9477 / 2027

Lesson 2 of 5 / Genomes and gene regulation

Non-coding does not mean useless

How can DNA influence a cell without encoding a polypeptide?

I can explain introns, centromeres, telomeres and transcriptional control regions.

  • I can explain the specified non-coding DNA regions and their functions.Syllabus

    Syllabus 9477, Core 2(h). Explain the specified non-coding DNA regions and their functions.

About 6 min

Make a guess. You are not marked.

Only a small fraction of human DNA codes for protein. Is the rest useless?

  1. Much of it controls expression or protects chromosomes.
  2. DNA that codes for nothing does nothing.
  3. It is left over from viruses and can be lost.
Show the answer

Much of it controls expression or protects chromosomes.

Promoters, enhancers, centromeres and telomeres code for no protein, yet each has a clear role.

The key idea

Non-coding regions can control expression, support chromosome segregation or protect chromosome ends.

Explore the idea

What can this non-coding region do?

A linear chromosomeOne expanded control/gene regionEnh.Sil.Pro.IntronPromoter

Transcription machinery assembles here. A change may alter initiation and protein amount even if the coding sequence is unchanged.

Enh. = enhancer; Sil. = silencer; Pro. = promoter. Their positions and distances are schematic: real control elements need not occur in this order or all be upstream. The intron is between retained exon regions. The centromere and telomeres belong to the whole-chromosome view above.

Explanation

A promoter is a DNA region where transcription machinery assembles to initiate transcription. Enhancers and silencers bind regulatory proteins that can increase or decrease transcription of a target gene. They can act at a distance through DNA looping and interactions with the transcription complex.

Introns are intervening sequences transcribed into pre-mRNA but removed during splicing. They do not normally contribute directly to the final polypeptide sequence, and their processing can contribute to expression control. Distinguish an intron within a transcription unit from a promoter that helps determine whether transcription starts.

Centromeric DNA and associated proteins establish the region where a kinetochore forms for spindle attachment. This supports reliable chromosome segregation. Telomeric repetitive DNA and proteins protect linear chromosome ends and help address the consequences of end replication.

The syllabus category focuses on portions that do not encode protein or functional RNA. Elsewhere, some genes do encode functional RNA such as rRNA or tRNA, so not every non-protein-coding sequence belongs to the same functional category. Avoid claiming that all non-coding DNA is junk or that every base has a fully established function.

Step by step
  1. 1

    Locate the region

    Within a transcript, at a control site or at a chromosome structure?

  2. 2

    Name the interacting machinery

    Use splicing, transcription factors or spindle attachment.

  3. 3

    Predict a specific effect

    A control-region mutation can change amount rather than sequence of protein.

Worked example

Work through the evidence

A promoter mutation reduces transcription but leaves the coding sequence intact. What protein change is most directly expected?

One way to explain it

Less protein may be produced, while the amino-acid sequence of any normally translated product can remain unchanged.

Why this answer works
  • Expression amount and protein sequence are separate variables.
  • A control-region mutation need not alter a codon.

Watch out for this

A student says: "Only DNA that codes for amino acids can affect phenotype." What is wrong with this?

Show the answer

Regulatory and structural DNA can affect expression, segregation and chromosome stability.

You may meet this in school papers

Not required by the 2027 syllabus, but school papers still ask it. Learn it after the main idea.

Transposons (jumping genes)

Some school papers mention transposons: DNA sequences that can move from one place in the genome to another. A transposon that lands inside a gene can disrupt it, causing a mutation, and transposon copies make up a large share of many eukaryotic genomes. Treat this as background; the 2027 syllabus asks you to explain promoters, enhancers, silencers, introns, centromeres and telomeres.

Check your understanding

Which region is most directly associated with spindle attachment?

  1. An intron in every gene
  2. A poly-A tail
  3. The centromere and its associated kinetochore
Show the answer

The centromere and its associated kinetochore

This region connects chromosomes to spindle microtubules.

Quick recall

Card 1 of 3. Answer in your head, then check.

How can DNA influence a cell without encoding a polypeptide?

You can return to this lesson any time.