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DNA notes
9/19/10 6:00 PM
Outline: Key points on DNA Structure and Replication
A) DNA is a double helix molecule
• The two strands are held together by hydrogen bonds between the
base pairs A=T (2 bonds) and G=C (3 bonds)
• The base pairs, like the steps on a spiral staircase, extend in to the
center of the molecule
• The "frame" of the double helix comes from the phosphatedeoxyribose linkages that connect nucleotides together in each strand
• The strands run in opposite directions, that is, the end with a free 5'
phosphate on one strand is matched with the free 3'OH of the
complementary strand
B) DNA is replicated in a semi-conservative fashion
• Each strand serves as the template for making a new complementary
strand
• Nucleotides, in the form of dATP, dCTP, dGTP and dTTP align to the
old strand s by base pairing
• DNA polymerases connect nucleotides in a 5' to 3' direction only, and
only to a pre-existing string of nucleotides (primer)
*Replication is continuous in one strand (leading strand) as the
old double helix unwinds
*Replication in the lagging strand is discontinuous, resulting in
a series of "Okazki fragments" that must be connected together
by DNA ligase
*The primers for both strands are first made of RNA by an RNA
polymerase (primase), and then extended using DNA bases by
DNA polymerase III ( E. coli)
*In E. coli, the RNA primers are removed and replaced with
DNA bases by DNA Polymerase I
*Both pol I and pol III can also back up and remove a misspaired base (exonuclease activity)
• Unwinding of a double helix requires a helicase and also a
topoisomerase to relieve supercoiling
• Single stranded regions of DNA formed during replication are
protected by SSBs, (single strand binding proteins)
C) Replication begins at structures called "origins" or just "ori"
http://www.tamu.edu/classes/plan/magill/gene603/Lecture%20outlines/Molecular/DNA_notes.html
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DNA notes
9/19/10 6:00 PM
• Origins have repeated sequence motifs, including an AT rich region
• Circular DNAs are replicated under several models
*E. coli uses bidirectional replication from a single origin
(Theta)
*T2 replicates using a rolling circle (Sigma)
*Mitochondrial DNA starts replicating one strand and the other
starts when its origin is opened (D-loop)
• Linear chromosmes replicate from many origins
D) The ends of linear DNA molecules (telomeres) can't replicate using DNA
polymerase alone
•The telomeres are 50 -100 repeats of simple sequences, eg. (TTAGGG)
•Telomerase, an enzyme that includes complementary RNA 'rebuilds'
the ends
http://www.tamu.edu/classes/plan/magill/gene603/Lecture%20outlines/Molecular/DNA_notes.html
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