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. This is illustrated with the diagram and notes below.

The correct enzyme protein might not be formed - can have serious consequences!

A The protein structure of the enzyme is correctly formed form correctly coded DNA i.e. no mutations have had an adverse effect. The amino acid sequence correct and so the protein coils into its correct 3D structure and the active site can accept the substrate molecule.

B The chemical change can take place because the protein structure of the active site is the correct 3D shape to accept the substrate molecule which 'docks in' - the 'key and lock' mechanism.

C One or more mutations has caused a change in the amino acid sequence, leading to a change in protein shape at the active site, so the substrate molecule cannot 'dock' in and be chemically changed by the enzyme.

Note that all the rest of the enzyme structure is correct, and even if not due to a mutation affecting the amino acid sequence, it might not affect the active site. One reason why mutations do not always have a detrimental effect on the protein-enzyme structure and function.

(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.

 

Note that most mutations have no effect on an organism's phenotypes.

Some mutations can have a small effect, but there are rare mutations that can produce a new phenotype in a species - see evolution.

 

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.

 

For more details see Introduction to the inheritance of characteristics and genetic diagrams (including Punnett squares)  including technical terms, Mendel's work and inherited genetic disorders, genetic testing