Molecular basis of inheritance

miescher's experiment

discovered nucleic acids - he purified nuclei from WBCs and found a precipitate rich in phosphorus and nitrogen

nucleic acid structure

5 carbon sugar, nitrogenous base, phosphate group

griffith's experiment - 2 strains used

streptococcus pneumoniae. R - benign, destroyable by immune system. S - virulent, outer layer prevents detection by host immune system

mice + S form =

mice die

mice + R form =

mice live

mice + heat killed S form =

mice live

mice + heat killed S form + R form

mice die, blood contains pathogenic strains of S. pneumoniae

transforming principle in griffith's experiment

genetic material could reprogram the R form into S form and cause the disease

how did he know that the transforming principle was DNA

it was resistant to proteases, lipases, ribonucleases etc, so couldn't be protein, lipid, RNA

hershey-chase method and results

used bacteriophages and electron microscopy - showed that the virus doesn't enter the cell but the genetic material is injected

when phage protein is labelled with radioactive material …

most of the radioactivity was in the supernatant

when the phage DNA is labelled with radioactive material ..

most of the radioactivity was in the pellet

hershey-chase conclusions

DNA is injected into the cell, not protein

chargaff's rule

the nitrogenous bases must contain the genetic code because the sugar and phosphate are invariant, ratio of bases isn't 1:1:1:1 and pairs are always A+T and C+G

purines

adenine and guanine

pyrimidines

cytosine and thymine

conclusions of DNA structure from x-ray diffraction

helical, 2nm wide, length of each turn = 3.4nm, distance between repeating units = 0.34nm, 10 nucleotide pairs per turn

a+t = ? H bonds

2 so weaker

c+g = ? H bonds

3 so stronger

semi conservation replication

each strand of DNA acts as a template for new strand synthesis

meselson-stahl experiment

bacteria cultured in medium with heavy N isotope, then transferred to medium with lighter N isotope, found all DNA had intermediate density - 1 light and 1 heavy strand

dna polymerase requires

single stranded DNA, all 4 nucleoside triphosphates, free 3' hydroxyl - primer

leading strand synthesis

continuous, 5' to 3', helicase separates strands, single stranded binding protein prevent reannealing, RNA primer synthesised and added by primer

lagging strand synthesis

discontinuous, RNA primer for fragment 1 binds, replication occurs - okazaki fragment 1 forms, RNA primer for fragment 2 binds - repeats, DNA pol1 replaces RNA with DNA, DNA ligase joins okazaki fragments

6 proteins used in dna replication

helicase, single stranded binding proteins, primase, dna polymerase III, dna polymerase I, dna ligase