Biolchem 415: Lec. 36
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What are characteristics of the genetic code?
- 1. Codon encode an amino acid
- 2. Non-overlapping code
- 3. Code has no internal punctuation
- 4. Directional (reads 5'-3')
- 5. Degenerate code (some AA encoded by >1 codon)
- 6. Genetic code isn't nearly universal
What does degeneracy do for the genetic code
Reduces the deleterious effects of mutation. Makes more changes silent mutations.
What types of mutation in which gene causes cancers?
Missense in Ras causes the normal codon to code for a different amino acid. Results in constitutively active Ras GTPase which promotes cancer cell proliferation.
What is the purpose of tRNA?
Adaptor molecule between codon and amino acid
How many tRNA/amino acid?
At least one tRNA/amino acid
What is tRNA composed of?
Single chain of RNA (73-93 ribonucleotides) with unusual modified bases (methylation, deamination, pseudo-uridylation) like iodine, methyl-Cytidine, and dihydrouridine
What are the 2D and 3D structures of tRNA?
- 2D: cloverleaf
- 3D: L-shaped
Where do amino acids attach on tRNA?
Attached to hydroxyl group of adenosine in CCA region of acceptor stem. Anticodon loop near center of sequence
How should you read anticodon-codon base pairing?
5' to 3'. Anticodon that pairs with AUG is written CAU
What is the wobble effect in base pairing?
Greater degree of pairing of the third base of the codon with the first base of the anticodon
What determines the extent of the wobble effect?
First anticodon base determines whether a tRNA can read 1, 2, or 3 codons. Ex: inosine can pair with 3 different third bases of codon
What is inosine? What can it form wobble base-pairs with?
Anticodon formed by deamination of adenosine. C, U, and A
How are amino acids activated for incorporation into a protein?
Ester linkage between carboxyl group of AA and either 2' or 3' OH of terminal adenosine of tRNA
Amino acid attached to tRNA: ammoniacyl tRNA/charged tRNA
What does ammoniacal tRNA synthetase do?
Catalyzes activation of amino acids
What are the two steps for attachment to tRNA?
- 1. Formation of amionacyl adenylate
- 2. Transfer of the aminoacyl group to a specific tRNA
T/F: Aminoacyl tRNA synthetases do not have highly discriminating amino acid activation sites.
F: ex: in active site of threonyl-tRNA synthetase, zinc at active site only interacts with OH of threonine
What happens if the wrong amino acid is added to the tRNA?
Threonyl-tRNA synthetase has an editing site to remove a serine inappropriately joined to Thr tRNA
What is the mechanism for tRNA editing?
CCA arm of tRNA swings into editing site where incorrect AA is removed.
What is the effect of the editing site?
Proofreading by aminoacyl tRNA synthetase increases the fidelity of protein synthesis
Why are synthetases true translators of the genetic code?
They assign a particular amino acid to a specific tRNA by recognizing anticodon loops and acceptor stems of tRNAs
What are some defects of amino-acylation of tRNA?
Charcot-Marine-Tooth Disease, Ataxia, cancers, auto-immune diseases, and inflammation
What is Charcot-Marine-Tooth Disease? What is is caused by?
Peripheral nerve disorder--degeneration of motor and sensory nerve function caused by mutations in genes for glycine or tyrosine tRNA synthetases
What is ataxia? What is it caused by?
Lack of voluntary muscle movements. Mutations editing activity of alanine tRNA synthetase leads to misincorporation of serine and glycine
What organelle catalyzes translation?
What is the actual catalyst for protein synthesis?
Ribosomal RNA. Ribosomal proteins make only minor contributions
How can co-transcriptional translation happen in bacteria?
Transcription and translation both happen in 5' to 3' direction, so bacterial protein synthesis begins before transcription is complete
How are polyribosomes/polysomes formed?
Several ribosomes can translate mRNA at the same time
Does transcription and translation happen at the same time in eukaryotes?
No. Transcription and most RNA processing in nucleus. mRNAs then exported to cytoplasm where translation into protein occurs
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