DNA (deoxyribonucleic acid)—molecule in the shape of a
double helix. Made up of long chains of nucleotides
consisting of a nitrogen base, a sugar molecule, and a
phosphate group. The order that these nucleotides occur in
determines the structure of proteins during protein synthesis.
Francis Crick and James Watson won a Nobel Prize in 1962
for their discovery of the structure of the DNA molecule in
Nucleotides are named for the nitrogen base that is contained
in them. There are four nitrogen bases:
1. Adenine (A)
2. Thymine (T)
3. Guanine (G)
4. Cytosine (C)
Base-pairing rule—when rungs in the ladder-like structure of
DNA form, adenine always pairs with thymine, and guanine
always pairs with cytosine.
Replication—the process of the DNA molecule making a copy
of itself. DNA replicates during the S stage of the cell cycle so
that each new cell will have a complete new copy of the
genetic code after mitosis occurs. It also replicates before
the first meiotic division.
The steps DNA goes through during replication are:
1. Chemical bonds between nitrogen bases break and
the molecule unzips.
2. Enzymes cause free nucleotides in the nucleus to pair
with complementary bases on original DNA strands.
3. Other enzymes link the new nucleotides to form new
4. Thus, each original strand serves as a template, or
pattern, for each new strand.
5. Each completed DNA molecule contains one old and
one new strand.
6. The process is powered by ATP and enzyme