Chapter 19.1 & 19.3:
Genetics of Viruses and Bacteria
Structure and Genome of Viruses
• Viruses are smaller and more simple
that prokaryotes
• Lack structures and metabolic
machinery found in most cells
• Most are a nucleic acid enclosed in a
protein coat
• Genomes have many possibilities
-double stranded DNA
-single stranded DNA
-double stranded RNA
-single stranded RNA
• Most have genome in linear
sequence or circular ranging from 4
genes to several hundred
Structure and Genome
• Protein shell enclosing genome is called a
capsid
*many possible shapes - rod, polyhedral,
phage
Viral envelope – derived from (membrane) of
host cell, and an accessory structure used to
aid in attachment while infecting host cell
*may contain proteins, glycoproteins
phospholipids and even some enzymes
Chapter 19.2
Viral Replication
Viral Reproductive Cycles
• Isolated virus cannot produce or survive long
without a host cell
• Each virus has a host range of cell it can infect
-lock and key fit occurs between outside of
virus and specific receptor molecules on
the surface of the cell
-some have wide range, Ex: West Nile
-target specific tissues – Ex: cold virus in
upper respiratory tract
Reproductive cycle (General)
1. Virus enters cell and
uncoats, releasing viral DNA
and capsid proteins
2. Host enzymes replicate the
viral genome
3. Host enzymes transcribe
viral genome to make mRNA
then translate to make viral
proteins
4. Viral genomes and capsids
self assemble into new virus
particles and exit cell
Reproductive cycle of Phages
• Lytic cycle – reproductive cycle that results in
death of host cell
-refers to last stage in which bacterium
lyses and releases phages produced
Reproduction of Phages
• How have bacteria survived?
1. Natural selection has allowed bacterial
mutants to modify receptor sites that are
not recognized by phages
2. DNA from the phage that enters host
bacterium can be recognized as foreign
and cut up by restriction endonucleases
3. Some may be able to coexist with
phages
Reproduction of Phages
• Lysogenic cycle – replication of phage genome without
destroying host
-viral DNA is incorporated by genetic
recombination into specific site in host’s
chromosome – creates a prophage
-prophage remains silent but is passed each time
the host cell divides and synthesizes its DNA
*prophage can switch to lytic cycle and release
from host
-triggers include exposure to radiation or
presence of certain chemicals
Lytic and Lysogenic Cycles
• Virulent phages – reproduce only by lytic cycle
• Temperate phages – can reproduce by both modes of
lytic and lysogenic
Animal Viruses
• Two key variables separate animal viruses
from phages:
1. nature of genome – DNA or RNA
- double stranded or single
stranded
2. Presence or absence of membrane envelope
- used to aid attachment
- glycoproteins bind to receptors
Animal Viruses
• List of
animal
viruses
• Cycle
of RNA
virus
RNA Viruses
• Retroviruses – most
complicated
-contains enzyme –reverse
transcriptase which will
transcribe RNA to DNA
EX: HIV (Human
Immunodeficiency Virus)
*Provirus – permanent resident
of host cell after newly made
viral DNA enters nucleus and
integrates into the DNA of
chromosome
Retrovirus cycle
Chapter 19.3
Viruses , Viroids, and Prions
Viral Diseases
• Quick facts:
-most viruses may damage or kill cells by
release of hydrolytic enzymes from lysosomes
-some can cause infected cells to produce
toxins that lead to symptoms
Vaccines – harmless variants or derivatives of
pathogenic microbes that stimulate the
immune system
Emerging Viruses
• Emerging viruses are classified as suddenly appearing or
suddenly come to the attention of medical scientists
Where do they come from?
Three processes contribute to the emergence of viral
diseases:
1. mutation of existing viruses- high rate of
errors in replication with no proofreading
2. Spread of existing viruses form one host species
to another
3. Dissemination of viral disease from small isolated
population can lead to widespread epidemics
Plant Viruses
• Plant viruses have same mode of replication as
animal viruses
• Symptoms include bleached or brown spots on
leaves or fruit, stunted growth, damaged roots
and flowers, and diminished crop quality and
yield
• Two major routes of spreading virus
1. horizontal transmission - plant infected from
external source
- insects, pruning sheers, irrigation
2. vertical transmission – plant inherits virus from
infected parent
Viroids and Prions
• Viroid – circular RNA molecules that do not
encode proteins but can replicate in host plant
cells
-cause errors that lead to abnormal development
or stunted growth
- single molecule can cause spread of disease
Prions
• Prions- infectious protein formed from a misfolded protein
found in brain cells
- very slow acting agents with long incubation periods
(up to 10 years)
-virtually indestructible
- causes a number of degenerative brain diseases in
animals
- possibly spread by transmission in food