DNA
THE GENETIC MATERIAL OF LIFE
UNIT 3
What is DNA?
• Stands for: DeoxyriboNucleic Acid
• Molecule that stores genetic information
in all organisms
• DNA is stored in the nucleus of your cells
DNA stands for:
D: Deoxyribose
N: Nucleic
A: Acid
DNA is too
small to see,
but under a
microscope it
looks like a
twisted up
ladder!
Every living thing has DNA. That means
that you have something in common with a
zebra, a tree, a mushroom and a beetle!!!!
STRUCTURE OF DNA
DNA Structure
• DNA is a long chain made up of units called
nucleotides
• Each nucleotide has three parts:
– Phosphate group (one phosphorus/3 oxygens)
– Deoxyribose (ring-shaped sugar)
– Nitrogen-containing base
• There are only four types of nucleotides—
based on what type of nitrogen base they
have
Structure of Nucleotides
Phosphate group
Nitrogen Base:
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Deoxyribose
One molecule of DNA contains BILLIONS of
nucleotides, but there are only 4 types:
– Adenine (A)
– Guanine (G)
– Thymine (T)
– Cytosine (C)
The letter refers to the DNA bases as well
as the nucleotide
Base Pairing
• DNA is a double helix structure—this
means that there are two strands of DNA
that wind together like a twisted ladder
• The strands are bonded together at the
bases
The bases always pair up in the same way
based on base pairing rules:
Adenine bonds with Thymine
Adenine
Thymine
Cytosine bonds with Guanine
Cytosine
Guanine
Pairing up
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
THE DOUBLE HELIX
bases
sugarphosphate
chain
The structure of DNA consists of two strands wrapped
around each other in a double helix
So if one strand is…
rules?
A
T
C
G
A
T
A
G
C
T
A
-T
-A
-G
-C
-T
-A
-T
-C
-G
-A
-T
what is the other strand, based on base pairing
Try it out…
1. DNA structure worksheet!
2. See it DNA structure in action:
http://youtu.be/zwibgNGe4aY
3. DNA candy model!
DNA REPLICATION
DNA Replication
• Before a cell divides, it must make a copy
of its DNA
The steps of DNA Replication:
1. DNA is unwound
2. DNA polymerase adds nucleotides,
according to base pair rules
3. The strand is proofread as nucleotides are
added, and mistakes can be fixed if the
wrong base was added
 (A mistake is called a mutation)
4. DNA polymerase continues adding
nucleotides until a STOP signal is reached
DNA Replication
in action
Try it out!
1. DNA replication interactive:
http://www.pbs.org/wgbh/aso/tryit/dna/s
hockwave.html
Make sure to click on DNA Replication NOT
protein synthesis because we have not
learned about protein synthesis yet!
2. DNA Paperclip Replication Activity
TRANSCRIPTION
RNA (this molecule is used during transcription)
• RNA: RiboNucleic Acid
– Chain of nucleotides made of:
• Sugar
• Phosphate group
• Nitrogen-containing base
– It’s a temporary copy of DNA that is used
and then destroyed
DNA vs. RNA
Both have a sugar, phosphate, & base
DNA
• Its sugar is deoxyribose
• Bases are: Adenine,
Thymine, Guanine,
Cytosine
• Double strand of
nucleotides
RNA
• Its sugar is ribose (has
one extra oxygen)
• Bases are: Adenine,
Uracil, Guanine, Cytosine
(uracil pairs w/ adenine)
• Single strand of
nucleotides
Transcription
• Copying a sequence of DNA to make a
complementary strand of RNA
• This is catalyzed (allowed to happen) by
RNA polymerase
Basic steps of Transcription in Cells
1. DNA strands unwind
2. RNA polymerase copies one strand of
DNA and creates a complementary
strand of RNA nucleotides (known as
mRNA)
3. Once the gene has been copied, mRNA
detaches from DNA
4. mRNA moves out of the nucleus and
into the cytoplasm
Step 1
start site
transcription complex
nucleotides
Step 2
RNA copying
DNA
Step 3
RNA
Types of RNA
• Three types of RNA are formed during
transcription, they will be used in
translation:
– Messenger RNA (mRNA)
– Ribosomal RNA (rRNA)
– Transfer RNA (tRNA)
Watch transcription in action
