DNA- Deoxyribonucleic acid
A large polymer used to carry the genetic
code of all living organisms
DNA – Heredity & Structure
What we know about DNA was not
discovered overnight! Many different
scientists contributed information. Because
of the efforts of all these scientists, we now
have a model of DNA that consistently fits
the observations we make. It also allows us
to make useful predictions!
Oswald Avery (1944)
genes are composed of DNA
studied the DNA molecule using a technique called X-ray
HERSHY – CHASE – DNA & Viruses
James Watson/Francis Crick (1953)
Developed the double helix model of DNA structure
Griffith‘s Experiment- 1928
Was trying to develop a vaccination for the
Vaccine- a prepared substance from killed or
weakened disease causing agents used to prevent
He was working with two strains of bacteria.
Rough - bacteria had a rough appearance in
culture, non-virulent (doesn't kill)
Smooth - bacteria had a smooth appearance in
culture, virulent (kills)
He discovered that something was being
transferred between the ―dead‖ smooth bacteria
and the living rough bacteria that caused them
to undergo transformation.
Avery, MacLeod, McCarthy identified that DNA was
being transferred killing the rats in 1944
Avery, MacLeod and McCarthy- 1944
1. Avery, MacLeod, McCarthy (1944)- proved that
the transfer of DNA is what killed Griffith‘s rats
1. took extract (from heated smooth bacteria) and
treated it with DNAase (destroys DNA) - then mixed
with rough bacteria and injected into rats -> the rats
2. in other side of experiment, treated extract with
protease (digests proteins) -then mixed with rough
bacteria and injected into rats -> rat died
This showed that DNA, not protein, has ability to transform cells
Dies of pneumonia
Heat-killed, diseasecausing bacteria
Dies of pneumonia
bacteria (smooth colonies)
Erwin Chargaff- 1950
Base pairing rule is A-T and
Thymine is replaced by Uracil
Bases are bonded to each
other by Hydrogen bonds
Discovered because of the
relative percent of each base;
(notice that A-T is similar and
C-G are similar)
Source of DNA
Hershey and Chase- 1952
Hershey and Chase proved that the genetic material
is DNA in 1952. Previously, scientists thought that
proteins were the hereditary molecule
Hershey and Chase used radioactively labeled
bacteriophages (viruses) to determine that DNA was
being injected by the viruses instead of proteins.
This proved that DNA was the hereditary material of
Martha Chase (left) & Alfred Hershey (right)
DNA is located
in the head.
The outside and tail
of the virus is
made out of
Hershey – Chase Experiment –
DNA in Viruses
sulfur-35 in protein
No radioactivity inside
Wilkins & Franklin- 1952
MHF Wilkins and Rosalind Franklin studied
the structure of DNA crystals using X-rays.
They found that the crystals contain
regularly repeating subunits.
The pattern generated by the diffraction of
the x-rays suggested that the overall
structure of DNA was a double helix.
Watson & Crick- 1953
James Watson and
Francis Crick used
Chargaff's base data and
Franklin‘s X-ray diffraction
data to construct a model
Their model showed that
DNA is a double helix with
backbones on the outside
and the paired nucleotide
bases on the inside, in a
structure that fit the
spacing estimates from
the X-ray diffraction data.
The paired bases can
occur in any order, giving
an overwhelming diversity
Watson & Crick with their model of DNA
There are 3 main components
of a strand of DNA
1. DNA is a large polymer (macromolecule)
Made up of monomers called nucleotides.
A nucleotide is made of
phosphate functional group
Nitrogen base (A, T, G, C)
Deoxyribose sugar (in DNA)
Dna twists into a double helix
due to the attraction between the
negatively charged phosphates
and net positive charge of the
hydrogen bonds between the
- DNA has an overall negative charge
due to the phosphates of the
• ―Rails‖ of the ladder are made
of alternating sugar and
The nitrogen base (A, T, G, C)
are always attached to the
Purines (two rings)
Pyrimidines (one ring)
Adenine and Guanine
Thymine and Cytosine
Weak HYDROGEN bonds form
between the Nitrogen Base Pairs.
The backbone of it all…
The backbone is made of alternating
sugars and phosphates.
- Remember: Sugar ALWAYS
attaches to the Nitrogen base
DNA & RNA continued!
1. Before mitosis (during S phase of
interphase) , a complete copy of a cell‘s
DNA is made through a process called
2. When a cell divides, each daughter cell
gets one complete copy of the DNA.
Similar to photocopying a document – the
end result is two identical documents that
contain the same information.
1) DNA must unwind and break the hydrogen
bonds. DNA Helicase ―unzips‖ the strands.
DNA Helicase- the enzyme that unzips DNA like a
zipper so it can be copied. The area where the DNA
is split is called the replication fork.
