Ch 10. Biochemisty of the Genome

key experiments between 1928 & 1952 that explored which biological molecules stored heritable information

• Griffith: discovery of transformation in bacteria

• Avery-MacLeod-McCarty: DNA can pass traits between organisms

• Hershey-Chase: DNA is genetic material

discovery of genetic material

molecular basis of inheritance was unknown

• known that cells contained nucleic acids, lipids, polysaccharides, & proteins

Griffith (1928)

discovery of transformation

• streptococcus pneumonia

  • strain R is non-virulent

  • strain S is virulent (it has a capsule)

• the presence or absence of the capsule gives colones a smooth or rough appearance

Griffith experiment results

a chemical factor from the virulent strain transforms the nonvirulent strain

• live R → survival

• live S → death

• heat killed S → survival

• heat killed S + live R → death

Avery-MacLeod-McCarthy experiment (1944)

evidence for DNA as genetic material

• streptococcus pneumonia

  • strain R: non-virulent

  • strain S: virulent (has capsule)

• treat heat-killed cells with enzymes to break down DNA, RNA or protein, then test transformation

Avery-MacLeod-McCarthy results

first clear evidence that genetic molecules is DNA

• DNA + RNA + protein → transformation

• DNA + RNA → transformation

• DNA + protein → transformation

• RNA + protein → no transformation

Hershey and Chase (1952)

use E. coli and phase T2 as model system

• phage have only protein & DNA

• only DNA contains P

• only protein contains S

prepared batches of phage with either:

• DNA labeled ³²P

• protein label ³⁵S

allowed phage to infect bacteria

• used blender to detach phage

• used centrifuge to separate phage and E. coli

Hershey and Chase experiment results

• ³⁵S radioactivity (protein) remains with detached phage

• ³²P radioactivity (DNA) transferred to bacteria

*DNA must be storage molecule

nucleic acids

there are 4 nitrogenous bases in DNA nucleotides

• C, T, A, G

DNA structure

• nucleotides polymerize to form strands of nucleic acid

• nucleic acids are directional

  • 5’ C at one end

  • 3’ C at other end

  • generally written 5’ → 3‘

• new nucleotide added at 3’ hydroxyl

• phosphodiester bond links phosphate to ribose 3’ carbon → phosphodiester backbone

• high density of negative charge at phosphates

• relatively hydrophobic nitrogen bases

• polar groups (hydrogen bond donors & acceptors)

DNA double helix

formed by double stranded DNA

• phosphodiester backbone along outside → strongly negatively charged

• base pairs in the center

• 2 distinct grooves between strands

  • major

  • minor (exposes sides of bases)

where can a protein interact with DNA & with the nucleotide sequence

more space in major groove

denaturing & re-hybridizing DNA

• heat + OH- → denatures double stranded DNA to single stranded state

• renaturation (special conditions) → denatures DNA to double strands

DNA structure: base pairs

allows nucleic acid strands to hybridize when base pairs are complementary → strands run in antiparallel directions

• A:T

• G:C

RNA

• bases: A, G, C, U

• sugar: ribose

• most RNA molecules single stranded, some double stranded

• ATP & GTP is a ribonucleotide

RNA: flow of info in the cell

central dogma

• DNA → RNA → protein

  • DNA: heritable genetic information is stored in DNA

  • RNA: instructions for protein syntheses are transcribed into mRNA

  • protein: mRNA translated into protein by ribosome, which incorporates rRNA

    • translation involves tRNA

role of mRNA

structure: largely unstructured

• short, unstable, single-stranded RNA corresponding to a gene encoded within DNA

function:

• serves as intermediary between DNA & protein; used by ribosome to direct synthesis of protein it encodes

roles of tRNA

structure: critical to function

• short (70-90 nucleotides), stable RNA with extensive intramolecular base pairing; contains an amino acid binding site & mRNA binding site

function

• carries the correct amino acid to site of protein synthesis in the ribosome