Protein Translation

• Genetic information is stored within chromosomes must be read and converted into proteins inside the cytosol for utilization.
• Messenger RNA, or mRNA, molecules are exported from the nucleus into the cytosol and translated into proteins using ribosomes.
• Transcription and translation are regulated with energy-dependent steps to ensure high fidelity.

Protein Translation

• Mature mRNA is exported to the cytosol and translated into protein by the ribosome.
• Transcription is the process of of DNA being converted to RNA.
• Transcription involves a one-to-one correspondence of subunits.
• Transcription is essentially the same language with minor changes such as, U for T, ribose for deoxyribose.
• Translation is the process of RNA being converted to protein.
• Translation has no one-to-one correspondence because there are 20 amino acids but only 4 bases and requires a different chemical language.
• The genetic code uses combinations of at least 3 nucleotides to accommodate the 20 different amino acids.
• A double-nucleotide code provides 16 different possible combinations, calculated as 4 x 4.
• A triple-nucleotide code provides 64 different possible combinations, calculated as 4 x 4 x 4.
• Each set of 3 nucleotides is called a codon.
• Most amino acids have multiple codons because there are more codons than amino acids.
• Transfer RNA or tRNA, matches amino acids with codons as amino acids cannot interact directly with mRNA so therefore an intermediary is required.
• tRNAs contain a distinctive 3-D structure consisting of short RNAs.
• Amino acid is covalently attached at the 3' end of the tRNA.
• Each tRNA contains a looped anticodon, that is complementary to the appropriate amino acid's codon.
• An amino acid is coupled to tRNA via aminoacyl-tRNA synthetase, which is distinct for each amino acid, and proofreads for accuracy once complete.
• The amino acid is first "activated" by conjugation to AMP, costing energy as high-energy phosphates are used up in the process then is transferred later to tRNA from AMP.
• The resulting conjugate between the amino acid and tRNA has a high-energy bond.
• Protein synthesis occurs in the N-terminal to C-terminal direction.
• New amino acids are added to the C-terminal end of the growing chain.
• The peptide chain is attached to the tRNA that was most recently added.
• A new aminoacyl tRNA replaces the old tRNA, extending the chain by one residue.
• The RNA message is decoded by the ribosome with rRNAs that create a structural and catalytic core called a ribozyme.
• The ribosome has two subunits: large (L) and small (S).
• Ribosome reads mRNA from 5' -> 3', reading 3 bases at a time, or one codon at a time, and staying consistent to avoid translation errors.
• Each mRNA possesses 3 potential "reading frames".
• Translation starts with the codon AUG which uses a special "initiator" tRNA, that is different from the Met tRNA utilized for the rest of translation.
• Elongation factors such as EF-Tu/Ef-G in prokaryotes and EF1/EF2 in eukaryotes facilitate translational elongation.
• Elongation factors use GTPase activity to allow proofreading and speed up ribosome translocation.
• Translation concludes when the ribosome encounters one of three "stop" codons: UAA, UAG,or UGA.
• Instead of a tRNA, a release factor replaces it and binds to the ribosome, triggering hydrolysis of peptidyl tRNA and therefore releasing the now completed protein.
• The ribosome dissociates into separate small and large subunits and releasing the mRNA, release factor, and remaining tRNA.
• Each ribosome can only synthesize one peptide chain at a time.
• Cells don't have to wait for one ribosome to finish before initiating the next one starting.
• Once the first ribosome has moved far enough along, another can bind to the cap of the mRNA.
• A single mRNA may have several ribosomes translating simultaneoulsy.
• A single mRNA with several ribosomes translating simultaneously is called a polyribosome or polysome.

Antibiotics and Protein Synthesis

• Many of the important antibiotics are protein synthesis inhibitors.
• Most antibiotics only affect prokaryotes caused by differences between prokaryotic and eukaryotic ribosomes, and can sometimes affect ribosomes in mitochondria and chloroplasts through this mechanism too.
• Some exceptions can block eukaryotic ribosomes and are useful in cell biology research.
• Cycloheximide blocks the translocation reaction in eukaryotes only.
• Puromycin mimics aminoacyl-tRNA and integrates into the growing peptide chain resulting in a premature termination in both eukaryotes and prokaryotes.
• RNA and protein synthesis are energetically costly.
• Adding each subunit requires the conversion of an NTP into an NMP.
• The conversion of an NTP into an NMP is equivalent to hydrolyzing 2 ATPs to ADP.
• Elongation steps and mRNA splicing or protein proofreading use up multiple ATP/GTP molecules.

Information content at each level

• Protein sequence provides information for a protein's 3-D structure, cellular location,and protein function.
• mRNA carries the same information as protein (in code), as well as information about initiation and termination.
• DNA holds the information in mRNA, information for initiating/terminating transcription and splicing, and anything useful in the introns themselves.