Download Chapter 12 - North Mac Schools

CHAPTER 12
DNA and RNA
12-1: DNA

How was DNA discovered?
 Fredrick
Griffith
 Oswald Avery
 Hershey & Chase
 Watson & Crick
Fredrick Griffith



Experimented with bacteria and mice
Cultured both harmless bacteria and
pneumonia causing bacteria.
Exposed mice to a mixture of both harmless
and heat-killed pneumonia causing bacteria

Mice came down with disease
page 288
Frederick Griffith
Griffith’s Conclusion
 Griffith
called it transformation
 Cells
can be transformed when
coming into contact with other types
of cells
 Harmless bacteria transformed into
disease causing strains.
 What
was this disease causing
agent and how did it transform the
other cell?
Oswald Avery
Set out to answer the previous question.
 Repeated Griffith's work, took extract from
heat-killed bacteria.
 Treated extract with enzymes that break down:

Proteins
 CHO’s
 Lipids
 Even RNA

Same results
 Then repeated with enzymes that break down DNA
 Result:


No transformation
Hershey and Chase
 Used
radioactive markers to
determine if proteins or DNA was
injected by viruses
 The tracers could be followed from the
virus to the bacteria.
 Injected with:
 Phosphorus-32
(in DNA, not in proteins)
 Sulfur-35 (in proteins, not in DNA)
 Results:
 Only
Phosphorus-32 was transferred
Martha Chase and Alfred Hershey
How did DNA work?


At this point, scientists knew DNA was where genes
were contained.
We still needed to know:
1.
2.
3.
How did DNA carry genes generation to generation?
How did DNA code for traits?
How was copied?
Text p. 291
The Components and Structure of DNA

Nucleotides
 5-carbon
sugar
 Deoxyribose
 Ribose
 Phosphate
 Nitrogen
base
Example of a nucleotide:
Deoxyribose (sugar) + Phosphate group + Cytosine (nitrogen base)
Component of DNA
5-Carbon sugar
Deoxyribonucleic Acid (DNA)
Chargaff’s rule
 Determined
complementary nature of DNA.
Chargaff’s Rule of Base Pairing
http://fig.cox.miami_edu/~cmallery/150/gene/chargeff.
Rosalind Franklin


X-rays were used to “see” the general structure of
DNA
Appeared like this:
Watson & Crick
 Francis
Crick and James
Watson were working on
the shape of DNA
 After Watson saw
Franklin’s X-ray, he knew
the shape had to be a
double helix
 Two strands with
complementary base pairs
in between that are
bonded together.
12-2: Chromosomes and DNA Replication

Chromosome structure
 Histones
 Proteins
that chromatin
is wrapped around
 Chromatin
 Condensed
 Nucleosome
DNA
=
histones + chromatin
Duplicating DNA
“Replication”
 After
mitosis, each new
cell gets an exact copy
1. Steps
of replication
DNA is “unzipped” by a protein called DNA helicase
(breaks hydrogen bonds between complimentary bases)
2. Steps
in replication
DNA polymerase
reads the sequence and adds base pairs that complement.
3. Steps
in Replication
DNA polymerase also proofreads along the existing strand.
4.
Steps in Replication
DNA ligases “put together” chunks of copied base segments.
RNA and Protein Synthesis
12-3
 Structure
of RNA
 Single-stranded
 Ribose-phosphate
backbone
 Contains

nitrogen base uracil instead of thymine
It bonds with adenine on DNA
Klug & Cummings 1997
Types of RNA

mRNA – messenger RNA


rRNA – ribosomal RNA


transcription
Make up components of the ribosomes
tRNA – transfer RNA

translation
Text p. 300
mRNA = messenger RNA

Transcription
DNA gives code to RNA for
making proteins
 Similar to replication except
now code is copied to RNA
(has uracil)
 RNA polymerase unzips the
DNA strand and begins to
add bases that complement
one of the strands.

