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MOLECULAR GENETICS
DNA
What is the significance of DNA? Why
is it so important?
• Because it is universal. It is the genetic
material for all forms of life.
• It points to a common evolutionary origin
of all living things.
• Why is it relevant in your life today?
The knowledge of DNA have changed our
lives drastically. From forensics to the
biotechnology industry, in agriculture and
in medicine to name just a few areas.
Where is DNA and what does it do?
• DNA is packed in the chromosomes
and the chromosomes are in the nucleus of
all eukaryotic cells.
What is DNA’s function?
DNA’s function is to store and transfer
genetic information from one generation to
the next.
To do this we need mitosis and meiosis so we can copy our
own DNA and pass it on.
What kind of information is encoded in
DNA?
• Instructions of how to make proteins.
What does DNA look like?
• DNA is a nucleic acid.
• Nucleic acids are made up of
nucleotides.
( remember that a nucleotide consists of three parts: a 5
carbon sugar, a phosphate group and a nitrogen base)
DNA looks like a twisted staircase. The
handrails or sides are also called its backbone and are
made up of sugars and phosphate groups connected to
each other. The paired bases made up the rungs of the
ladder connecting the two strands.
– The structure of DNA
• Consists of two nucleotide strands wrapped around
each other in a double helix
Figure 10.3C
Twist
•DNA and RNA are polymers of nucleotides
– DNA is a nucleic acid
• Made of long chains of nucleotide monomers
Sugar-phosphate backbone
Phosphate group
A
C
Nitrogenous base
A
Sugar
DNA nucleotide
C
Nitrogenous base
(A, G, C, or T)
Phosphate
group
O
H3C
O
T
T
O P
O
CH2
O–
G
C
HC
O
C
N
N
C
H
O
Thymine (T)
O
C H
G
H
C
HC
CH
H
Sugar
(deoxyribose)
T
T
DNA nucleotide
Figure 10.2A
DNA polynucleotide
Fig. 16.5
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
– DNA has four kinds of nitrogenous bases
• A, T, C, and G
A pairs with T and
H
O
H3C
H
C
H
N
H
N
C
N
C
C
C
C
H
O
Thymine (T)
Cytosine (C)
Pyrimidines
Figure 10.2B
N
H
H
H
C
H
C
N
G pairs with C
N
H
O
N
H
O
C
C
N
C
C
N
H
C
N
N
H
N
C
C
N
H
Adenine (A)
Guanine (G)
Purines
H
C
N
H
C
C
N
H
H
– RNA is also a nucleic acid
• But has a slightly different sugar
• And has U instead of T
Nitrogenous base
(A, G, C, or U)
O
Phosphate
group
H
O
O
P
N
C
C
H
O CH2
N
H
O
Uracil (U)
O–
O
C H
H C
H C
C H
O
Figure 10.2C, D
C
C
OH
Sugar
(ribose)
Key
Hydrogen atom
Carbon atom
Nitrogen atom
Oxygen atom
Phosphorus atom
A little history…
• In April 1953, James Watson and Francis
Crick
– published a model for the structure of
deoxyribonucleic acid or DNA.
– won the Nobel prize for it
• Your genetic material is the DNA you
inherited from your parents.
• Nucleic acids are unique
– direct their own replication.
• The resemblance of offspring to their parents
– depends on the precise replication of DNA
– transmission from one generation to the next.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The Search for Genetic Material
• Once Thomas Morgan’s group showed that
genes are located on chromosomes, the two
constituents of chromosomes - proteins and
DNA - were the candidates for the genetic
material.
• Until the 1940s, abundance and variety of
proteins seemed to indicate that proteins
were the genetic material.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Early 1900”s everyone “knew” that the genetic material was
protein. In 1928 Fred Griffith trying to produce a
vaccine against pneumonia made scientist doubt .
• Griffith said that protein was not the
genetic material because heat denatures
proteins and in his experiments heat did
not denatured the genetic material
• Still nobody knew what was the genetic
material
• Griffith called this phenomenon transformation,
a change in genotype and phenotype due to the
assimilation of a foreign substance (now known
to be DNA) by a cell.
Fig. 16.1
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
1944, Oswald Avery, McCarty and
MacLeod announced that the
transforming substance was DNA.
• Still, many biologists were skeptical.
– this reflected a belief that the genes of bacteria
could not be similar in composition and
function to those humans.
