• Identify the different kinds of viruses and their structures.

Section Objectives:

• Compare and contrast the replication cycles of viruses.

• You’ve probably had the flu—influenza—at some time during your life.

• Viruses are composed of nucleic acids enclosed in a protein coat and are smaller than the smallest bacterium.

What is a virus?What is a virus?

• Nonliving particles called viruses cause influenza.

• Most biologists consider viruses to be nonliving because they don’t exhibit all the criteria for life.

• They don’t carry out respiration, grow, or develop. All viruses can do is replicate—make copies of themselves—and they can’t even do that without the help of living cells.

What is a virus?What is a virus?

• A cell in which a virus replicates is called the host cell.

• Viruses, such as rabies viruses and polioviruses, were named after the diseases they cause.

• Other viruses were named for the organ or tissue they infect.

What is a virus?What is a virus?

What is a virus?What is a virus?• Today, most viruses are given a genus

name ending in the word “virus” and a species name.

• However, sometimes scientists use code numbers to distinguish among similar viruses that infect the same host.

• A virus that infects a bacterium is called a bacteriophage (bak TIHR ee uh fayj), or phage for short.

Viral StructureViral Structure• A virus has an

inner core of nucleic acid, either RNA or DNA, and an outer protein coat called a capsid.

Capsid

Nucleic acid

Envelope

Viral StructureViral Structure• Some relatively

large viruses, such as human flu viruses, may have an additional layer, called an envelope, surrounding their capsids.

Capsid

Nucleic acid

Envelope

Viral StructureViral Structure• Envelopes are

composed primarily of the same materials found in the plasma membranes of all cells.

Capsid

Nucleic acid

Envelope

• Viral nucleic acid is either DNA or RNA and contains instructions for making copies of the virus.

• Some viruses have only four genes, while others have hundreds.

Nucleic acid

Capsid

Viral StructureViral Structure

• The tobacco mosaic virus has a long, narrow helical shape.

Nucleic acid

Capsid

Viral StructureViral Structure

Viral StructureViral Structure• The arrangement of

proteins in the capsid of a virus determines the virus’s shape.

Nucleic acidCapsid

• Polyhedral viruses resemble small crystals.

Viral StructureViral Structure• The protein

arrangement also plays a role in determining what cell can be infected and how the virus infects the cell.

Nucleic acidCapsid

Attachment to a host cellAttachment to a host cell

• Before a virus can replicate, it must enter a host cell.

• A virus recognizes and attaches to a host cell when one of its proteins interlocks with a molecular shape that is the receptor site on the host cell’s plasma membrane.

Attachment to a host cellAttachment to a host cell

• A protein in the tail fibers of the bacteriophage T4 recognizes and attaches the T4 to its bacterial host cell.

Capsid

Nucleic acid

Tail

Tail fiber

Attachment to a host cellAttachment to a host cell

• In other viruses, the attachment protein is in the capsid or in the envelope.

Capsid

Nucleic acid

Tail

Tail fiber

Attachment is a specific processAttachment is a specific process

• Each virus has a specifically shaped attachment protein. Therefore, each virus can usually attach to only a few kinds of cells.

• In general, viruses are species specific, and some also are cell-type specific. For example, polio viruses normally infect only intestinal and nerve cells.

Viral Replication CyclesViral Replication Cycles

• Once attached to the plasma membrane of the host cell, the virus enters the cell and takes over its metabolism.

• Only then can the virus replicate.

Disease symptoms of provirusesDisease symptoms of proviruses

• Having chicken pox, which usually occurs before age ten, gives lifelong protection from another infection by the virus. However, some chicken pox viruses may remain as proviruses in some of your body’s nerve cells.

Disease symptoms of provirusesDisease symptoms of proviruses

• Later in your life, these proviruses may enter a lytic cycle and cause a disease called shingles—a painful infection of some nerve cells.

Release of virusesRelease of viruses

• Either lysis, the bursting of a cell, or exocytosis, the active transport process by which materials are expelled from a cell, release new viruses from the host cell.

Release of virusesRelease of viruses

• In exocytosis, a newly produced virus approaches the inner surface of the host cell’s plasma membrane.

• The plasma membrane surrounds the virus, enclosing it in a vacuole that then fuses with the host cell’s plasma membrane.

• Then, the viruses are released to the outside.

RetrovirusesRetroviruses• Many viruses, such as the human

immunodeficiency virus (HIV) that causes the disease AIDS, are RNA viruses—RNA being their only nucleic acid.