2. Each strand of original DNA is used as a
template (blueprint). DNA Primase ―flags‖ or
―marks‖ the spot for it to begin
DNA Primase- Begins DNA replication by attaching a
short fragment of RNA called a primer to the place
where replication will begin. This primer ―tells‖ DNA
polymerase where to start copying
Created by DNA Primase
1. DNA Polymerase- makes the new strand of DNA like a
copying machine. It can only read in one direction 3‘ to 5‘ just
like how we read a page from left to right. As a result, the new
strand it makes is made in the 5‘ to 3‘ direction.
Leading strand- the continuous strand that DNA polymerase makes in the
5‘3‘ direction. It never stops once it starts until it reads the entire strand of
DNA. DNA replication 5' to 3‗
Lagging strand- DNA polymerase can only read in the 3‘ to 5‘ direction, it
must make the new strand in small chunks. These small chunks are called
Okazaki fragments. They
are normally between
Direction of replication
100-200 base pairs long.
2. DNA Ligase- connects
okazaki fragments together on
the lagging strand to make a
1. Because of Chargaff‘s rule, only the
correct, complementary bases will fit, so
chances are good that the DNA
polymerase will make a perfect copy.
2. What would happen if DNA polymerase
made a mistake?
How long do you think these animals will
DNA has genes that code for the synthesis
(creation) of specific PROTEINS
Here‘s the problem…
Where is DNA located?
Where does Protein Synthesis occur?
At ribosomes in the cytoplasm
Can DNA ever leave the nucleus?
Sugar is ribose
Thymine is replaced by URACIL
3 types of RNA
1) Messenger RNA (mRNA)
carries information from DNA to ribosome
2) Transfer RNA (tRNA)
Carries amino acids
3) Ribosomal RNA (rRNA)
Makes up ribosomes
which functions to
which functions to
to make up
amino acids to
Differences between DNA and RNA
Transcription1. Transcription- creating a strand of mRNA from
an original strand of DNA
occurs in the nucleus!!!
Steps of transcription
1. Just as DNA polymerase copies DNA, a similar enzyme
called RNA polymerase makes new RNA from the DNA
2. RNA polymerase temporarily separates the strands of a
small section of the DNA molecule which exposes some
of the bases of the DNA molecule.
3. Along one strand of the DNA, the RNA polymerase
binds complementary RNA nucleotides to the exposed
DNA bases and makes a strand of mRNA.
It is called messenger RNA because it carries DNA‘s message
out of the nucleus and into the cytoplasm.
mRNA is SINGLE STRANDED!
5. When the RNA polymerase is done
reading the gene in the DNA, it seperates
from the DNA
6. The separated DNA strands
reconnect, ready to be read again when
7. mRNA moves out of the nucleus and finds
Translation- (also known as protein synthesis) making
a protein from the instructions found on mRNA.
These instructions are originally found in genes.
A gene is a region of DNA that contains the
instructions for making proteins. This is why we refer
to DNA as the ―blueprints‖
Where does this happen?
Where is the DNA
Where are proteins
made in the cell?
Genetic code – the language of mRNA
Read in three bases (codon) at a time by a ribosome
Codon found on mRNA; consists of three
bases (one right after the other)
There are 64 different codons that code for 20 amino acids
Each codon ―codes‖ for a specific amino acid
• Ex: Consider the following RNA sequence:
• The sequence would be read three base pairs at a time:
UCG – CAC – GGU
• The codons represent the amino acids:
Serine – Histidine - Glycine
Special codons- Start and Stop
AUG – start codon which codes for the
amino acid Methionine. All protein chains
begin with this
UAA, UAG, UGA – These three codons
are ―stop‖ codons. When a ribosome
reaches these codons it tells the ribosome
to end the protein chain.
Ribosomes- the protein ―factory‖
1. Ribosomes are organelles in the cell designed
to make proteins by reading mRNA made
2. Ribosomes are found in two main locations in a
Freely floating in the cytoplasm
3. Ribosomes are made of rRNA
4. Ribosomes have two main parts or ―subunits‖
that attach to mRNA to ―read‖ it.
5. A ribosome can fit two ―codons‖ inside of it at a
tRNA (transfer RNA)
tRNA carries (or transfers) the
correct amino acid to the
codon on the mRNA.
One end of the tRNA has an
ANTICODON that is paired
with the codon on the mRNA
There are 1000‘s of tRNA‘s
floating around in the
cytoplasm to be used for
Step 1 of Translation (protein synthesis)
1. mRNA is made during transcription. It then leaves the
nucleus and combines with a ribosome. The ribosome
then reads the mRNA to make a protein
GUA UCU GUU ACC GUA
•Codon: a sequence of 3 nitrogen bases on
mRNA that code for 1 amino acid
•It‘s a TRIPLET code
•Example: This strand of mRNA has 5
codons, so it would code for 5 amino acids.
