Transcript The Living World

Chapter 7
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
7.1 The Griffith Experiment
The work of Sutton and Morgan established that
genes reside on chromosomes
But chromosomes contain proteins and DNA
So which one is the hereditary material
Several experiments ultimately revealed the
nature of the genetic material
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
7.1 The Griffith Experiment
In 1728, Frederick Griffith discovered transformation
while working on Streptococcus pneumoniae
The bacterium exists in two strains
S
Forms smooth colonies in a culture dish
Cells produce a polysaccharide coat and can cause
disease
R
Forms rough colonies in a culture dish
Cells do not produce a polysaccharide coat and are
therefore harmless
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
7.2 The Avery and Hershey-Chase
Experiments
Two key experiments that demonstrated conclusively
that DNA, and not protein, is the hereditary material
Oswald Avery and his coworkers Colin MacLeod and
Maclyn McCarty published their results in 1744
Alfred Hershey and Martha Chase published their
results in 1752
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The Avery Experiments
Avery and his colleagues prepared the same mixture
of dead S and live R bacteria as Griffith did
They then subjected it to various experiments
All of the experiments revealed that the properties of
the transforming principle resembled those of DNA
1.
2.
3.
4.
Same chemistry and physical properties as DNA
Not affected by lipid and protein extraction
Not destroyed by protein- or RNA-digesting enzymes
Destroyed by DNA-digesting enzymes
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The Hershey-Chase Experiment
Viruses that infect bacteria have a simple structure
DNA core surrounded by a protein coat
Hershey and Chase used two different radioactive
isotopes to label the protein and DNA
Incubation of the labeled viruses with host bacteria
revealed that only the DNA entered the cell
Therefore, DNA is the genetic material
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
7.3 Discovering the Structure of DNA
DNA is made up of nucleotides
Each nucleotide has a central sugar, a
phosphate group and an organic base
The bases are of two main types
Purines – Large bases
Adenine (A) and Guanine (G)
Pyrimidines – Small bases
Cytosine (C) and Thymine (T)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 7.3 The four nucleotide subunits that make up DNA
Nitrogenous
base
5-C sugar
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Erwin Chargaff made key DNA observations that
became known as Chargaff’s rule
Purines = Pyrimidines
Rosalind Franklin’s
X-ray diffraction
experiments
revealed that DNA
had the shape of a
coiled spring or helix
A = T and C = G
Fig. 7.4
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Rosalind
Franklin
(1720-1758)
In 1753, James Watson and Francis Crick deduced
that DNA was a double helix
They came to their conclusion using Tinkertoy
models and the research of Chargaff and Franklin
Fig. 7.4
James Watson
(1728)
Francis Crick
(1716-2004)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
7.4 How the DNA Molecule Replicates
The two DNA strands are held together by weak
hydrogen bonds between complementary base pairs
A and T
C and G
If the sequence on one strand is
The other’s sequence must be
ATACGCAT
TATGCGTA
Each chain is a complementary mirror image of the
other
So either can be used as template to reconstruct the other
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
There are 3 possible methods for
DNA replication
Fig. 7.5
Daughter DNAs
contain one old
and one new
strand
Original DNA
molecule is
preserved
Old and new
DNA are
dispersed in
daughter
molecules
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Thus, DNA replication
is semi-conservative
Fig. 7.6
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
How DNA Copies Itself
The process of DNA replication can be summarized
as such
The enzyme helicase first unwinds the double
helix
The enzyme primase puts down a short piece of
RNA termed the primer
DNA polymerase reads along each naked single
strand adding the complementary nucleotide
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 7.7 How nucleotides are added in DNA replication
Template strand
New strand
HO 3’
HO
5’
C
Sugarphosphate
backbone
Template strand
P
O
P
T
A
T
P
O
A
P
O
O
P
T
A
T
A
P
P
DNA polymerase
O
O
O
O
P
P
C
G
C
P
O
O
G
P
O
O
P
P
A
3’
OH
A
O
A
T
O
O
T
P
5’
P
G
O
P
P
5’
C
O
O
O
O
3’
P
G
New strand
P
P
P
Pyrophosphate
P
P
O
A
O
OH
P
5’
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3’
OH
DNA polymerase can only build a strand of DNA in
one direction
The leading strand is made continuously from one primer
The lagging strand is assembled in segments created
from many primers
Fig. 7.8
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
RNA primers are removed and replaced with DNA
Ligase joins the ends of newly-synthesized DNA
Fig. 7.7
Mechanisms exist for DNA proofreading and repair
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
7.5 Transcription
The path of genetic information is often called the
central dogma
DNA
RNA
Protein
A cell uses three kinds of RNA to make proteins
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
7.5 Transcription
Gene expression is the use of information in DNA to
direct the production of proteins
It occurs in two stages
Fig. 7.7
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
7.5 Transcription
The transcriber is
RNA polymerase
It binds to one DNA
strand at a site
called the promoter
It then moves along
the DNA pairing
complementary
nucleotides
It disengages at a
stop signal
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 7.11
7.6 Translation
Translation converts the order of the nucleotides of
a gene into the order of amino acids in a protein
The rules that govern translation are called the
genetic code
mRNAs are the “blueprint” copies of nuclear genes
mRNAs are “read” by a ribosome in threenucleotide units, termed codons
Each three-nucleotide sequence codes for an
amino acid or stop signal
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 7.7
The genetic code is (almost) universal
Only a few exceptions have been found
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Ribosomes
The protein-making factories of cells
They use mRNA to direct the assembly of a protein
A ribosome is
made up of two
subunits
Each of which
is composed
of proteins
and rRNA
Fig. 7.13
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Sites play key
roles in
translation
Transfer RNA
tRNAs bring amino
acids to the ribosome
They have two
business ends
Anticodon which is
complementary to
the codon on
mRNA
3’–OH end to
which the amino
acid attaches
Hydrogen
bonding causes
hairpin loops
3-D shape
Fig. 7.14
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Making the Protein
mRNA binds to the
small ribosomal
subunit
The large subunit
joins the complex,
forming the
complete ribosome
mRNA threads
through the
ribosome producing
the polypeptide
Fig. 7.16
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 7.15 How translation works
The process continues until a stop codon enters the A site
The ribosome complex falls apart and the protein is released
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
7.7 Architecture of the Gene
Most eukaryotic genes exist in multiple copies
Clusters of almost identical sequences called
multigene families
As few as three and as many as several
hundred genes
Transposable sequences or transposons are DNA
sequences that can move about in the genome
They are repeated thousands of times, scattered
randomly about the chromosomes
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display