(2.6) Extra notes on non-coding DNA

Explaining the significance of non-coding DNA.

In humans the coding for proteins is only about 1.5% of our DNA involving 20 000 to 25 000 genes, most of the rest is non-coding DNA, but it isn't there just to make up the numbers!

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 switched on and used to make a protein that contributes to a phenotype - what is produced in a particular characteristic.

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 - the gene expression.

 

Some specific examples of non-coding DNA

Fruit flies have an enzyme XDH which is involved in producing a red pigment.

Fruit flies with normal XDH enzyme activity have red eyes.

Fruit flies with no XDH activity have brown eyes because no red pigment is produced.

So there are parts of the DNA strands that do NOT code for any proteins, but they are of great importance.

More and more scientific research is showing that 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.

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

Before transcription can occur, the RNA polymerase has to bind to a non-coding section of DNA adjacent to the specific gene (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.

A summary from DNA and RNA structure and Protein Synthesis  gcse biology revision

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Key biology points Source of information is based on the syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points and more examples of non-coding DNA

Non-Coding DNA: Function and Importance in Human Genetics

Not all DNA sequences code for proteins - non-coding DNA makes up a significant portion of the human genome.

While it was once thought to have little function, researchers have discovered that non-coding DNA plays a crucial role in gene regulation, genome stability, and evolutionary processes.

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What Is Non-Coding DNA?

Non-coding DNA refers to sections of the genome that do not directly encode proteins. In humans, about 98% of DNA is non-coding.

While some of these sequences were previously called “junk DNA,” scientists now understand their essential roles in genetic function and regulation.

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Functions of Non-Coding DNA

Non-coding DNA serves multiple important functions:

1. Regulatory Elements

• Promoters and Enhancers – Control when and where genes are switched on or off.

• Silencers and Insulators – Prevent unnecessary gene activation.

• Transcription Factor Binding Sites – Help proteins bind to DNA and influence gene expression.

2. Structural DNA

• Centromeres – Help chromosomes align properly during cell division.

• Telomeres – Protect chromosome ends from degradation, preventing aging-related damage.

3. Non-Coding RNAs (ncRNAs)

Some non-coding DNA (ncRNA) is transcribed into RNA but does not make proteins. Instead, it has regulatory functions:

• MicroRNAs (miRNAs) & Small Interfering RNAs (siRNAs) – Control gene expression by targeting messenger RNA (mRNA).

• Ribosomal RNA (rRNA) & Transfer RNA (tRNA) – Essential for protein synthesis.

4. Introns

Introns are non-coding sequences within genes that are removed during RNA processing. They may play a role in alternative splicing, which allows a single gene to produce multiple proteins.

5. Repetitive DNA and Evolutionary Significance

• Satellite DNA and Tandem Repeats – Found in centromeres and telomeres, contributing to genome stability.

• Transposons & Retrotransposons – “Jumping genes” that may impact genetic variation and evolution.

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Importance in Understanding Human Genetics

Non-coding DNA is essential for studying genetics and medicine:

• Gene Regulation & Disease – Abnormalities in non-coding DNA can contribute to diseases such as cancer and genetic disorders.

• Evolution & Genetic Diversity – Helps scientists trace evolutionary history and understand genetic adaptation.

• Medical Research & Therapy – Used in developing treatments such as gene therapy and personalized medicine.

Even though non-coding DNA does not create proteins, it plays a vital role in genetic functions, making it a key focus in modern biology.

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Summary of learning objectives and key words or phrases

Know about the significance of non-coding DNA and the consequences of mutations which may affect the switching on and off of genes involved in protein synthesis.

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