IB Biology HL D2.2 Gene expression [HL only] Notes

Gene expression is the route from stored DNA information to phenotype. The main stages are transcription to make mRNA, translation to make a polypeptide, and protein function that affects cell activity or phenotype.

Expression connects information to phenotype.

A major control point in gene expression is transcription. Transcription factors bind specific DNA sequences; promoters, enhancers, activators, and repressors change RNA polymerase activity, so the cell makes more or less mRNA from a gene.

Transcription regulation changes mRNA amount.

Gene expression can be controlled after transcription by changing mRNA lifetime. If an mRNA is degraded quickly, translation stops sooner; poly-A tail shortening and nucleases help remove mRNA after use.

mRNA lifetime changes protein output.

Epigenesis explains how an undifferentiated zygote can give rise to specialized cells. Epigenetic changes alter gene activity without changing the DNA base sequence, so different cell types can keep different expression patterns while sharing the same genome.

Differentiation can be controlled epigenetically.

Genome, transcriptome, and proteome describe different layers. The genome is all genetic information, the transcriptome is the set of expressed mRNAs, and the proteome is the dynamic protein set that changes with cell type, time, and environment.

Expression outputs change while the genome may stay the same.

Epigenetic tags regulate expression by changing access to genes. Promoter DNA methylation usually represses downstream transcription, while histone methylation or acetylation changes chromatin access and therefore gene expression.

Tags change access to the gene.

Epigenetic inheritance means a gene-expression state can be passed on without changing the DNA sequence. Persistent DNA methylation or histone tags can survive cell division or, in some cases, gamete formation.

Inherited tags can preserve expression states.

External conditions can alter gene expression. Diet, oxygen, light, drugs, temperature, mutagens, and pollution may affect expression; air pollution is a key example because it can modify DNA and histone methylation linked to lung disease.

Environment can act through expression control.

Most epigenetic tags are reset during human egg and sperm development, which limits inheritance of many expression states. The exception to remember is retained imprints: they can silence one parental allele and affect offspring phenotype.

Resetting is common; imprint retention is the exception.

Monozygotic twins share a genome, so they help test genetic versus environmental effects. If twins age in different environments, epigenetic differences can accumulate and may help explain differences in expression or phenotype.

Shared genome makes environmental/epigenetic differences easier to study.

External factors can regulate expression through different systems. In eukaryotes, hormones can act through receptors and transcription factors; in bacteria, lac and trp operons show inducible and repressible gene control.

Different systems control expression in different organisms.

Gene expression uses DNA information to affect phenotype through proteins; main stages are transcription, translation, and protein function. Transcription factors bind specific DNA sequences to regulate transcription; promoters, enhancers, activators, and repressors alter RNA polymerase activity. mRNA degradation controls how long translation can continue; poly-A tail shortening and nucleases help remove mRNA after use. Epigenesis develops differentiation patterns from an undifferentiated zygote; epigenetic changes alter gene activity without changing DNA base sequence. Genome is all genetic information; transcriptome is expressed mRNA set; proteome is the dynamic protein set produced by cell type, time, and environment. Promoter DNA methylation usually represses downstream transcription; histone methylation or acetylation changes chromatin access and gene expression. Epigenetic inheritance passes gene-expression changes without DNA sequence change; persistent DNA methylation or histone tags can survive cell division or gamete formation. Diet, oxygen, light, drugs, temperature, mutagens, and pollution can alter expression; air pollution can modify DNA and histone methylation linked to lung disease. Most epigenetic tags are reset during human egg and sperm development; retained imprints can silence one parental allele and affect offspring phenotypes. Monozygotic twins share a genome, helping test genetic versus environmental effects; epigenetic differences can accumulate with age and different environments. Hormones regulate eukaryotic expression through receptors and transcription factors; lac and trp operons show inducible and repressible gene control in bacteria.

Use this for HL questions that combine transcription regulation, mRNA degradation, epigenesis, genome/transcriptome/proteome, methylation/histone tags, epigenetic inheritance, environmental effects, imprinting, twin studies, hormones, and operons.