TMC 2 Flashcards
Explain how RNA synthesis works
RNA synthesis is carried out by RNA Pol II
RNA polymerase uses one strand of DNA as a template to synthesise a complementary strand of RNA
It can synthesise RNA de novo without the need of a primer
RNA polymerase can only attach an incoming NTP to the 3’ OH group on a ribose (5’ to 3’ direction)
RNA Pol catalyses the formation of a 5’-3’ phosphodiester bond between the phosphate on the 5’ carbon of incoming NTP and 3’ OH on another nucleotide
High energy NTP is consumed, and pyrophosphate is released
Template indicates the nucleotides incorporated into the RNA
Explain how a gene is transcribed into mRNA.
RNA polymerase initiates the transcription by recruiting NTPs
DNA strands within the nucleus are melted open to a width of 14 bp and the transcription starts at a promoter site. NTPs are joined with the release of pyrophosphate
In eukaryotes, there are introns transcribed which do not encode for proteins
Introns removed by splicing which rejoins exons in varying ways to create different versions of the protein - very common in higher eukaryotes
mRNA strand is cleaved at the polyA adenylation site usually about 10 - 30 bps downstream AAUAAA
polyA polymerase adds hundreds of adenines
7 methyl guanine is added to the 5’ end by means of a 5’ to 5’ triphosphate bond making the mRNA capped
Cap serves as a ribosome binding site in eukaryotes and RBS serves as one in prokaryotes
In eukaryotes, mRNA is transported out of the nucleus into the cytosol where translation occurs in prokaryotes the process of transcription and translation are as one, ribosomes start translation at the first AUG downstream
What is a gene promoter?
The start point of a gene to which RNA polymerase binds
What is a gene terminator?
The end point of a gene where RNA polymerase stops
What is an exon and what is an intron?
Exon - sections of DNA that encode a protein, spliced together during RNA splicing
Intron - Sections of DNA that do not encode anything in proteins, removed during RNA splicing, very common in human and other mammalian genes, intervening sequences
Describe RNA splicing with the help of a diagram.
DNA is transcribed into pre mRNA with both introns and exons
Introns are removed by RNA splicing and exons are spliced together generating the mature mRNA
Describe alternative splicing with the aid of a diagram.
When different combinations of exons are spliced together.
This generates different proteins from the same genes. Therefore, one gene can encode several versions of a protein
What is the function of alternative splicing?
To generate different proteins from the same genes
One gene can give rise to a number of mRNAs with different exon combinations thus, one gene can encode several versions of a protein
Where do ribosomes bind to mRNA in prokaryotes and where do they start translation?
Ribosome binding site
translation begins at the first AUG downstream from the RBS
Where do ribosomes bind to mRNA in eukaryotes and where do they start translation?
The CAP - 7-methylguanine added via a 5’ to 5’ triphosphate linkage
Translation begins at the first AUG downstream from the CAP
What is the direction of transcription and translation?
5’ to 3’
5’ to 3’
What is the +1 site?
The exact position on DNA where RNA Polymerase begins to synthesise RNA
What is the mRNA CAP?
7-methylguanine that is added to the 5’ end of the mRNA via a 5’ to 5’ triphosphate linkage in eukaryotic cells
Acts to protect mRNA against degradation by 5’ exonucleases
Helps in transport of mRNA to cytosol for translation
Acts as a ribosome binding site therefore initiating translation
Plays a role in splicing and polyadenylation
What is the polyA tail?
PolyA adenylation cleavage site - AAUAAA polyA polymerase
100-200 adenines added to end of celaved mRNA that play a role in protection. termination of translation and transport into the cytosol
What is the terminator site?
Where transcription ends
Prokaryotes’ RNA form a specific shape which causes the polymerase to dismantle
Causes the RNA to be released from the RNA-DNA hybrid bubble at the termination point and the RNA Pol dissociates
