From DNA to Protein I

Review

- DNA Composed of 4 nucleotides (ATCG)

- nucleotides arranged in a double helix, each strand running in opposite directions

- DNA replicated with DNA polymerase

• This is one of the fundamental concepts of molecular biology
• All visible phenotypes are the result of the actions of enzymes, which are protein catalysts
• Any alteration of this process will affect the formation of proteins, hence the phenotype

The players

• DNA
• RNA - similar to DNA, but less stable. Four nucleotides (AUCG)
• The gene - a series of bases on one strand of DNA which codes for protein
• RNA polymerase - an enzyme which reads DNA and makes a complementary messenger RNA strand (mRNA) during transcription
• mRNA - the RNA product of transcription
• ribosome - reads mRNA message and aids in synthesis of protein
• tRNA - used in synthesis of protein - carries amino acid

Basic Anatomy of a Gene

• Promoter - protein which binds to DNA at promoter region and initiates DNA unwinding and transcription
• Gene - area of DNA which codes mRNA

Transcription

• Promoter binds to promoter region and DNA unwinds
• RNA polymerase binds to DNA and begins to synthesize mRNA
• When RNA polymerase is finished, mRNA leaves and is either modified or immediately used to form protein

The Genetic Code

• universal - with very few exceptions, all organisms use the same code in the formation of protein
• mRNA bases read as triplets (codons). The codons are non-overlapping
• The genetic code is redundant (64 combinations, only 20 aa's)

• Example: CCC, CCU, CCA, CCG all code for Proline
• Example: AUG CAU UAC UAA
Codes for: Met---His---Tyr---Stop

Translation

• Ribosome has two sites - P site and A site (P = peptidyl; A = aminoacyl)

• Initiation - Ribosome + mRNA + tRNA come together at AUG start codon

• Elongation - tRNA with anticodon comes into A site
• Enzymes create peptide bond and break bond between pp chain and tRNA in P site


• Translocation - whole system ratchets down so that the tRNA in the A site is now in the P site

• The cycle repeats itself until as STOP codon is reached

• Termination - when a stop codon is encountered
• Release factors bind to codon
• Whole polypeptide chain falls off

Mutation

• DNA: CCA is changed to TCA
• Then RNA: GGU [Gly] is changed to AGU [Ser]

However, a change in DNA does not always lead to a change in the coded protein

• DNA: GAT is changed to GAA
• RNA: CUA [Leu] changed to CUU [Leu]

Note that even though the RNA was changed, both codons still coded for Leu, so there was no net change in the protein