– Avery confirmed Griffith discovery and that the
genetic material was a nucleotide and not a
protein
THE STRUCTURE OF THE GENETIC MATERIAL
• Experiments showed that DNA is the
genetic material
– The Hershey-Chase experiment showed that
certain viruses reprogram host cells
• To produce more viruses by injecting their DNA
Head
DNA
Tail
Tail fiber
Figure 10.1A
What did Hershey and Chase do?
• They showed that the genetic material of
a virus is DNA.
• They separated the protein coat and the DNA of
the virus (a phage known as T2) and tagged it with a
radioactive isotope and then traced it. The
protein coat remained outside and only the DNA
when inside the bacteria.
• In 1952, Alfred Hershey and Martha Chase showed
that DNA was the genetic material of the phage
T2.( a phage is a virus that attacks bacteria)
• The T2 phage (the virus), consisting almost entirely of
DNA and protein, attacks Escherichia coli (E. coli),
a common intestinal bacteria of mammals.
• This phage can quickly
turn an E. coli cell into
a T2-producing factory
that releases phages
when the cell ruptures.
Fig. 16.2a
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
– The Hershey-Chase experiment
Phage
Radioactive
protein
Bacterium
Empty
protein shell
Radioactivity
in liquid
Phage
DNA
DNA
Batch 1
Radioactive
protein
Centrifuge
Pellet
1 Mix radioactively
labeled phages with
bacteria. The phages
infect the bacterial
cells.
2 Agitate in a blender to
separate phages outside
the bacteria from the cells
and their contents.
3 Centrifuge the mixture
so bacteria form a pellet
at the bottom of the test
tube.
4 Measure the
radioactivity in
the pellet and
the liquid.
Radioactive
DNA
Batch 2
Radioactive
DNA
Centrifuge
Pellet
Figure 10.1B
Radioactivity
in pellet
Chargaff’s Rules
• By 1947, Erwin Chargaff already knew that DNA was a
polymer of nucleotides consisting of a nitrogenous base,
deoxyribose ( a sugar), and a phosphate group.
By 1947 Chargaff had developed rules saying that
The bases could be adenine (A), thymine (T),
guanine (G), or cytosine (C).
Chargaff rule:
A always pairs with T and
C always pairs with G
• In any one species, the four bases are found in
characteristic, but not necessarily equal, ratios.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• He found a regular ratio of nucleotide bases
which are known as Chargaff’s rules.
• The number of adenines was approximately
equal to the number of thymines (%T = %A).
• The number of guanines was approximately
equal to the number of cytosines (%G =
%C).
– Human DNA is 30.9% adenine, 29.4% thymine,
19.9% guanine and 19.8% cytosine.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
By 1950 DNA was accepted by scientists as
the genetic material but nobody knew its
structure
• Many scientists wanted to be the first to
discover it
Race for the Prize
• By the beginnings of the 1950’s, the race
was on to move from the structure of a single
DNA strand to the three-dimensional
structure of DNA.
– Among the scientists working on the problem
were Linus Pauling, in California, and Maurice
Wilkins and Rosalind Franklin, in London.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The story of DNA :The social nature of science
•
•
•
•
•
•
•
In 1951 James Watson went to Europe (after completing his PhD) and met Crick.
Francis Crick was a graduate student working in hemoglobin in Cambridge, England.
They talked about DNA
Crick and Watson teamed up to work on the structure of DNA.
Watson went to London to hear a lecture by Rosalind Franklin on DNA. She and
Maurice Wilkins were authorities on X-ray diffraction and had been working on the
DNA structure on the same lab. R.Franklin made observations about her X ray
diffraction of DNA.
Watson didn’t understand about X-ray crystallography so he didn’t get much out of
Franklin’s lecture.
Rosalind Franklin was a well know physical chemist because of her previous work on
carbon fiber technology. She was invited to King’s College in London to set up an Xray diffraction lab. She was not told of Wilkins interest in her DNA work.
Wilkins did not know about Franklin. He treated her as a lab technician or a
secretary, attacked her behind her back.
Watson and Crick made a model of DNA that was all wrong because they did not
have Franklin’s information. Watson believed that the sugar-phosphate backbone
was on the inside and the bases sticking out. Franklin knew it was on the inside.
The story continues….
• In December 1952 Watson and Crick learned that Linus Pauling was
working on DNA structure. They asked Pauling for a copy of his paper and
Pauling sent one to Watson. They also got the information from Chargaff
about the bases A-t ans C-G.
