DNA

The Blueprint

• DNA contains a code unique to every individual

  • It can be passed down from generation to generation with little or no change

• Every cell excepting red blood cells has the same DNA molecules

  • Humans have 99.9% common DNA—the 0.1% gives differentiating features (e.g. hair colour, eye colour, etc.)

• The code acts as a blueprint for an organism’s structure and functions

DNA Structure

The Nucleotide

• DNA is a polymer—it is made of small subunits called nucleotides

• Nucleotides consist of three components:

  • A nitrogen base (aka just ‘base’)

  • A sugar molecule (called ‘deoxyribose’)

  • a phosphate molecule

• There are four different nitrogen bases, each represented by the first letter of their name:

  • adenine (A)

  • guanine (G)

  • cytosine (C)

  • thymine (T)

These bases differentiate and define the four different nucleotides that make up DNA

Polynucleotide Chains

• Nucleotides form a long polynucleotide chain (nucleic acid) when they join together

  • DNA is a nucleic acid

• They are joined via their sugar and phosphate groups

  • A sugar of nucleotide #1 joins to the phosphate of nucleotide #2, etc.

  • This forms a sugar-phosphate backbone, like the sides of a ladder

The Double Helix

• DNA’s double helix shape is formed as the sugar-phosphate backbone of one nucleic chain is attracted to a second backbone

  • Think of the DNA double helix as a twisted ladder

    • Sugar and phosphate = sides of ladder

    • Pairs of nitrogen bases = rungs

• The “rungs” are pairs of nitrogen bases bonded together by hydrogen bonds

  • A large base (A or G) will always bond to a small base (T or C) to ensure proper space between strands (chains)

  • Adenosine always pairs with thymine (A-T)

  • Cytosine always pairs with guanine (G-C)

  • A-T and G-C are complementary pairs

The two chains then wind into the double helix formation

Vital Properties

• DNA molecules have two vital properties:

  • DNA can replicate itself

    • If two strands unwind, each can be used to make a new DNA molecule

  • DNA can carry information

    • The order of the nitrogen bases along a strand is a code for making proteins