Translation (don‘t copy)
GUA UCU GUU ACC GUA
•The mRNA molecule travels to the
ribosomes where the mRNA codes are
―read‖ by the ribosomes
•Ribosomes hold the mRNA so another type
of RNA, transfer RNA (tRNA) can attach to
Step 2 of translation
GUA UCU GUU ACC GUA
CA U A G A
1. As the ribosome reads down the
mRNA strand, it will pair each
mRNA codon with the correct
2. Remember, only 2 tRNA‘s can fit
in a ribosome at a time
3. After it has been paired, a
covalent bond will form between
the amino acids creating a chain
of amino acids also known as a
GUA UCU GUU ACC GUA
CA U A G A CA A
Translation- step 3
The ribosome will read through the entire strand of mRNA making a protein in the
process until it reaches a ―stop‖ codon. Once it reaches a stop codon, the
ribosome releases the mRNA and the protein is completed.
Protein Synthesis Video
As the ribosome reads the mRNA strand, amino
acids linked together to form a protein. The new
protein could become cell part, an enzyme, a
Protein synthesis in prokaryotes
Prokaryote vs eukaryote protein synthesis
Prokaryotes lack a nucleus. While RNA
polymerase begins making the strand of mRNA
from the template DNA, the ribosome floating
around in the cytoplasm can simultaneously
read the mRNA strand that‘s being made and
translate it into a protein
In Eukaryotes, the mRNA strand must first
exit the nucleus through a nuclear pore
before it can be translated into a protein
Point Mutations- Substitutions
Point mutations – mutations involving changes in one
or a few nucleotides in a DNA sequence. Point mutations
come from a substitution in the copied DNA strand
Substitutions – one base is changed to another
3 types of point mutations:
Silent mutation- No change in the protein
Missense mutation- changes one amino acid (missense)
Sickle-cell anemia is caused by this
Nonsense mutation- Inserts a pre-mature STOP codon
A frameshift mutation occurs when the “reading” frame of
the ribosome is changed.
How frameshift mutations can affect the protein:
This may change every amino acid that follows the point of the
Can alter a protein so much that it cannot perform its function.
Frameshift mutations can come from 2 different
changes to the DNA sequence
Insertion – a extra base is inserted into the original strand of DNA
Deletion – a base is removed from the original strand of DNA
Frameshift due to insertion
Frameshift due to deletion
Guess the mutation…
Significance of mutations
Mutations can be neutral, beneficial, or harmful
Generally have little or no effect on an organism.
May produce proteins with new or altered activities
Useful to organisms in different or changing environments
Plant an animal breeders take advantage of these
Polyploidy often results in larger, stronger plants.
Bananas and other citrus fruits have been made polyploid.
Can cause dramatic change in protein structure or gene
Defective proteins can disrupt normal biological activities
May result in genetic disorders
Fruit fly face
Mutations & Inheritance
Mutations in somatic (body) cells affect only that
organism, but the effects can be dramatic.
Harmful mutations cause many forms of cancer.
Mutations in gametes (sperm & egg) are passed along
These mutations become the basis for new genetic
variation within a species, which is important to
Mutations can also occur when a
chromosome is changed. A chromosomal
mutation is a change in the number or
structure (genes) of chromosomes.
4 main types of chromosomal mutations:
Part 3- Genetic Techniques
What is Genetic Engineering?
• Genetic Engineering- Making
changes in the DNA code of living
organisms in an effort to achieve a
more desirable trait
Techniques in Genetic
Removal of DNA from a cell
Small sections are cut from the DNA using Restriction
DNA is separated in a technique called Gel Electrophoresis
(separates according to size)
CSI- crime scene investigation- DNA is often used to link
criminals to crime scenes by matching DNA fingerprints of a
suspect with DNA found at the crime scene.
Many copies of DNA can be made in a technique known as
Polymerase Chain Reaction (PCR)
13-6 Gel Electrophoresis
DNA plus restriction
Mixture of DNA
EcoRI cuts the DNA
DNA from different species that is cut and
recombined; usually human DNA is cut
and combined with bacterial DNA
Applications of Genetic Engineering
Transgenic – organisms that contain
genes from other species (recombinant
Easy to grow
Produces a host of important useful
substances such as human forms of
proteins such as insulin, growth hormone,
and clotting factor
Figure 13-9 Making Recombinant DNA
Gene for human
Gene for human
Bacterial cell for
containing gene for
human growth hormone
Used to study genes and improve food
Mice have been produced with human
genes that make immune system act
similar to human
Many contain genes that produce natural
Others resist weed-killing chemicals
Eventually produce human antibodies
Member of a population of genetically
identical cells produced from a single cell
Cloned sheep – DOLLY
Ethical and moral issues
Figure 13-13 Cloning of the First Mammal
A donor cell is taken
from a sheep‘s udder.
These two cells are fused
using an electric shock.
The nucleus of the
egg cell is
An egg cell is taken
from an adult
The fused cell
normally into a
The embryo is
placed in the uterus
of a foster mother.