Fourth Edition
9
PowerPoint ® Lectures prepared by Johnny El-Rady
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Mendel’s work left a key question unanswered:
What is a gene?
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
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In 1928, 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
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Fig. 9.1 How Griffith discovered transformation
Thus, the dead S bacteria somehow “transformed” the live R bacteria into live S bacteria
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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 1944
Alfred Hershey and Martha Chase published their results in 1952
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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. Same chemistry and physical properties as DNA
2. Not affected by lipid and protein extraction
3. Not destroyed by protein- or RNA-digesting enzymes
4. Destroyed by DNA-digesting enzymes
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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
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Fig. 9.2 The
Hershey-Chase
Experiment
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Thus, viral DNA directs the production of new viruses

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)
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Fig. 9.3 The four nucleotide subunits that make up DNA
Nitrogenous base
5-C sugar
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Erwin Chargaff made key DNA observations that became known as Chargaff’s rule
Purines = Pyrimidines A = T and C = G
Fig. 9.4
Rosalind Franklin’s
X-ray diffraction experiments revealed that DNA had the shape of a coiled spring or helix
Rosalind
Franklin
(1920-1958)
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In 1953, 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. 9.4
James Watson
(1928- )
Francis Crick
(1916-2004)
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Fig. 9.4 The DNA double helix
Dimensions suggested by
X-ray diffraction
The two possible basepairs
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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
Each chain is a complementary mirror image of the other
So either can be used as template to reconstruct the other
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There are 3 possible methods for
DNA replication
Fig. 9.5
Daughter DNAs contain one old and one new strand
Original DNA molecule is preserved
Old and new
DNA are dispersed in daughter molecules
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These three mechanisms were tested in 1958 by
Matthew Meselson and Franklin Stahl
Fig. 9.6
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Thus, DNA replication is semi-conservative
Fig. 9.6
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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
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Sugar- phosphate backbone
Fig. 9.7 How nucleotides are added in DNA replication
Template strand
HO 3’
C
G
New strand
5’
P
O
Template strand
HO 3’
C
G
P
O
T
A
P
P
O
T
A
O
O O
P P
A
T
P
T
O DNA polymerase
A
O
New strand
5’
P
O
O
P
P
O O
P P
C C
G
P
G
P
O
O O
O
P
P
A 3’ OH A T
O
P
Pyrophosphate
P T
O O
P P P
P
A
O
A 3’
OH
P
5’
O
OH
P
O
5’
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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. 9.8
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RNA primers are removed and replaced with DNA
Ligase joins the ends of newly-synthesized DNA
Fig. 9.9
Mechanisms exist for DNA proofreading and repair
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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)
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Gene expression is the use of information in DNA to direct the production of proteins
It occurs in two stages
Fig. 9.10
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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
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Fig. 9.11

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
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Fig. 9.12
The genetic code is (almost) universal
Only a few exceptions have been found
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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. 9.13
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Sites play key roles in translation

