Transcript Lecture 1

Chapter 13
Lecture 1
Viruses
Angelika Stollewerk
Associate Professor of
Evolutionary Developmental Biology
Viruses
Aims:
•To appreciate the basic structure of
viruses
•To be able to discuss the reproduction
of bacteriophages
•To appreciate the differences in viruses
in humans
Viruses
Aims:
• To appreciate the basic structure of viruses
• To be able to discuss the reproduction of bacteriophages
• To appreciate the differences in viruses in humans
This lecture forms part of the
knowledge required for learning
outcome 2:
Describe basic organism structure
and diversity (LOC2).
Viruses
Essential reading
•pages 282 - 289
Influenza virus particles
(brown) invade cilia
(blue) in the airways of
the human lung.
13 The Genetics of Viruses and Prokaryotes
• 13.1 How Do Viruses Reproduce and
Transmit Genes?
13.1 How Do Viruses Reproduce and Transmit Genes?
Prokaryotes and viruses make good
model organisms:
• Small genomes
• Reproduce quickly
• Usually haploid
Figure 13.1 Model Organisms – Relative Sizes
Bacteriophage T4
Escherichia coli
13.1 How Do Viruses Reproduce and Transmit Genes?
Viruses are acellular.
Most are composed only of nucleic acids
and some proteins.
Viruses do not:
• Regulate transport of materials into and
out of themselves
• Perform any metabolic functions
Table 13.1 Relative Sizes of Microorganisms
13.1 How Do Viruses Reproduce and Transmit Genes?
The first virus was discovered in the
1890s—it was an agent that causes
tobacco mosaic disease.
The “agent” could pass through a filter
that retained bacteria, and could diffuse
through an agar gel.
The agent was crystallized in 1930s.
13.1 How Do Viruses Reproduce and Transmit Genes?
Viruses are obligate intracellular
parasites.
They use the host cell’s DNA replication
and protein synthesis machinery to
reproduce themselves.
13.1 How Do Viruses Reproduce and Transmit Genes?
Viruses outside the host cell are called
virions.
They consist of a central core of DNA or
RNA, surrounded by a capsid of
proteins.
Viruses are not affected by antibiotics
that target bacterial cell walls or
ribosomes.
13.1 How Do Viruses Reproduce and Transmit Genes?
Classification:
• Genome of DNA or RNA
• Nucleic acid is single- or double-stranded
• Simple or complex shape
• Whether virion is surrounded by a
membrane or not
• Type of organism it infects
• Manner of the infection
Figure 13.2 Virions Come in Various Shapes
13.1 How Do Viruses Reproduce and Transmit Genes?
Viruses that infect bacteria are called
bacteriophage or phage.
The Phage binds to a receptor on the
host cell wall, injects the nucleic acid,
then one of two things happens:
13.1 How Do Viruses Reproduce and Transmit Genes?
• Phage reproduces immediately and kills
the host cell—lytic cycle—cell bursts
and releases progeny viruses.
• Postpones reproduction by integrating
into the host cell’s genome—lysogenic
cycle.
Figure 13.3 The Lytic and Lysogenic Cycles of Bacteriophage
The Lytic Cycle of Bacteriophage
Bacteriophage attack Escherichia coli
13.1 How Do Viruses Reproduce and Transmit Genes?
A virulent virus reproduces only by the lytic
cycle.
Early stage: The virus genome has a
promoter that attracts host RNA
polymerase. Viral genes adjacent to the
promoter are transcribed.
Products are proteins that shut down host
transcription, stimulate viral transcription,
and digest the host’s chromosomes to
provide nucleotides.
13.1 How Do Viruses Reproduce and Transmit Genes?
Late stage: The viral genes that code for
the capsid and proteins to lyse the host
cell are transcribed.
Sequence is controlled so that lysis does
not occur prematurely.
Figure 13.4 The Lytic Cycle: A Strategy for Viral Reproduction
13.1 How Do Viruses Reproduce and Transmit Genes?
Two viruses can infect one cell.
With two different viral genomes in the
same cell, there is the possibility of
genetic recombination by crossing
over—producing new strains.
13.1 How Do Viruses Reproduce and Transmit Genes?
Temperate viruses have a lysogenic
cycle. Bacteria harboring them are called
lysogenic bacteria.
The viral genome is a prophage,
incorporated into the bacterial genome.
Activation results in phage entering the
lytic cycle.
13.1 How Do Viruses Reproduce and Transmit Genes?
Bacteriophage have been tested as
possible control agents for bacteriacaused diseases.
13.1 How Do Viruses Reproduce and Transmit Genes?
Animal viruses:
In invertebrates, viruses are common only
in arthropods.
Arboviruses are transmitted to vertebrates
through e.g. insect bites. The insect is
the vector, virus does not harm the
vector.
13.1 How Do Viruses Reproduce and Transmit Genes?
Enveloped viruses have a membrane
derived from the host cell’s plasma
membrane.
13.1 How Do Viruses Reproduce and Transmit Genes?
Animal viruses enter cells in several ways:
• A naked virion is taken up by endocytosis.
• The enveloped virus has glycoproteins that
bind to receptors on host cell; also taken in
by endocytosis (e.g., influenza).
• The membrane of the host cell and
enveloped virus fuse (e.g., HIV).
Figure 13.5 The Reproductive Cycle of the Influenza Virus
13.1 How Do Viruses Reproduce and Transmit Genes?
After reproduction, enveloped viruses
escape the cell by a budding process.
An envelope is acquired from the host
cell’s plasma membrane in the process.
13.1 How Do Viruses Reproduce and Transmit Genes?
HIV is a retrovirus, it has reverse
transcriptase, which transcribes RNA
into DNA.
A DNA provirus is produced that is
integrated permanently into the host’s
genome.
When proviral DNA is activated, new
virions are produced.
Figure 13.6 The Reproductive Cycle of HIV
13.1 How Do Viruses Reproduce and Transmit Genes?
Plant viruses may be passed horizontally,
from one plant to another.
Or vertically, from parent to offspring.
Virus must pass cell wall and plasma
membrane—usually associated with
vectors, often insects.
In the plant, viruses may spread through
the plasmodesmata.
13.1 How Do Viruses Reproduce and Transmit Genes?
Wheat streak mosaic virus:
• The vector is a tiny mite.
• Destruction of photosynthetic tissue
causes yellow streaks in leaves—
reduces grain production.
Figure 13.7 Wheat Streak Mosaic Virus
Viruses
Check out 8th editon:
13.1 Recap, page 289
13.1 Chapter summary, page 303
Check out 10th edition:
26.4 Recap, pages 543-548
Self-Quiz (8th edition)
Page 304: questions 2 & 3
For Discussion (8th edition)
Page 305: questions 1 & 2
Viruses
Key terms:
Acellular (non-cellular), adenovirus, bacteriophage
(phage), capsid, intracellular, lysis, lysogenic
cycle, lytic cycle, methanogen, obligate, parasites,
prophage, provirus, replication, retrovirus,
reverse transcriptase, temperate virus, vector,
virion, virulence, virus (viruses)