RNA synthesis Flashcards
How many chromosomes do humans have?
23 pairs of chromosomes
22 pairs of autosomes
1 pair of sex chromosomes
Give the two definitions of a gene
- Unit of heredity; contains instructions for making an organism’s phenotype
- DNA segment containing instructions for making a particular product
What is the promoter region of a gene?
The promoter region is a short base sequence which determines whether a gene is going to be turned off or on which is really important in terms of gene expression
What are exons and intron?
Exons are expressed sequences in the DNA
Introns are sequences in the DNA that are spliced out
Where are the untranslated regions?
At the beginning of the first exon and the end of the last exon
What are the differences between transcription and translation?
Transcription is the synthesis of mRNA transcript from DNA
It occurs in the same ‘language’- nucleic acids
Translation is the protein production from mRNA transcript
It occurs in different ‘languages’- nucleic acid to protein
What is the function of RNA polymerase II?
RNA polymerase II catalyses the synthesis of the phosphodiester backbone of the RNA strand
What genes does each type of RNA polymerase transcribe?
RNA polymerase I transcribes mostly ribosomal RNA (rRNA)
RNA polymerase II ia protein coding and also transcribes microRNA (miRNA), non-coding RNA
RNA polymerase III Transfer RNA (tRNA), 5S rRNA and other small RNAs
What does RNA synthesis require to begin?
DNA template RNA polymerase II Ribonucleotides A buffer Transcription factors
What are transcription factors and what do they do?
Transcription factors are proteins required to initiate or regulate transcription in eukaryotes
They assemble on promoter to position RNA pol II
Pull apart DNA helix and expose template strand
How do the different transcription factors assemble (what does each one do)?
- TATA box is recognised by the TATA-binding protein (TBP) a subunit of TFIID
- TFIIA and TFIIB bind; TFIIA stabilises the complex
- The other general transcription factors bind, E and H, bind
- Then RNA Pol II assembles at the promoter , forming the transcription initiation complex
- TFIIH pulls apart the DNA helix and phosphorylates RNA Pol II
- Phosphorylated RNA Pol II is released from the complex and begins transcription
Are UTRs transcribed and why?
UTRs are transcribed but not translated
5’ UTR for regulation of translation
3’ UTR for mRNA stability and miRNA binding
What three processes does the primary transcript have to go through before leaving the nucleus?
Capping
Polyadenylation
Splicing
Recall the capping process
Capping is done by the capping enzyme complex
The 5’-5’ triphosphate bridge is formed and methylated at the 7 position
(This is a co-transcriptional modification, meaning the capping is happening while the mRNA is still being transcribed)
The capping enzyme complex is recruited by the phosphorylated form of the RNA Pol II
What is the purpose of capping?
The cap can act as a marker
It can stimulate splicing and is used for recognition of messenger RNA by the protein’s translation machinery in the cytoplasm
Recall the process of polyadenylation
The cleavage signal is recognised and cleaved by specific endonucleases After cleavage we get the addition of a poly A tail by an enzyme called poly(A) polymerase The poly(A) polymerase adds about 200 As to the end of the RNA chain (this process uses ATP)
Why do capping and polyadenylation occur?
- Stability- the nucleus contains many nuclease enzymes ready to degrade RNA and the poly(A) tail prevents it from being degraded
- Transport to cytoplasm- RNA must be fully formed before it leaves the nucleus and processes are in place to check that this has happened
- Integrity prior to translation- the cell doesn’t want to waste energy resources translating an mRNA that is incomplete or incorrect
Describe introns
Non-coding sequences within genes
Size: <100->700,000 nucleotides
Median: 1800 nucleotides (exon: 123 nucleotides)
Number/gene: 0 (e.g. histones) - 78 (dystrophin)
Median: 7-9
Recall the process of splicing
All of this is done by a large enzyme complex called a spliceosome The GU (sequence) is recognised by an RNA within the spliceosome and the other proteins are binding The pyrimidine rich region also gets binding of proteins The proteins undergo conformation change so that you get cleavage at the 5' end of the intron The ends join up with the A site to form a lariat structure Then you have cleavage of the 3' end of the intron and finally ligation of the two exons together The intron gets degraded in the nucleus
What is the reason for introns existing?
Alternative splicing
This means there are many more proteins than there are genes so you can make a bunch of proteins from a couple of genes
How does the mRNA leave the nucleus?
The CBC (cap-binding complex) recognises the cap
TREX (transcription-coupled export complex) joins up and then unbinds to make a large complex
The EJC recognises successful splicing events
