The Genetic Material DNA Structure

A. Introduction eukaryotes (animals, plants, some single-celled organisms) have DNA in nucleus prokaryotes (bacteria) have DNA in cytoplasm

B. Structure of DNA DNA (deoxyribonucleic acid) is a polymer composed of many monomers called nucleotides. Each nucleotide consists of three subunits: five-carbon sugar (deoxyribose) phosphate group (PO4) Nitrogen-containing base

NITROGEN -CONTAINING BASE DNA (Polymer) Made up of NUCLEOTIDES (monomer) contain PHOSPHATE Group 5-CARBON SUGAR NITROGEN -CONTAINING BASE

A nucleotide looks like this (draw it)! PO4 Phosphate group A or T C or G deoxyribose Nitrogen base 5 Carbon sugar

nitrogen base may be one of four different kinds: sugar (deoxyribose) and phosphate (PO4) molecules do not change in each nucleotide so are often referred to as the “backbone” nitrogen base may be one of four different kinds: Adenine (A) Guanine (G) Thymine (T) Cytosine (C) purines pyrimidines

Watson and Crick (and Franklin) determined that the DNA molecule consists of two strands twisted around each other into a double helix resembling a double spiral staircase.

If DNA were to be flattened out, it would look like a ladder. 2 strands are held together by hydrogen (H) bonds between pairs of bases.

Sugar-phosphate molecules in the nucleotide are like side rails of the ladder (the backbone) nitrogen-containing (nitrogenous) bases are similar to steps or rungs of ladder with 1 purine bound to 1 pyrimidine Nitrogenous bases always pair so that: T pairs with A with 2 hydrogen bonds G pairs with C with 3 hydrogen bonds A T G C

Hydrogen Bonds

C. Base-Pairing Principle base pairing of A-T and C-G is called complementary because 2 specific bases bond together to make a complete unit. Specificity of this bonding results in the 2 DNA strands running in opposite directions.

A completed double strand of DNA looks like the picture to the left. Note that the sugars point in opposite directions. Each strand is antiparallel to the other strand. This orientation is crucial in DNA replication (next).