• Wilkins allowed Crick to get Franklin’s information from her lab’s annual
report without asking Franklin. He gave Franklin’s X ray pictures to Crick.
• They learned all the positions and measurements from Franklin’s picture and
build a model in which everything fit except the bases.
• Examining Chargaff rules they build an accurate model of DNA in March
1953.
• When Watson and Crick wrote their results in a scientific journal Franklin
was given no credit
• Franklin died in 1958 (breast cancer) without ever knowing her data was
used to figure the structure of DNA.
• Franklin’s failure was due to her social isolation because Watson and Crick
had many contacts in her lab, lots of friends and people supporting and
consulting with them.
•DNA is a double-stranded helix
– James Watson and Francis Crick
• Worked out the three-dimensional structure of DNA,
based on work by Rosalind Franklin
Figure 10.3A, B
• Maurice Wilkins and Rosalind Franklin used
X-ray crystallography to study the structure
of DNA.
– In this technique, X-rays are diffracted as they passed
through aligned fibers of purified DNA.
– The diffraction pattern can be used to deduce the threedimensional shape of molecules.
• James Watson learned
from their research
that DNA was helical
in shape and he deduced
the width of the helix
and the spacing of bases.
Fig. 16.4
Watson and Crick
• James Watson and his colleague Francis Crick
began to work on a model of DNA with two strands,
the double helix. They decided on a helix shape
after seeing Franklin’s X ray.
• They build their model to scale to conform to the
X ray data.
Using molecular models made of wire, they first tried
to place the sugar-phosphate chains on the inside..
However, this did not fit the X-ray measurements and
other information on the chemistry of DNA. In 1953
Watson and Crick won the Nobel prize for
discovering the structure of DNA
The Molecule Comes Together
• The key breakthrough came when Watson
put the sugar-phosphate chain on the
outside and the nitrogen bases on the inside
of the double helix.
– The sugar-phosphate chains of each strand are
like the side ropes of a rope ladder.
– Pairs of nitrogen bases, one from each strand,
form rungs.
– The ladder forms a twist every ten bases.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The nitrogenous bases are paired in specific combinations:
adenine with thymine and guanine with cytosine.
• Pairing like nucleotides did not fit the uniform diameter
indicated by the X-ray data.
– A purine-purine pair would be too wide and a pyrimidinepyrimidine pairing would be too short.
– Only a pyrimidinepurine pairing would
produce the 2-nm
diameter indicated
by the X-ray data.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In addition, Watson and Crick determined that chemical
side groups off the nitrogen bases would form hydrogen
bonds, connecting the two strands.
– Based on details of their
structure, adenine would
form two hydrogen bonds
only with thymine and
guanine would form three
hydrogen bonds only with
cytosine.
– This finding explained
Chargaff’s rules.
Fig. 16.6
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
– The structure of DNA
• Consists of two nucleotide strands wrapped around
each other in a double helix
Figure 10.3C
Twist
Fig. 16.5
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Protein synthesis
DNA RNA protein
What type of chemical bonds hold the DNA
molecule together?
• The sugar-phosphates are joined by
covalent bonds which are strong
• The base pairs that form the rungs of the
ladder are hydrogen bonds.
• Hydrogen bonds are easily broken which is
good when the DNA molecule is going to
replicate.
Replication
• When? Replication occurs during the “S”
phase of the cell cycle
• It requires lots of enzymes
• Ex: enzymes unwinds the double helix
breaks the hydrogen bonds between the
paired bases and other enzymes separate
the two strands and add new nucleotides.
• The main team of enzymes are the
DNA polymerases
Form Predicts Function
• The specific pairing of nitrogenous bases in DNA was the
flash of inspiration that led Watson and Crick to the correct
double helix.
• The possible mechanism for the next step, the accurate
replication of DNA, was clear to Watson and Crick from their
double helix model.
• In a second paper Watson and Crick published their
hypothesis for how DNA replicates.
– Essentially, because each strand is complementary to each other,
each can form a template when separated.
– The order of bases on one strand can be used to add in
complementary bases and therefore duplicate the pairs of bases
exactly.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
– DNA replication is a complex process
• Due in part to the fact that some of the helical DNA
molecule must untwist
G C
A
T
G
C
C
G
A
T
T
A
A
C
G
T
C
G
C
C
A
A
T
A
A
Figure 10.4B
G
G
T
G
C
G
T
T
G
C
C
G
C
T
A
A
A
A
G
T
T
T
A
C
T
T
A
Replication is Relatively Fast and
Very Accurate
• It takes E. coli less than an hour to copy 5 million
base pairs in its chromosome and divide to form
two identical daughter cells.
