Q1 vs. Q2 Grades
Grade
A
B
C
D
F
1st Quarter
55
32
21
7
19
2nd Quarter
75
27
16
8
5
Figure 18.6 The lytic cycle of phage T4, a virulent phage
1 Attachment. The T4 phage uses
its tail fibers to bind to specific
receptor sites on the outer
surface of an E. coli cell.
5 Release. The phage directs production
of an enzyme that damages the bacterial
cell wall, allowing fluid to enter. The cell
swells and finally bursts, releasing 100
to 200 phage particles.
2 Entry of phage DNA
and degradation of host DNA.
The sheath of the tail contracts,
injecting the phage DNA into
the cell and leaving an empty
capsid outside. The cell’s
DNA is hydrolyzed.
Phage assembly
4 Assembly. Three separate sets of proteins
self-assemble to form phage heads, tails,
and tail fibers. The phage genome is
packaged inside the capsid as the head forms.
Head
Tails
Tail fibers
3 Synthesis of viral genomes
and proteins. The phage DNA
directs production of phage
proteins and copies of the phage
genome by host enzymes, using
components within the cell.
Figure 18.7 The lytic and lysogenic cycles of phage ,
a temperate phage
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
Phage DNA
circularizes
Phage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Bacterial
chromosome
Lytic cycle
Lysogenic cycle
Certain factors
determine whether
The cell lyses, releasing phages.
Lytic cycle
is induced
New phage DNA and
proteins are synthesized
and assembled into phages.
or
Lysogenic cycle
is entered
Prophage
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Figure 18.10 The reproductive cycle of HIV, a retrovirus
HIV
Membrane of
white blood cell
1 The virus fuses with the
cell’s plasma membrane.
The capsid proteins are
removed, releasing the
viral proteins and RNA.
2 Reverse transcriptase
catalyzes the synthesis of a
DNA strand complementary
to the viral RNA.
HOST CELL
3 Reverse transcriptase
catalyzes the synthesis of
a second DNA strand
complementary to the first.
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
4 The double-stranded
DNA is incorporated
as a provirus into the cell’s
DNA.
0.25 µm
HIV entering a cell
DNA
NUCLEUS
Chromosomal
DNA
Provirus
5 Proviral genes are
transcribed into RNA
molecules, which serve as
genomes for the next viral
generation and as mRNAs for
translation into viral proteins.
RNA genome
for the next
viral generation
mRNA
6 The viral proteins include capsid
proteins and reverse transcriptase
(made in the cytosol) and envelope
glycoproteins (made in the ER).
New HIV leaving a cell
9 New viruses bud
off from the host cell.
8 Capsids are
assembled around
viral genomes and
reverse transcriptase
molecules.
7 Vesicles transport the
glycoproteins from the ER to
the cell’s plasma membrane.
The Genetics of Viruses & Bacteria
1.
2.
3.
4.
5.
6.
7.
8.
What do you know about viruses?
How big are viruses?
What are the components of a virus?
How do viruses identify appropriate cells to infect?
What is the lytic cycle of a bacteriophage?
What is the lysogenic cycle of a bacteriophage?
How do retroviruses (like HIV) reproduce?
How do “new” viruses emerge?
- Mutation of an existing virus since there is no proofreading
- Spread of an existing virus from 1 host species to another
- Spread of viral disease from a small isolated population
9. What is the difference between horizontal & vertical transmission?
- Horizontal – 1 organism spreads to another
- Vertical – 1 organism inherits disease from parent
10. What are viroids & prions?
- Viroids – tiny molecules of naked, circular RNA that infect plants,
several hundred nucleotides long
- Prions – infectious proteins (NO genetic material)
- Slow incubation period – at least 10 yrs
- Virtually indestructible
- 1997 Nobel Prize in Medicine – Stanley Prusiner
The Genetics of Viruses & Bacteria
1. What do you know about viruses?
2. How big are viruses?
3. What are the components of a virus?
4. How do viruses identify appropriate cells to infect?
5. What is the lytic cycle of a bacteriophage?
6. What is the lysogenic cycle of a bacteriophage?
7. How do retroviruses (like HIV) reproduce?
8. How do “new” viruses emerge?
9. What is the difference between horizontal & vertical transmission?
10. What are viroids & prions?
11. How is bacterial DNA different from eukaryotic DNA? (refer to Ch. 19 notes)
Bacterial
Eukaryotic
Circular chromosome
Linear chromosomes
Nucleoid region
Nucleus
No introns (all exons)
Introns & exons
Transcription coupled w/ translation Transcription & translation separate
More mutations
Fewer mutations (proofreading)
12.How does bacterial DNA replicate its circular chromosome?
