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Ch. 12 DNA and RNA
What kind of DNA do clones have?
Xeroxyribonucleic Acid
What kind of DNA do joggers have?
Reeboxyribonucleic Acid
Chapter 12 Outline

12-1: DNA
Griffith and Transformation
 Avery and DNA
 The Hershey-Chase Experiment
 The Components and Structure of DNA


12-2: Chromosomes and DNA Replication
DNA and Chromosomes
 DNA replication

Chapter 12 Outline

12-3 RNA and Protein Synthesis
The structure of RNA
 Types of RNA
 Transcription
 RNA Editing
 The Genetic Code
 Translation
 The Roles of RNA and DNA
 Genes and Proteins

Chapter 12 Outline

12-4: Mutations
Kinds of Mutations
 Significance of Mutations


12-5: Gene Regulation
Gene Regulation: An Example
 Eukaryotic Gene Regulation
 Development and Differentiation

Griffith and Transformation


The discovery of how the molecular structure of a
gene began in 1928. Frederick Griffith was trying
to figure out how bacteria make people sick.
Griffith’s Experiments:


Griffith injected mice with one of two strains of bacteria
(one harmless and one that caused pneumonia). The
harmful bacteria caused the mice to die.
Next, Griffith heated some of the disease-causing
bacteria and injected it, but the mice lived. This
confirmed that the bacteria did not produce a poison.

What happened?
Griffith’s Experiment (con’t)
Finally, Griffith injected some of the heated
disease-causing bacteria with some of the
harmless bacteria into the mice, and, to his
amazement, many of them died.
 Griffith named this process transformation
because the bad bacteria had transferred
their disease causing abilities to the
harmless bacteria.

Griffith’s Experiment
Heat-killed,
disease-causing
bacteria (smooth
colonies)
Disease-causing
bacteria (smooth
colonies)
Harmless bacteria Heat-killed, disease(rough colonies) causing bacteria
(smooth colonies)
Dies of pneumonia
Lives
Lives
Control
(no growth)
Harmless bacteria
(rough colonies)
Dies of pneumonia
Live, disease-causing
bacteria (smooth colonies)
Avery and DNA
In 1944, Griffith’s work was repeated by
Oswald Avery and added enzymes to digest
molecules. The digested macromolecules
then couldn’t be the molecule passing on
information in their experiments.
 Avery and other scientists discovered that
the nucleic acid DNA stores and transmits
the genetic information from one generation
to another.

The Hershey-Chase Exp.

Alfred Hershey and Martha Chase worked
together in 1952 to study DNA in viruses.
They studied one particular type of virus
called a Bacteriophage.


Bacteriophage: Virus that infects Bacteria
Their research supported the fact that
genetic material was DNA
The Hershey-Chase Exp.
Bacteriophage with
phosphorus-32 in
DNA
Phage infects
bacterium
Radioactivity inside
bacterium
Bacteriophage with
sulfur-35 in protein
coat
Phage infects
bacterium
No radioactivity inside
bacterium
Hershey-Chase Exp.
Bacteriophage with
phosphorus-32 in
DNA
Phage infects
bacterium
Radioactivity inside
bacterium
Bacteriophage with
sulfur-35 in protein
coat
Phage infects
bacterium
No radioactivity inside
bacterium
The Hershey-Chase Exp.
Bacteriophage with
phosphorus-32 in
DNA
Phage infects
bacterium
Radioactivity inside
bacterium
Bacteriophage with
sulfur-35 in protein
coat
Phage infects
bacterium
No radioactivity inside
bacterium
The Components and
Structure of DNA

Scientists knew that DNA has 3 functions:
1.
2.
3.

Genes had to carry info from one generation to the
next.
Genes put information to work by determining the
inheritable characteristics of an organism.
Genes have to be easily copied because genes are
replicated every time a cell divides
DNA is made up of monomers called
nucleotides. Components of a nucleotide:



5-carbon sugar (deoxyribose)
Phosphate Group
Nitrogenous Base
The Components and
Structure of DNA

Four types of nitrogenous bases:





Purines:



Double-Ring structure
Adenine and Guanine
Pyrimidines:



Guanine
Adenine
Cytosine
Thymine
Single Ring
Cytosine and Thymine
Backbone of DNA:

Sugar-Phosphate
The Bases
Purines
Adenine
Guanine
Phosphate
group
Pyrimidines
Cytosine
Thymine
Deoxyribose
Chargaff’s Rules
After finding out that DNA was a series of
nucleotides, with the nitrogen bases in
random and different orders, scientists still
worked to figure out the complete structure
of DNA.
 Erwin Chargaff discovered that the
percentage of A’s and T’s was equal and C’s
and G’s was equal. A=T and C=G became
known as Chargaff’s rules