• A human cell can copy its 6 billion base pairs and
divide into daughter cells in only a few hours.
• This process is remarkably accurate, with only one
error per billion nucleotides.
• More than a dozen enzymes and other proteins
participate in DNA replication.
Enzymes: Helicase, DNA polymerase and
ligase
• Helicase: Is the enzyme that untwists the
double helix at the replication forks
• DNA polymerase: is the enzyme that
adds nucleotides to the existing chain.
It catalizes the elongation of the new DNA
strand. It also does proof-reading to
remove mistakes.
• Ligase: a linking enzyme. It bonds
fragments, like glue
DNA REPLICATION
• DNA replication depends on base pairing
– DNA replication starts with the separation of DNA strands (done by
an enzyme)
– Then enzymes use each strand as a template
to assemble new nucleotides into complementary
strands.( DNA polymerase and ligase)
A with T
C with G
A
T
A
T
A
T
A
T
A
T
C
G
C
G
C
G
C
G
C
G
G
C
G
C
G
C
G
C
A
T
A
T
A
T
A
T
T
A
T
A
T
A
T
A
Parental molecule
of DNA
Figure 10.4A
C
A
Nucleotides
Both parental strands serve
as templates
Two identical daughter
molecules of DNA
• When a cell copies a DNA molecule, each strand serves
as a template for ordering nucleotides into a new
complimentary strand.
– One at a time, nucleotides line up along the template
strand according to the base-pairing rules.
– The nucleotides are linked to form new strands.
Each strand serves as a template (blueprint) for
the synthesis of a new strand
• DNA polymerase can only work in one
direction so each new molecule is made
up of an old and a new strand.
• Since DNA is a very long molecule
replication occurs simultaneously at many
sites called replication forks the enzyme
ligase joins them
Errors
• Errors are made frequently during
replication. DNA polymerase may skip a
nucleotide or add an extra one or put one
in the wrong place.
• Very few errors remain because a system
of enzymes ( repair nuclease) that detect
and repair the errors.
• Those errors that remain are called
MUTATIONS
• Watson and Crick’s model, semiconservative
replication, predicts that when a double helix replicates
each of the daughter molecules will have one old strand
and one newly made strand.
• Other competing models, the conservative model and the
dispersive model, were also proposed.
Fig. 16.8
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The first replication in the 14N medium produced a
band of hybrid (15N-14N) DNA, eliminating the
conservative model.
• A second replication produced both light and hybrid
DNA, eliminating the dispersive model and supporting
the semiconservative model.
Fig. 16.9
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
– Each strand of the double helix
• Is oriented in the opposite direction
5 end
P
3 end
HO
5
4
3
2
2
1
A
T 1
5
P
P
C
G
P
P
G
C
P
P
T
OH
3 end
Figure 10.5B
3
4
A
P
5 end
– Using the enzyme DNA polymerase
• The cell synthesizes one daughter strand as a continuous
piece
– The other strand is synthesized as a series of short
pieces
• Which are then connected by the enzyme DNA ligase
DNA polymerase
molecule
5
3
3
5
Daughter strand
synthesized
continuously
Parental DNA
3
5
5
3
DNA ligase
Figure 10.5C
Overall direction of replication
Daughter
strand
synthesized
in pieces
Protein synthesis
DNA RNA protein
THE FLOW OF GENETIC INFORMATION FROM
DNA TO RNA TO PROTEIN
• The DNA genotype is expressed as proteins,
which provide the molecular basis for
phenotypic traits
• The information constituting an organism’s
genotype
– Is carried in its sequence of its DNA bases
• A particular gene, a linear sequence of many
nucleotides
– Specifies a polypeptide
• Genetic information written in codons is
translated into amino acid sequences
– The “words” of the DNA “language”
• Are triplets of bases called codons
– The codons in a gene
• Specify the amino acid sequence of a polypeptide
DNA molecule
Gene 1
Gene 2
Gene 3
DNA strand
A A A C C G G C A A A A
Transcription
U U U G G C C G U U U U
RNA
Codon