- Figure 16.16
Figure 18.16 Generalized transduction
Phage DNA
1
Phage infects bacterial cell that has alleles A+ and B+
2
Host DNA (brown) is fragmented, and phage DNA
and proteins are made. This is the donor cell.
A+ B+
A+ B+
Donor
cell
3
A bacterial DNA fragment (in this case a fragment with
the A+ allele) may be packaged in a phage capsid.
A+
4
Phage with the A+ allele from the donor cell infects
a recipient A–B– cell, and crossing over (recombination)
between donor DNA (brown) and recipient DNA
(green) occurs at two places (dotted lines).
Crossing
over
A+
A– B–
Recipient
cell
5
The genotype of the resulting recombinant cell (A+B–)
differs from the genotypes of both the donor (A+B+) and
the recipient (A–B–).
A+ B–
Recombinant cell
Figure 18.17 Bacterial conjugation
Sex pilus
1 m
Figure 18.18 Conjugation and recombination in E. coli
F Plasmid
Bacterial chromosome
F+ cell
F+ cell
Mating
bridge
1
F+ cell
Bacterial
chromosome
F– cell
2
A cell carrying an F plasmid
(an F+ cell) can form a
mating bridge with an F– cell
and transfer its F plasmid.
4
3
DNA replication occurs in
both donor and recipient
cells, using the single
parental strands of the
F plasmid as templates
to synthesize complementary
strands.
A single strand of the F
plasmid breaks at a
specific point (tip of blue
arrowhead) and begins to
move into the recipient cell.
As transfer continues, the
donor plasmid rotates
(red arrow).
The plasmid in the
recipient cell
circularizes. Transfer
and replication result
in a compete F plasmid
in each cell. Thus, both
cells are now F+.
(a) Conjugation and transfer of an
F plasmid from an F+ donor to
an F– recipient
Hfr cell
F+ cell
F factor
1
The circular F plasmid in an F + cell
can be integrated into the circular
chromosome by a single crossover
event (dotted line).
2
The resulting cell is called an Hfr cell
(for High frequency of recombination).
Plasmid – extra-chromosomal, small, circular, self-replicating DNA
B+
A+
C+
D+
C+
B+
D+
D+
A+
C+
B+
A+
D+
C+
B+
A+
A+
B+
F– cell
3
B–
C–
A–
Since an Hfr cell has all
the F-factor genes, it can
form a mating bridge with
an F– cell and transfer DNA.
D–
B–
A–
B+
B–
C–
A–
B–
C–
B+
D–
A–
5
A single strand of the F factor
breaks and begins to move
through the bridge. DNA
replication occurs in both donor
and recipient cells, resulting in
double-stranded DNA
A+
A+
D–
4
Temporary
partial
diploid
7
C–
A+
The location and orientation
of the F factor in the donor
chromosome determine
the sequence of gene transfer
during conjugation. In this
example, the transfer sequence
for four genes is A-B-C-D.
B–
D–
A+
B+
C–
A–
D–
6
B–
C–
A–
The mating bridge
usually breaks well
before the entire
chromosome and
the rest of the
F factor are transferred.
Recombinant F–
bacterium
D–
Figure 18.21 The trp operon: regulated synthesis of repressible
enzymes
trp operon
Promoter
DNA
Promoter
Genes of operon
trpD
trpC
trpE
trpR
trpB
trpA
Operator
Regulatory
gene
mRNA
5
3
RNA
polymerase
Start codon
Stop codon
mRNA 5
E
Protein
Inactive
repressor
D
C
B
Polypeptides that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the
promoter and transcribes the operon’s genes.
A
DNA
No RNA made
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off. As tryptophan
accumulates, it inhibits its own production by activating the repressor protein.
Figure 18.22 The lac operon: regulated synthesis of inducible enzymes
Promoter
Regulatory
gene
DNA
Operator
lacl
lacZ
No
RNA
made
3
mRNA
Protein
RNA
polymerase
5
Active
repressor
(a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and in
the absence of lactose it switches off the operon by binding to the operator.
lac operon
DNA
lacl
lacz
3
mRNA
5
lacA
RNA
polymerase
mRNA 5'
5
mRNA
-Galactosidase
Protein
Allolactose
(inducer)
lacY
Permease
Transacetylase
Inactive
repressor
(b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses
the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.
