DNA Structure and Function

The Components of DNA DNA is a nucleic acid made up of nucleotide's joined into long strands or chains by covalent bonds. Nucleotides may be joined in any order.

A DNA nucleotide is a unit made of a nitrogenous base, a 5-carbon sugar called deoxyribose, and a phosphate group.

DNA has four kinds of nitrogenous bases: adenine, guanine, cytosine, and thymine.

Solving the Structure of DNA

Erwin Chargaff showed that the percentages of adenine and thymine are almost always equal in DNA. The percentages of guanine and cytosine are also almost equal.

Rosalind Franklin’s X-ray diffraction studies revealed the double-helix structure of DNA.

James Watson and Francis Crick built a model that explained the structure of DNA.

The Double-Helix Model The double-helix model explains Chargaff’s rule of base pairing and how the two strands of DNA are held together. The model showed the following:

The two strands in the double helix run in opposite directions, with the nitrogenous bases in the center.

Each strand carries a sequence of nucleotides, arranged almost like the letters in a fourletter alphabet for recording genetic information.

Hydrogen bonds hold the strands together. The bonds are easily broken allowing DNA strands to separate.

Hydrogen bonds form only between certain base pairs–adenine with thymine, and cytosine with guanine. This is called base pairing.

Chapter 4 Sections 1 & 2 textbook PDF