DNA structure and replication

genetic material must

• store biological info

• be accurately replicated

• transmitted from parent to offspring

• capable of change via mutation

Griffith’s transformation experiment (1928)

strains

1. infectious: smooth (S) strain

2. noninfectious: rough (R) strain

3. heat killed smooth strain

mice

• smooth only = mouse dies

• rough only = mouse lives

• rough + heat killed smooth = mouse dies

conclusion: a transforming principle existed

Avery et al. experiment (1944)

determining which biomolecule was genetic material

• protein

• DNA

• RNA

enzymes helped isolate candidates

• protease: digested proteins, R strain became S strain

• DNase: digested DNA, R strain remained R strain

• RNae: digested RNA, R strain became S strain

conclusion: DNA is the transformative substance

Hershey-Chase experiment (1952)

setup

• T2 bacteriophages contained labeled DNA and protein

  • DNA was labeled with ³²P (radioactive element that labels solely DNA)

  • protein was labeled with ³⁵S (radioactive element that labels solely proteins)

result

• T2 bacteriophages born from infected e-coli contained radioactive DNA but not (extremely little) radioactive protein

conclusion: DNA is hereditary material

DNA overview

DNA — deoxyribonucleic acid

DNA contains

1. deoxyribose

2. phosphate

3. one nitrogenous base

polarity

• one end has a 5’phosphate

• one end has a 3’OH

  • DNA polymerase only adds new nucleotides to 3’OH

• backbone alternates between sugar and phosphate group

nitrogenous bases

• purines (double ring, larger): adenine (A) and guanine (G)

• pyrimidines (single ring, smaller): cytosine (C) and thymine (T)

bonds

1. phosphodiester bonds: bonds the 5-carbon sugar of 1 nucleotide to the phosphate group on the next nucleotide

2. N-glycosidic bonds: bonds the 5 carbon sugar to its nitrogenous base

3. hydrogen bonds: holds nitrogenous base pairs together

nitrogenous bases

purines pair with pyrimidines:

• A = T: adenine pairs with thymine using 2 hydrogen bonds

• G = C: guanine pairs with cytosine using 3 hydrogen bonds

• hydrogen bonding holds base pairs together

• this pairing maintains consistent helix width

discovery of DNA’s double helix (1953)

Rosalind Franklin

• xray diffraction revealed DNA’s helical structure

• critical for building double the helix model

Watson and Crick

• used Franklin and other findings to develop the double helix model

• did not acknowledge Franklin

Watson-Crick model

• DNA consists of 2 antiparallel polynucleotide bonds

• strands twist to form a double helix

• nitrogenous bases pair at the center (AT and GC)