What does DNA look like?
• Inherited characteristics are determined by
genes, which are parts of chromosomes.
• Chromosomes are structures in the nucleus of
most cells.
• Chromosomes are made of protein and DNA.
(deoxyrybonucleic acid)
Nucleotides
• DNA is made of subunits called nucleotides,
which consists of a sugar, a phosphate, and a
base.
• Nucleotides are identical except for their base,
which all have a different shape.
• There are Four Bases.
–
–
–
–
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
Important discoveries
• In the 1950’s, Erwin Chargaff found that the amount of
adenine in DNA always equals the amount of thymine,
and that the amount of guanine always equals the
amount of cytosine. This is known as Chargaff’s Rules.
• British scientist Rosalind Franklin was able to make
images of DNA molecules using a process known as Xray diffraction. This revealed that DNA has a spiral
shape.
Watson and Crick
• Once Franklin had determined
the spiral shape of DNA,
James Watson and Francis
Crick were able to determine
that DNA must look like a
long, twisted ladder.
• Their model perfectly fit with
both Chargaff’s and Franklin’s
findings and eventually was
able to help explain how DNA
is copied and how it functions
in the cell.
DNA’s Double Structure
• The twisted ladder shape
of DNA is known as a
double helix.
• The sides of the ladder are
made of alternating sugars
and phosphates. The rungs
are made of a pair of
bases.
• Remember, because of
Chargaff’s Rules, Adenine
always pairs with Thymine,
and Guanine always pairs
with Cytosine.
Making copies of DNA
• DNA copies itself by the process of replication.
• Because the bases are complimentary and can only pair with a single
other base, one side of a strand of DNA will tell you what the other side is.
For instance the sequence CGAC will bond with GCTG.
• During Replication, DNA splits down the middle, and the base for each
side of the strand are used as a pattern for a new strand.
• DNA is copied every time a cell divides. Each cell gets a complete copy of
all the DNA.