Lecture 6: Transcription Flashcards
Compare the overview of transcription vs translation
Transcription is transfer of information from DNA into mRNA. vs Translation is nucleotide sequence of mRNA translated into amino acid sequence of a protein.
What is the difference between transcription in eukaryotes vs prokaryotes
The mRNA molecule doesn’t need to undergo any RNA processing in transcription before translating in prokaryotic cells
Describe 3 steps of Initiation- transcription
RNA polymerase binds to a nucleotide sequence called the promoter. This also determines which strand will be the template.
- DNA strands unwind
- RNA polymerase initiates RNA synthesis at start point within promoter of template strand
Compare initiation prokaryote vs eukaryote
P: RNA polymerase can bind directly to promoter itself
E: transcription factor proteins facilitate the binding of RNA polymerase to the promoter and initate transcription
Describe elongation- transcription
RNA polymerase catalyses RNa nucleotides being added complementary to DNA template to the 3’ end.
The New RNA peels away from the template strand, dissociating from the DNA strands which can come back together to wind back.
Describe Termination- transcription
Complete RNA transcript is released. RNA polymerase detaches from DNA
Compare Termination in prokaryotes vs eukaryotes
P: transcription happens because of termination sequence in transcribed RNA. This makes a signal causing polymerase to release DNA and transcript.
E: When the polyadenylation signal sequence is transcribed into RNA, proteins in the nucleus bind to it and these cut the RNA from the polymerase.
In DNA A-T, in RNA
A-U
Give an example of some genes may overlap
HIV-1 genomes consists of nine genes, some of which are overlapping.
What are the modifications done to premRNA before it leaves in Eukaryotes
Enzymes add 5’ cap (has guanine base with triP linkage to 5’UnTranslatedRegion. 3’ poly A tail to pre mRNA.
Splicing: Introns (non coding regions) are cut out, and exons are ligated together to form the mature mRNA - UTR coding segmentUTR .
What is the function of 5’ cap and 3’ poly A tail.
Promotes the export of mRNA from nucleus through nuclear pores and protects mRNA from degradation from hydrolytic enyzmes . 5’ cap facilitates ribosome attachment at the start of translation
What splices the premRNA
snRNP: small nuclear ribonucleoproteins (snRNA ribozyme and protein) + other proteins make the Spliceosome which splices pre mRNA
How does the spliceosome splice
snRNA in the spliceosome pairs with bases in the pre mRNA intron (splice sites)
The spliceosome cuts the premRNA, releasing the intron for degradation and splices the exons together. Then releasing the mRNA
How does each exon get expressed in the protein
Different exons code for specific domains of proteins which have distinct functions.
What catalyses the splicing
snRNA
Describe initiation (translation) eukaryote
- initiator tRNA binds to small ribosomal subunit
- 5’ cap mRNA binds to the small ribosomal subunit
- initiator tRNA H-binds to start codon AUG- methionine on mRNA .
- Large ribosomal subunit the binds and intiator tRNA is positioned into P site
Describe elongation (translation)
- tRNA pairs complementary to mRNA codon in A site
- An rRNA molecule of the large ribosomal unit (ribozyme) catalyses the formation of peptide bond between the new amino acid and the growing peptide
- The ribosome moves tRNAs into neighbouring sites which moves the mRNA along and expose the next codon into A site
Describe Termination (translation)
When a stop codon is pulled into A site, it binds to a release factor- a protein shaped like tRNA. This promotes release of polypeptide from tRNA in P site by hydrolysis. Polypeptide released through exit tunnel and the mRNA + ribosome complex dissociate
What is the signal mechanism for targetting proteins to the ER
Signal peptides on the newly generated polypeptides can be met by SRP which docks with the receptor protein on the ER and lets the protein be released into the ER where are they modified- later golgi.
What are polyribosomes - for efficiency
A single mRNA strand along which many ribosomes are travelling. Each of the ribosomes is synthesising growing polypeptide chains
How are specific amino acids joined to specific tRNA
Enzymes (1 for each amino acid) called aminoacyl tRNA synthetases catalyse the covalent bonding of correct amino acid to tRNA molecule that binds to active site, using ATP.
Where does the 5’ cap bind at the start of translation
Small subunit of ribosome under E site
Compare P site, A site and E site of tRNA binding sites on ribsome
P site: holds the tRNA carrying the growing polypeptide chain
A site: holds the tRNA carrying the next amino acid to be added
E site: where discharged tRNAs leave the ribosome.
Where is the energy for translation come from
GTP
Why can coupled translation happen in prokaryotes not in Eurkaryotes
In Eukaryotic cells transcription and translation separated by physical barrier of the nucleus + pre mRNA modification needs to happen
What is coupled transcription
translation begins while mRNA is still be transcripted
