DNA
DNA (Deoxyribonucleic acid) is a
long polymer of repeating units.
 The monomers are called
nucleotides.
 Each nucleotide has three
components:

A phosphate group
 A ring-shaped sugar, known as
deoxyribose.
 A nitrogen-containing base

One molecule of DNA contains billions of
nucleotides.
 However, there are only four nucleotides that
make up DNA.
 The four nucleotides are:

Cytosine
 Guanine
 Thymine
 Adenine

PYRMIDINES
Thymine & Cytosine
 Single-ring structures
 Cytosine pairs with
guanine

PURINES
Adenine & Guanine
 Double-ring
structures
 Adenine pairs with
thymine

In a single strand of DNA, nucleotides join
together through covalent bonds.
 The sugar base of one nucleotide bonds to the
phosphate group of another nucleotide.

DOUBLE HELIX
DNA forms a twisted staircase when the covalent
bonds form.
 The structure of DNA is a double helix; meaning
that the DNA is double stranded, with the two
strands wrapping around each other.

The bases of each strand pair together through
hydrogen bonds.
 A and T form two hydrogen bonds, while G and C
form three hydrogen bonds.

DNA REPLICATION


Replication ensures that every cell has a
complete set of identical genetic information.
A single strand of DNA serves as a template to
make another identical copy.

Proteins and enzymes do all the actual work of
replication.
Enzymes begin the process by unwinding the
double strands of DNA.
 Specific proteins hold the DNA strands apart
while each strand serves as a template.
 Free floating nucleotides in the nucleus can pair
up with the existing strands.
 DNA polymerases bond the new nucleotides with
the old strands.


The result is two new strands of genetically
identical DNA
FAST & ACCURATE



DNA replication occurs very quickly and
accurately. In humans, about 50 nucleotides are
added every second.
Replication proceeds from hundreds of origins of
replications along a chromosome.
DNA polymerases are also able to detect errors,
take out the incorrect nucleotide and replace it
with the proper one.