DNA is one of the strangest molecules known to science.

For one thing, the DNA molecule is incredibly complex. atoms

Also, DNA is incredibly long – the longest molecule we have ever discovered. Lebron James – 6’8” Lebron James’ DNA – 9’8”

How the (@#$^* does that all fit inside the nucleus of a cell?

Even when DNA is not condensed into chromosomes, it is coiled to allow it to fit into the nucleus of a cell.

Look at it this way: you have 23 feet of intestines, coiled inside your abdominal cavity.

Also, DNA is the only molecule we know of that contains instructions for making other molecules. R group R group R group R group R group R group R group R group DNA Molecule Amino Acids Protein

Most diagrams of DNA look like this:

However, we can see the molecule more clearly if we un-twist it:

Looking at it this way, we can see that the DNA molecule is actually double-stranded.

Furthermore, we can see that the DNA molecule is made from smaller, repeating molecules which are labeled A, T, C, and G.

DNA Nucleic acid R group R group R group R group Long, double-stranded, polymer molecule that tells ribosomes how to arrange amino acids into protein. L D P R A A P

The DNA polymer is made from nucleotides.

Nucleotide Monomer of DNA and another nucleic acid, RNA. New page M D

There are 4 nucleotides in DNA: adenine, thymine, cytosine, and guanine.

Nucleotides can join together in any order.

The order of nucleotides in DNA determines the protein that is made (the “genetic code.”) ATTACGACA = AGCGGATAT= GCCATAAGC= O P C

Most diagrams of DNA are highly simplified. If this is a molecule, where are the bonds? We can see that DNA is made from nucleotides… but what are the nucleotides made from?

In reality, DNA looks more like this: 3 parts nucleotide Strand #1 Strand #2 Bond Bond

Diagram of a Nucleotide DNA = deoxyribose Diagram of a Nucleotide Phosphate group Nitrogenous base (contains phosphorus) (contains nitrogen) Deoxyribose (a 5-carbon sugar)

Nucleotides join together to form a strand.

DNA, column 3: Each DNA strand is held together by bonds between the deoxyribose and phosphate groups of each nucleotide. S B D P N “sugar -phosphate backbone”

Sugar-phosphate backbone Sugar-phosphate backbone Genetic code

What holds the 2 strands together?

The two strands are held together by bonds between the nitrogenous bases. “Base pair” N B

Sugar-phosphate backbone Sugar-phosphate backbone Base pair A T Base pair G C T A

In DNA, A always bonds with T, and C always bonds with G; this is called the rule of base-pairing.

There are two types of bonds in the DNA molecule.

The covalent bonds in the backbone are very strong, to prevent the genetic code from changing. Hydrogen bond (weak)

The hydrogen bonds holding the base pairs together are relatively weak… Covalent bond (strong) Hydrogen bond (weak)

…which allows the two strands to separate during replication.

The 4 nucleotides have different nitrogenous bases. Adenine Cytosine Guanine Thymine

Looks more like a pyramid A and G are purines (base has 2 rings). C and T are pyrimidines (base has 1 ring). P P Adenine Cytosine Looks more like a pyramid Guanine Thymine Purines Pyrimidines

Purines always form base pairs with pyramidines. Pyrimidine Purine Pyrimidine Purine Purine Pyrimidine Purine Pyrimidine

The two strands in DNA are not identical The two strands in DNA are not identical. Instead, they are complementary to each other (opposite). I O Pyrimidine Purine Pyrimidine Purine Purine Pyrimidine Purine Pyrimidine

Complimentary: Complementary:

If you know one DNA strand, you can figure out the complementary strand using base-pairing rules.

Now you know the structure of DNA: two long, complementary strands of nucleotides, held together in the middle by weak hydrogen bonds.

There’s one last thing you need to know about DNA structure…

The two strands of DNA form a double helix.

Double helix Shape of the DNA molecule; formed by two parallel strands that twist around each other. M T S T

Single helix Double helix

Nucleic acids are twisted molecules because the covalent bonds in their backbones are bent.

Bent bonds = spiral shape