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DNA AND ITS STRUCTURE
DNA is located inside the
nucleus of eukaryotic cells
DNA is stored in chromosomes; a gene is a
segment of DNA which codes for proteins
A karyotype is a picture of all the chromosomes in
a cell. Humans have 22 pairs of chromosomes and
two sex chromosomes. This is an abnormal
karyotype. Can you find the mistake?
Female and Male Sex Chromosomes. Everyone needs an X
chromosome as it contains critical genes that make us human.
DNA
DNA
Heredity material
for life
Deoxyribonucleic
Acid
Double helix
Molecule
What has
DNA?
Bacteria
-People
-Animals
-Plants- yes, this
means the fruit
and vegetables
you eat contain
DNA
--
-Basically, if it is
alive, or once was
alive, it has DNA
DNA
• Your DNA contains a set of instructions for
building a human. It is responsible for all
our inherited characteristics and is
passed down to us from our parents
• It directs all the cell’s activities
• Instructions in DNA codes for proteins
(proteins are responsible for thousands of
chemical reactions that take place in our
bodies and also codes for our traits)
So, DNA is like a recipe or blue
print for making us human
A dog’s DNA has instructions different than
ours-it “builds” a dog
Did you know that your DNA is 99.9% like everyone
else’s in this room? So, just a tiny bit of difference in
DNA can make us look very different.
But your DNA is MORE than 99.9% like
your relative’s DNA
50%
Did you know that
of the genes in a banana are
the same as your genes? That’s right! You have lots in
common with this banana which is probably why he is
smiling and waving at you. 
The DNA in a flower
codes for their special
traits….making them
unique to other flowers
Just as we get our DNA
from our parents, plants
inherit their DNA from the
plants they came from.
• DNA shows us how living
organisms can pass down
information to their offspring
• For example, it tells us how a
child can be born with “his
mother’s eyes” or “his father’s
nose”
• DNA is compact, yet complex
enough to carry all the
information needed to guide
the development of an entire
organism
DNA USES
• Help to solve crimes (find the criminal)
• Helps to identify victims of a crime or
accident
• Has been used to identify genetic illnesses
• Used to determine blood/biological
relatives (such as in paternity tests)
• Used in cloning, gene therapy and genetic
engineering
Where Can You Get A Sample
of DNA From People?
• Skin cells
• White blood cells (white blood cells only,
not red blood cells because they do not
contain a nucleus)
• Bones
• Semen
• Saliva
• Hair
Do Identical Twins Have the Same Fingerprints?
• The same DNA- yes!
• The same fingerprints? No!
• True, fingerprints are a phenotype (physical
characteristic) that are determined by the
interaction of genes, but also the development of
fetus in uterus has an influence here
• The shape of fingerprints is mostly influenced
by environmental factors during pregnancy, like
nutrition, blood pressure, position in the womb,
and the growth rate of fingers in the first
trimester. SO TWINS WILL HAVE similarities in
fingerprints, but differences, too!
Exciting DNA!
• What makes DNA so exciting is that it shows how
living organisms store information in biological
molecules
• The structure of DNA is nicely suited for this task
• The structural backbone creates a simple,
consistent chain upon which many, many bases can
be laid out in an orderly sequence
Double Helix Molecule-DNA
James Watson
and Francis
Crick
In 1953, these
two scientists
made history
by discovering
the molecular
structure of
DNA.
What is this structure? DNA is a double helix
nucleotide made up of four bases, a phosphate,
and the sugar, deoxyribose
The Nitrogen
(nucleotides)Bases
• These bases make up the “rungs” of the
ladder:
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
DNA is made up of a long sequence of
these units strung together.
So the genetic code is written in a series of A’s, T’s, G’s,
and C’s. It’s a “secret” code that scientists are trying to
crack- to figure out just how it is that DNA makes us all
unique .
• The capital letters represent the bases:
Adenine= A
Thymine= T
Guanine= G
Cytosine= C
Pairing of the
Nucleotide
Bases
Adenine
always pairs
with thymine
Guanine
always pairs
with cytosine
Each base has a
particular shape
that causes this
base pairing
The back bone of DNA
• Made up of alternating molecules of
phosphate and a sugar called deoxyribose
• Base pairs are complementary to each
other because each base only bonds with
the other base pair
• CCGA will bond to the sequence
__________
• GGCT
• AATTCGAT will have a complementary
strand of:
• AATTCGAT
TTAAGCTA
Before a cell divides, the DNA in
every chromosome is copied.