X-ray Evidence
Around this same time (late 1940’s – early
1950’s), Rosalind Franklin discovered that
DNA was a double helix.
 She used a process called X-ray Diffraction


She shot X-rays at the DNA and recorded where
the X-rays reflected. Her picture did not reveal
the exact structure, but it did paint the picture.
The Double Helix
In addition to Franklin and Chagraff’s work,
two scientists named Watson and Crick were
determined to discover the structure of DNA.
 Once given Franklin’s results, they
discovered the actual structure of DNA and
made a model of it. This model is a double
helix, in which the nitrogenous bases are
connected in the middle by hydrogen bonds.

The Double Helix

Bonding Pattern:

A and T are joined by two hydrogen bonds and
C and G are joined by 3 hydrogen bonds. They
always bond in this pattern, which explain’s
Chargaff’s rules. This is called base pairing.
Structure of DNA
Nucleotide
Hydrogen
bonds
Sugar-phosphate
backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
DNA and Chromosomes
Prokaryotes: DNA molecules out in
cytoplasm, most bacteria have circular DNA,
called a plasmid.
 Eukaryotes: much more complicated
because there is about 1000x as much DNA
as a prokaryote.

DNA not found out in cytoplasm, it’s in the
nucleus in the form of chromosomes
 The Number of chromosomes varies between
organisms

DNA and Chromosomes

Chromosome Structure
Chromatin: long, stringy DNA in the cell
 DNA is wrapped around proteins called
histones
 DNA cell division, the chromatin condenses to
form tightly packed structure called a
chromosome.

Prokaryote DNA Structure
Chromosome
E. coli bacterium
Bases on the chromosome
Chromosome structure
Chromosome
Nucleosome
DNA
double
helix
Coils
Supercoils
Histones
DNA replication
Watson and Crick’s model of DNA became a
quick success because it revealed the
mechanism by which DNA can copy itself.
 Each strand of DNA can be used to make
another strand. Because of this we say that
DNA is “complementary”.
 Replication: The process of duplicating
DNA

DNA replication

How DNA replicates:
DNA separates into two strands
 Two new strands are produced using the base
pairing rules


What separates the DNA?


An enzyme called helicase separates the DNA
(by breaking apart the hydrogen bonds)
What builds the new strands of DNA?

An enzyme called DNA polymerase
DNA replication

DNA replication occurs at hundreds of
places at once until the entire strand is
copied.

Replication fork: The site where replication
begins in each section of DNA
RNA and Protein Synthesis
Genes: Coded DNA instructions that control
the production of proteins within a cell
 In order to make a protein, DNA is copied
into RNA (ribonucleic acid). RNA then carries
the code to make a protein.

The Structure of RNA


Monomer: Nucleotides
Differences between RNA and DNA:
1.
2.
3.

Single Stranded
Ribose Sugar
Uracil instead of Thymine
Types of RNA:



Messenger RNA (mRNA)
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)
Concept Map
RNA
Section 12-3
can be
Messenger RNA
Ribosomal RNA
also called
which functions to
mRNA
Carry instructions
Transfer RNA
also called
which functions to
also called
which functions to
rRNA
Combine
with proteins
tRNA
Bring
amino acids to
ribosome
from
to
to make up
DNA
Ribosome
Ribosomes
Structure of RNA
All three types of RNA are involved with
making proteins
 mRNA – carries the DNA copy of genes
 tRNA – carries amino acids that link together
to make a protein
 rRNA – makes up ribosomes (the site of
protein synthesis)

Transcription


Transcription is the process of making an mRNA
copy of DNA.
Why make a copy of DNA to make proteins?


Because DNA never leaves the nucleus, it’s protected
there! mRNA goes out into the cytoplasm where
proteins are made.
An enzyme called RNA polymerase (very similar
to DNA polymerase) is required for transcription to
happen. RNA polymerase binds to DNA and
separates the strands. It also reads one strand of
DNA (the template) and assembles nucleotides to
make a corresponding strand of RNA.
Transcription

How does RNA polymerase “know” where to
start and stop?

Promoters: specific base sequences on DNA
that signal RNA polymerase to start transcribing.
 A similar
process occurs to signal stopping.
RNA editing

DNA is extremely long and contains a lot of
sequences that don’t code for any amino
acids. Only small sections of DNA actually
code for proteins.
Introns – Non-coding sections of DNA
 Exons – DNA segments that code for proteins


When mRNA is made from DNA, both
introns and exons are copied
RNA Editing
Introns are cut out of mRNA and the
remaining exons are “spliced” together so
the mRNA can read it to make a protein
 So what is the purpose of introns?