• Before the invention of antibiotics, the clean
modern hospitals of India, which were mostly
reserved for Europeans, reported cholera
death rates of 86%. Meanwhile, in other more
crowded and less hygienic hospitals, the death
rate from cholera was only 27%. WHY? Based
on knowledge gained from the case study,
explain the most likely process that occurred
in the crowded hospitals that led to such a low
death rate from cholera.
(a) Animal development. Most
animals go through some
variation of the blastula and
gastrula stages. The blastula is
a sphere of cells surrounding a
fluid-filled cavity. The gastrula
forms when a region of the blastula
folds inward, creating a
tube—a rudimentary gut. Once
the animal is mature,
differentiation occurs in only a
limited way—for the replacement
of damaged or lost cells.
Cell
movement
Zygote
(fertilized egg)
Eight cells
Blastula
(cross section)
Gut
Gastrula
(cross section)
Adult animal
(sea star)
Cell division
Morphogenesis
(b) Plant development. In plants
with seeds, a complete embryo
develops within the seed.
Morphogenesis, which involves
cell division and cell wall
expansion rather than cell or
tissue movement, occurs
throughout the plant’s lifetime.
Apical meristems (purple)
continuously arise and develop
into the various plant organs as
the plant grows to an
indeterminate size.
Observable cell differentiation
Seed
leaves
Shoot
apical
meristem
Zygote
(fertilized egg)
Root
apical
meristem
Two cells
Figure 21.4a, b
Embryo
inside seed
Plant
Chapter 21: The Genetic Basis of Development
1. How do we study development in the genetics-based lab?
2. How does a zygote transform into an organism?
3. How do cells become differentiated?
-All cells have the same DNA, so differential gene expression
must be the explanation!
APPLICATION This method is used to produce cloned
animals whose nuclear genes are identical to the donor
animal supplying the nucleus.
TECHNIQUE
Shown here is the procedure used to produce
Dolly, the first reported case of a mammal cloned using the nucleus
of a differentiated cell.
The cloned animal is identical in appearance
RESULTS
and genetic makeup to the donor animal supplying the nucleus,
but differs from the egg cell donor and surrogate mother.
Egg cell
donor
Mammary
cell donor
1
2
Egg cell
from ovary Nucleus
Nucleus
3 Cells fused removed
removed
Cultured
mammary cells
are semistarved,
arresting the cell
cycle and causing
dedifferentiation
4 Grown in culture
Nucleus from
mammary cell
Early embryo
5 Implanted in uterus
of a third sheep
6 Embryonic
development
Figure 21.7
Surrogate
mother
Lamb (“Dolly”)
genetically identical to
mammary cell donor
Chapter 21: The Genetic Basis of Development
4. What is a stem cell?
-a relatively unspecialized cell
-can differentiate into cells of different types under specific conditions
-Embryonic = totipotent
-Adult = pluripotent (can produce some, but not all, cell types)
Embryonic stem cells
Early human embryo
at blastocyst stage
(mammalian equivalent of blastula)
Adult stem cells
From bone marrow
in this example
Totipotent
cells
Pluripotent
cells
Cultured
stem cells
Different
culture
conditions
Different
types of
differentiated
cells
Figure 21.9
Liver cells
Nerve cells
Blood cells
Chapter 21: The Genetic Basis of Development
5. What type of genetic signal leads to cell differentiation?
-Step 1: Cell receives signals from other cells
-Step 2: A regulatory gene is turned “on”, and a protein is made that
activates other genes. (“point of no return”)
-Step 3: Activated genes make proteins that determine cell type/
structure/behavior.
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
OFF
Embryonic
precursor cell
Determination. Signals from other
cells lead to activation of a master
regulatory gene called myoD, and
the cell makes MyoD protein, a
transcription factor. The cell, now
called a myoblast, is irreversibly
committed to becoming a skeletal
Myoblast
(determined) muscle cell.
OFF
1
2
Differentiation. MyoD protein stimulates
the myoD gene further, and activates
genes encoding other muscle-specific
transcription factors, which in turn
activate genes for muscle proteins. MyoD
also turns on genes that block the cell
cycle, thus stopping cell division. The
nondividing myoblasts fuse to become
mature multinucleate muscle cells, also
called muscle fibers.
OFF
mRNA
MyoD protein
(transcription
factor)
mRNA
MyoD
Muscle cell
(fully differentiated)
mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell-cycle
blocking proteins
Epidermis
Gonad
Anchor cell
Signal
protein
Vulval precursor cells
Outer vulva
ADULT
Inner vulva
Epidermis
Figure 21.16b
(b) Induction of vulval cell types during larval
development.
