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Chapter 8
8-1 DNA
How do genes work? What are they made of? How do
they determine the characteristics of an organism?
Griffith and Transformation
A. Griffith finds a ‘transforming principle.’
B. Griffith experimented with the bacteria that cause
pneumonia.
C. He used two forms: the S form (deadly) and the R form
(not deadly).
D. A transforming material passed from dead S bacteria to
live R bacteria, making them deadly.
Avery identified DNA as the transforming principle.
A. Avery isolated and purified Griffith’s transforming
principle.
B. Avery performed three tests on the transforming
principle.
Hershey and Chase confirm that DNA is the genetic
material.
A. Hershey and Chase studied viruses that infect bacteria,
or bacteriophages.
a. Virus – non-living particles smaller than a cell that
can infect living organisms.
b. Radioactive markers – used in viruses to
determine that the genetic material of the
c. bacteriophage was DNA, not protein.
8-2 Structure of DNA
* genes had to carry information from one
generation to the next
* genes had to put that information to work by
determining heritable characteristics for
organisms
* genes had to be easily copied because all
genetic information is copied before every cell
division
Scientists who contributed to our knowledge of DNA
structure:
Chargaff’s Rules:
The amount of A = amount of T
amount of C = amount of G
Franklin – used x-ray diffraction to get information on the
structure of DNA
A. strands of DNA are twisted around each other like
coils of a spring (helix)
B. 2 strands
C. nitrogenous bases are near the center of the
molecule
Watson and Crick – working on the structure of DNA,
saw Franklin’s x-ray pattern of DNA
which led to their breakthrough with DNA structure
A. DNA – double helix where 2 strands are wound
around each other
B. Hydrogen bonds from between certain nitrogenous
bases and provide the force to hold the two strands
together
C. Base pairing rule – hydrogen bonds can only form
between
a. A-T and C-G
DNA is composed of four types of nucleotides.
A. DNA is made up of a long chain of nucleotides.
B. Each nucleotide has three parts.
–a phosphate group
–a deoxyribose sugar
–a nitrogen-containing base (A, T, C, G)
8-3 DNA Replication
Replication copies the genetic information.
A. A single strand of DNA serves as a template for a
new strand.
B. The rules of base pairing direct replication.
C. DNA is replicated during the S (synthesis) stage of
the cell cycle.
D. Each body cell gets a complete set of identical
DNA.
Proteins carry out the process of replication.
A. DNA serves only as a template.
B. Enzymes and other proteins do the actual work of
replication.
C. DNA Polymerase – polymerizes individual
nucleotides to produce DNA. Also “proof reads”
the new DNA
D. Enzymes “unzip” the DNA molecule (hydrogen
bonds are broken, 2 strands unwind)
E. Free-floating nucleotides form hydrogen bonds
with the template strand.
Replication is fast and accurate.
8-4 Transcription converts a gene into a singlestranded RNA molecule.
RNA and Protein Synthesis
DNA → RNA → protein
RNA – long, single-strand of nucleotides; carries out
DNA’s instructions
5C sugar (Ribose), phosphate group,
nitrogenous base
· Uracil replaces thymine in RNA
· RNA is like a disposable copy of DNA (“working
copy” of a single gene)
Transcription
RNA polymerase binds to DNA and separates the DNA
strands. It then uses 1 strand of DNA as a template
from which nucleotides are assembled into a strand of
RNA.
Binds to regions of DNA called promoters (signals in
DNA that indicate to the enzyme where to bind to make
RNA)
Transcription makes three types of RNA.
• Messenger RNA (mRNA) – carry copies of
instructions
• Ribosomal RNA (rRNA) – site of protein assembly at
ribosome
• Transfer RNA (tRNA) – transfers each amino acid to
the ribosome
Transcription and replication both involve complex
enzymes and complementary base
pairing.
•The two processes have different end results.
–Replication copies all the DNA; transcription
copies a gene.
–Replication makes one copy; transcription can
make many copies.
