The nucleus of a cell contains the chromosomes. The chromosomes all contain nuclei acids, usually DNA which controls heredity in cells. Watch a DNA Video!

DNA is made up of a twisted double helix or ladder made of phosphates and sugars. The helix has steps made of four nitrogen bases that are paired; adenine with thymine and cytosine with guanine. Different organisms have different amounts of each base. Humans have more adenine than thymine. The way the bases are lined up on a twisted helix determines heredity traits or genes which are found lined up on different chromosomes. Just one gene can be made of hundreds of bases.

DNA

Replication of DNA

        The replication of the chromosomes and DNA in mitosis is the key to genetic continuity. This is how DNA creats a copy of itself. First, the helix untwists itself as shown here.

Then the bases of the DNA ladder begin to separate down the middle
   

As the bases separate, other bases ( with sugars and phosphates attached ) float into the area of the opened ones and join with the exposed bases to eventually create two DNA helices necessary for two cells.
   

Two identical DNA strands are made
   

During DNA replication, occasional errors are made in the new DNA strand. These errors result in what we know as a mutation.

CELLS  |  CHROMOSOMES  | DNA  |   QUIZ