The Watson and Crick Wing

In 1953 James Watson and Francis Crick discovered the structure of DNA.

DNA is located in the nucleus of eukaryotic cells. Remember Prokaryotic cells do not have a nucleus. But, they do have DNA material. It is spread throughout the cell.

This is a model of DNA (Deoxyribonucleic Acid). It is a double helix (2 strands twisted together). Think of it as a twisted ladder. DNA consists of nitrogenous bases and sugar-phosphate units. The bases are in the center like the rungs on a ladder. Hydrogen bonds hold these bases together in the DNA molecule. The sugar-phosphate units are along the sides. Together a nitrogenous base, sugar, and phosphate make up a nucleotide. The nucleotide is a group that repeats over and over to make a very large molecule of DNA. The strands are complementary

This is a drawing of the complementary strands.

The following are nitrogenous bases that create the steps of the ladder:

A= Adenine
G= Guanine
T= Thymine
C= Cytosine

The following is a VERY important fact.

A (Adenine) always pairs with T (Thymine).
G (Guanine) always pairs with C (Cytosine).

Because these 4 molecules always match up this way, it allows for DNA to copy itself without error (under normal conditions).

When DNA makes copies of itself it is called replication. This process produces two double-stranded DNA molecules.

Here's how replication occurs:

Step 1. The DNA double helix "unzips." An enzyme called DNA helicase attaches to the DNA molecule (double helix) and moves along the molecule and "unzips" the 2 strands of DNA. It does this by breaking the hydrogen bonds between the base pairs (or rungs on the ladder). For example, it will break the hydrogen bond that holds Adenine and Thymine together.

Step 2. There are complementary nucleotides floating around in the nucleus. That means there are Adenine, Guanine, Thymine and Cytosine floating around the nucleus. These bases form hydrogen bonds with the bases in the DNA strands.

Step 3. The enzyme called DNA polymerase catalyzes the formation of the sugar-phosphate units along the side of the "ladder." The sugar-phosphate units connect one base pair (or rung on the ladder) to the next one.

Result: Two new DNA molecules result from the replication - each molecule with a strand of old DNA and a strand of new DNA.

Cells have special molecules that help to check and correct DNA replication if something doesn't line up correctly. But, what if it doesn't copy exactly? It can happen. A change in DNA is called a mutation. Mutations can be good or bad. If the new DNA produces a protein that allows a trait to form that helps the organism survive then the mutation can be good. We will be looking at some mutations in the next lesson.

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