• This is called DNA replication
How Does DNA Replicate?
First, in replication, a DNA molecule is split
down the middle by the enzyme, DNA helicase
The bases on each side of the molecule are
used as a pattern for a new strand
• As bases on the original
molecule are exposed,
complementary
nucleotide bases (from
the nucleus) are added.
DNA polymerase brings
them together then
proofreads it for
mistakes
• DNA ligase is an enzyme that seals in the
sugar and phosphate backbone.
So half of the new molecule is
old DNA and half is new DNA
What are Mutations?
• It is a change in the nucleotide- base
sequence of DNA, in other words: a
mistake with the pairing of the A,T,C,
and G’s
• It can be caused by random errors in the
copying of DNA
• Or by things in the environment
Do Mutations Matter?
• Changes in DNA can cause an improved
trait, no change, or a harmful trait
• If the mutation does not change the
protein that the gene makes then there will
be no change to the trait
Can you think of a mutation that could
benefit an organism?
When a nucleotide is deleted or inserted, the result
is a polypeptide that codes for the wrong sequence
of amino acids
• Consider this for ex:
THE CAT ATE THE RAT. If the letter C is deleted
from this sentence, then it would read:
THE ATA TET HER AT- something that doesn’t
make sense!
Mutations can lead to proteins that function
poorly or not at all- this may or may not be of
concern, it depends on what protein it is coding
for. If there is a mutation in the hemoglobin of
red blood cells that affects its shape, this could
cause sickle cells that lead to blood clots (sickle
cell anemia).
Errors in DNA replication are rare, because DNA
polymerase proof reads the new and old strand,
but still they sometime do occur
Where the Mutation Occurs ..
Mutations is a person’s skin cell won’t
get passed down to an offspring, but
mutations in a sperm or egg (gamete)
can get passed down to one’s offspring
and cause a genetic condition.
Known Harmful Mutagens (a mutagen is
something from the environment that can
cause a mutation in DNA)
Some mutations in DNA lead to
• Diseases such as cancer (UV light can
case changes in skin cell’s DNA)
• Physical signs in aging
A mutagen might have caused
this frog to develop six legs
Note Check
Is a mutation always harmful?
No they are not always harmful- sometimes they
are beneficial, other times they cause no change in
the organism
*Does a mutation always produce a change in the
individual? Explain.
No, only if it changes a protein that a gene codes for
will there be a change in a trait
• But there is so much information that needs to
be stored about us. How is DNA able to provide
enough information for a living organism?
• The simple answer is that DNA molecules are
very, very long
• Did you know a simple bacteria, E. Coli, is 4
million nucleotides sequences long! This
corresponds to the information storage capacity
of an 8 MB hard drive! Lots of information for a
tiny bacteria!
The human genome is
approximately 3
billion nucleotides long!
The genome is all the DNA in
our cell.
The information
storage capacity of a 3GB
hard drive on your computer!
We can think of DNA as a
“genetic
database” for living organisms
. We have somewhere
between 25,000 and 30,000
genes.
Genes can be isolated and studied, and used to try and treat
diseases/conditions.
What is the human genome project?
• A project designed by the U.S. department of
Energy and National Institute of Health to:
• Their goal is to identify all the approximately
30,000 genes in human DNA
• To determine the sequence of the 3 billion
chemical base pairs (nucleotides)
• Store the information in databases
• improve tools for data analysis
• Transfer data to private sector
• Address ethical issues stemming from the
project’s findings
In other words, scientists want to locate every gene in
our genome (all our DNA). Then this information will be
mapped out and stored in a database or book- kind of
like an encyclopedia !
A cloning cartoon
DNA Extracted From
Strawberries
Uses of Protein in the Body
• Proteins play a role in the anatomy and physiology of
cells
• The plasma membrane contains protein that carry out
various functions
• Many proteins are enzymes that are responsible for
thousands of chemical reactions in our bodies
• Some proteins are hormones that regulate the body’s
functions
• Antibodies in blood and hemoglobin are proteins…..
