Biological polymerization of amino acids into polypeptide chains
Requirements for translation
Amino acids
Messenger RNA (mRNA)
Ribosomes
Transfer RNA (tRNA)
tRNA
Adaptor molecule that adapts genetic information present as specific triplet codons in mRNA to corresponding amino acid
tRNA
Small in size and very stable
75-90 nucleotides
Transcribed as larger precursors and cleaved into mature tRNA molecules
Several regions of complementary nucleotide stretches in tRNA primary sequence yield 'stem loops' and determine structure
Modified bases in tRNA
Inosine, dihydrouridine, and pseudouridine are common modified bases
All tRNA have ACC at the 3' end and G at the 5' end
tRNA anticodon
Complementarily base-pairs with codon in mRNA
Corresponding amino acid is covalently linked to CCA sequence at 3' end of all tRNAs
Aminoacylation: tRNA charging
1. Amino acid is activated by reaction with ATP to create an aminoacyl adenylic acid (5' phosphate and carboxyl group of amino acid)
2. Aminoacyl tRNA synthetase enzyme catalyzes this process
Aminoacyl tRNA synthetase
Enzyme that catalyzes aminoacylation
~31 tRNAs and 20 different tRNA Aminoacyl tRNA synthetases
Highly specific; recognize only one amino acid
Mischarged tRNAs are "edited" by removing the incorrect amino acid
Ribosome
Have an essential role in expression of genetic information
Consist of ribosomal proteins and ribosomal RNAs (rRNAs)
Consists of large and small subunits
Prokaryote ribosomes are 70S, Eukaryote ribosomes are 80S
rRNAs provide for important catalytic functions associated with translation
Bacterial cell contains approximately 10,000, many more in eukaryotic cells
rRNA genes highly redundant
Translation of mRNA can be divided into three steps
1. Initiation
2. Elongation
3. Termination
Initiation of translation
1. Requires small and large ribosomal subunits, mRNA molecule, GTP, charged initiator tRNA, Mg2+, Initiation factors
2. Translation starts with AUG (fmet)
Shine-Dalgarno sequence
Ribosomal binding site in prokaryotes, precedes by 8 bases upstream of AUG start codon in mRNA of bacteria, base-pairs with region on 16S rRNA of 30S small subunit, facilitating initiation
A and P sites in ribosomes
Charged tRNAs enter the A site, peptidyl transferase catalyzes peptide bond formation between the amino acid on the tRNA at the A site and the growing peptide chain bound to the tRNA in the P site
The uncharged tRNA moves to the E (exit) site
The tRNA bound to the peptide chain moves to the P site
mRNA is read in the 5'-3' direction, the polypeptide chain is synthesized in the N-C direction
23S rRNA
Is the peptidyl transferase catalyst, catalyzes peptide bond formation between amino acid on tRNA at A site and growing peptide chain bound to tRNA in P site
Termination
Signaled by stop codons (UAG, UAA, UGA) in A site, GTP-dependent release factors stimulate hydrolysis of polypeptide from peptidyl tRNA—released from translation complex
Polyribosomes (polysomes) are mRNAs with several ribosomes translating at once
Translation in eukaryotes
Ribosomes are larger than in bacteria
Transcription and translation are spatially and temporally separated
Presence of 7-methylguanosine essential, 3' poly-A required
No 'Shine-Delgarno like sequence', small ribosomal subunit associates with m7G cap (cap-dependent translation)
Requires more factors for initiation, elongation, and termination
Does not require formylmetionine, uses a unique tRNA tRNAimet
Ribosomes can be associated with the endoplasmic reticulum
Kozak sequence
Eukaryotic mRNAs contain a purine (A or G) three bases upstream from the AUG initiator codon, followed by G, considered to increase efficiency of translation by interacting with initiator tRNA
Closed loop translation in eukaryotes: The mRNA forms a loop that is closed where the cap and tail are brought together
Errors in metabolism like alkaptonuria and phenylketonuria result from mutations that lead to metabolic blocks, hundreds of medical conditions caused by errors in metabolism due to mutant genes
Sir Archibald Garrod's work on inborn errors of metabolism set the foundation for the "one gene-one enzyme" hypothesis
Individuals with alkaptonuria secrete homogentisic acid (2,5-dihydrophenylacetic acid) in urine, which turns black when exposed to air and also accumulates in cartilaginous tissues
Inborn Errors of Metabolism
Errors in metabolism that result from mutations leading to metabolic blocks
Inborn Errors of Metabolism
Alkaptonuria
Phenylketonuria
Alkaptonuria
Individual cannot metabolize alkapton
Phenylketonuria
Individual is unable to convert phenylalanine to tyrosine
Hundreds of medical conditions caused by errors in metabolism due to mutant genes
Sir Archibald Garrod studied inborn errors of metabolism early in the 20th century
Garrod's work set the foundation for the "one gene-one enzyme" hypothesis
Homogentisic acid
Also called alkapton (or alcapton), turns black when urine is exposed to air, also accumulates in cartilaginous tissues
Garrod also studied pentosuria, albinism, cystinuria and other diseases
Alkaptonuria
Reflects the absence of homogentisic acid oxidase activity
Inherited as a single gene recessive trait
Garrod was working at the same time as Mendel's work was being rediscovered
Garrod didn't fully understand Mendelian genetics, but understood the significance of multiple alkaptonurics in families and many parents of alkaptonurics being first cousins
Phenylketonuria (PKU)
Phenylalanine hydroxylase is inactive, so phenylalanine is not converted to tyrosine, resulting in mental retardation
Beadle and Tatum demonstrated a link between gene and enzyme well in advance of the discovery of DNA structure, transcription or translation