Translation
Polypeptide
Figure 10.7
Amino acid
• The genetic code is the Rosetta stone of
life
– Nearly all organisms
• Use exactly the same genetic code
Second base
U
U
C
UUU
Phe
UUC
UCU
UCC
UUA
UCA
UUG
Leu
CUU
C
CUC
First base
CUA
CUG
A
Leu
GUA
GUG
Figure 10.8A
UGA Stop A
UGG Trp
CCU
CAU
His
CAC
CGU
CGC
CAA
CAG Gln
CGA
CGG
AAU
Asn
AAC
U
AGU
Ser
AGC
C
AAA
AGA
A
AGG Arg
G
U
GGU
C
GGC
Gly
GGA
A
CCC
CCA
CCG
ACA
ACC
Pro
Thr
AAG
GAU
GCU
GCC
Val
UAC
UGU Cys U
UGC
C
UAA Stop
AUA
GUC
G
Ser
Tyr
UAG Stop
ACU
ACC
GUU
UAU
G
UCG
AUU
AUC Ile
Met or
AUG
start
A
GCA
GCG
GAC
Ala
Lys
Asp
GAA
GAG
Glu
GGG
G
U
C
Arg
A
G
G
– An exercise in translating the genetic code
Strand to be transcribed
T
A
C
T
T
C
A
A
A
A
T
C
A
T
G
A
A
G
T
T
T
T
A
G
U
A
G
DNA
Transcription
A
U
G
A
A
G
U
U
U
RNA
Start
condon
Stop
condon
Translation
Figure 10.8B
Polypeptide
Met
Lys
Phe
• Transcription produces genetic messages
in the form of RNA
– A close-up view of transcription
RNA nucleotides
RNA
polymerase
T C C A
A U
A
T
T
A
C C A
T A G G T
Direction of
transcription
Figure 10.9A
Newly made RNA
Template
Strand of DNA
– In the nucleus, the DNA helix unzips
• And RNA nucleotides line up along one strand of the
DNA, following the base pairing rules
– As the single-stranded messenger RNA
(mRNA) peels away from the gene
• The DNA strands rejoin
– Transcription of a gene
RNA polymerase
DNA of gene
Promoter
DNA
Terminator
DNA
1 Initiation
2 Elongation
3 Termination
Completed RNA
Figure 10.9B
Area shown
In Figure 10.9A
Growing
RNA
RNA
polymerase
• Eukaryotic RNA is processed before leaving
the nucleus
– Noncoding segments called introns are spliced
out
• And a cap and a tail are added to the ends
Exon Intron
Exon
Intron
Exon
DNA
Cap
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
Nucleus
Cytoplasm
Figure 10.10
• Transfer RNA molecules serve as
interpreters during translation
– Translation
• Takes place in the cytoplasm
– A ribosome attaches to the mRNA
• And translates its message into a specific
polypeptide aided by transfer RNAs (tRNAs)
Amino acid attachment site
Hydrogen bond
RNA polynucleotide chain
Figure 10.11A
Anticodon
• Ribosomes build polypeptides
– A ribosome consists of two subunits
• Each made up of proteins and a kind of RNA called
ribosomal RNA
tRNA
molecules
Growing
polypeptide
Large
subunit
mRNA
Figure 10.12A
Small
subunit
– The subunits of a ribosome
• Hold the tRNA and mRNA close together during
translation
tRNA-binding sites
Large
subunit
Next amino acid
to be added to
polypeptide
Growing
polypeptide
tRNA
mRNAbinding site
mRNA
Small
subunit
Codons
Figure 10.12B, C
• An initiation codon marks the start of an
mRNA message
Start of genetic message
End
Figure 10.13A
– mRNA, a specific tRNA, and the ribosome
subunits
• Assemble during initiation
Met
Met
Large
ribosomal
subunit
Initiator tRNA
P site
U
A C
A U G
U
A C
A U G
Start
codon
1
mRNA
Figure 10.13B
A site
Small ribosomal
subunit
2
• Elongation adds amino acids to the
polypeptide chain until a stop codon
terminates translation
– Once initiation is complete
• Amino acids are added one by one to the first amino
acid
– Each addition of an amino acid
• Occurs in a three-step elongation process
Amino
acid
Polypeptide
P site
A site
Anticodon
mRNA
Codons
1 Codon recognition
mRNA
movement
Stop
codon
2 Peptide bond
formation
New
Peptide
bond
Figure 10.14
3 Translocation
– The mRNA moves a codon at a time
• And a tRNA with a complementary anticodon pairs
with each codon, adding its amino acid to the
peptide chain
– Elongation continues
• Until a stop codon reaches the ribosome’s A site,
terminating translation
• Review: The flow of genetic information in
the cell is DNARNAprotein
– The sequence of codons in DNA, via the
sequence of codons
• Spells out the primary structure of a polypeptide
– Summary of transcription and translation
DNA
Transcription
1 mRNA is transcribed
from a DNA template.
mRNA
RNA
polymerase
Amino acid
2 Each amino acid
attaches to its proper
tRNA with the help of a
specific enzyme and ATP.
