DNA
Ch 8
Standard 4
History of DNA
1. Hershey & Chase  proved DNA is the
genetic material found in cells
Erwin Chargaff
DNA composition


all 4 bases not in equal quantity
bases present in characteristic ratio



A = T ~ 40%
G = C ~ 60%
varies from species to species
Rosalind Franklin  took x-ray photos of
DNA that showed it’s 3-D structure
Watson & Crick
made the first 3-D model of DNA in 1953
Won the Nobel Prize in 1962
Why is DNA important?
It stores genetic information in cells
& this information is used to make
proteins
DNA Structure
DNA is a double helix (twisted ladder)
DNA is
made of
nucleotides
3 nucleotide parts
1. Deoxyribose sugar
2. Phosphate
3. Nitrogen base
- adenine (A)
- thymine (T)
- guanine (G)
- cytosine (C)
Base Pair Rules:
-
-
Adenine binds only with
Thymine (AT)
Cytosine binds only with
Guanine (CG)
----Straight letters together
Curvy letters together-------
• Deoxyribose Sugars &
phosphates make up the sides
of the ladder

• Phosphodiester bonds hold the
sugars to the phosphates
• Nitrogen bases are held
together by hydrogen bonds
A  T = 2 hydrogen bonds
G C = 3 hydrogen bonds
• The Base pairs make up the
steps (rungs) of the DNA Ladder
Base Pair Shape
Purine
Double ring bases
(Adenine or Guanine)
Pyrimadine
Single ring bases
(Thymine or Cytosine)
Difference in their shapes causes the DNA to twist
Each Base Pair is made of one Purine and one Pyrimadine
Hydrogen Bonds
H
Thymine
H
H
O
C
Thymine
H
Adenine
H
H
C
C
C
N
C
C
N
H
C
N
N
C
O
Adenine
C
N
C
H
N
N
H
H
Cytosine
Cytosine
H
N
H
C
Guanine
H
C
N
C
N
C
C
C
H
C
N
C
H
Guanine
C
N
O
H
N
O
N
H
N
Practice
If one side of DNA has this base
sequence, what does the other side
have?
ATGGACTAC
TACCTGATG
DNA Replication
How Does DNA Replicate
Process discovered by Meselson & Stahl
Semi-conservative replication

Half old strand, Half new strand
Anti-parallel

New strands are made in opposite directions
When does DNA replicate?

During the cell cycle before mitosis & meiosis
Why does DNA replicate?
So new cells
will have their
own copy of DNA
Six Steps of DNA Replication
1)
2)
3)
DNA is unzipped by the Helicase enzyme
(breaks hydrogen bond)
DNA polymerase enzyme binds DNA
begins attaching complimentary nucleotides
Same thing happens on the other side, but in
opposite direction
4) Bonds Reform
Helicase reforms Hydrogen bonds
(between bases)
Ligase reforms Phosphodiester
bonds (sugar & phosphate)
5) DNA polymerase “proofreads”
checking each strand for errors.
6) DNA rewinds itself into two
identical double helix strands
Let’s watch
http://www.lpscience.fatcow.com/
jwanamaker/animations/DNA%2
0Lecture.html
Once more time…..
Protein Synthesis
DNA RNA Proteins
DNA
1.sugar=deoxyribose
2.bases = A, C, G, T
3.double strand
4.stays in nucleus
RNA
1.sugar = ribose
2.Bases=A, C, G,U
3.single strand
4.leaves nucleus
3 Types of RNA
1.
mRNA  made from DNA in nucleus
“messenger”
2.
tRNA  brings amino acids to the
ribosomes; found in cytoplasm
“transfer”
3. rRNA  part of the ribosome;
this is where proteins are made
“ribosomal”
DNA vs. RNA
TRANSCRIPTION
How RNA is made from DNA
Transcription Steps
1.
DNA is unzipped by Helicase

2.
3.
4.
all genes start with the code “TAC”
RNA polymerase adds RNA
nucleotides to the DNA strand
mRNA is complete when it reaches a
stop code on DNA
mRNA then leaves nucleus & carries
code into the cytoplasm

DNA never leaves the nucleus
Don’t Confuse
Replication with Transcription!
DNA Replication
Transcription
A–T
C–G
G–C
T-A
A–U
C–G
G–C
T-A
Transcribe this……
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Translation
Decoding the Message
Steps
1.
2.
3.
4.
5.
mRNA leave nucleus and enters ribosome
mRNA codons read & tRNA brings matching
amino acid to the ribosome
Amino acids are strung together like beads on
a necklace
Amino Acids are held together by peptide
bonds
1000 or more Amino Acids = protein
WHEN WE DECODE DNA


Use the “Genetic Code”

Convert mRNA 3 letter
groupings called codons
Example:
AUG= Methylamine (Start)

The mRNA Code tells us
what amino acid each
codon codes for.
Transcribe and Translate this
DNA Strand
without looking at your notes
TACAGTACCATAATC
NOW, Label the DNA, mRNA, tRNA, rRNA
Which part is transcription
Which is translation ?
TACAGTACCATAATC
mRNA AUGUCAUGGUAUUAG
tRNA UACAGUACCAUAAUC
DNA
rRNA
MET-SER-TRP-TYR-STOP
DNA
RNA
Transcription
A, T, C, G
Double
Strand
A, U, C, G
Single Strand
In The Nucleus
Protein
Translation
Amino Acids
Codons &
Anti-codons
In Cytoplasm
Is it all DNA?

Entire chromosome is made of DNA
Humans~ 3 billion base pairs

Only part of the DNA codes for Genes
Gene: The DNA that codes for a
Protein.
There are different parts of the
chromosome
 Exons and Introns

Introns are cut out before translation
 Exons are left, and get Translated to
make Proteins

GENETIC VARIATION
 There are some variations in genes
 ~every 1350th bp (average)
 Known as polymorphisms “different
forms”
 Different forms of genes are called
alleles
 Effects
 How well the protein works. EX: Melanin
 How the protein interacts with another
protein or substrate. EX: Enzymes
Proteins do the work of the cell
 Proteins
are chains of amino
acids.
 The amino acids are coded
by the bases that make up
that gene
 Each gene codes for a
different protein.
Gene Expression
 All
cells within an organism have the
same DNA and genes.
 What makes cells different from each
other is that different genes are turned
on and turned off in different cells.
Ex: Pigment in eyes or skin
Ex: Keratin in nails or hair
How can organisms be different
from each other if their DNA is
made of the same nucleotides?
•Two individuals DNA are different because of the
order of nitrogen bases
•The more closely related two organisms are, the
more alike the order of nucleotides in their DNA will
be.