EXPERIMENT
Spemann and Mangold transplanted a piece of the dorsal lip of a pigmented newt gastrula to the
ventral side of the early gastrula of a nonpigmented newt.
Pigmented gastrula
(donor embryo)
Dorsal lip of
blastopore
Nonpigmented gastrula
(recipient embryo)
RESULTS
During subsequent development, the recipient embryo formed a second notochord and neural tube in
the region of the transplant, and eventually most of a second embryo. Examination of the interior of the double embryo
revealed that the secondary structures were formed in part from host tissue.
Primary embryo
Primary
structures:
Secondary
structures:
Notochord (pigmented cells)
Secondary (induced) embryo
Neural tube
Notochord
Neural tube (mostly nonpigmented cells)
CONCLUSION
Figure 47.25
The transplanted dorsal lip was able to induce cells in a different region of the recipient to form
structures different from their normal fate. In effect, the dorsal lip “organized” the later development of an entire embryo.
Chapter 21: The Genetic Basis of Development
-cytoplasmic determinants in the unfertilized egg regulate gene
expression in the zygote that affects differentiation/development
Unfertilized egg cell
Molecules of a
a cytoplasmic
determinant
Fertilization
Nucleus
Zygote
(fertilized egg)
Mitotic cell division
Two-celled
embryo
(a)
Cytoplasmic determinants in the egg. The unfertilized egg cell has molecules in its cytoplasm,
encoded by the mother’s genes, that influence development. Many of these cytoplasmic
determinants, like the two shown here, are unevenly distributed in the egg. After fertilization
and mitotic division, the cell nuclei of the embryo are exposed to different sets of cytoplasmic
determinants and, as a result, express different genes.
Figure 21.11a
Chapter 21: The Genetic Basis of Development
-Cytoplasmic determinants from mother’s egg initially establish the axes of
the body of Drosophila.
-bicoid gene
Tail
Head
T1 T2
T3
A1 A2 A3 A4 A5
A6 A7
A8
Wild-type larva
Tail
Tail
A8
A7
Mutant larva (bicoid)
A8
A6
A7
(a) Drosophila larvae with wild-type and bicoid mutant phenotypes. A mutation
Figure 21.14a
in the mother’s bicoid gene leads to tail structures at both ends (bottom larva).
The numbers refer to the thoracic and abdominal segments that are present.
Egg cell
Nurse cells
1 Developing
egg cell
bicoid mRNA
2 Bicoid mRNA
in mature
unfertilized egg
Fertilization
Translation of bicoid mRNA
100 µm
3 Bicoid protein in
early embryo
Anterior end
(b) Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo.
Figure 21.14b
Chapter 21: The Genetic Basis of Development
-7. How does morphogenesis (pattern formation) occur in animals?
After the body’s axes are determined (by cytoplasmic determinants)…
-Segmentation genes produce proteins that direct formation of
body segments.
-Then, the development of specific features of the body segments is directed
by HOMEOTIC GENES (Hox genes.)
Hierarchy of Gene Activity in Early Drosophila Development
Maternal effect genes (egg-polarity genes)
Gap genes
Pair-rule genes
Segmentation genes
of the embryo
Segment polarity genes
Homeotic genes of the embryo
Other genes of the embryo
Chapter 21: The Genetic Basis of Development
8. What is the relationship among the genetic basis of development
across organisms?
-Molecular analysis of the homeotic genes in Drosophila has shown that they
all include a sequence called a homeobox
-An identical (or very similar) DNA sequence has been discovered in the
homeotic genes of vertebrates and invertebrates
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse
Figure 21.23
Chapter 21: The Genetic Basis of Development
9. What is apoptosis?
-programmed cell death (cell suicide)
Ced-9
protein (active)
inhibits Ced-4
activity
Death
signal
receptor
Mitochondrion
Ced-4 Ced-3
Inactive proteins
Cell
forms
blebs
(a) No death signal
Ced-9
(inactive)
Death
signal
Active Active
Ced-4 Ced-3
Activation
cascade
(b) Death signal
Other
proteases
Nucleases
Figure 21.18 Molecular
basis of apoptosis in C.
elegans
Chapter 21: The Genetic Basis of Development
8. What is apoptosis?
-programmed cell death (cell suicide)
-necessary for development of hands/feet in vertebrates
Interdigital tissue
1 mm
Figure 21.19