DNA, AND IN SOME CASES
RNA, IS THE PRIMARY SOURCE
OF HERITABLE INFORMATION
Genetic information is transmitted from one
generation to the next through DNA or RNA
Structure of DNA

Building Blocks

Nucleotide




Uprights / Backbone


Deoxyribose sugar
Phosphate group
Nitrogenous bases
Alternating DNA and
Phosphate
Rungs / Core




Nitrogenous bases
A-T and G-C
A and G – purines (double
ring)
T, C and U – pyrimidines
(single ring)
Structure of DNA

Double helix
2
polynucleotides
bonded together by
hydrogen bonding
between nitrogen
bases

Antiparallel
 one
strand runs 5’ to 3’
 one strand runs 3’ to 5’
DNA replication

Replication is a
semiconservative
process
 Each
strand serves as
the template for a new
strand
 Ends with two identical
strands of DNA; each
having one old and
one new strand

http://youtu.be/teV62
zrm2P0
DNA replication
Helicase unwinds DNA
forming a Y-shaped
region where new DNA
strands are produced replication fork/bubbles.
There can be 100's or
even 1,000's of
replications forks/bubbles
in a eukaryotic
chromosome.
Single-stranded binding
proteins bind to and
stabilize the singlestranded DNA so that it
can be used as a
template (see above).
Topoisomerases correct
"overwinding" ahead of
the replication fork by
breaking, swiveling, and
rejoining DNA strands.
Primase adds a short RNA
primer (5 - 10 nucleotides
long) to add an initial
starting point for the new
DNA strand. DNA
polymerase III can then
add complementary DNA
nucleotides to the 3' end
of the RNA primer (at a
rate of about 50/sec in
humans.)
DNA Replication
Each new nucleotide that is added
to the growing DNA strand is a
nucleoside triphosphate (see right).
As each nucleoside triophosphate
monomer is added it loses two
phosphate groups - hydrolysis of
2 phosphates is exergonic &
powers the polymerizeration of
monomers.
One strand is continuously
replicated. This strand is referred
to as the leading strand and it is
the strand that is replicated in a 5'
to 3' direction.
The lagging strand on the other
hand has discontinuous replication.
The lagging strand is synthesized
as a series of segments called
Okazaki fragments, which are
joined by DNA ligase (see right).
When the replication fork opens a
RNA primer is needed to change
the change the direction of
replication so that replication can
run 5' to 3'.
When the entire replication is
complete DNA polymerase I
removes RNA primers and
replaces them with DNA. DNA
polymerases also proofread
newly made DNA replacing any
incorrect nucleotides.
Replication of Telomeres






Tips of linear chromosomes
are called telomeres
DNA polymerase works in a
5’ to 3’ direction
Tips of lagging strand stops
short of the end during
replication
Telomerase is an enzyme that
extends the lagging strand
Telomerase prevents the
premature shortening of
telomeres / chromosomes
Length of telomeres has been
linked to cell death
(apoptosis)

http://youtu.be/AJNoTmWsE
0s
Enzymes of Replication that you should
know…

Helicases


Topoisomerases


synthesizes an RNA primer at 5' end of leading strand and of each Okazaki
fragment of lagging strand
DNA Polymerase


corrects "overwinding" ahead of replication forks by breaking, swiveling, and
rejoining DNA strands
RNA Primase


enzymes that untwist the double helix at the replication forks
using parental DNA as a template synthesizes new DNA strand by covalently
adding nucleotides to the 3' end of a pre-existing DNA strand or RNA primer;
removes RNA nucleotides of primer from 5' end and replaces them with DNA
nucleotides
DNA Ligase

joins Okazaki Fragments