DNA structure DNA machinery DNA FAQ Commentary

Francis Crick, James Watson genetics

DNA structure

• Deoxyribonucleic acid is
• a double helix with antiparallel chains
• of hydrophilic sugar-phosphate backbones
• and hydrophobic bases packed into the core.
• Maurice Wilkins and Raymond Gosling
• prepared X-ray diffraction images of DNA
• showing clear “X shaped” patterns.
• Wilkins and Alex Stokes believed its structure was helical.
• Rosalind Franklin provided X-ray diffraction images
• and told Crick and Watson that the backbones
• had to be on the outside of the structure,
• but disputed that its structure was helical.
• Erwin Chargaff discovered that in DNA
• the number of guanine units equals the number of cytosine units
• and the number of adenine units equals the number of thymine units
• and he explained this to Crick and Watson.
• Crick and Watson recognized the structural simularity
• of guanine-cytosine and adenine-thymine pairs
• and predicted that ten nucleotide bases
• were stacked into the core for each turn of the double helix.

DNA machinery

• Helicases split a double helix into strands
• like opening a zipper.
• Nucleases cut the resulting strands
• at specific sites.
• Polymerases make copies
• and proofread the results
• and remove bases that don’t match
• the DNA template.
• Ligases rejoin the cut strands.

DNA FAQ

• DNA encodes the instructions
• for the development, functioning, and reproduction
• of all known living organisms.
• Tends to makes me wonder
• how this complex molecule
• could be so basic.
• Multiple strands in a nucleus
• are long, about 220 million base pairs
• in the largest human chromosome.
• However, most of it,
• 98 percent in human DNA,
• doesn’t code information for making proteins.
• In the vast random universe
• (if the universe were random)
• a little something has clicked.