8.2 Transcription: From DNA to RNA Flashcards
What catalyses transcription?
RNA polymerase
What are promoters?
They are DNA sequences that signal RNA polymerase to start transcription
Which of the two DNA strands serves as the template strand for transcription?
The 3’-5’ direction strand
In which direct does RNA polymerase add nucleotides ?
5’-3’ direction
What provides energy for transcription?
Hydrolysis of bonds in NTPs
What are terminators?
They are RNA sequences that signal RNA polymerase to stop transciption
Briefly explain Initiation (in prokaryotes)
RNA polymerase binds to the promoter sequence located near the beginning of the gene. Sigma factor binds to RNA polymerase. Region of DNA is unwound to form open promoter complex. Phosphodiester bonds formed between first two nucleotides
Briefly explain elongation(contructing RNA copy of the gene)
sigma factor separates from RNA polymerase (core enzyme). Core RNA polymerase loses affinity to promoter, moves in 3’-5’ direction on template strand. Within transcription bubble, NTPs added to 3’ end of nascent mRNA.
Briefly explain termination (The end of transcription in prokaryotes)
There are two kinds of terminators in bacteria:
1. Extrinsic: require rho factor
2. Intrinsic: do not require rho factor
Terminators usually form hairpin loops (intramolecular H-bonding)
How does the initiation of eukaryotes differ from that of prokaryotes
Eukaryotic genes often have enhancers:
1. Can be thousands of base pairs away from the promoter
2. Require for efficient transcription
How do the products of transcription differ between prokaryotes and eukaryotes
Prokaryotes: The primary trnascript is mRNA
Eukaryotes: The primary transcript is processed to make an mRNA in the following ways:
1. 5’ methylated cap
2. 3’ poly-A-tail (poly-A polymerase)
3. Introns removed by RNA splicing
Define exon
A coding region of a gene that remains in the final mRNA and is translated into protein.
Define intron
A non-coding region of a gene that is removed during RNA splicing and does not appear in the final mRNA.
Splicing removes _______ and joins adjacent ______-
introns, exons
What three short sequences in the primary transcript determine where splicing occurs?
- Splice donor
- Branch site
- Splice acceptor
How many sequential cuts remove an intron
2
What catalyses RNA splicing
the spliceosome
Translation takes place on _________ that coordinate movements of tRNA carrying specific amino acids
ribosomes
What is tRNA
they are short single-stranded RNAs of 74-95 nt
Each tRNA has an _________ that is complementary to an mRNA codon
anticodon
What is a charged tRNA
A specific tRNA that is covalently coupled to a specific amino acid
How many levels of tRNA structure?
three
Explain the primary structure of tRNA
it is the nucleotide sequence
Explain the secondary structure of tRNA
(cloverleaf shape) is formed because of complementary sequences within the tRNA
Explain the tertiary structure of tRNA
(L shape) is formed by 3D folding
What enzymes catalyse attachment of tRNAs to specific amino acids
Aminoacyl-tRNA synthetases
How do Aminoacyl-tRNA synthetases perform their function
By recognizing a specific amino acif and the structural features of its corresponding tRNA
The tRNA anticodon base pairs with ________
the mRNA codon
What did Wobble discover?
Some tRNAs recognise moe than one codon coding for the same amino acid
How many subunits does a ribosome have
2
Prokaryotic vs eukaryotic ribosomal subunits
P(70S): large =50S
small = 30S
E (80S): large= 60S
small= 40S
true or false: different parts of a ribosome have different functions
true
Where does the small subunit of a ribosome bind?
it binds to the mRNA
Wqhat is the function of the large subunit of a ribosome
It has peptiyl transferase activity
What are the 3 distinct tRNA binding areas
- Aminoacyl (A) site
- Peptidyl (P) site
- Exit (E) site
In bacteria, initiator tRNA has___________
formylated methionine (fMet)
Briefly explain initiation of translation in prokaryotes
Ribosomes bind to a shine-Dalgarno box and an AUG.
Three sequential steps:
1. small ribosomal subunit binds to mRNA
2. fMet-tRNA is positioned in P site
3. Large subunit binds
Briefly explain initiation of translation in Eukaryotes
Small ribosomal subunit binds to 5’ methylated cap, then migrates to the first AUG. Initiator tRNA carries Met and not fMet
Explain Elongation of translation
Addition of amino acids to the C-terminus of the growing polypeptide chain. (Charged tRNAs ushered into A site by elongation factors)
What direction do ribosomes move along the mRNA
5’-3’ direction
Explain termination of translation
No normal tRNAs carry anticodons for the stop codons. Release factors bind to the stop codons. Release of ribosomal subunits, mmRNA, and polypeptide
What consists of several ribosomes translating the same mRNA, thus causing simultaneous synthesis of many copies of a polypeptide from a single mRNA
polyribosomes
What are the two types of posttranslational processing?
- Enzymatic cleavage
- Addition of chemical constituents
Explain enzymatic cleavage
It may remove an amino acid (eg N-terminal Met removal), split a polyprotein(into multiple smaller polypeptides), or activate a zymogen
Explain what addition of chemical constituents do
They may alter protein structure, activity, or cellular location
Name the 4 processes that add chemical constituents
- Phosphorylation
- Glycosylation
- Lipidation
- Ubiquitination
