Nucleic Acids
Information
storage
AP Biology
2006-2007
Nucleic Acids
 Function:

genetic material
 stores information
 genes
 blueprint for building proteins

DNA
DNA  RNA  proteins
 transfers information
 blueprint for new cells
 blueprint for next generation
AP Biology
proteins
Nucleic Acids
 Examples:

RNA (ribonucleic acid)
 single helix

DNA (deoxyribonucleic acid)
 double helix
 Structure:

AP Biology
monomers = nucleotides
DNA
RNA
Nucleotides
 3 parts
nitrogen base (C-N ring)
 pentose sugar (5C)

 ribose in RNA
 deoxyribose in DNA

phosphate (PO4) group
Are nucleic acids
charged molecules?
AP Biology
Nitrogen base
I’m the
A,T,C,G or U
part!
Types of nucleotides
 2 types of nucleotides
different nitrogen bases
 purines

 double ring N base
 adenine (A)
 guanine (G)

pyrimidines
 single ring N base
 cytosine (C)
 thymine (T)
 uracil (U)
AP Biology
Purine = AG
Pure silver!
Nucleic polymer
 Backbone
sugar to PO4 bond
 phosphodiester bond

 new base added to sugar of
previous base
 polymer grows in one direction

N bases hang off the
sugar-phosphate backbone
Dangling bases?
Why is this important?
AP Biology
Pairing of nucleotides
 Nucleotides bond between
DNA strands
H bonds
 purine :: pyrimidine
 A :: T

 2 H bonds

G :: C
 3 H bonds
Matching bases?
Why is this important?
AP Biology
DNA molecule
 Double helix

H bonds between bases
join the 2 strands
 A :: T
 C :: G
H bonds?
Why is this important?
AP Biology
Copying DNA
 Replication

2 strands of DNA helix are
complementary
 have one, can build other
 have one, can rebuild the
whole
Matching halves?
Why is this
a good system?
AP Biology
When does a cell copy DNA?
 When in the life of a cell does DNA have
to be copied?

cell reproduction
 mitosis

gamete production
 meiosis
AP Biology
DNA Replication
AP Biology
2007-2008
Directionality of DNA
 You need to
PO4
nucleotide
number the
carbons!

it matters!
N base
5 CH2
This will be
IMPORTANT!!
O
4
3
AP Biology
1
ribose
OH
2
The DNA backbone
 Putting the DNA
backbone together

refer to the 3 and 5
ends of the DNA
 the last trailing carbon
Sounds trivial, but…
this will be
IMPORTANT!!
5
PO4
base
5 CH2
O
4
1
C
3
O
–O P O
O
5 CH2
2
base
O
4
1
2
3
OH
AP Biology
3
Anti-parallel strands
 Nucleotides in DNA
backbone are bonded from
phosphate to sugar
between 3 & 5 carbons
5
3
3
5
DNA molecule has
“direction”
 complementary strand runs
in opposite direction

AP Biology
Bonding in DNA
5
hydrogen
bonds
3
covalent
phosphodiester
bonds
3
5
….strong or weak bonds?
AP
Biology
How
do the bonds fit the mechanism for copying DNA?
Base pairing in DNA
 Purines
adenine (A)
 guanine (G)

 Pyrimidines
thymine (T)
 cytosine (C)

 Pairing

A:T
 2 bonds

C:G
 3 bonds
AP Biology
Copying DNA
 Replication of DNA
base pairing allows
each strand to serve
as a template for a
new strand
 new strand is 1/2
parent template &
1/2 new DNA

 semi-conservative
copy process
AP Biology
DNA Replication
Let’s meet
the team…
 Large team of enzymes coordinates replication
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Replication: 1st step
 Unwind DNA

I’d love to be
helicase & unzip
your genes…
helicase enzyme
 unwinds part of DNA helix
 stabilized by single-stranded binding proteins
helicase
single-stranded binding proteins
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replication fork
Replication: 2nd step
 Build daughter DNA
strand
add new
complementary bases
 DNA polymerase III

DNA
Polymerase III
AP Biology
But…
Where’s the
We’re missing
ENERGY
something!
for the bonding!
What?
Okazaki
Leading & Lagging strands
Limits of DNA polymerase III

can only build onto 3 end of
an existing DNA strand
5
3
5
3
5
3
5
5
5
Lagging strand
ligase
growing
3
replication fork
Leading strand
3
Lagging strand


Okazaki fragments
joined by ligase
AP Biology
 “spot

3
welder” enzyme

5
3
DNA polymerase III
Leading strand

continuous synthesis
Replication fork
DNA
polymerase III
lagging strand
DNA
polymerase I
5’
3’
ligase
primase
Okazaki
fragments
5’
3’
5’
SSB
3’
helicase
DNA
polymerase III
5’
3’
leading strand
direction of replication
AP Biology
SSB = single-stranded binding proteins
DNA polymerases
 DNA polymerase III
1000 bases/second!
 main DNA builder

Thomas Kornberg
 DNA polymerase I
20 bases/second
 editing, repair & primer removal

DNA polymerase III
enzyme
AP Biology
Arthur Kornberg
Editing & proofreading DNA
 1000 bases/second =
lots of typos!
 DNA polymerase I

proofreads & corrects
typos

repairs mismatched bases

removes abnormal bases
 repairs damage
throughout life

AP Biology
reduces error rate from
1 in 10,000 to
1 in 100 million bases