DNA
Deoxyribonucleic Acid
Components, Structure and
Replication
Components and Structure

DNA looks like a twisted
ladder.
 Two strands of DNA spiral
around one another to form
a double helix.
 The strands are held
together like two sides of a
zipper by hydrogen bonds.
 The double helix model was
developed in 1953 by
Watson and Crick
Components and Structure
 Each
strand is
made up of units
called nucleotides




5-carbon sugar
called deoxyribose
Phosphate group
Nitrogenous base
Label the picture on
your notes
Components and Structure
4
kinds of bases: A, G, C and T
 Purines


Adenine and Guanine
Two rings
 Pyrimidines


Cytosine and Thymine
One ring
Components and Structure
 The
bases hydrogen bond with each
other to form base pairs in the double
helix, creating the “rungs” of the
ladder.



Adenine pairs with Thymine
Cytosine pairs with Guanine
Chargaff’s Rule - The percentage of A and T
will always be equal in a sample of DNA. The
percentage of C and G will also always equal.
DNA Replication

During S Phase of the Cell Cycle, a cell must
replicate its genetic material in preparation for
cell division.
 Why does the cell need two copies?
Replication – the process of copying or
duplicating DNA
 The strands of a double helix are
complementary; because of the base pairing
rules, the sequence of one strand can be used
to determine the sequence of the other strand.

DNA Replication

The two strands are “unzipped” at the replication
fork.



Prokaryotes start at one point and replicate in
both directions.
Eukaryotes start at many points and have several
replication forks at once since they have so much
DNA to copy.
An enzyme called DNA polymerase uses an old
strand of DNA as a template to create a new
strand of DNA. Each old strand is a template for
a new strand.
 DNA polymerase proofreads its work to make
sure that it has created an accurate
complementary strand.
DNA Replication

The end result of replication – two double
helices, each with one old strand and one new
strand.
Replication
 http://www.youtube.com/watch?v=rpwjZX_
z5rg
Question
 Why
is it important for DNA polymerase to
proofread its copies?
 What
would happen if DNA polymerase
had a greater frequency for mistakes?
Let’s Practice!
 Template
DNA:
AGCTAA
 Complementary DNA: TCGATT
 Template
DNA:
TTGCGA
 Complementary DNA: AACGCT
 Template
DNA:
AGCATG
 Complementary DNA: TCGTAC