Transcript BIOLOGY CHAPTER 12

Chapter 12 DNA & RNA
The genetic code is the way in
which the cell stores
information that it passes on
from generation to generation.
GRIFFITH & TRANSFORMATION
Griffith studied ways in which
types of bacteria cause pneumonia.
His main concern was to make sure
that pneumonia causing bacteria did
not produce toxins.
He worked with 2 slightly different
strains of bacteria.
Both strains of bacteria grew
well in his lab but only one of
them actually caused the
disease.
*The disease causing bacteria
grew into SMOOTH colonies on
the culture plates…Page 288
Figure 12-2
The harmless strain grew into rough
TEXTURED colonies.
GRIFFITH’S EXPERIMENT:
1. He injected mice with the
SMOOTH bacteria & they
developed pnuemonia and died.
2.He injected mice with the ROUGH
non-disease causing bacteria and
the mice LIVED.
??Do disease causing bacteria
produce a poison???
3. Griffith heat killed the
disease causing bacteria and
injected them into the mice…the
mice survived!
??Maybe Pneumonia is not caused
by a toxin released by the disease
4. In his next experiment
Griffith injected a mixture of
harmless bacteria & heat-killed
bacteria into the mice…the
mice developed pneumonia.
When mixed, the heat-killed
harmful strain had passed on its
disease causing ability to the
harmless rough bacteria.
Due to this surprise Griffith recovered
the bacterial strains from the animals
that had developed pneumonia & grew
them on culture plates…the colonies that
grew were SMOOTH!
CONCLUSION: One strain of bacteria
had been transformed into another
strain. Griffith referred to this as
TRANSFORMATION.
The Transforming Factor
GRIFFITH’S HYPOTHESIS: When
live harmless bacteria & heat-killed
bacteria were mixed, a factor was
transformed from the heat-killed
cells to the live cells.
In 1944 a group of scientists
lead by Oswald Avery
(@Rockefeller Institute, NY)
repeated Griffith’s experiment
& identified DNA as the
transforming factor.
DNA- deoxyribonucleic acid
The nucleic acid that stores &
transmits the genetic
information from one generation
of an organism to the next…DNA
CARRIES THE GENETIC CODE!
Alfred Hershey & Martha Chase
studied the viruses that infect
bacteria. Viruses are non-living
particles smaller than a cell that
can infect living organisms.
BACTERIOPHAGE- “bacteriaeater”…a virus that infects
bacteria.
Bacteriophages are composed of :
1. A DNA Core
2.A Protein Coat (Capsid)
The DNA core & Protein Coat make
up the head of the
bacteriophage.
The bacteriophage also has a tail.
How a Bacteriophage infects a
bacterial cell:
1-The bacteriophage attaches to
the bacterial cell & injects a
material into the bacterium.
2. The injected material
reproduces using the DNA of the
bacterial cell…more
bacteriophages are made.
3. The bacterial cell becomes so
filled with the bacteriophages that
it LYSES (breaks open) & infects
other cells.
Hershey & Chase wanted to find
out what part of the infecting virus
was the material that entered the
bacterium…was it the protein coat,
the DNA or both?
Hershey-Chase Experiment
Radioactive Tagging
(Page 290)
Two groups of bacteriophages were
prepared: one had radioactive
sulfur-35 added to it & the other
had radioactive phosphorus-32
added.
The radioactive isotopes were used
to trace the pathway of certain
materials….this was a way of
“labeling” the viruses.
The protein coat was labeled with
sulfur-35 & the DNA was labeled
with Phosphorus-32.
Proteins contain little or no
phosphorus & DNA does not contain
sulfur…
If sulfur was found in the bacteria
this would mean that the viruses
protein coat was injected into the
cell.
If phosphorus was found, then it
was the DNA that had been
injected.
RESULTS: Nearly all of the
radioactive sulfur remained with
the viruses…nearly all of the
radioactive phosphorus had
entered the bacteria.
CONCLUSION: The virus’s DNA
enters a bacteria & the protein coat
remains behind on the outside of
the bacteria…This was convincing
evidence that DNA is the molecule
that contains genetic code!
THE STRUCTURE OF DNA
Page 291
How can DNA store information
AND duplicate itself?
DNA is a polymer found in units
called NUCLEOTIDES.
NUCLEOTIDE- a molecule made
up of three basic parts:
1-A 5 carbon sugar = Deoxyribose
2-A Phosphate Group
3-A nitrogen containing base
Nucleotide
DNA contains 4 Nitrogen Bases:
1. Adenine & 2.Guanine
*Both belong to a group of
compounds known as purines.
3. Cytosine & 4. Thymine
*Belong to pyrimidine group;
Individual nucleotides are joined
together to form a long chain.
The sugars & phosphate groups form
the BACKBONE of the chain. The
nitrogen bases form the MIDDLE
PART of the chain.
X-RAY EVIDENCE
Rosalind Franklin, in the early 1900’s
purified a sample of DNA, stretched it
out & made X-ray diffraction
pictures of the
molecule.
