Protein Synthesis in Detail

Transcription (in eukaryotes)

1. DNA --> RNA
• Binds RNA polymerase to promoter of a gene
2. Transcription elongation
• Adds RNA nucleotides
• DNA in front of RNA unwinds and RNA nucleotides are added to the 3' end of the RNA transcript
• Has a short DNA-RNA hybrid, 8 base pairs, where RNA is temporarily hydrogen-bonded to the DNA template strand
• mRNA can involve multiple RNA polymerases, so numerous mRNA's are produced from a single gene
• also involves proofreading mechanism that can replace RNA nucleotide
3. Termination of transcription (eukaryotes) 
• Adds string of A's to mRNA 3' end
• Proteins cut RNA transcript from polymerase
• Produces pre-mRNA step
4. Pre-mRNA processing
• Splicing
• exons: region of a gene that contains code for producing a protein
• introns: long regions of DNA that have no identified function
• splicing: introns are removed by spliceosome
• 5' cap addition
• modified guanine nucleotide added to the 5'-end of the mRNA
• crucial for recognition and proper attachment of mRNA to the ribosome
5. Polyadenylation
• addition of poly-A tail to 3' end of mRNA
• protects mRNA from degradation by exonucleases
6. Genetic code
• start and stop codons
• start: AUG
• stop: UAG, UGA, UAA
7. Reading frame
• starts reading in a frame of 3 nucleotides
• frameshift mutations: insertions or deletions of 3 nucleotide bases
• If reading frame is disrupted, mRNA may not be translated correctly
• results in premature stop codon = smaller protein with no function

Translation (RNA-protein)

1. Ribosomes
• has three binding sites (E, P, A) 
• initiation, elongation, and termination
2. Initiation (eukaryotes) 
• tRNA binds to AUG codon on mRNA
• translation can begin at all AUG codons
• only in-frame AUG will produce functional polypeptide
3. Elongation
• start tRNA sitting on AUG codon in P site next available codon at A site
• tRNA binds to codon and peptide bond joins between AUG and next amino acid
• entire ribosome complex moves along mRNA
• sends first tRNA to E site and tRNA into P site
4. Termination
• occurs when ribosome comes to one of stop codons