Scientists continue to research that question.
The believe it might be leftover DNA from
Evolution (they call it “junk DNA”)
Genetic Code
Proteins are made by joining amino acids
into long chains called polypeptides. The
properties of proteins are determined by the
order in which different amino acids are
joined together.
 So how do the order of nitrogen bases in
DNA and RNA code for the order of amino
acids in a protein?


The “language” of mRNA instructions is called
genetic code.
The Genetic Code
Four bases in the code: A, U, C, G
 How do these four letters code for 20
different Amino acids?



The Genetic Code is read three letters at a time.
For example, “GUC” Codes for the amino acid,
serine.
The three letter “word” in the genetic code is
called a codon.

Codon: three consecutive nucleotides that
specify a single amino acid.
The Genetic Code

Example:
mRNA – UCGCACGGU
 Codon – UCG-CAC-GGU
 Amino Acids – Serine-histadine-glycine


There are 64 possible codons in the genetic
code. Therefore, one amino acid can be
specified by more than one codon

Ex. Lysine (AAG or AAA)
The Genetic Code

Three of these codons signal “stop” and
don’t code for an amino acid. One signal
“start” (AUG) or the amino acid, methionine.
Genetic “Decoder”
Translation
The sequence of nucleotide bases in an
mRNA molecule serves as instructions for
the order in which amino acids line up to
make the primary structure of a protein.
 Translation: the decoding of an mRNA
message into a protein
 Location: this all takes place on a ribosome

Translation

To understand the process of translation, we
must first learn the structure of tRNA.
tRNA’s carry amino acids
 Each tRNA can only carry one amino acid
 At the bottom of every tRNA molecule is an
anticodon

 Anticodon:
a three-nucleotide sequence that is
complimentary to an mRNA codon
Translation

The process:







mRNA attaches to a ribosome in the cytoplasm
The codons are read by the tRNA
If the tRNA anticodon matches up with the codon, then
tRNA delivers its amino acid.
The Amino acid is transferred to the growing polypeptide
chain
The empty tRNA molecule (without any Amino acid) then
leaves
The Ribosome then shifts down so that another codon
can be read
The Process repeats until a stop codon is reached.
Translation
Translation
Mutations
Sometimes there are mistakes in copying
DNA.
 Mutations: Changes in the genetic material
 Kinds of Mutations


Gene Mutations
 Changes

in a single gene
Chromosome Mutations
 Changes
in whole chromosomes
Gene Mutations

Point mutations – a change in one or a few
nucleotides




Substitution
Insertion
Deletion
Insertion and deletion mutations can be more
dangerous than a simple change in one amino acid
(sub). The code is still read in groups of three.
Inserting an extra nitrogen base will throw off the
entire “reading” of the code.
Gene Mutations

Frameshift Mutations – shift the reading
frame of the genetic message

These can lead to a completely different protein
Deletion
Substitution
Insertion
Chromosome Mutations
This involves a change in an entire
chromosome, not just one or a few bases.
 Four types:

Deletion
 Duplication
 Inversion
 Translocation

Chromosomal Mutations
Deletion
Duplication
Inversion
Translocation
Significance of Mutations

Many mutations are neutral and have no
effect on the expression of genes (making of
proteins!). Some have very harmful effects
(defective proteins). Others are beneficial.


Ex. Polyploidy – the condition of having an
extra set of chromosomes (good for plants)
Mutations are a source of genetic variation,
and variation can be beneficial
Gene Regulation
How does an organism “know” when to turn
a gene on (and make a protein) or off?
 An example of gene regulation:


In E. coli, there is a cluster of 3 genes that are
turned on or off together. This cluster is called
the lac operon (operon: cluster of genes that
work together) because it is turned on to help
bacteria use lactose as food.
Gene Regulation


The lac genes are turned off by the repressors and
turned on by the presence of lactose.
On one side of the 3 genes there are two important
regulatory regions: Promoter and operator



Promoter (P) – RNA Polymerase binds and transcription
begins
Operator (O) – Contains repressor protein (blocks
transcription).
Transcription can only happen when the lac
repressor protein is removed. It is removed when
lactose binds to it.
Eukaryotic Gene Regulation
The lac operon represents a simple version
of gene regulation. It is often much more
complicated in eukaryotic cells.
 Before many eukaryotic genes, there is a
sequence of nucleotides “TATATATA” or
“TATAAA”. This marks where genes will
begin so the RNA polymerase knows where
to bind. It is so common in eukaryotic cells
that it has a name  the “TATA box”
