Structure and Replication of DNA
Watson and Crick:
James Watson and Francis Crick announced and discover the structure of DNA in 1953.
They compiled all the information that other scientists had done in the past to come up
with the structure. Instead of performing experiments, they built 3-D models using the
information from experiments done by Edwin Chargaff, as well as this x-ray photograph
of DNA by Rosalind Franklin to figure out the structure of DNA. The actual structure and
description of DNA are explained ahead.
Meselson and Stahl:
Matthew Messelson and Franklin Stahl tested the three hypotheses as to
the way DNA replicates. The three hypotheses were that DNA
replication was either: 1) Conservative, meaning that the original strand
remained intact, while an exact copy was made 2) Dispersive, meaning
the new and old pieces of the DNA were mixed together and 3) Semiconservative, meaning that one of the original strands was conserved and
used as a template to make the other strand, the complementary, new
strand. You do not need to know the details of their experiment, but you
must know the results. They concluded that the semi-conservative
process was consistent with their experimental findings as to how DNA
replicates. Remember, DNA replication takes place during the Synthesis
phase (S-phase) of interphase, during the cell cycle.
The structure of DNA
As you have learned before, DNA is made up of nucleic acids (nucleotides).
DNA is then arranged into genes, and many genes make chromosomes. Lets
look at this a bit more detailed. Deoxyribonucleic acid (DNA) is made up of
two main parts: the backbone and the bases. The backbone is made up of
sugar (deoxyribose) and phosphates. The bases are attached to the sugars,
one base for every sugar. The two sugar-phosphate backbones are arranged in
such a way that the bases pair up, with their complimentary bases (explained
below). These bases are attracted to each other by hydrogen bonds. This
attraction between the nucleotides and the fact that DNA is double stranded,
gives DNA the appearance of a ladder. DNA is arranged in what we call a
double helix, that is two strands of DNA that are twisted together to look like a twisted ladder.
There are 4 nucleotides (bases) in DNA: Adenine (A), Thiamine (T), Guanine (G) and Cytosine (C).
Because of their shape and their size, A always pairs with T and G always pairs with C. The
complementary strands of DNA arrange so they are paired up correctly. Also, when DNA is replicating
or copied, the sequence is conserved, due to this complimentary arrangement of the nucleotides and
bases. DNA replication has three main steps. First, the enzyme DNA Helicase breaks apart the weak
hydrogen bonds between the complimentary bases, separating the DNA into two strands. A second
enzyme, DNA polymerase, attaches the corresponding nucleotides using as a template the nucleotides
of the original strand. Then, the enzyme DNA ligase forms bonds between sugars and phosphates of the
free nucleotide bases to build a new backbone. This forms two complete strands of DNA, each made of
one old and one new strand, hence the name semi-conservative replication. This also makes it so that
each strand of DNA is not the same as the other, but complimentary (the opposite of one another).