HIV virus

RetrovirusesRetroviruses

• The RNA virus with the most complex replication cycle is the retrovirus

HIV virus

RetrovirusesRetroviruses

• Once inside a host cell, the retrovirus makes DNA from its RNA.

• To do this, it uses reverse transcriptase, an enzyme it carries inside its capsid.

• Once inside a human host, HIV infects white blood cells.

• Newly made viruses are released into the blood stream by exocytosis and infect other white blood cells.

Normal white blood cells

HIV: An infection of white blood cellsHIV: An infection of white blood cells

HIV: An infection of white blood cellsHIV: An infection of white blood cells• Most people with an HIV infection

eventually get AIDS because, over time, more white blood cells are infected and produce new viruses.

• Because white blood cells are part of a body’s disease-fighting system, their destruction interferes with the body’s ability to protect itself from organisms that cause disease, a symptom of AIDS.

Plant virusesPlant viruses• The first virus to be identified was a

plant virus, called tobacco mosaic virus, that causes disease in tobacco plants.

Tobacco mosaic virus causes yellow spots on tobacco leaves, making them unmarketable.

Question 1

Which of the following is NOT a reason that viruses are considered to be nonliving?

D. Viruses don’t develop.

C. Viruses don’t grow.

B. Viruses don’t respire.

A. Viruses don’t replicate.

The answer is A.

Question 2

Which is NOT a component of a virus?

D. phage

C. DNA

B. capsid

A. RNA

The answer is D.

Question 4What two ways do viruses have of getting into host cells?

Answer

The virus can inject its nucleic acid into the host cell, or attach to the host cell’s membrane and become surrounded by the membrane and placed in a vacuole. The virus then bursts out of the vacuole and releases its nucleic acid into the cell.

IN: 1.20

• There are three types of archaebacteria that live mainly in extreme habitats where there is usually no free oxygen available.

Archaebacteria: The extremistsArchaebacteria: The extremists

• One type of archaebacterium lives in oxygen-free environments and produces methane gas.

• These methane-producing archaebacteria live in marshes, lake sediments, and the digestive tracts of some mammals, such as cows.

Archaebacteria: The extremistsArchaebacteria: The extremists

Archaebacteria: The extremistsArchaebacteria: The extremists

• They also are found at sewage disposal plants, where they play a role in the breakdown of sewage.

• A second type of archaebacterium lives only in water with high concentrations of salt.

Archaebacteria: The extremistsArchaebacteria: The extremists

Dead Sea

• A third type lives in the hot, acidic waters of sulfur springs.

Archaebacteria: The extremistsArchaebacteria: The extremists

• This type of anaerobic archaebacterium also thrives near cracks deep in the ocean floor, where it is the autotrophic producer for a unique animal community’s food chain.

Archaebacteria: The extremistsArchaebacteria: The extremists

What is bacterium?What is bacterium?

• A bacterium consists of a very small cell.

• Although tiny, a bacterial cell has all the structures necessary to carry out its life functions.

The structure of bacteriaThe structure of bacteria

• Prokaryotic cells have ribosomes, but their ribosomes are smaller than those of eukaryotes.

• They also have genes that are located for the most part in a single circular chromosome, rather than in paired chromosomes.

The structure of bacteriaThe structure of bacteria Ribosome

Cytoplasm

Chromosome

Gelatinlikecapsule

Cell Wall

Cell Membrane

Flagellum

A Typical Bacterial CellA Typical Bacterial Cell• A typical bacterium, such as Escherichia coli

would have some or all of the structures shown in this diagram of a bacterial cell.

Capsule Cell Wall

Chromosome

Flagellum

PlasmidPilus

Plasma membrane

The structure of bacteriaThe structure of bacteria

• A bacterial cell remains intact as long as its cell wall is intact.

• If the cell wall is damaged, water will enter the cell by osmosis, causing the

cell to burst.

• Scientists used a bacterium’s need for an intact cell wall to develop a weapon against bacteria that cause disease.

The structure of bacteriaThe structure of bacteria

• In 1928, Sir Alexander Fleming accidentally discovered penicillin, the first antibiotic—a substance that destroys bacteria—used in humans.

The structure of bacteriaThe structure of bacteria

• Later, biologists discovered that penicillin can interfere with the ability of some bacteria to make cell walls.

• When such bacteria grow in penicillin, holes develop in their cell walls, water enters their cells, and they rupture and die.

Identifying bacteriaIdentifying bacteria• Bacterial cell walls also give bacteria

different shapes.

• Shape is another way to categorize bacteria.

