L295 Regulation of Gene Expression Flashcards
How many % of the genome do exons make up?
~1-2%
Hetero vs euchromatin (4)
- Heterochromatin: condensed dark bands (Giemsa), mostly inactive, A + T rich, late S-phase rep
- Euchromatin: light bands, more active, C + G rich, early S-phase rep
Gene expression can be … (2 types)
- Constitutive
- ‘housekeeping’ genes: products made by all cells all the time
- Expression is constant
- Regulated
- Time (developmental), place (cell type), amount, in response to signals
- Can be under very tight, complex control
- Time (developmental), place (cell type), amount, in response to signals
Different classes of RNA genes
- Coding RNA (mRNA) → translated into protein
- Non-coding RNAs e.g. roles in RNA maturation and gene reg
- rRNA and tRNA: involved in protein synthesis
RNA splicing of protein-coding genes: how does it occur?
- Spliceosome binds to intron at donor splice and acceptor splice sites
- Cleavage of splice sites by spliceosome → splicing of gene
What is the spliceosome?
= large ribonucleoprotein complex that splices primary transcripts to remove introns
How might splicing contribute to protein diversity?
Alternative splicing: process by which different combos of introns/exons can be spliced together from one gene
Points of regulation of gene expression: which were covered in the lecture (4)?
- Conformation of chromatin (localised only)
- Transcription
- Post-transcriptional processing
- Non-coding RNA regulation
Chromatin =
= DNA and associated protein (histone and non-histone)
Which conformation of chromatin is needed for gene expression?
Open conformation
How is the conformation of chromatin regulated?
Epigenetic changes
Examples of epigenetic changes regulating conformation of chromatin
- Histone variants
- e.g. acetylation, methylation, ubiquitination, phos…
- DNA methylation
- Nucleosome occupancy
- Nuclear localisation of chromosome
Describe ‘nucleosome occupancy’ and how it is regulated
- Distances between nucleosomes can vary to allow greater access
- Regulated by ATP-driven chromatin modifying complexes, that change position of nucleosomes along DNA
‘Nuclear localisation of chromosome’: where are non-transcriptionally active chromosomes located?
Chromosomes that are not transcriptionally active are located peripherally in the nucleus
DNA methylation generally means…
↓ gene expression
Where at DNA does methylation occur?
at cytosine of CpG islands
CpG islands - where are they often clustered and in what form? What does this mean?
- Often surround promoter regions, but are usually unmethylated there - even when genes aren’t expressed
- i.e. not all CpG motifs are methylated
DNA methylation and DNA replication: what happens to the methylation pattern with new DNA strands?
- New DNA strands initially lack methyl groups, but original pattern is soon transmitted to daughter cells
Examples of transcriptional regulation of gene expression
- Multiple promoter sites
- RNA polymerase and transcription factors
- Interaction with other cis-acting DNA sequences, ncRNAs and trans-acting proteins
What does interaction with other cis-acting DNA sequences entail?
- Looping DNA together brings regions on the same chromosome together, and allow interaction with other cis-acting DNA sequences (‘enhancers’ or ‘silencers’ of transcription)
What does ‘cis-acting’ mean?
On the same chromosome as the gene to be transcribed
Example of post-transcriptional regulation of gene expression
Alternative splicing of different exons can lead to different protein products
What must we note about alternative splicing?
- alternative splicing does not necessarily just involve taking out an intervening intron between exons
- Parts of introns can be included, and parts of exons can even be excluded!
Examples of non-coding RNAs regulating gene expression
- Long non-coding RNAs (lncRNAs)
- Short interfering RNAs (siRNA)
- Micro RNAs (miRNA)
4 possible ways long non-coding RNAs might act to regulate gene expression
- Decoy - takes away enzymes that would usually act for transcription
- Scaffold - can bring proteins together
- Guide - guide molecules together
- Enhance - make proteins bind better
How do siRNAs act to regulate gene expression?
- siRNA is ds - each has sequence complementary to specific mRNA
- Form complex with RNA-induced silencing complex (RISC) and becomes ss
- Binds to complementary sequence on mRNA
- RISC cleaves mRNA before translation
- → mRNA degradation
How do miRNAs act to regulate gene expression?
• miRNA only partially complementary to mRNA sequence
1. miRNA forms complex with RISC 2. Multiple proposed mechanisms of repression 3. → repressed mRNA translation
