7-1
Topic: DNA
Aim: How does DNA work?
(A) FUNCTIONS
o Carries genetic information
o Determines what proteins are made
o Transmits genetic information from parents to offspring
(B) STRUCTURE
o Watson and Crick first to discover that DNA was a double helix
 Two strands wound around each other “twisted ladder”
o DNA is a nuclei acid which is a long molecule made up of nucleotides
o Each nucleotides are made up of
 Pentose (5 carbon) sugar (deoxyribose)
 Phosphate group
 Nitrogenous base
o
o
o
o
Nitrogenous bases:
 Adenine—A
 Guanine—G
 Cytosine—C
 Thymine—T
Adenine and guanine are purines
Thymine and cytosine are pyrimidines
Purines and pyrimidines are different groups of nitrogenous bases—
they differ based on their structure
o
o
The sides of the ladder are made up of alternating sugar and phosphate
The steps are made up of nitrogenous bases held together by hydrogen
bonds
o
Base Pairing Rules state that adenine always pairs with thymine and cytosine
always with guanine
A=T
C=G
o
o
(C)
DNA REPLICATION
o DNA is copied in a process called DNA replication
o Without DNA replication new cells would only have half the DNA
o During Interphase of both mitosis and meiosis
o New copies of DNA are exactly like the original molecules
1. Separation of Strands
 The two nucleotide strands separate when the DNA
helicase unwinds the helix leaving the bases exposed
 DNA polymerases move along each DNA strand and
add nucleotides to the exposed bases according to the
base pairing rules.
 Two DNA molecules form that are identical to the
original. The DNA polymerase also proofreads DNA
during its replication so that very few errors occur.
**THE OLD DNA STRAND SERVES AS A TEMPLATE FOR THE NEW DNA STRAND.**
Check out the following website for an animation of DNA Replication.
http://www.lewport.wnyric.org/JWANAMAKER/animations/DNA%20Replication%20%20long%20.html