Hydrogen bonding causes hairpin loops 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
3-D shape
Fig. 9.14
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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. 9.16
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Fig. 9.15 How translation works
The process continues until a stop codon enters the A site
The ribosome complex falls apart and the protein is released
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In eukaryotes, genes are fragmented
They are composed of
Exons – Sequences that code for amino acids
Introns – Sequences that don’t
Eukaryotic cells transcribe the entire gene, producing a primary RNA transcript
This transcript is then heavily processed to produce the mature mRNA transcript
This leaves the nucleus for the cytoplasm
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Fig. 9.17 Processing eukaryotic mRNA
Protect from degradation and facilitate translation
Different combinations of exons can generate different polypeptides via alternative splicing
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6. The polypeptide chain grows until the protetin is completed.
Amino acid
7. Phosphorylation or other chemical modifications can alter the activity of a protein after it is translated.
Completed polypeptide tRNA
5’
Ribosome moves toward 3’ end
Fig. 9.18 How protein synthesis works in eukaryotes
Cytoplasm
DNA
5. tRNAs bring their amino acids in at the A site of the ribosome. Peptide bonds form between amino acids at the P site, and tRNAs exit the ribosome from the E site.
Ribosome
4. tRNA molecules become attached to specific amino acids with the help of activating enzymes.
Amino acids are brought to the ribosome in the order dictated by the mRNA.
3’
RNA polymerase
1. In the cell nucleus, RNA polymerase transcribes
RNA from DNA
3’
Poly-A tail
Introns
3’
Nuclear membrane
5’
5’
Primary
3’
RNA transcript
Exons
5’
Cap mRNA
Poly-A tail
2. Introns are excised from the RNA transcript, and the remaining exons are spliced together, producing mRNA
3’
Small ribosomal subunit
Nuclear pore mRNA
5’
Cap
Large ribosomal subunit
3. mRNA is transported out of the nucleus. In the cytoplasm, ribosomal subunits bind to the mRNA
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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
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Genes are typically controlled at the level of transcription
In prokaryotes, proteins either block or allow the
RNA polymerase access to the promoter
Repressors block the promoter
Activators make the promoter more accessible
Most genes are turned off except when needed
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lac
An operon is a segment of DNA that contains a cluster of genes that are transcribed as a unit
The lac operon contains
Three structural genes
Encode enzymes involved in lactose metabolism
Two adjacent DNA elements
Promoter
Site where RNA polymerase binds
Operator
Site where the lac repressor binds
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lac
In the absence of lactose, the lac repressor binds to the operator
RNA polymerase cannot access the promoter
Therefore, the lac operon is shut down
Fig. 9.19
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lac
In the presence of lactose, a metabolite of lactose called allolactose binds to the repressor
This induces a change in the shape of the repressor which makes it fall off the operator
RNA polymerase can now bind to the promoter
Transcription of the lac operon is ON
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Fig. 9.19
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lac
What if the cell encounters lactose, and it already has glucose?
The bacterial cell actually prefers glucose!
The lac operon is also regulated by an activator
The activator is a protein called CAP
It binds to the CAP-binding site and gives the
RNA polymerase more access to the promoter
However, a “low glucose” signal molecule has to bind to CAP before CAP can bind to the DNA
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Fig. 9.20 Activators and repressors of the lac operon
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DNA sequences that make the promoters of genes more accessible to many regulatory proteins at the same time
Usually located far away from the gene they regulate
Common in eukaryotes; rare in prokaryotes
Fig. 9.21
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The genetic material can be altered in two ways
Recombination
Change in the positioning of the genetic material
Mutation
Change in the content of the genetic material
Bithorax mutant
Fig. 9.22
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Mutation and recombination provide the raw material for evolution
Evolution can be viewed as the selection of particular combinations of alleles from a pool of alternatives
The rate of evolution is ultimately limited by the rate at which these alternatives are generated
Mutations in germ-line tissues can be inherited
Mutations in somatic tissues are not inherited
They can be passed from one cell to all its descendants
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Mutations are caused in one of two ways
Errors in DNA replication
Mispairing of bases by DNA polymerase
Mutagens
Agents that damage DNA
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The sequence of DNA can be altered in one of two main ways
Point mutations
Alteration of one or a few bases
Base substitutions, insertion or deletion
Frame-shift mutations
Insertions or deletions that throw off the reading frame
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Fig. 9.23
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The position of genes can be altered in one of two main ways
Transposition
Movement of genes from one part of the genome to another
Occurs in both eukaryotes and prokaryotes
Chromosomal rearrangements
Changes in position and/or number of large segments of chromosomes in eukaryotes
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Agents that cause cancer are called carcinogens
These are typically mutagens
The hypothesis that chemicals cause cancer was first advanced in the 18 th century
Many investigations since then have determined that chemicals can cause cancer in both animals and humans
For example, tars and other chemicals in cigarette smoke can cause cancer of the lung
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