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