YouTube - Transcription
How does transcription know where to start?
 Promoters
 Specific
sequences of base pairs that RNA polymerase can only
bind to in order to initiate transcription.
RNA editing
 Introns
and Exons
 Introns

Sequences that code for nothing
 Exons


Sequences that directly code for sections of proteins
Sequences that are “expressed” as protein
 Eventually
enzymes go back and cut the introns out and splice
together the exons to have a fully functioning mRNA.
The Genetic Code


20 different amino acids
Each is coded for by a segment of
3 base pairs



Codon
Most amino acids have multiple
codons
Also codons for starting and stopping
transcription
Translation
 Copying
mRNA into a sequence of amino acids.
 mRNA attaches to ribosome
 Ribosome “reads” looks for a “start codon”
 Two tRNA with “anticodon” that complement the strand are
attached to mRNA by the ribosome.
Peptide synthesis

Temporary hydrogen bonds allow the tRNA to be bound to
mRNA long enough to form a “peptide bond” between the
two amino acids.
YouTube - Translation
Genes and Proteins

The proteins that are produced ultimately go to make:
 Structural
proteins (eye color, physical features)
 Enzymes (control all cellular activities, ex: digesting lactose)
 Hormones (producing testosterone/estrogen)
Combining all of these
factors ultimately make
us what we are.
12-4

Mutations
Mutations – changes in the genetic sequence
Point mutations
 Occur
in one (or few)
base(s) of the DNA
sequence
 Includes substitutions
 Sometimes little to no
effect on
amino acid sequence,
however sometimes
effect can be cataclysmic
 Sickle-cell
anemia
Frameshift Mutations
 Caused
by insertions
or deletions of bases,
shifting the way the
is read.
 Shifts
or
mRNA
the “reading frame”
 Usually
has dramatic effects on the formation of the protein –
often rendering it useless
Chromosomal mutations
 Deletions
 Duplications
 Inversions
 Translocations
Significance of mutations
 Most
often, mutations ultimately show little to no effect on the
protein that is supposed to be made.
 However, when it does have an effect, the new protein formed
can be:
1.
Detrimental - Increases organisms chance
of dying and not passing the mutated
gene on.
2. Beneficial - Increases the organisms
chance of survival and reproducibility –
therefore passing it down
CREATES GENETIC VARIABILITY!
This is often how asexually reproducing organisms evolve...
a slow process
12-5: Gene Regulation

How does an organism “know” when to turn the
gene on or off?
 Example:
gene?
How does your body turn on the lactase
Regulation of the lac operon in E.Coli
 Operon
 Groups
of genes that code for specific protein
 lac operon in E.Coli codes for protein that breaks down
lactose
Transcription
 Again,
transcription begins at the sequence called
the promoter
 Just “below” the promoter are “operator” (O) sites
 These
sites are areas where “repressors” can bind
 In most cases repressors are bound to O site, preventing
transcription
 Turning off the gene
 Just like a room, when you are not in it – TURN OFF
THE LIGHT!
Lac Operon
 When
lactose is
present, it binds with
the repressor
changing its shape,
forcing it off the O
site
 Allows RNA
polymerase to begin
transcription
 YouTube - Lac Operon
TATA box

How is this done in eukaryotic cells?
 Much
more complex than lac operon.
 Involves sequence of base pairs near promoter called
“TATA box”
 Sequence

of TATATATA… or TATAAA….
Positions RNA polymerase
 All
cells contain the entire Genetic Code BUT any one
cell will use a small fraction of those genes.
 Heart
cells use different genes than brain cells.
Development and Differentiation



Cells that change into specific types of specialized cells
Hox genes
 Genes that control this differentiation early in development
 Mutations involving hox genes can have HUGE effect on
outcome of organism
Pax 6
 Gene found in Drosophila and mice that controls eye
development
 Inserted mouse Pax 6 gene into the “knee” of Drosophila
embryo

Grew an eye on its leg
 YouTube
- Evolution Genetic Tool Kit