1. The Genetic Material
DNA Structure

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

3. 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

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

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

6. 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

7. 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.

8. 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.

9. 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

10. Hydrogen Bonds

11. 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.

12. 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).