Identifying bacteriaIdentifying bacteria• The three most common shapes are spheres,

called coccus; rods, called bacillus; and spirals, called spirillum.

Reproduction by binary fissionReproduction by binary fission

• Bacteria reproduce asexually by a process known as binary fission.

• To reproduce in this way, a bacterium first copies its chromosome. Then the original chromosome and the copy become attached to the cell’s plasma membrane for a while.

Reproduction by binary fissionReproduction by binary fission• The cell grows larger, and eventually the two chromosomes separate and move to opposite ends of the cell.

Reproduction by binary fissionReproduction by binary fission• Then, a partition forms between the

chromosomes. This partition separates the cell into two similar cells.

Reproduction by binary fissionReproduction by binary fission• Because each new cell has either the

original or the copy of the chromosome, the resulting cells are genetically identical.

Reproduction by binary fissionReproduction by binary fission

• Under ideal conditions, some bacteria can reproduce every 20 minutes, producing enormous numbers of bacteria quickly.

• But bacteria don’t always have ideal growing conditions. They run out of nutrients and water, they poison themselves with their own wastes, and predators eat them.

The Importance of BacteriaThe Importance of Bacteria

• Bacteria help to fertilize fields, to recycle nutrients on Earth, and to produce foods and medicines.

• Disease-causing bacteria are few compared with the number of harmless and beneficial bacteria on Earth.

Nitrogen fixationNitrogen fixation

• Yet few organisms, including most plants, can directly use nitrogen from the air.

• All organisms need nitrogen because the element is a component of their proteins, DNA, RNA, and ATP.

Nitrogen fixationNitrogen fixation

• Other bacteria then convert the ammonia into nitrite (NO2

–) and nitrate (NO3–),which plants

can use.

• Several species of bacteria have enzymes that convert N2 into ammonia (NH3) in a process known as nitrogen fixation.

• Bacteria are the only organisms that can perform these chemical changes.

Nitrogen fixationNitrogen fixation

• Farmers grow legume crops after the harvesting of crops such as corn, which depletes the soil of nitrogen.

• Some nitrogen-fixing bacteria live symbiotically within the roots of some trees and legumes.

Recycling of nutrientsRecycling of nutrients

• This food is passed from one heterotroph to the next in food chains and webs.

• Autotrophic bacteria and also plants and algae, which are at the bottom of the food chains, use the nutrients in the food they make.

• In the process of making food, many autotrophs replenish the supply of oxygen in the atmosphere.

Food and medicinesFood and medicines• Some foods that you eat—mellow Swiss

cheese, crispy pickles, tangy yogurt—would not exist without bacteria.

Food and medicinesFood and medicines

• Specific bacteria are used to make different foods, such as vinegar, cheeses, and sauerkraut.

• Bacteria also inhabit your intestines and produce vitamins and enzymes that help digest food.

Food and medicinesFood and medicines

• Streptomycin, erythromycin, bacitracin, and neomycin are some of these antibiotics.

• In addition to food, some bacteria produce important antibiotics that destroy other

types of bacteria.

Bacteria cause diseaseBacteria cause diseaseDiseases Caused by Bacteria

Disease Transmission Symptoms Treatment

Strep throat

(Streptococcus)

Inhale or ingest throughmouth

Fever, sore throat,swollen neck glands

Antibiotic

Tuberculosis Inhale Fatigue, fever, nightsweats, cough, weight loss, chest pain

Antibiotic

Tetanus Puncturewound

Stiff jaw, musclespasms, paralysis

Open and clean wound,antibiotic; give antitoxin

Lyme disease Bite ofinfected tick

Rash at site of bite,chills, body aches,joint swelling

Antibiotic

Dentalcavities (caries)

Bacteriain mouth

Destruction of toothenamel, toothache

Remove and fill thedestroyed area of tooth

Diptheria Inhale orclose contact

Sore throat, fever,heart or breathingfailure

Vaccination to prevent, antibiotics

What part of a bacterial cell is most affected by penicillin?

Question 2

D. cell wall

C. flagellum

B. plasmid

A. pilus

IN: 1.20

The answer is D, cell wall.

Cell Wall

IN: 1.20

What is a pilus used for in a bacterium?

Question 5

IN: 1.12

What causes anthrax?

Question 8

Question 9

Describe the process in which bacteria make nitrogen in the air accessible for use by plants.

Several species of bacteria have enzymes that convert nitrogen gas into ammonia. Other bacteria then convert the ammonia into nitrite and nitrate that plants can use.

Answer