8-5 Translation converts mRNA messages into
polypeptides.
The Genetic Code - the “language” of mRNA
instructions.
•A codon is a sequence of three nucleotides that codes
for an amino acid. They are on mRNA. RNA contains
A, U, C and G
Ex.
U C G C A C G G U
Is read as
UCG – CAC – GGU
these represent
serine – histidine – glycine
3 amino acids in a protein
A. Because of the 4 different bases (a, u, c, g) there are
64 possible 3-base codons on the mRNA. Some amino
acids are specified by more than one codon. There are
also some codons, like AUG that can code for
methionine, or “start” codon for protein synthesis.
Some codons are “stop” codons that do not code for
any amino acid.
B. The sequence of amino acids in an mRNA are the
instructions for the order the amino acids should be in
for a certain protein. There must be something to read
the instructions, and assemble the correct sequence of
amino acids. In the cell, this is the ribosome (rRNA +
protein).
•Amino
acids are linked to become a protein.
•An anticodon is a set of three nucleotides that is
complementary to an mRNA codon.
•An anticodon is carried by a tRNA.
A. mRNA must be transcribed from DNA in the nucleus
and released into the cytoplasm.
B. Translation begins when an mRNA molecule in the
cytoplasm attaches to a ribosome.
a. As each codon is “read” by the ribosome, the
proper amino acid is brought to
b. the ribosome by the tRNA
C. The ribosome forms a peptide bond between the first
and second amino acids, breaks the bond that held the
first tRNA to its amino acid, and releases the tRNA
molecule. The ribosome then moves to the third codon,
where a tRNA molecule brings it the amino acid
specified by the third codon.
D. The polypeptide chain continues to grow until the
ribosome reaches a stop codon on the mRNA
molecule.
a. When a stop codon is reached, the ribosome
releases the newly formed
b. polypeptide and the mRNA molecule completing
the process of translation.
8-6 Prokaryotic cells turn genes on and off by
controlling transcription.
A. A promotor is a DNA segment that allows a gene to
be transcribed.
B. An operator is a part of DNA that turns a gene “on” or
off.”
C. An operon includes a promoter, an operator, and one
or more structural genes that code for all the proteins
needed to do a job.
–Operons are most common in prokaryotes.
–The lac operon was one of the first examples of
gene regulation to be discovered.
–The lac operon has three genes that code for
enzymes that break down lactose.
•Eukaryotes
regulate gene expression at many
points.
A. Different sets of genes are expressed in different
types of cells.
B. Transcription is controlled by regulatory DNA
sequences and protein transcription factors.
•RNA processing is also an important part of gene
regulation in eukaryotes.
A. Many RNA molecules have sections that are
“edited” out before the molecules become
functional.
B. Introns – intervening sequences of bases that are
cut out of RNA in the cell nucleus.
C. Exons – expressed sequences of bases that are
spliced together to form the final mRNA (after
“cap” and “tail” added)
D. 3 Steps:
1)Introns are removed and exons are spliced
together.
2)A cap is added.
3)A tail is added.
8-7 Some mutations affect a single gene, while others
affect an entire chromosome.
A. A mutation is a change in an organism’s DNA.
B. Many kinds of mutations can occur, especially during
replication.
C. A point mutation substitutes one nucleotide for
another.
Mutations may or may not affect phenotype.
A. Chromosomal mutations tend to have a big effect.
B. Some gene mutations change phenotype.
–A mutation may cause a premature stop codon.
–A mutation may change protein shape or the
active site.
–A mutation may change gene regulation.
Mutations can be caused by several factors.
A. Replication errors can cause mutations.
B. Mutagens, such as UV ray and chemicals, can cause
mutations.
C. Some cancer drugs use mutagenic properties to kill
cancer cells.
Roles of DNA and RNA
DNA – the “master plan”
RNA – the “blue print”
Genes and Proteins
Most genes contain nothing more than instructions for
assembling proteins.
**Proteins are the keys to almost everything that living
cells do.