What are exons and introns?
• exons are coding regions, and
• introns are non-coding regions of the mRNA transcript
• exons and introns are found in most, but not all, eukaryote genes
• introns have to be spliced out before the mRNA is translated
• splicing is by snRNA's acting as enzymes, or ribozymes, an example of the catalytic function of RNA
DNA replication, transcription and translation:
• Synthesis of a linear polymer of amino acids from a linear polymer of nucleotides
Where does it occur?
On the ribosome, a rRNA-protein complex that provides:
• a scaffold for mRNA
• sites for the docking of tRNA charged with a specific amino acid
• an enzyme for peptide bond synthesis between amino acids
• an enzyme for translocation of the mRNA through the ribosome
What is the function of tRNA?
• Carrier of a specific amino acid during translation
What is the structure of tRNA?
• secondary structure has some base-pairing --> cloverleaf
• information transfer at the anti-codon loop, complementary to the codon
• note the importance of H-bonds in the genetic code
• tertiary structure is L-shaped which places the amino acid far from the codon-anticodon site
• degeneracy of the code produces wobble
What is the genetic code?
A sequence of 3 nucleotides forms a codon
• unambiguous, each codon specifies an amino acid, or start, or stop
• degenerate, some amino acids have multiple codons
• 2-letters often sufficient, specifiy hydrophobic and hydrophillic amino acids
What is the enzyme that charges tRNA with an amino acid?
An aminoacyl-tRNA synthetase
• it has proof reading capabilities through the precise fit of amino acid and tRNA
• energy provided by ATP: energy for the formation of aminoacyl-tRNA and for proof reading
• there are at least 20 synthetases, isoaccepting for the tRNA's coding for a single amino acid
What is the mechanism of translation?
• mRNA forms a large complex with the ribosome and protein factors
• together they guide in the correct aminoacyl-tRNA
• correct amino acid specified by codon-anticodon base pairing (H-bonds)
• protein factors have proof reading capability--energy provided by GTP
• an enzyme catalyzes polymerization of two amino acids, peptide (amide)bond formation between two amino acids
• an enzyme catalyzes movement of mRNA through the polymerization site: energy provided by GTP
• mRNA translated from 5'--> 3', same direction as it is synthesized
• Flow of information: central dogma
• DNA--> RNA-->linear amino acid sequence --> 3D-conformation of protein
But some viruses have only RNA as their genome: no DNA.
How do they carry out information transfer? How do they get around the unidirectional flow of information in the central dogma?
• Use an enzyme called reverse transcriptase to transcribe RNA into DNA.
• Example: HIV, a retrovirus
• Then, use central dogma.
• For HIV:
RNA-->DNA--> mRNA --> linear amino acid sequence --> 3D-conformation of protein.