What is transcription and why is it important? (2)
- Transcription is the process by which encoded genetic information in DNA is used as a template to form mRNA
- Different genes can make multiple RNA based on protein. E.g Alpha Tropomyosin gene can make striated muscle, smooth muscle, fibroblast and brain mRNA.
Describe Transcription into pre-mRNA. (4)
- Basal transcription factors bind first to DNA at ‘promoter region’ (TATA box) then RNA polymerase attaches to same area and unzips strand.
- RNA polymerase moves from 5’ to 3’ end, adding complementary nucleotides to coding strand.
- It reaches a terminator code, which causes a conformational change to make a ‘hair-pin’ in RNA strand so RNA polymerase lets go. This is pre-mRNA.
- This is capped at 5’ end and poly-adelynated tail added to other end.
Describe how mRNA is made from pre-mRNA (3)
- pre-mRNA has introns (non-coding region) and exons (coding region).
- Introns are spliced out, leaving exons.
- This forms the actual mRNA. Cap and poly A tail remain
What are the different types of RNA polymerase? (3)
- RNA polymerase I (most rRNA genes)
- RNA polymerase II (used for most mRNAs). Mushroom Alpha amantin is a poison as it inhibits this polymerase causing death by liver failure.
- RNA polymerase III (used for tRNA, some rRNA and other small RNAs). This polymerase often increases during cancer progression.
What is the C-terminal domain and what does it do?
- The C-terminal domain is part of the RNA polymerase II.
- It is phosphorylated to allow RNA polymerase to start making RNA.
- It serves as an assembly line for capping, splicing and polyadenylation of pre-mRNA.
What are examples of Transcription factors needed for transcription? (3)
- eukaryotic activator protein
- This attaches to the mediator
- Which attaches to basal transcription factors.
***Transcription factors can be regulatory, therefore binding sites can be far away from gene they are controlling.
Example of condition caused by lack of transcription in a gene? (3)
Transcription factors cannot bind properly due to >200 CGG repeats in promoter region. Therefore RNA polymerase cannot start transcription.
- It is mostly common in males as only one X-chromosome. If passed down paternally, no further expansion. If passed down maternally risk for an increase in mutation.
Symptoms: Large protruding ears, hyperextensible finger joints and double jointed thumbs, Large testes, autism etc.
Example of condition caused by lack of transcription in multiple genes? (3)
Alpha-thalassemia (deformities in facial bones, jaudice, slow growth, fatigue etc):
- caused by mutation in ATRX, a protein associated with centromeres that can unwind DNA. It is part of large complex that controls local structure of chromatin.
- Lack of ATRX shuts down transcription in multiple genes including alpha-globin.
What protein splices Introns from pre-mRNA?
- Removed by spliceosome. A complex set of small nucleolar RNAs (snRNAs) and proteins.
- Accuracy of spliceosome is conferred by snRNAs.
Example of condition from incorrect intron splicing (which causes exon skipping)?
- Beta thalaseemia (Anaemia from about 6 months)
Intron spliced incorrectly due to premature stop codon (terminator).
- Cystic Fibrosis - from deletion mutation in nucleotides, exon 9 often skipped.
- IGHD (isolated growth hormone deficiency) - Substitution mutation on growth hormone mutation. This shortens mRNA, leading to short stature.