Overview
Gene expression in eukaryotes is controlled at many levels, from chromatin and transcription to the processing, transport, stability and translation of mRNA. This topic covers the post-transcriptional steps, including splicing, mRNA transport and stability, RNA interference and translational control.
Eukaryotic genome and chromatin
- Most of the DNA in eukaryotic cells is stored inside the nucleus.
- Chromatin is a mix of DNA and proteins that helps organize and compact the genome.
- Around 90 percent of eukaryotic DNA is non-coding, meaning it does not directly code for proteins.
Types of DNA Sequences
- Unique sequences
- Found in only one or two copies
- Include nearly all structural and regulatory genes
- Moderately repeated DNA
- Found in tens to thousands of copies
- Includes histone and rRNA genes
- Some have regulatory roles, but many have unknown functions
- Highly repeated DNA (Satellite DNA)
- Short sequences repeated many times in tandem
- Found in centromeres and telomeres
- Mostly structural; function often unclear
- Examples in humans:
- Alpha (alphoid) DNA: 171 bp units, present on all chromosomes
- Beta satellite DNA: 68 bp, found on certain centromeres
- Satellite 1, 2, 3: 5 to 48 bp, found in heterochromatin and centromeres
- Nucleolar organizers
- Chromosome regions that contain rRNA gene repeats
- Located on the short arms of chromosomes 13, 14, 15, 21, and 22
- Chromatin structure directly affects gene expression
- Euchromatin
- Loosely packed
- Active for transcription
- Heterochromatin
- Densely packed
- Inactive for transcription
- Includes:
- Constitutive heterochromatin: always condensed
- Facultative heterochromatin: can switch between active and inactive states
- Heterochromatinization
- Used to silence large regions of the genome
- Position effect
- When a gene is relocated near heterochromatin, it may get silenced
- X-chromosome inactivation in females
- One X chromosome becomes heterochromatin (Barr body)
- The other X remains active
- In some white blood cells, the Barr body appears as a drumstick structure (Davidson body)