DNA stores genetic instructions in the form of a code made up of four nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C). DNA has a double-stranded helical structure, with the strands bonded together via complementary base pairing between A and T and between G and C. According to Chargaff's rule, the percentage of A bases is always equal to the percentage of T bases, and the percentage of G bases is always equal to the percentage of C bases within a DNA sample. The two strands of DNA run in opposite directions of each other (anti-parallel) and form a double helix.

1. Unit 4
notes:
DNA Structure

2. (1) What is DNA?
• DNA = DeoxyriboNucleic Acid
• Stores genetic instructions for producing
proteins.
• Stored in the Nucleus of Eukaryotes.

4. (2) Overall DNA Structure
• Double stranded
• Helix
• Outside / Backbones Made Of:
– Alternating Phosphates and Sugars
• Inside / Steps Made of:
– Nitrogenous Bases

6. (Yes You Need To Draw This…)

7. (3) DNA Nucleotide
• A nucleotide is the monomer for DNA.
• Structure:
– Phosphate
– Deoxyribose Sugar
– Nitrogenous Base
• Looks Like:

8. (4) Nitrogenous Bases
• The sequence of these bases ARE your genetic
code/instructions.
• 4 Types of Bases:
– A = Adenine
– T = Thymine
– G = Guanine
– C = Cytosine
• A’s and T’s bond together.
• G’s and C’s bond together.

9. ADENINE
THYMINE
GUANINE
CYTOSINE

11. (5) Chargaff’s Rule
• Chargaff discovered how the bases pair
together in DNA.
• He found that:
– % of A bases = % of T bases
– % of G bases = % of C bases
• We can use Chargaff’s Rule to find exact
concentrations of bases:
(%A + %T) + (%G + %C) = 100%

12. Do you see
how all of the
%’s of A and T
are about
equal?
Do you see
how all of the
%’s of G and C
are about
equal?
…And that if we take %A + %T + %G +
%C it equals 100%!

13. (6) Chargaff’s Rule Example:
Within the DNA of yeast cells, it is found that
16% of the bases are Adenine…
• What % are Thymine?
• What % are Guanine or Cytosine?

14. (7) Anti-Parallel Strands:
• Anti-Parallel refers to the two strands in
DNA being positioned in opposite
directions.
• The strands are upside down mirror images
of one another!