• Genotypes and phenotypes are determined by proteins
Before we talk about protein synthesis let’s talk about
another important nucleic acid in the body..RNA
• DNA
• Double stranded
molecule
• Contains the sugar
deoxyribose
• Contains the base
pairs: A,T,G,C
• RNA
• Single stranded
molecule
• Contains the sugar
ribose
• Contains the base
pairs: A,U,G,C
Types of RNA
• There are three types
of RNA:
1.Messenger RNA
(mRNA)
2. Ribosomal RNA
(rRNA)
3.Transfer RNA
(tRNA)
Protein Synthesis
• Protein synthesis takes place in the ribosomes
• A gene is a segment of DNA that specifies the
amino acid sequence of a protein
• DNA stores this information in a nucleotide code
• A difference in base sequences can determine
whether you have blue eyes or long fingers and
more!
• A gene does not directly control protein
synthesis, however, it passes its genetic
information to RNA which is more directly
involved.
Gene expression requires two
steps
Transcription
Genetic information is
passed from DNA to RNA
One type of polynucleotide
(DNA) is transcribed letter by
letter into another type of
polynucleotide (RNA)
This takes place in the
nucleus
Translation
Here the RNA transcript
directs the sequence of
the amino acids in the
newly formed protein
This step takes place in
the cytoplasm and in the
ribosomes
Step 1 Transcription
• Transcription begins when
the enzyme RNA polymerase
binds tightly to a promoter, a
region of DNA that contains a
special sequence of
nucleotides.
• This enzyme opens up the
DNA helix so that
complementary base pairing
can occur
• Then RNA polymerase joins
the RNA nucleotides and an
mRNA molecule results
The nucleotide code comes in a triplet code, in
order to supply 64 different triplets, far more than
needed to code for the 20 known amino acids!
A codon is a group of three base pairs that code
for a specific amino acid on an mRNA
molecule…What two amino acids are coded for
here?
Anticodons
• Anticodons are a group of
three bases that are
complementary to a
specific codon of mRNA
• There is at least one
tRNA for each of the 20
amino acids found in
proteins
• tRNA bring amino acids
to the ribosomes and pair
them up accordingly with
the codon on mRNA
Use chart on P.492 worksheet
Codon (mRNA) Anticodon (tRNA) Amino Acid
CAA
GUU
glutamine
Translation takes place in the
cytoplasm and new proteins are
made in the ribosomes
Translation requires 3 Steps
• Chain initiation
• Chain elogation
• Chain termination
A small ribosomal subunit binds to mRNA; an initiator tRNA
with the anticodon UAC pairs with the codon AUG
A tRNA amino acid approaches the second binding site of
the ribosome. Peptide bond formation attaches the
polypeptide chain to the newly arrived amino acid
The ribosome comes to a stop codon on the mRNA. A
release factor binds to the site. The release factor
hydrolyzes the bond between the last tRNA at the Psite
and the polypeptide, releasing them. The ribosomal units
dissociate.
Transcription and Translation
What are some practical benefits to
learning about DNA?
• Knowledge of the variations of DNA among
individuals can help diagnose, treat and
someday prevent thousands of disorders that
affect us
• Useful in crime solving cases and paternity
suits
• Help us understand the natural capabilities of
our non- human friends- that can be applied in
solving challenges in agriculture, health care,
energy production, etc
Questions for guess who?
• That’s right…for you!
Questions
1.Which two scientists are well known for
discovering the structure of the DNA
molecule?
Francis Crick and James Watson
3.The nitrogen base pairing pattern is key
to understanding how______occurs.
DNA replication
• DNA replication ensures that each
daughter cell will have____________.
All of the genetic information it needs to
carry out its activities
4.DNA replication begins when
DNA___________unzips the two sides of
the DNA molecule causing it to unwind
and separate, like a zipper unzipping in a
jacket.
6.The molecule separates between
the__________on each rung.
paired nitrogen bases
Place the steps of DNA replication
in order
1. The nitrogen bases that are floating in
the nucleus pair up with the bases on each
half of the DNA molecule
2.The two sides of the ladder unwind and
separate, like a zipper unzipping
3.The new bases are attached
4.The order of the bases in each new DNA
molecule will match the order of the old
DNA
• 2, 1, 3, 4
Warm-up
If this was a sequence of nitrogen bases in
a DNA strand, what would its matching
strand look like?
A T G C G A T C A A C
T A C G C T A GT T G
Warm Up
1. List three facts about DNA (book pages
57-59).
• DNA is shaped like a twisted ladder. This
shape is known as a double helix.
• The two sides of the DNA ladder are made
up of molecules of sugar, called
deoxyribose, alternating with molecules
called phosphates
• The rungs of the ladder are made of
nitrogen bases