lecture 21 - translation Flashcards
cellular messengers
receptor proteins stud the outside of your cells and transmit signals to partner proteins on the inside of the cells
muscles
muscle protein called actin and myosin enable all muscular movememt. from blinking to breathing to rollerblading
hair and nails
protein called alpha-keratin froms your hair, fingernals and also is the major component of feathers, wool, claws, scales, horns, hooves
brain and nerves
ion channel proteins control brain signaling by allowing small molecules into and out of nerve cells
enzymes
in you saliva, stomach, and small intestine are proteins that help you digest food
cellular construction workers
huge clusters of proteins form molecular machines that do your cells heavy work such as copying genes during cell division and making new proteins
The genetic code
- the DNA sequence encoding a protein is read in triplets, or codons
- 61/64 codons are sense codons
- three stop codons, signal termination of translation
- the code is degenerate (redudant): a single AA can be encoded by more than one codon (20 common aa)
- AUG codes for methionine, usually start of translation
ORF (open reading frame)
a run of sense codons before a stop codon is encountered
translation requires 4 primary components
- ribosomes - site of translation
- mRNA template
- tRNAs - adaptors
- aminoacyl-tRNA synthetases
Synthesis of a single polypeptide chain requires a coordinated action of atleast 100 proteins and RNAs
transfer RNA (tRNA) as adaptor
Adaptor molecules recognize codons in mRNA and carry specific amino acids
tRNA 2D structure
the structure of the tRNA molecyke reveals how it is capable of functioning as an adaptor
- a small molecule, single stranded, 73-93 nt
- each tRNA represents a single aa which it can covalently bind (3’ terminal A residue)
- coverleaf - secondary structure, held together by H-bonds
amino acid attachment site on tRNA
3’-OH end
T loop
ribothymidine and pseudoridine
D loop
dihydrouridine
each tRNA contains anticodon
complementary to the codon, representing its aa
3D structure of tRNA
3D (twisted) L shaped structure
- formed by 2 extended helices perpendicular to each other
- D and (TpsiC) T loops come “together”
- the shape must be the same for each tRNA so it can “fit into ribosome”
- anticodon and the acceotir arm are at opposite ends of the L structure
each tRNA has an anticodon sequence
that will base pair with a codon in mRNA (antiparallel)
2 first nucleotides of each codon
are the primary determinants of specificity, and the difference between cofons usually lies at the third position
wobble enables one tRNA to recognize two or more codons
a min. of 32 tRNAs are required to recognize a;; the amino acid codons
31 tRNAs for the amino acids and one for initiation
watson crick base pairing
61 different tRNAs would be required
wobble base pairing
the 3rd base of codon and 1st of anticodon can form “unusual” base pairing - the rules are more relaxed because of the “geometry”.
The wobble hypothesus was formalized by F. Crick
the 5’ nucleotide in the anticodon is in the wobble position
the wobble bases allow a single tRNA anticodon to bind more than one mRNA codon
in the wobble position ..
- G can pair with U
- I (inosine) can pair with U,C,A
charging of rRNAs
aminoacyl-tRNA synthetases charge tRNAs (attach correct aa)
tRNA(ala)
is an uncharged tRNA specific for alanine
Ala-tRNA(ala)
is a tRNA specific for alanine that is charged with alanine
aminoacyl-tRNA synthetases:
amino acids are activated and linked to specific tRNAs
- two step process that uses ATP
- takes place in the cytoosol (the enzymes active site)
steps:
1. Adenylylation step = the AA is linked to adenylate, forming aminoacyl-AMP
2. tRNA charging step
amino acyl mechanism
aminoacyl-tRNA synthetases attach the correct amino acids to their tRNAs
- 20 different aminoacyl-tRNA synthetases: one per amino acid
- each enzyme is specific for one amino acid but can recognize ALL tRNAs for that amino acid (isoaccepting or cognate)
- recognition of tRNAs
1. anticodon arm
2. acceptor arm
