RNA Synthesis Flashcards
How many types of RNA are found in the cell?
3
What are the differences between RNA and DNA?
RNA has ribose not deoxyribose
RNA has uracil instead of thymine
RNA is single stranded
RNA is shorter than DNA
Where is mRNA made?
In the nucleus
How are amino acids attached to tRNA?
Aminoacyl tRNA synthases
How does RNA polymerase know where to start transcribing?
in prokaryotic and eukaryotic DNA there are regions called promoter regions which are specific sequences of nucleotides which bind with RNA polymerase
Where is the TATA box found?
25 nucleotides to the start of where the gene is found
What are the requirements for transcription?
RNA polymerase II rNTPs Promoter regions Transcription factors Enhancers and silencers
Describe initiation
→TFIID binds to the promoter region (TBP is a subunit of TFIID tata binding protein)
→TFIIB binds to the TBP and it links RNA polymerase to the TFIID
→WHEN RNA polymerase binds to the TFIIB it is already associated with TFIIF
→TFIIE and TFIIH associate with RNA polymerase
→TFIIH unwinds the DNA around the initiation site (cistron) and phosphorylated the RNA polymerase II
ATP is needed
Describe elongation
→RNA Polymerase moving along DNA synthesizing the RNA which is released at the other end.
→Transcription Bubble formed which is 12 - 14 bps.
Describe termination
→near the stop codon there is a sequence called AAUAAA which is the polyadenylation signal or the termination signal which is after the stop codon.
→A specific endonuclease recognizes the termination signal. The CSPF is the enzyme that does the cleavage after the termination signal.
→Polyadenylate polymerase adds 250 adenines to the end of the 3’ end of the pre-mRNA and this is called the poly A tail.
What is the use of the poly A tail?
IT protects pre-mRNA from degradation and promotes the nuclear transport of pre-mRNA and promotes translation in the ribosome.
The longer the polyA tail the more stable the RNA is.
Describe how splicing occurs
→Splicing is the removal of introns and the joining of exons.
→There is cleavage at the 5’ site by a splicosome.
→formation of a lariat-like intermediate (GU joining to A at the branch point, making an AGU)
→Then, ligation joining together of exons occurs.
What is alternative splicing?
→The ligation of exons can differ and so the sequence of the mRNA also changes.
→The proteins made may have similar functions as there are common exons, but different function as the sequence of exons differs.
what do the 3 RNA polymerases do?
→RNA polymerase I= transcribes rRNA genes
→RNA polymerase II= transcribes mRNA
→RNA polymerase III–transcribes tRNA and other small RNAs
How does 5’ capping occur and why is it important?
→After 25 nucleotides are synthesized the 5’ end becomes modified.
→the enzyme 7methyltransferase adds a methyl group at position 7 on the guanine so it becomes a 7 methyl guanine cap.
→The cap protects the 5’ end from the activity of 5’ exonucleases