Lecture 16 Flashcards
Explain the “One Gene, One Protein” hypothesis and how it was initially demonstrated
Genes function by encoding enzymes and each gene encodes a separate enzyme; more specifically this can be called the one gene, one polypeptide hypothesis
Initially demonstrated in an experiment shown on slides 7-10
How was the specificity of codons determined?
- Homopolymer added to a test tube containing cell-free translation system, 1 radioactively labeled amino acid, and 19 unlabeled amino acids
- Tube was incubated at 37 degrees C
- Translation takes place
- Protein is filtered, and the filter was checked for radioactivity
- Procedure was repeated in 20 tubes, with each tube containing a different labeled amino acid
- Tube in which the protein was radioactively labeled contained newly synthesized protein with the amino acid specified by the homopolymer; in this case, UUU specified phenylalanine
Another experiment shown on slide 20
What is the initiation codon?
AUG/ATG
What is an auxotroph?
Organism with a mutation that prevents synthesis of a nutritional component (i.e. amino acid) necessary for growth
What is minimal media?
Contains the minimum nutrients possible for colony growth, generally without the presence of amino acids
How many stop codons do we have?
3
What is a degenerate code?
Amino acid may be specified by more than one codon
What are synonymous codons?
Codons that specify the same amino acid
What are isoaccepting tRNAs?
Different tRNAs that accept the same amino acid but have different anticodons
What is the wobble hypothesis?
mRNA and tRNA pair in antiparallel fashion; pairing at the first and second codon positions follows Watson and Crick pairing, but at the third position, a G on the anticodon can pair with either U or C on the codon, for example
What can a C in the first position of an anticodon pair with on the mRNA codon?
G
What can a G in the first position of an anticodon pair with on the mRNA codon?
U and C
What can a A in the first position of an anticodon pair with on the mRNA codon?
U
What can a U in the first position of an anticodon pair with on the mRNA codon?
A and G
What can a I (inosine) in the first position of an anticodon pair with on the mRNA codon?
Inosine is a modified base found in tRNA
A, U, and G
What primarily determines the secondary and tertiary structures of a protein?
Primary structure
Through wobble, a single _____ can pair with more than one _____
anticodon; codon
What is the reading frame?
Three ways in which the sequence can be read in groups of three; each different way of reading encodes a different amino acid sequence
Not being in the correct/functional reading frame almost always results in nonsense; there is usually only one functional reading frame that will result in a functional protein
What is nonoverlapping?
A single nucleotide may not be included in more than one codon
How is translation initiated?
Initiation factor-3 (IF-3), initiator tRNA with N-formylmethionine attache to form fmet-tRNA; the energy molecule in this step is GTP
What is the Kozak sequence?
Sequence that facilitates the identification of the start codon; commonly the sequence from 5’ to 3’ is ACCAUGG
What are the 3 steps in elongation of translation?
- Charged tRNA binds to A site of ribosome
- Formation of peptide bond between P and A site
- Ribosome translocates to next codon
What are release factors?
Proteins that are triggered by the presence of the stop codon to bind to the ribosome and facilitate release of the polypeptide when a stop codon is encountered
What is a polyribosome?
An mRNA with several ribosomes attached
What does the universality of the code refer to?
The code is near universal with some exceptions
What are the steps in translation?
- tRNA charging (binding of amino acids to tRNAs)
- Initiation of translation
- Elongation of the peptide chain
- Termination