See page 292 Time line…
(It was noted that the fibers that
make up the DNA molecule are
twisted like strands of a rope…see
pg 292…the X near the center)
**It was also noted that large
groups of molecules in the fiber
are spaced out at regular intervals
along the length of the fiber.
*The X-ray did not allow us to
determine the structure of
DNA…it was more like a
fingerprint to give scientists
evidence to work with.
BUILDING A MODEL OF DNA
Francis Crick (British physicist)
& James Watson (25 year old
American biochemist) tried to
determine the structure of
DNA by building a 3-D models
of the atomic groups in DNA.
After viewing Franklin’s x-ray
pattern…they realized
something important about the
pattern…& within weeks Watson
& Crick determined the
structure of DNA.
THE DOUBLE HELIX
The shape that Watson & Crick
used for their model was a helix
similar to a spiral but it was
twisted.
Franklin’s idea of DNA containing 2
strands was also useful in
determining the structure.
Watson & Crick decided that the
structure would be a double helix.
Characteristics Described by the
Watson & Crick Model:
1. The nitrogen bases on each of
the strands of DNA were
positioned exactly opposite each
other.
2. Nitrogen base positioning allows
weak Hydrogen bonds to form
between the nitrogen bases
ADENINE & THYMINE…and
between CYTOSINE & GUANINE.
Erwin Chargaff observed that in
any sample of DNA, the # of
adenine molecules was equal to the
# of thymine molecules.
This was also true for cytosine and
guanine.
This observation allowed Watson &
Crick to determine that adenine
bonds only to thymine and cytosine
bonds only to guanine.
BASE PAIRING- the attraction
of one base for the other.
Base pairing holds the 2 strands
of a DNA double helix together.
Waston & Crick’s model was
quickly accepted because it also
showed how DNA could replicate.
(The model explained much about the
structure of DNA, including
placement of the nitrogen bases & the
formation of hydrogen bonds!)
In 1962 Watson, Crick, & Maurice
Wilkins were awarded the Nobel
Prize for medicine/physiology…for
their discoveries concerning the
DNA molecule.
Rosalind Franklin was not included
because she had already died.
`
12-2 Chromosomes & The
Replication of DNA
Before a cell divides, the DNA
must be duplicated in a process
called REPLICATION that is
carried out by a series of
enzymes.
Duplicating the DNA ensures
that each resulting cell will have
a complete set of DNA
DNA Replication/Synthesis
Defined: the process in which DNA
is duplicated before a cell divides.
The 2 strands of a DNA double
helix are COMPLEMENTARY…the
2 halves are not identical but one
half tells us what the other half
looks like.
DNA contains 4 nitrogen bases..and
each half of the DNA strand can
specifically direct the sequence of
the other half by complementary
base pairing.
Each strand of DNA serves as a
template or pattern from which a
new strand is made.
The enzymes that carry out DNA
replication separate or unzip the
DNA molecule, insert the
appropriate bases, & produce sugar
phosphate links to extend the
growing DNA chains.
The enzymes also proof read the
bases to make sure that they have
been paired correctly.
`
Replication begins when the
DNA molecule unzips.
The unzipping occurs when the
hydrogen bonds between the
base pairs are broken and the
molecule unwinds.
(Template paper example)
EX: T-A-C-G-T-T will produce
complementary bases
A-T-G-C-A-A
The 2 DNA molecules that are
produced are identical to each
other…and identical to the original
molecule.
12-3
RNA & Protein Synthesis
RNA
The DNA model that was developed
by Watson & Crick did not explain
how the information contained in
the DNA molecule was used by the
cell.
This process involves a 2nd nucleic
acid called RNA (ribonucleic acid).
TRANSCRIPTION
Defined: the process of copying
DNA to form a complementary
RNA molecule called :
Messenger RNA (mRNA)….
this is the name given because of
how the RNA strand is made from a
DNA pattern.
RNA is the nucleic acid that acts
as a messenger between the DNA
in the nucleus & the ribosomes
during the process of making
proteins from amino acids in the
cell.
Transcription is the 1st step in the
synthesis (making) of cell proteins.
In transcription, information is
transferred from DNA to RNA.
This transfer of information is
important because DNA is found
only in the nucleus & a messenger
or carrier must bring the genetic
information from the nucleus out
to the ribosomes in the
cytoplasm.
mRNA-Messenger RNA- 1 of
the 3 main types of RNA…
(Carries information from the
nucleus to the cytoplasm!)
RNA is similar to DNA in structure
because it too is a long chain made
up of nucleotides.
mRNA is the blueprint of the
genetic code.
3 Major Differences Between RNA &
DNA
1. The sugar in RNA is ribose…in DNA it
is deoxyribose.
2. RNA consists of a single strand of
nucleotides although it can form
double stranded sections by folding
back on itself in loops…DNA is double
stranded.
