From Gene to Protein

1
From Gene to Protein

• ie
• Transcription Translation

2
Protein Synthesis overview

• One gene-one enzyme hypothesis
• (Beadle and Tatum)
• One gene-one polypeptide (protein) hypothesis
• The linear sequence of DNA determines the
  sequence of Amino Acids
• Transcription synthesis of mRNA under the
  direction of DNA
• Translation actual synthesis of a polypeptide
  under the direction of mRNA
• Difference in types of cells
• Prokaryotes- DNA not segregated from ribosomes.
  Transcription Translation occurs in rapid
  succession
• Eukaryotes- mRNA is modified and translocated to
  the cytoplasm for translation

3
The Triplet Code

• The genetic instructions for a polypeptide chain
  are written in the DNA as a series of
  3-nucleotide words called.
• Codons the basic unit of genetic code
• Dont forget!
• U (uracil) replaces T in RNA
• Xn number of amino acids that can be coded
  for.
• where X of bases nbases per codon
• For life on Earth 43 64

4
Transcription

• Only one strand of DNA is transcribed The
  Template Strand
• RNA polymerase pries DNA apart and hooks RNA
  nucleotides together from the DNA code
• Promoter region on DNA where RNA polymerase
  attaches and where initiation of RNA begins
• Terminator region sequence that signals the end
  of transcription
• Transcription unit stretch of DNA transcribed
  into an RNA molecule

5
Transcription continued

• Initiation transcription factors mediate the
  binding of RNA polymerase II to an initiation
  sequence (TATA box)
• Elongation RNA polymerase continues unwinding
  DNA and adding nucleotides to the 3 end.. Uses
  ATP this time!
• Termination RNA polymerase reaches terminator
  sequence signals the RNA polymerase to stop
  transcription and release the mRNA

6
Transcription overview

• See Campbell animations!!!

D\bc_campbell_biology_7\0,7052,3117225-,00.html
7
mRNA modification

• 1) 5 cap modified guanine protection
  recognition site for ribosomes
• 2) 3 tail poly(A) tail (adenine) protection
  recognition transport
• 3) RNA splicing exons (expressed sequences)
  kept,introns (intervening sequences) spliced out
  spliceosome

D\bc_campbell_biology_7\0,7052,3117225-,00.html
8
Translation

• mRNA from nucleus is read along its codons by
  tRNAs anticodons at the ribosome
• Each codon on the mRNA has a complimentary
• tRNA anticodon (nucleotide triplet)
• Each tRNA has been previously linked to an amino
  acid using the enzyme
• Aminoacyl-tRNA-synthetase
• Wobble the ability of one tRNA to recognize 2-3
  different mRNA codons. Example
• CC _ anticodon recognizes
• GGC, GGA, GGG, and GGU but all code for Glycine.
• Redundancy not Ambiguity!

9
Translation continued

• Ribosome made of rRNA) site
  of mRNA codon tRNA anticodon coupling
• P site holds the tRNA carrying
  the growing polypeptide chain
• A site holds the tRNA carrying
  the next amino acid to be added to the chain
• E site discharged tRNAs

10
Translation continued (switch to transparency?)

• Initiation union of mRNA, tRNA,
  small ribosomal subunit followed by large
  subunit
• Elongation codon recognition
  peptide bond formation translocation
• Termination stop codon reaches A
  site
• Polyribosomes translation of mRNA by many
  ribosomes (many copies of a polypeptide very
  quickly)

11
Translation

• See Campbell animations!!

D\bc_campbell_biology_7\0,7052,3117225-,00.html
12
Mutations genetic material changes in a cell

• Point mutations.
• Changes in 1 or a few base pairs in a single gene
• Base-pair substitutions
• Silent mutations no effect
  on protein
• Missense mutations ?
  to a different amino acid (different protein)
  
• Nonsense mutations
  ? to a stop codon and a nonfunctional protein
• Base-pair insertions or deletionsadditions or
  losses of nucleotide pairs in a gene alters the
  reading frame of tripletsframeshift mutation
• Mutagens physical and chemical agents that
  change DNA. Examples include
• Radiationor
• Base Analogs