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