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D2.2.3 (HL)—Control of mRNA degradation

mRNA degradation controls expression duration by removing transcripts so translation stops when proteins are no longer needed by cells later.

Syllabus
First assessment 2025
Objective
D2.2.3
Level
HL

mRNA Stability Controls How Long a Message Is Available

HL only

Regulating mRNA degradation controls how long an mRNA remains available for translation and therefore how much protein can be produced.

Human mRNAs can persist from minutes to days. Shortening of the protective 3′ poly-A tail helps initiate degradation, after which nucleases break down the transcript.

Longer mRNA lifetime → more opportunities for ribosomes to translate it. Faster degradation → fewer translation rounds and a shorter-lived protein-production response.

If two cells transcribe equal numbers of mRNA molecules but one transcript is degraded within minutes while the other persists for hours, the longer-lived transcript can usually support more translation.

mRNA abundance reflects both its rate of transcription and its rate of degradation; high transcription alone does not guarantee high protein output.

HL Gene Expression Control

HL only
  • Transcription factors, promoters, enhancers, activators and repressors control RNA polymerase activity.
  • mRNA lifetime limits translation; poly-A shortening and nucleases help remove transcripts.
  • DNA methylation and histone modification alter chromatin access without changing base sequence, creating epigenetic patterns during differentiation.
  • The genome is all genetic information; the transcriptome and proteome vary with cell type, time and environment.
  • Some epigenetic marks persist through cell division or inheritance, although most are reset during gamete formation; genomic imprinting is an exception.
  • Hormones regulate eukaryotic transcription through receptors and transcription factors; lac and trp operons illustrate bacterial control.
  • Twin studies and environmental exposures help separate genetic, epigenetic and environmental effects on phenotype.

Concept essentials

  • Stable mRNA can be translated repeatedly.
  • mRNA degradation reduces the time available for translation.
  • Poly-A tail shortening can help mark mRNA for removal.
  • Post-transcriptional control changes protein production after mRNA has been made.