DNA Structure and
Replication
Mrs. Gamari
Oswald and Avery cont’d
Oswald and Avery
Experimental Evidence
Nucleic Acids

Compounds that contain phosphate and
nitrogen in addition to carbon, hydrogen,
and oxygen.

Examples of nucleic acids:
 DNA
 RNA
DNA’s shape is often referred to as
a “double helix”

1.8 meters of DNA must fit into the nucleus of
human cells.

Since cells are so small, and the nucleus even
moreso, the DNA must curl up and coil to fit into
such a small space.

Think about curly hair vs. straight hair… when you pull at the
piece of hair, it becomes straight and longer. When
released, the piece of hair appears to get shorter when it
curls up, taking up less space.
Structure of DNA

Long chain of repeating units called nucleotides.

A nucleotide has 3 main parts:
 5-carbon

sugar
Deoxyribose (DNA) or ribose sugar (RNA)
 Phosphate
group
 Nitrogenouse base

A, C, G, T/U
A Nucleotide
Science Aid: DNA Structure and Replication
scienceaid.co.uk/biology/genetics2/dna.html
DNA

DNA is double-stranded and its shape
resembles a ladder.

The sides of the ladder are made up of
repeating sugar and phosphate groups.

The rungs of the ladder are made of 2 bases
bonded together by hydrogen bonds.
The DNA molecule needs to coil and foldup so that it can fit into the small nucleus.
The Double
Helix
The Ladder
Latest from the Labs: Cells and DNA
info.cancerresearchuk.org/.../cellsanddna/
Base Pairing Rules

DNA has 4 different nitrogenous bases: adenine,
guanine, cytosine, and thymine.

The bases always pair the same way in a DNA
molecule…unless a mutation occurs.

Adenine always pairs with Thymine (2 H bonds)


A-T
Cytosine always pairs with Guanine. (3 H bonds)

C-G
A DNA Sequence
DNA is really a
pattern of
repeating
nucleotides.
Science Aid: DNA Structure and Replication
scienceaid.co.uk/biology/genetics2/dna.html
http://68.90.81.6/ScienceTAKS/Integration/DNA%20&%20RNA.htm
Purines and Pyrimidines
Guardian Angels are Pure, with Two Wings
Orientation of Molecule

Antiparallel – Read strand 5’ to 3’. The
other strand is oriented in the opposite
direction with the 3’ end across from the 5’
end.
Concepts in Motion
Try it out!

Below is one strand of a DNA molecule
 GATTACA

Determine the “complementary” strand of
that same DNA molecule
Practice – use your “Nucleotide Kits” to build
DNA molecules with complementary strands
to the sequences below.

GTATAGCCAGAG

CTGTAGAGCTAGCAT

CGCCTAACTAGTTAGG
CHECK FOR UNDERSTANDING:

Name the 3 parts of a nucleotide.

Describe the structure of DNA using the
ladder analogy.
Adenine pairs with __________
 Guanine pairs with __________

DNA Replication

What does the term “replication” mean?

Why would the cell need to replicate its
DNA?

Where does DNA replication take place?
Semiconservative Replication

Parental strands
separate and serve
as templates for new
strands of DNA.

(Two new molecules
of DNA are identical
and each one has a
new strand and an old
strand)
Steps of Replication

DNA molecule unwinds with the help of
enzyme.

Enzymes add complementary bases to
build the new strands of DNA.

Enzymes seal the two strands (one old
and one new) together.
Replication Process
DNA Replication Animation
Prokaryote and Eukaryote
Replication

Replication Fork:
location where DNA
molecule is untwisted
and the strands
separate from one
another.

How do prokaryote and
eukaryote replication
differ from one
DNA Replication
Honors Biology
Mrs. Gamari
DNA Replication

What does the term “replication” mean?

Why would the cell need to replicate its
DNA?

Where does DNA replication take place?
Semiconservative Replication

Parental strands
separate and serve
as templates for new
strands of DNA.

(Two new molecules
of DNA are identical
and each one has a
new strand and an old
strand)
Steps of Replication

DNA molecule unwinds with the help of
enzyme.

Enzymes add complementary bases to
build the new strands of DNA.

Enzymes seal the two strands (one old
and one new) together.
Unwinding

DNA helicase – unwinds double helix
(hydrogen bonds broken)

RNA primase adds a short segment of
RNA, called an RNA primer, on each DNA
strand
Base Pairing
DNA Polymerase adds the appropriate
nucleotides to the new DNA strand
according to the code of the template
strand.
 Nucleotides are added to the 3’ end of the
new DNA strand. Extended/Built 5’ to 3’..


Leading strand (5’-3’) – strand is
continuously built by the addition of
nucleotides to the 3’ end towards the
replication fork.

Lagging strand (3’-5’) – strand is
discontinuously built. Elongates away from
the replication fork. Small segments
(Okazaki fragments ) are synthesized along
template– could be 100 to 200 nucleotides
long

Semidiscontinuous replication?
Replication Process
DNA Replication Animation
Advanced DNA Replication Animation
Joining

DNA polymerase removes the RNA
primase.

DNA ligase links the segments/fragments
together.

2 DNA molecules wind themselves back
up into double helix
Prokaryote and Eukaryote
Replication

Replication Fork:
location where DNA
molecule is untwisted
and the strands
separate from one
another.
How do prokaryote and
eukaryote replication
differ from one
DNA Keychain Model
Label original DNA molecule with 5’ and 3’
ends. Label the leading/lagging strand.
 Mimic replication process by adding DNA
nucleotides continuously or
discontinuously according to the strand.
 Draw the correct number of hydrogen
bonds between nitrogenous bases.
 Label where covalent bonds are located.
 Draw key for your DNA model. Include
bonds.
