1. Chapter 13
DNA and Its Role in Heredity

2. Review of DNA Structure
Four key features of DNA structure:
• It is a double-stranded helix of uniform
diameter
• It is right-handed
• It is antiparallel
• Outer edges of nitrogenous bases are
exposed in the major and minor
grooves

3. Review of DNA Structure
Complementary base pairing:
• Adenine (A) pairs with thymine (T) by
two hydrogen bonds
• Cytosine (C) pairs with guanine (G) by
three hydrogen bonds
• Every base pair consists of one purine
and one pyrimidine

4. Review of DNA Structure
Two Copies of the
same sequence, just
reverse compliments

5. Four Important Functions of DNA
1) Stores genetic information
2) Is susceptible to mutation
3) Is precisely replicated in cell
division
4) Is expressed as the phenotype

6. DNA Replication
Three possible replication
patterns:
• Semiconservative: Parent
serves as a template and new
molecules have one old and
one new strand
• Conservative:
Original helix only serves as a
template
• Dispersive:
Parent fragments serve as
templates, assembling old and
new parts into molecules

7. Semi-Conservative
Replication
Two steps in DNA replication:
• The double helix is unwound, making
two template strands
• New nucleotides are added to the new
strand at the 3′ end and joined by
phosphodiester linkages. Sequence is
determined by complementary base
pairing

8. Strands Grow from the 3’ End

9. How does Replication Initiate
A large protein complex called the
“replication complex” interacts with
the template strands.
All chromosomes have a region called
origin of replication (ori).
Proteins in the replication complex bind
to a DNA sequence in ori.

10. Main Components in DNA
Replication
Primase synthesizes RNA primers to start
replication
DNA polymerase (I and III) adds nucleotides to
the 3′ end.
DNA helicase uses energy from ATP hydrolysis
to unwind the DNA.
Single-strand binding proteins keep the
strands from getting back together.
DNA ligase “glues” together any gaps in the
newly synthesized sequence

11. So how does it work?
Helicase will unwind the DNA Strand
SS-Binding Proteins keep the strands apart
Primase hops on and lays down a 10-20bp RNA
primer
DNA polymerase III recognizes the primers and
continues to extend the growing strand by
reading the complimentary base pairs
DNA polymerase I hopes on and replaces RNA
primer with DNA and proofreads the new
sequence
DNA Ligase then glues together any gaps

12. DNA Polymerase
• Proof Reading
Ability
• Can NOT start
without a primer or
existing template
sequence

13. Not all DNA Strands are Equal
• Leading Strand
“Easy Replication”
• Lagging Strand is
slower and more
difficult
• Replication Fork

14. Leading Strand
• Primer is created on
the 3’ end of the
existing sequence
• Replication occurs
following the
Replication fork
until it falls off the
other end

15. Lagging Strand
• Primer is created near
the replication fork
• The strand is then
replicated moving away
from the Replication
fork
• Forms Okazaki
Fragments

16. After Replication…
• DNA pol I replaces all
RNA primers with DNA
• DNA ligase glues all
Okazaki Fragments
together
• DNA pol can check for
errors in sequencing
(proofreading)

17. PCR
Copies of DNA sequences can be made
by the polymerase chain reaction (PCR)
technique.
PCR is a cyclical process:
• DNA fragments are denatured by
heating
• Primers, plus dNTPs and DNA
polymerase are added
• New DNA strands are synthesized

18. PCR

19. PCR
PCR results in many copies of the DNA
fragment—referred to as amplifying the
sequence.
The base sequence at the 3′ end of the
DNA fragment must be known.
Complementary primers, about 15–30
bases long, are made in the laboratory.

20. PCR
An initial problem with PCR was its
temperature requirements.
The heat needed to denature the DNA
destroyed most DNA polymerases.
A DNA polymerase that does not
denature at high temperatures (90 °C)
was taken from a hot springs
bacterium, Thermus aquaticus.

21. Steps in a typical PCR Cycle
1) Denature
2) Annealing of the Primers
3) Elongation of new sequence DNA Pol
Leads to exponential amplification of
your target…