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Genetics Part-III

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:
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

Reprise:

• 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.

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