Lecture #7 Flashcards
What is Translation?
Genetic information is converted into protein information.
- Generation of peptide strand from a nucleotide sequence.
- Template: mRNA
Properties of Translation
Not a 1:1 association:
- 4 nucleotides vs. 20 amino acids
Rule: a codon of nucleotides is needed for each amino acid.
- A group of three consecutive nucleotides in an mRNA strand.
- 64 possible combinations
Some amino acids use more than one codon.
Redundancy
Properties of Translation Continued
A given mRNA sequence can be translated using one of three different “reading frames”.
- Why does it matter which reading frame is used?
Rule: reading frame begins at the AUG site.
- Start codon: AUG
Translation requires the presence of “adaptor” proteins.
- tRNAs
tRNAs act as adaptors during protein synthesis
tRNAs
- transfer RNAs
- Made during transcription
- Small RNA molecules
Form double helix regions
- Base pairing
- provides additional folding
- anti-codon loop
tRNAs act as adaptors during protein synthesis continued
tRNAs
- One tRNA can base pair with more than one codon.
- Only need ~ 31 tRNAs to “read” the possible codons.
- “Wobble Position”
Position 3( 5’ position)
- “Wobble Position”
Aminoacyl-tRNA synthetases attach amino acids to tRNAs
aminoacyl-tRNA synthetase
- Attaches amino acid to the 3` phosphate group on the tRNA.
- Different synthetase is used for each amino acid.
- Recognition based on nucleotides in the anticodon and in the amino-acid-accepting arm.
Ribosomes and Translation Part I
Ribosomes facilitate the rapid and accurate addition of amino acids to the growing polypeptide chain.
Ribosomes and Translation Part II
Ribosome
- The site of protein translation
- Cytoplasm of the cell or the ER
- Millions present within the cell
4.2 MDa
2/3 RNA + 1/3 Protein
- 50 proteins
- 4 rRNA molecules
Large Subunit
Small Subunit
Ribosomes and Translation Part III
Ribosome
Core - rRNA
- Determines ribosome structure.
Proteins located on the surface
- Fills in the gaps
- Stabilize the rRNA core
23S rRNA catalyzes the addition of an amino acid to the growing strand – peptidyl transferase
- Ribozyme
Ribozyme: RNA molecules capable of catalytic activity.
The ribosome and polypeptide chain growth
The ribosome contains…
One binding site for the mRNA
- small subunit
Three binding sites for three tRNAs
A-site – aminoacyl tRNA
P-site – peptidyl tRNA
E-site – Exit site
The Four Steps of Translation Step One
Step One aminoacyl tRNA binds in the A-site tRNA is “charged” - tRNA is "charged" - Anticodon sequence is complimentary to mRNA codon sequence.
“A” and “P” sites are close together
- prevents base pair skipping
tRNA in E-site is ejected
The Four Steps of Translation Step Two
Step Two
- Amino acid chain is uncoupled from the P-site tRNA
- Peptide bond formed between the peptide chain and the amino acid attached to the tRNA in the A-site
- Catalyzed by the rRNA within the large subunit
The Four Steps of Translation Step Three
Step Three
Large subunit translocates 5→3
tRNA in the P-site moves to E-site of the large subunit
tRNA in the A-site moves to the P-site of the large subunit
The Four Steps of Translation Step Four
Step Four
Small subunit translocates
- Mediated by base pairing in E-site and P-site.
Empty A-site is available for next charged t-RNA
**Remember** Translation - 5'- 3' Peptide strand generation - N-terminis--> C-terminis
How do we get started? Part I
Translation initiated by an initiator tRNA
Carries amino acid Methionine
- Different from others
Charged initiator tRNA + translation initiator factors bind to the small ribosomal subunit
- P site
Binds to the 5` end of the mRNA
- How long does it know which end?