• http://study.com/academy/lesson/transc
ription-of-messenger-rna-mrna-fromdna.html
TRANSLATION
Translation
• Process that converts (translates) mRNA
into a protein —called protein synthesis
• mRNA is a message that codes for a
protein when it attaches to a ribosome in
the cytoplasm of the cell
• Each combination of 3 nucleotides
(known as a codon) on the mRNA codes
for an amino acid —groups of amino
acids become proteins in the cell
Steps of Translation
1. mRNA moves out of nucleus and into
the cytoplasm
2. mRNA attaches to ribosome
3. Transfer RNA (tRNA) decodes the mRNA
into amino acids, which link together
into proteins
4. Protein (chain of amino acids) detaches
from ribosome and goes off to work in
the cell
Check it out…
• Go to the following website to see
translation (also called PROTEIN SYNTHESIS)
in action
• http://www.pbslearningmedia.org/resource
/tdc02.sci.life.gen.proteinsynth/from-dnato-protein/
• http://www.pbs.org/wgbh/aso/tryit/dna/sh
ockwave.html
• Make sure to click on Protein Synthesis!!
Genetic Code
• tRNA matches each codon in mRNA to three
nucleotide bases called anticodons on tRNA
• This anticodon has a specific amino acid
attached that will eventually form a chain of
amino acids to make a protein :
• There are 64 possible 3 letter combinations
BUT only 20 amino acids….
• SO, some Codons code for more than one
amino acid
Codon
Anticodon
Different codons code for different amino acids!!!
Transcription vs. Translation Review
Transcription
Translation
• Process by which
• Process by which
genetic information
information encoded in
encoded in DNA is
mRNA is used to
copied onto
assemble a protein at a
messenger RNA
ribosome
• Occurs in the nucleus • Occurs on a Ribosome
• DNA
mRNA
• mRNA
protein
MUTATIONS
Mutation
A Mutation is a change in an organism’s DNA
It’s a mistake made during replication or
transcription
Can be:
– Harmful: diseases, disorders or deformities
– Helpful: help the organism to survive
– Neutral: organism is unaffected
Passing on Mutations
• If a mutation occurs in a sperm or egg
cell, the mutation IS PASSED on to
offspring
• If a mutation occurs in a body cell, it only
affects the organism and IS NOT PASSED
on to offspring
Causes of Mutations
1. During DNA Replication:
• Base-pairing mistakes can happen but most are
fixed
2. Exposure to powerful Chemicals:
• Cigarette smoke, smokeless tobacco, exhaust from
burning petroleum fuels, pesticides, herbicides,
alcohol, sawdust from CCA lumber, paints, mineral
spirits & oils, asbestos, etc.
3. Exposure to powerful radiation:
• Gamma Rays (nuclear fuel & bombs)
• X rays (at hospitals & dental offices)
• UV rays (from sunlight and tanning beds)
Sickle Cell Anemia
What do you know?
• Mutations concept map
Point Mutations
• An alteration in DNA sequence caused by
a single nucleotide base change,
insertion, or deletion
Types of Mutations
Insertion-- changes the number of DNA bases in a gene by
adding a piece of DNA. The protein made by the gene
may not function properly.
Deletion-- changes the number of DNA bases by removing a
piece of DNA. This could be removing a few base pairs
within a gene or an entire gene/several neighboring
genes. This may alter the function of the resulting
protein(s).
Duplication-- consists of a piece of DNA that is mistakenly
copied one or more times. This may alter the function
of the resulting protein.
What happens when a protein is altered due
to a mutation?
• To function correctly, each cell depends on
thousands of proteins to do their jobs in the
right places at the right times.
• A mutation can cause the protein to
malfunction or to be missing entirely.
• An altered protein can disrupt normal
development or cause a medical condition.
Examples: Breast Cancer, Down’s syndrome,
Crohn’s disease, Hemophilia, Duchenne Muscular
Dystrophy