THE HISTORY OF DNA FRIEDRICH MIESCHER (1869) Discovered a nonprotein substance in the nuclei of pus cells Named the substance “nuclein”

Joachim Hammerling (1930’s) Experiments with the green algae Acetabularia showed that the regeneration of appendages required the nucleus which was located in the foot Suggested that genetic information is stored in the nucleus

Chemical analysis showed that: A = T and G = C A + G = T + C Complementary base-pairing Erwin Chargaff (1949)

Alfred Hershey and Martha Chase (1952)

Their experiments showed that DNA tagged with radioactive phosphorus (32P) was found inside infected bacterial cells and directed the synthesis of new viral DNA and new viral protein coats Protein coats tagged with radioactive sulfur (35S) did not enter bacterial cells Suggested that DNA is the hereditary material

Rosalind Franklin (1953) X-ray diffraction showed that DNA was in the shape of a double helix

Watson and Crick (1953)

Watson and Crick deduced the model of DNA that persists today: Double helix, clockwise (right-handed helix) 1 complete turn every 10 nucleotides (3.4 nm full helical twist) Sugar-phosophate backbone Complementary base-pairing between nucleotides – hydrogen bonding Constant diameter of 2nm Antiparallel strands

DNA STRUCTURE

The Chemical Composition of DNA Composed of nucleotides held together by phosphodiester bonds Each nucleotide consists of a deoxyribose sugar, a phosphate group and a nitrogenous base

Deoxyribose sugar

4 Nitrogenous Bases Pyrimidines (single rings) Thymine Cytosine Purines (double rings) Adenine Guanine

Sugar-Phosphate backbone Phosphodiester bonds hold the sugar-phosophate backbone together Each DNA strand has a 5’ end (attached to a phosphate group) and a 3’ end (attached to a hydroxyl group)

Hydrogen bonding occurs between complementary base pairs.

DNA Molecule DNA is a double helix The two strands run “antiparallel” to one another Hydrogen bonds hold the complementary bases together

Writing DNA sequences: Every DNA molecule has “directionality” and bases sequences are written as follows: e.g.) 5’ – ATGCCGTTA – 3’ 3’ - TACGGCAAT – 5” (Usually, only the 5’ to 3’ strand is written since the complementary strand can be deduced.)

What attributes of DNA’s structure enable it to be replicated? Complementary base-pairing Hydrogen bonds between base pairs Double stranded, therefore, each strand can serve as template for the replication of the complementary strand

Gene Organization and Chromosome Structure

Define These Terms Gene Chromosome Histone Chromatin Metaphase chromosome Chromatid Centromere

Gene – a sequence of nucleotides that codes for a protein Chromosome – one continuous double-stranded DNA molecule Histone – a positively charged protein that binds to negatively charged DNA Chromatin – complex of DNA and histones located in the nucleus of eukaryotic cells Metaphase chromosome – a duplicated chromosome Chromatid – one half of a duplicated chromosome Centromere – a constricted region that holds two replicated chromosomes together