RNA Synthesis

Question 1

What is transcription and why is it important? (2)

Answer 1

1. Transcription is the process by which encoded genetic information in DNA is used as a template to form mRNA
2. Different genes can make multiple RNA based on protein. E.g Alpha Tropomyosin gene can make striated muscle, smooth muscle, fibroblast and brain mRNA.

Question 2

Describe Transcription into pre-mRNA. (4)

Answer 2

1. Basal transcription factors bind first to DNA at ‘promoter region’ (TATA box) then RNA polymerase attaches to same area and unzips strand.
2. RNA polymerase moves from 5’ to 3’ end, adding complementary nucleotides to coding strand.
3. 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.
4. This is capped at 5’ end and poly-adelynated tail added to other end.

Question 3

Describe how mRNA is made from pre-mRNA (3)

Answer 3

1. pre-mRNA has introns (non-coding region) and exons (coding region).
2. Introns are spliced out, leaving exons.
3. This forms the actual mRNA. Cap and poly A tail remain

Question 4

What are the different types of RNA polymerase? (3)

Answer 4

1. RNA polymerase I (most rRNA genes)
2. RNA polymerase II (used for most mRNAs). Mushroom Alpha amantin is a poison as it inhibits this polymerase causing death by liver failure.
3. RNA polymerase III (used for tRNA, some rRNA and other small RNAs). This polymerase often increases during cancer progression.

Question 5

What is the C-terminal domain and what does it do?

Answer 5

1. The C-terminal domain is part of the RNA polymerase II.
2. It is phosphorylated to allow RNA polymerase to start making RNA.
3. It serves as an assembly line for capping, splicing and polyadenylation of pre-mRNA.

Question 6

What are examples of Transcription factors needed for transcription? (3)

Answer 6

1. eukaryotic activator protein
2. This attaches to the mediator
3. Which attaches to basal transcription factors.

***Transcription factors can be regulatory, therefore binding sites can be far away from gene they are controlling.

Question 7

Example of condition caused by lack of transcription in a gene? (3)

Answer 7

Fragile-X syndrome:
Transcription factors cannot bind properly due to >200 CGG repeats in promoter region. Therefore RNA polymerase cannot start transcription.

1. 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.

Question 8

Example of condition caused by lack of transcription in multiple genes? (3)

Answer 8

Alpha-thalassemia (deformities in facial bones, jaudice, slow growth, fatigue etc):

1. 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.
2. Lack of ATRX shuts down transcription in multiple genes including alpha-globin.

Question 9

What protein splices Introns from pre-mRNA?

Answer 9

1. Removed by spliceosome. A complex set of small nucleolar RNAs (snRNAs) and proteins.
2. Accuracy of spliceosome is conferred by snRNAs.

Question 10

Example of condition from incorrect intron splicing (which causes exon skipping)?

Answer 10

1. Beta thalaseemia (Anaemia from about 6 months)
   Intron spliced incorrectly due to premature stop codon (terminator).
2. Cystic Fibrosis - from deletion mutation in nucleotides, exon 9 often skipped.
3. IGHD (isolated growth hormone deficiency) - Substitution mutation on growth hormone mutation. This shortens mRNA, leading to short stature.