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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
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What does degeneracy do for the genetic code
Reduces the deleterious effects of mutation. Makes more changes silent mutations.
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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.
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What is the purpose of tRNA?
Adaptor molecule between codon and amino acid
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How many tRNA/amino acid?
At least one tRNA/amino acid
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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
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What are the 2D and 3D structures of tRNA?
- 2D: cloverleaf
- 3D: L-shaped
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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
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How should you read anticodon-codon base pairing?
5' to 3'. Anticodon that pairs with AUG is written CAU
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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
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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
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What is inosine? What can it form wobble base-pairs with?
Anticodon formed by deamination of adenosine. C, U, and A
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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
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What does ammoniacal tRNA synthetase do?
Catalyzes activation of amino acids
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What are the two steps for attachment to tRNA?
- 1. Formation of amionacyl adenylate
- 2. Transfer of the aminoacyl group to a specific tRNA
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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
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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
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What is the mechanism for tRNA editing?
CCA arm of tRNA swings into editing site where incorrect AA is removed.
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What is the effect of the editing site?
Proofreading by aminoacyl tRNA synthetase increases the fidelity of protein synthesis
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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
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What are some defects of amino-acylation of tRNA?
Charcot-Marine-Tooth Disease, Ataxia, cancers, auto-immune diseases, and inflammation
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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
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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
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What organelle catalyzes translation?
Ribosomes
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What is the actual catalyst for protein synthesis?
Ribosomal RNA. Ribosomal proteins make only minor contributions
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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
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How are polyribosomes/polysomes formed?
Several ribosomes can translate mRNA at the same time
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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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