CourseCourseQ BankQuestion BankDocsDocuments

D2.2.1 (HL)—Gene expression mechanism

Gene expression uses transcription and translation to convert DNA information into proteins that influence phenotype and cellular behaviour in organisms.

Syllabus
First assessment 2025
Objective
D2.2.1
Level
HL

Gene Expression Converts DNA Information into Output

HL only

Gene expression is the mechanism by which information in a gene affects phenotype, most commonly through production and function of a protein.

The DNA base sequence is transcribed into mRNA, the mRNA sequence is translated into a polypeptide, and the folded protein performs a function such as catalysing a reaction. That function contributes to the cell's traits.

Gene information → transcription → mRNA → translation → protein → cellular function → phenotype.

Expression of a gene for a digestive enzyme produces mRNA, then enzyme protein; the enzyme's catalytic activity contributes to the digestive phenotype of that cell.

Possessing a gene does not mean it is expressed in every cell. Regulation can change the amount of mRNA and protein without changing the DNA sequence.

Gene expression mechanism

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain.

Command terms

Explain

What earns marks

Build the answer around this relationship: DNA information affects phenotype through gene products.

Representative question

Question 1

[Maximum number: 1]

One important chemical in the mobilization of stem cells is a protein, CXCL12, which maintains the stem cells inside the bone marrow. The breakdown of CXCL12 causes the mobilization of stem cells to the blood vessels.

The graph below shows the mobilization of stem cells and the production of mRNA for CXCL12 when the bone marrow is treated with two different chemicals (isoprenaline and clenbuterol).

Explain how the amount of mRNA for CXCL12 gives an indication of the amount of protein CXCL12 produced.

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

  • DNA information affects phenotype through gene products.
  • mRNA is produced by transcription from a gene.
  • mRNA can be translated into a polypeptide.
  • Measuring mRNA can indicate expression of the protein-coding gene.