Module 04 - Section 04 Flashcards
Translation initiation in prokaryotes and eukaryotes
What is the 2D shape of tRNA and its 3D shape
2D: clover leaf with 4 arms
3D: L-shape
Describe the amino acid arm of tRNA
Contains a trinucleotide sequence CCA at the 3’ terminus - the 3’-terminal A residue if the nucleotide to which the amino acids attach – each tRNA carry a specific amino acid making aminoacylated
Describe the anticodon arm of tRNA
three-nucleotide sequence that base pairs with the mRNA (complementary)
What is tRNA called when bound to its amino acid?
aminoacyl-tRNA or said to be charged?
What provides specificity to tRNA?
the enzymes that attach the amino acids to particular tRNAs - called aminoacyl-tRNA synthetases
What is the denoting convention for
a) uncharged tRNA specific to leucine
b) charged tRNA specific to leucine
a) tRNA^Leu
b) Leucyl-tRNA^Leu or Leu-tRNA^Leu
Which enzyme is responsible to make sure the correct amino acid is added?
aminoacyl-tRNA synthetases
What is Wobble base pairing?
Noncanonical base pairing that can only happen in the third position of the mRNA codon (5’ nucleotide in anticodon)
What is the wobble position?
nucleotide at the 3rd position of the codon which can vary without affecting the encoded amino acid
What is Inosine?
Adenoside that has been converted to inosine through the actions of adenosine deaminase action on RNA
Which nucleotides can Inosine form base pairs with?
A, C or U in the 3rd position of the mRNA codon
What are the 4 relationships of the Wobble hypothesis?
(1) the first 2 bases of an mRNA codon always form watson-crick base pairs and confer most of the coding specificity
(2) 1st base of the anticodon determines the number of codons recognized by tRNA
- when A or C - only one codon recognized
- when U or G = 2 codons recognize
- when I = 3 different codons
(3) When an amino acid is specified by several codon, codons that differ by either of the first two bases required different RNAs
(4) A minimum of 32 tRNAs are required to translate all 61 codons (31 for amino acids and 1 for initiation)
What is resposible of the functional activity of the ribosome?
rRNA
describe the subunits of ribosome
Small unit of 40S and Large unit of 60S which combine to form the functional 80S ribosome
Discuss the relation between amino acids, tRNAs and the ribosome
All 3 are intimately involved in the process of translation
Amino acids are the building blocks of proteins
tRNAs function as adaptor molecules to carry amino acids to the growing polypeptide
Ribosome are the protein factory within the cell
What catalyzes the peptide bond between adjacent amino acid?
Peptidyl tranferase center, located in the 60S subunit
What and where is the decoding center in ribosomes?
It is where aminoacylated tRNAs read the genetic code by base pairing with tripled codon in mRNA. It is located in the 40S subunit of the ribosome
What is the A site?
location where the Aminoacyl-tRNA binds (charged tRNA)
What is the P site?
Location of the Peptidyl-tRNA binding (tRNA that contains the growing polypeptide chain)
What is the E site?
the Exit site, occupied by the tRNA molecule released after the growing polypeptide hain is transferred to the aminoacyl-tRNA in the PO site
What is the sequence of “sites” a tRNA goes through in a ribosome?
A-P-E
What are the 5 steps of translation?
(1) initiator tRNA is charged with methionine
(2) Translation initiates with the assembly of mRNA and aminoacylated tRNA on the small ribosomal subunit, followed by joining with the large subunit = ribosome active
(3) Polypeptide elongation occurs in successive cycles of aminoacyl-tRNA binding and peptide bond formation
(4) Translation stops when the ribosome encounters a stop codon in the mRNA, leading to release of mRNA and dissociation of ribosome into 2 subunits
(5) Polypeptide fold during or immediately after transaltion
What are the 3 steps of initiation of translation?
(1) Alignment of mRNA on the small ribosomal subunit - IF-3 associates with the small unit to prevent premature assembly of ribosome
(2) Association of a charged initiator tRNA with the AUG start codon in the Psite
- this tRNA is shephered to the ribosome by IF-2 to ensure the correct alignment of the tRNA
(3) Recruitment of the large ribosomal subunit to form a complete initiation complex
- IFs dissociate from the complex which consumes GTP
What is considered the energy currency of translation
GTP
Which stage of translation is the most regulated?
Initiation
What is the Shine-Dalgarno sequence and what is its use?
Consensus sequence of 4-9 purines residues situated 8-13 nucleotides on the 5’ side of the start codon
It is an Initiation Signal, which base pairs with a complementary pyrimidine rich sequence near the 3’ end of the 16S rRNA ribosomal small subunit
What are the 2 bacterial tRNA specific for methionine?
tRNA^fmet = initiation codon tRNA^met = within the mRNA transcript
Why can’t methionine be added to the transcript of mRNA (internally aka anywhere else than the 1st amino acid)?
A transformylase enzyme transfers a formyl group to the amino group of Methionine – essentially, this blocks the N-terminus and thus it can only be the first amino acid because it can no longer be added at the C’ end
What is Polycistronic mRNA?
A contiguous mRNA with more than 2 gens that can be translated into proteins
What is a monocistronic mRNA
Encoding a single proteins
Match eukaryote and prokaryote to polycistronic and monocistronic
eukaryote = monocistronic prokaryote = polycistronic
What are Kozak sequences?
A sequence around the start codon in eukaryotic mRNA taht guides translation. The Kozak sequence has a purine nucleotide three residues before, and a G residue immediately after, the start codon
What are the functions of the interactions between the eukaryotic IF with the 5’ cap or poly(a) binding protein which are associated with the 3’ poly(A) tail of the mRNA?
(1) ensuring that mRNA processing is complete prior to translation
(2) Promoting translational efficiency
(3) enabling the sophisticated translation regulation of gene expression
