Lesson 2 of 5 / Genomes and gene regulation
Non-coding does not mean useless
How can DNA influence a cell without encoding a polypeptide?
In this lesson: Explain introns, centromeres, telomeres and transcriptional control regions.
About 6 min
The key ideaNon-coding regions can control expression, support chromosome segregation or protect chromosome ends.
Explore the idea
What can this non-coding region do?
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
Locate the region
Within a transcript, at a control site or at a chromosome structure?
- 2
Name the interacting machinery
Use splicing, transcription factors or spindle attachment.
- 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.
Is this true? "Only DNA that codes for amino acids can affect phenotype."
Regulatory and structural DNA can affect expression, segregation and chromosome stability.