3. RNA contains all of the same
nitrogen bases as DNA except
for one…RNA contains Uracil
in the place of thymine.
There are 3 types of RNA
molecules in cells: Messenger RNA
(mRNA), Transfer RNA (tRNA), &
Ribosomal RNA (rRNA).
PROCESS OF
TRANSCRIPTION
1-Portions of the DNA molecule
unwind & the 2 strands separate.
2-RNA nucleotides pair with
complementary bases on a DNA
strand forming a strand of
mRNA(complementary strand).
*The single strand of DNA
serves as a template/pattern
for synthesizing mRNA.
The hereditary information in
the sequence of DNA is copied
in complementary form into
the nucleotide sequence of
mRNA.
The enzyme RNA polymerase is
very important in transcription.
An enzyme is very specific as to
what substance it works on…this
particular enzyme works on the
polymers DNA & RNA.
(Polymer- a long strand of
molecules)
In transcription the RNA
polymerase attaches to a
specific place on the DNA
molecule & separates the two
strands of the double helix…a
mRNA strand is also made.
Amino acids are the building blocks
of proteins.
The genetic code for each amino
acid is a specific sequence of 3
nucleotides.
CODON- 3 nucleotide sequence in
mRNA that specifies a
certain/particular amino acid.
Certain DNA sequences serves as
START (GO) signals & are
recognized by the RNA
polymerase enzyme.
Other areas are recognized as
TERMINATION SITES (STOP)
where the RNA polymerase
releases the newly made mRNA
molecule.
PROTEIN SYNTHESIS
Genes contain instructions for
assembling proteins.
The sequence of nucleotides in
mRNA contain the genetic
code, which determines the
amino acid sequence of
proteins.
The information that DNA
transfers to mRNA is in the
form of a code.
The code is determined by the
way in which the 4 nitrogen
bases are arranged in DNA.
DNA & RNA are responsible
for making proteins…proteins
are responsible for cell
structure & movement;Most
enzymes are proteins; Proteins
direct the making of
carbohydrates, lipids, &
nucleotides.
The Nature of the Genetic Code
Proteins are made by stringing
amino acids together to form long
chains called POLYPEPTIDES.
Each polypeptide contains a
combination of the many different
amino acids.
DNA & RNA contain 4 nitrogen
bases, so the genetic code has a
4 letter alphabet.
In order to code for the
different amino acids, more than
one nucleotide has to make up
the code for the amino acid.
The smallest size for a code word
in DNA is 3 nucleotides.
CODON- a 3 nucleotide sequence
on mRNA that codes for an amino
acid (3 letter code word!)
Each codon specifies a particular
amino acid that is to be placed in
the polypeptide chain.
The genetic code consists of 64
codons along with their
corresponding amino acids. These
codons are found on mRNA.
There are 20 amino acids. This
is why it is possible for an amino
acid to be specified by more
than 1 kind of codon.
Of the 64 codons, 61 specify a
particular amino acid & 3 of them
are STOP CODONS which signify
the end of a polypeptide chain.
See page 303 Figure 12-17
TRANSLATION
Defined: the process in which a
message carried out by mRNA is
decoded into a polypeptide chain
known as a PROTEIN.
The message is being translated
from the language of the nucleic
acid to a polypeptide.
During translation, the type of
amino acid that is added to the
growing polypeptide depends on
the codon on the mRNA & the
anticodon on the tRNA to which
the amino acid is attached.
The cell uses information from
mRNA to produce proteins!
Transfer RNA
Defined: the type of RNA that
carries amino acids to the ribosomes
where amino acids are joined
together to form polypeptides. It is
a single strand of RNA that loops
back onto itself.
tRNA molecules are found in the
cytoplasm.
Their function is to carry amino
acid molecules to the ribosomes,
the sites of protein synthesis.
There are 20 different amino acids
in cells, & there is a different form
of tRNA for each amino acid.
Each type of tRNA has a 3
nucleotide sequence called an
ANTICODON, that is
complementary to mRNA.
(Homemade transparency…PROTEIN SYNTHESIS…)
The amino acids are joined together
in the ribosome to form a
polypeptide.
The Role of tRNA
Each tRNA has 2 important sites of
attachment:
1. One site called the anticodon binds to
the codon on the mRNA molecule.
ANTICODON-3 nucleotide sequence in
tRNA that base pairs with a
complementary sequence in mRNA
during protein synthesis.
2. The other site attaches to a
particular amino acid.
During protein synthesis the
anticodon of tRNA molecule base
pairs with the appropriate mRNA
codon.
ROLE OF THE RIBOSOME
rRNA- the type of RNA that
makes up the major part of the
ribosomes.
The ribosomes consisting of rRNA
& protein, release the new protein.
The ribosomes are essential to
making the genetic code work.
Proteins are made up of smaller
molecules of amino acids strung
together into long chains called
polypeptides.
Individual amino acids are
attached to the individual codon.
TEST CH 12-1 THRU 12-3