Translation
Enzyme
ATP
tRNA
Large
ribosomal
subunit
Initiator
tRNA
Start Codon
mRNA
Anticodon
3 Initiation of
polypeptide synthesis
The mRNA, the first tRNA,
and the ribosomal
subunits come together.
Small
ribosomal
subunit
New peptide
bond forming
4 Elongation
Growing
polypeptide
A succession of tRNAs
add their amino acids to
the polypeptide chain as the mRNA is
moved through the ribosome, one codon
at a time.
Codons
mRNA
Polypeptide
5 Termination
The ribosome recognizes a stop
codon. The poly-peptide is
terminated and released.
Figure 10.15
Stop codon
• Mutations can change the meaning of
genes
– Mutations are changes in the DNA base
sequence caused by errors in DNA replication
or recombination, or by mutagens
Normal hemoglobin DNA
C
T
T
mRNA
A
T
G U
A
C
mRNA
G
Figure 10.16A
Mutant hemoglobin DNA
A
A
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
– Substituting, inserting, or deleting nucleotides
alters a gene
• With varying effects on the organism
Normal gene
A U G A A G U U U G G C G C A
mRNA
Met
Protein
Lys
Phe
Gly
Ala
Base substitution
A U G A A G U U U A G C G C A
Met
Lys
Phe
Ser
Ala
U Missing
Base deletion
A U G A A G U U G G C G C A U
Figure 10.16B
Met
Lys
Leu
Ala
His
MICROBIAL GENETICS
• Viral DNA may become part of the host
chromosome
– Viruses
• Can be regarded as genes packaged in protein
– When phage DNA enters a lytic cycle inside a
bacterium
• It is replicated, transcribed, and translated
– The new viral DNA and protein molecules
• Then assemble into new phages, which burst from
the host cell
– In the lysogenic cycle
• Phage DNA inserts into the host chromosome and is
passed on to generations of daughter cells
– Much later
• It may initiate phage production
The AIDS virus makes DNA on an RNA
template
– HIV, the AIDS virus
• Is a retrovirus
Envelope
Glycoprotein
Protein coat
RNA (two
identical strands)
Reverse transcriptase
Figure 10.21A
– Inside a cell, HIV uses its RNA as a template
for making DNA
• To insert into a host chromosome
Viral RNA
CYTOPLASM
1
RNA
strand
NUCLEUS
Chromosomal
DNA
2
Doublestranded
DNA
3
Provirus
DNA
4
Viral
RNA
and
proteins
5
RNA
6
Figure 10.21B
• 10.22 Bacteria can transfer DNA in three
ways
– Bacteria can transfer genes from cell to cell
by one of three processes
• Transformation, transduction, or conjugation
Mating bridge
DNA enters
cell
Fragment of DNA
from another
bacterial cell
Bacterial
chromosome
(DNA)
Figure 10.22A–C
Phage
Phage
Fragment of
DNA from
another
bacterial cell
(former phage
host)
Sex pili
Donor cell
(“male”)
Recipient cell
(“female”)
– Once new DNA gets into a bacterial cell
• Part of it may then integrate into the recipient’s
chromosome
Donated DNA
Figure 10.22D
Recipient cell’s
chromosome
Crossovers
Degraded DNA
Recombinant
chromosome
• Bacterial plasmids can serve as carriers
for gene transfer
– Plasmids
• Are small circular DNA molecules separate from
the bacterial chromosome
– Plasmids can serve as carriers
• For the transfer of genes
F factor (plasmid)
F factor
(integrated)
Male (donor) cell
Origin of F
replication
Bacterial
chromosome
F factor starts replication
and transfer of chromosome
Male (donor) cell
Bacterial chromosome
F factor starts replication
and transfer
Only part of the chromosome
transfers
Plasmid completes transfer
and circularizes
Plasmids
Recombination
can occur
Cell now male
Figure 10.23A–C
Colorized TEM 2,000
Recipient cell