Topic 10: The Genetic Code and Translation

translation

the process which information in an mRNA is used to synthesizes polypeptides which will become a protein

protein

a macromolecule based on a single or multiple polypeptides

amino acid

monomers connected to form polypeptides

polypeptide

polymers of amino acids linked by peptide bonds

R group

the variable group in amino acids which gives each one unique properties

peptide bond

amino acids linked by the carboxyl of one amino acid bonding to the amino group of another via dehydration synthesis

amino group

NH2 (or NH3+) group on the end of a single amino acid

carboxyl group

COOH (or COO-) group on the end of a single amino acid

primary structure

sequence of amino acids specified by the gene

secondary structure

helices and folded sheets polypeptides formed spontaneously

tertiary structure

folding of the polypeptide upon itself by intramolecular interactions, including ionic bonds, disulfide bridges and more

quarternary structure

combination of multiple polypeptide subunits

3’ UTR

non-coding sequences at 3’end, from the end of the protein-coding sequence to the 3’end

5’ UTR

non-coding sequence at 5’ end, from the 5’ end to the beginning of the protein-coding sequence

genetic code

relationship between the base sequence of a nucleic acid coding sequence and the amino acid sequence of the polypeptide produced by translation

triplet code

each codon is three bases that specifies an amino acid

start codon

AUG, tells transcription to start

different mechanism in bacteria vs eukaryotes

stop codon

terminates translation

degenerate genetic code

more than one triplet sequence will specify for each amino acid and stop codon

exception: Met and START

reading frame

uninterrupted series of bases in mRNA used as codons

any DNA sequence has 3 possible, but each gene has only one determined by start codon

transfer RNA (tRNA)

one of the types of noncoding RNA made of one single RNA molecule

anticodon loop

contains 3 base anticodon which will pair with the codon of mRNA during translation in antiparallel orientation

ribosome

site of protein synthesis, catalyzes protein synthesis

small subunit

part of a ribosome that finds the start codon

different mechanisms in bacteria and eukaryotes 

large subunit

at the end of initiation this binds to the complex and translation can begin

ribosomal protein

a large number resides in ribosomes

ribosomal RNA

several reside in ribosomes

A site

where aminoacyl enters

P site

where peptidyl tRNA enters

E site

where free tRNA leaves (exits)

initiation of translation

gets translation started

1. ribosome small subunit binds an mRNA so that start codon is positioned in P site

2. initiator enters P site (fMet in bacteria, Met in eukaryotes)

3. large subunit binds to assemble the complete ribosome

Shine-Dalgarno sequence

pairs in the mRNA pair with a complimentary sequence in part of the ribosome

start codon in bacteria will be shortly downstream from this

initiator tRNA

formyl Met (fMet) in bacteria

unmodified Met in eukaryotes

elongation phase of translation

a repeated cycle that adds amino acids to a growing polypeptide 

1. aminoacyl enters A site

2. peptide bond is formed

3. shift: the ribosome moves exactly 3 bases down mRNA, taking the polypeptide with it

termination of translation

elongation proceeds until stop codon is reached

there is no stop tRNA, release factors bind to stop codon and release the polypeptide

release factor

bind to stop codon and release the polypeptide

Kozak sequence

a consensus sequence surrounding start codons in eukaryotes, affects efficiency of initiation

initation factor

found at 5’ cap

poly A binding protein

binds to 3’ poly(A) tail, then interacts with initiation factors at 5’ cap and stabilizes ribosome binding there