W9 - Transcription and Translation Flashcards
Where does transcription occur?
what is it?
in the nucleus
It is the copying of the messages in the DNA to produce mRNA
* Use mRNA instructions to build a new protein
What a gene?
What are the special DNA sequences in a gene which regulate transcription?
Gene - a specific section(nucleotides) of DNA within a chromosome that codes for a specific protein
- Exons - sequences of DNA within a gene that code for a protein
- Introns - spacer regions in between exons, that do not code for a protein
- The promotor region is the beginning of a gene
Describe DNA
- Formed of nucleotides
- Nucleotides are formed from pentose sugars(pentose ribose sugars)
- Phosphate, sugar and a base = nucleotide
- Strands of DNA are bound together by H bonds between nucleotides
- A-T, G-C
- DNA is kept in cells (chromatin/chromosomes)
- Cells do this by wrapping DNA around histones = called a nucleosome
- Chromatin - is when this is done many times
- DNA packaged up in chromatin = forms a chromosome structure
- 23 pairs of chromosomes in each cell
What is the thymine base in DNA replaced with in RNA?
Uracil
What sugar makes up the backbone of RNA?
What sugar makes up the backbone of DNA?
- RNA sugar backbone is ribose (no O2 to reduce reactivity, decreases breakdown of molecule)
- DNA sugar backbone is deoxyribose
Define the coding strand:
Define the template strand:
What directionality do each of these have?
- Coding strand - The strand of DNA which is to be copied (5’ to 3’)
- Template strand - a copy, single strand RNA molecule (3’ to 5’)
What are the roles of:
RNA polymerase I
RNA polymerase II
RNA polymerase III
- PI - rRNA genes
- PII - all protein-coding genes, makes mRNA
- RNA PI and II make ribosomes in the nucleus
PIII - tRNA genes
Describe how RNA Polymerase II works:
- The nucleoside triphosphate (e.g. ATP/GTP/UTP/CTP) is used as the building blocks of mRNA
- During the addition of the nucleotide to the RNA molecule, the triphosphate is hydrolysed to a monophosphate, releasing Ppi (pyrophosphate = 2 inorganic phosphate)
- This releases the free energy required for synthesis of mRNA
How does RNA Polymerase II know where to bind to the DNA?
Promotor lies just upstream of the gene - RNA binds here to start transcription
- promotor regulates transcription by showing the RNA Polymerase II where to bind
- promoter region has specific nucleotide sequences are recognisable = consensus sequence (brings in all the transcription machinery)
- These sequences lead to binding of General Transcription Factors
- TATAA box is a region of DNA in the promotor region of the gene that is rich in A’s and T’s, lots of proteins bind here
What are general transcription factors?
- Proteins other than RNA polymerase involved in transcription of any gene
- General transcription factors (GTFs) required for transcription initiation:
- Not subunits of RNA polymerase
- Required for RNA polymerase to bind avidly and specifically to promoters (c.f. sigma factor in prokaryote)
- GTFs for RNA polymerase II are called TFIIx, where x = A, B, D
- Can have multiple subunits
Describe transcription initiation:
A. Subunit of TFIID (GTF) called TBP recognises the TATA box
B. TFIID binds to the TATA box
C. In turn TFIIB (GTF) binds adjacently
TBP = TATA binding protein
D. The remaining GTFs and RNA polymerase II bind
E. TFIIH(GTF) uses energy from ATP hydrolysis to pry apart the DNA double helix(opening H bonds) = exposes the template strand. TFIIH is called a helicator
E. TFIIH(kinase) then phosphorylates RNA polymerase II, releasing it from the GTFs so that transcription can begin
Then Caping proteins bind to RNA polymerase
What are transcription factors? (activators)
- Proteins which bind to another regulatory region on DNA called the enhancer region
- Causes DNA to bend backwards, when touches GTF’s in promotor region Transcription occurs very fast
- Can bind a long way from the promoter (see video)
- Enhance or repress gene expression (activators or repressors - binds to repressive regions in DNA, to repress genes that we do not want to switch on)
- Transcription factors are specific for individual genes and can therefore control gene expression in a cell (e.g. turn neuron genes off in a liver cell)
- Separate from GTFs
Describe transcription elongation:
- Once RNAPII is released from GTFs it moves along the template strand (3’ – 5’) unwinding DNA one nucleotide at a time
- Therefore new RNA molecule is made from 5’ to 3’ direction
- Nucleoside triphosphates used as building blocks for mRNA synthesis
- RNAPII has a catalytic site which generates phosphodiester bonds between nucleotides (forming backbone)
- New mRNA is synthesised 5’ – 3’.
What 3 steps occur during mRNA processing?
mRNA Processing:
mRNA is termed pre-mRNA until it has been successfully processed:
1. Capping - addition of a 5’ cap
2. Splicing
3. Polyadenylation
These events occur during transcription (co-transcriptional)
If not processed will get degraded by the cell
What is the role of the 5’ cap?
- Once phosphorylated, capping proteins bind to RNAPII
- When the 5’ end of the mRNA emerge these proteins catalyse the addition of the 7-methylguanosine to the primary transcript(cap allows mRNA to be recognised and not degraded, plays a role in nuclear export for recognition and binding to leave cell.
- The cap has multiple roles, but initially it prevents mRNA degradation in the nucleus
