(5) The effects of mutations on coding DNA and non-coding DNA

Non-coding DNA

There are parts of the DNA strands that do NOT code for any amino acids, hence do NOT code for proteins.

However, some of these non-coding sections switch genes on and off, in other words, they control whether or not a gene is expressed to make a protein.

So, do NOT assume non-coding DNA is of no significance!

Therefore some of these non-coding regions of the DNA are involved in protein synthesis.

Before transcription can occur (involves reading the DNA code), the RNA polymerase (enzyme) has to bind to a non-coding section of DNA adjacent to the specific gene (coding for a specific protein).

If a mutation has occurred in this section of the DNA it can affect the ability of the RNA polymerase to bind to it - it might be harder or easier (or no effect).

The quantity and accuracy of how much mRNA is transcribed depends on how well this binding takes place - and therefore affects how well the protein is produced.

Therefore the production of the protein may be affected, and, depending on its function, that specific phenotype may also be affected.

This means that genetic variants in non-coding regions of DNA can affect the phenotypes exhibited by an organism, despite the fact that these non-coding sections of DNA done code for proteins themselves.

 

Example of a mutation in a triplet code and formation of variants

The original section of DNA with the original sequence of triplet codes

The original triplet code and amino acid sequence was: CCA for proline, TCG for serine and AGA for arginine.

If just one base has changed 'mutated', e.g. middle triplet from TCG to TGG, the 2nd amino acid is this sequence is changed.

The sequence is now: CCA for proline, TGG for threonine and AGA for arginine (diagram below).

Mutated section of DNA - a change in one of the triplet codes

 

The mutation causes the protein to have a slightly different molecular structure and maybe a different shape, and this can have consequences.

(i) It may not affect the function of the protein at all e.g. the enzyme might still work

(ii) It may enhance the function of the protein, possibly with an evolutionary advantage.

(iii) BUT, it may adversely affect the function or activity of the protein e.g. the enzyme might not work as efficiently or maybe not at all due to a significant change in shape.

Effectively the gene is changed to a genetic variant of that gene, known as an allele, and can result in a different gene expression - a different phenotype.

These genetic changes in the DNA structure can involve substituting one base for another, deletion of a base or addition of a base. All of these change the triplet code sequence.

Mutations can also occur in non-coding sections of the DNA.

 

More on variants in non-coding DNA

A mutation changes the base sequence in a DNA molecule in a gene i.e. the DNA is potentially a different genotype.

This produces a genetic variant that can lead to changes in an organisms phenotype - gene expression characteristics.

Variants in non-coding sections of the DNA molecule can also affect the phenotype of an organism, despite the fact that the non-coding DNA does not code for proteins.

This can happen because before transcription can take place, RNA polymerase needs to bind to a section of non-coding DNA in front of a gene sequence of bases.

If a mutation occurs in the region of DNA, then it can affect the ability of RNA polymerase to bind to it - it might have no effect, promote binding or inhibit binding - there are always several possibilities in these sorts of situations - including driving evolution!

Depending on how well RNA polymerase can bind to this non-coding section of DNA will affect how much mRNA is transcribed in the transcription process - therefore how much of the protein is synthesised.

Therefore the structure and function of the protein is changed and the final phenotype of the organism can be affected.

For more on the consequences of mutations see

Introduction to genetic variation and formation and consequence of mutations including cancer

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Key points about mutations

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Mutations and Their Effects

What Are Mutations?

• A mutation is a random change in the DNA sequence.

• These changes can occur spontaneously or be caused by external factors such as:

  • Ionising radiation (e.g. X-rays, gamma rays)

  • Chemical mutagens (e.g. tobacco smoke, pollutants)

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Mutations in Coding DNA

What is Coding DNA?

• Coding DNA contains genes that are transcribed and translated into proteins.

Effect of Mutations in coding DNA:

• Mutations in coding regions can change the order of amino acids in a protein:

  • This may alter the structure and function of the resulting protein.

• There are several types of mutation:

  • Substitution: One base is swapped for another

  • Insertion: An extra base is added

  • Deletion: A base is removed

• These changes may result in:

  • A non-functional protein

  • A protein with altered activity

  • Or have no effect if the mutation is "silent" (same amino acid is coded)

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Mutations in Non-Coding DNA

What is Non-Coding DNA?

• Non-coding DNA does not code for proteins, but it can regulate gene expression.

Effect of Mutations in non-coding DNA:

• Mutations here may affect when, where, and how much of a protein is produced.

• This can:

  • Increase or decrease gene activity

  • Result in too much, too little, or mis-timed protein production

  • Influence how a cell behaves or develops

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Formation of Genetic Variants

How Variants Arise:

• A genetic variant is a version of a gene that differs from the "normal" sequence due to mutation.

• If the mutation is passed on during reproduction, it becomes part of the gene pool.

Inheritance:

• Variants can be:

  • Harmful (e.g. cause genetic disorders)

  • Beneficial (e.g. increase survival chances)

  • Neutral (no significant effect)

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Consequences of Genetic Mutations

Harmful Consequences:

• May cause genetic disorders such as cystic fibrosis or sickle cell anaemia.

• Can lead to cancer if mutations affect genes that control cell division.

Beneficial Consequences:

• Sometimes lead to advantageous traits (e.g. resistance to disease).

• These may spread in a population through natural selection.

Neutral Consequences:

• Many mutations do not affect the organism because:

  • They occur in non-coding DNA

  • They result in the same amino acid (redundancy in the genetic code)

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Summary of learning objectives and key words or phrases

Be able to describe how variants are formed from the effect of mutations on coding DNA and non-coding DNA.

Appreciate, understand and describe how the mutation effects may be negative, neutral or positive on protein synthesis, and the DNA may code for the wrong sequence of amino acids in the protein - mutation may have no effect, may be harmful or may be useful for evolution of a species!

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