chapter 19.1 & 19.3: genetics of viruses and bacteria
TRANSCRIPT
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 phages2. DNA from the phage that enters host bacterium can be recognized as foreign and cut up by restriction endonucleases3. 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 stranded2. 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 proofreading2. Spread of existing viruses form one host species to another3. 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 virus1. horizontal transmission - plant infected from external source
- insects, pruning sheers, irrigation2. 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