usually slight and harmless and its function or appearance is relatively unaffected.

However, certain mutations can have quite an effect on a protein with serious consequences.

This result of this genetic variant may code for a different sequence of amino acids and consequently may change the shape of the final protein structure and its activity.

e.g. theoretically, for an enzyme (protein), its activity may be increased, decreased or completely inhibited its action.

A mutation might even lead to coding for a different amino acid and hence a different protein is produced.

The protein might not be useful or potentially harmful and treated as a 'foreign' substance by the immune system.

If the protein is no longer the right shape it might not be able to perform its function e.g.

(i) an altered shape might mean an enzyme E cannot perform its catalytic action because the substrate molecules S can't lock into the active site - see diagram below (from my Enzymes - structure and functions page).

(ii) If substances like collagen, the main structural protein molecule in the connective tissues of your body, isn't formed properly, muscle tissue can be weakened or completely useless in providing physical support for an organisms body.

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

See more on the consequences of mutations (on this page)

and  the effects of non-coding DNA (on this page)

and see evolution page for lots more notes on variation

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Summary of learning objectives and key words or phrases

Be able to explain, using diagrams, changes to the DNA of the genome in terms of a mutation and the formation of genetic variants and changes in chromosomes.

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