Viruses and

Prokaryotes

Viruses – Are they living things?

Viruses can reproduce, however, they cannot

reproduce without a host cell.

They also do not contain cytoplasmic materials and

they do not have a form of metabolism.

Therefore, viruses are not categorized as living

things.

However, viruses are still classified to identify them.

Viruses do have taxonomical classification into

orders, families, genera, and species.

Classification is based on size, shape, and type of

genetic material.

Viral Structure

Viruses are small particles that are capable of infecting the cells of a biological organism.

Viruses are simple in their structure. They are composed of genetic material (either DNA or RNA) contained within a capsule composed of protein (the capsid).

There is no cytoplasm within the capsid, though some viruses do have a membranous envelope around the capsid.

The DNA or RNA contains the genetic information for synthesizing proteins that can make new viruses.

Viruses will infect cells and take control of the cell to synthesize new viral particles.

Viral Infections

Viruses have specificity in terms of infection. They will

recognize certain cell types or specific organisms.

The disease can then spread from host to host.

A large-scale outbreak of disease which is confined to a

limited geographic region is known as an epidemic.

An outbreak that occurs over a widespread geographic

area (often global), is known as a pandemic.

Depending on the type of viral infection, viruses can

cause a variety of symptoms in the host organism.

Sometimes, they can even cause cancer in the host

organism

Viral Replication

Outside of a host cell, a virus does not carry out any

life functions.

To replicate, viruses must infect living cells by

inserting their genetic information into a host cell.

The host cell will read the genetic information and

replicate viral particles, creating new viruses.

The viruses will then destroy and exit the host cell to

infect other host cells. Thus, continuing the cycle of

infection.

There are two types of cycles:

The Lytic Cycle

The Lysogenic Cycle

Lytic Cycle

Steps in a lytic cycle:

1. Virus binds to the surface of a host cell and inserts its

genetic information (or the whole virus) into the cell.

2. Viral DNA/RNA acts as instructions for the host cell to

make new viral particles.

3. New viruses (copies of the original virus) are

assembled within the host cell.

4. Host cell lysis (rupturing of a cell) occurs and releases

new viral particles to infect other cells

Lysogenic Cycle

Steps in a lysogenic cycle:

1. Virus binds to the surface of a host cell and inserts its

genetic information (or the whole virus) into the cell.

2. Viral DNA/RNA inserts itself into the host chromosome.

3. Each time the host cell divides, a copy of the viral

information is replicated and passed on to the

daughter cells.

4. This can eventually result in the lytic cycle when the

viral DNA becomes active in the host cells.

This can result in some bacterial DNA being

transferred by the resulting viruses. This is known as

transduction.

Viral Applications

Gene therapy

A method of treating diseases in which genes are

introduced into cells to replace, supplement, or repair

a defective gene.

Since certain viruses are capable of inserting their

DNA into host chromosomes, this mechanism can

be used to insert wanted genes into a host.

Prokaryotes

Recall: Prokaryotes are single celled organisms that

lack membrane-bound organelles.

Prokaryotes have been on earth for more than 3.5

billion years.

The domains bacteria and archaea both consist of

prokaryotes.

Formerly, bacteria and archaea were thought to

be very similar since they were all prokaryotic.

However, since then, it has been discovered that

one group of the bacteria behave more like

eukaryotic cells than the other group.

The Domain Bacteria

Genetic studies shows that bacteria diverged early

into many branches with varying characteristics

(e.g. photosynthetic, pathogenic, etc.)

Bacteria contain a single chromosome. Some also

have plasmids which are small circular loops of

DNA. The plasmids usually contain information

which is not crucial but may be advantageous.

Bacterial Structure

Bacteria can be characterized and distinguished

generally by these four characteristics:

Cell shape

Cell wall structure

Motility

Nutritional Requirement

Bacteria Cell Shapes

The basic shape of bacteria are as follows:

Cocci (coccus for singular)

Spherical bacteria

E.g. Staphylococcus aureas

Bacilli (bacillus for singular)

Rod-shaped bacteria

E.g. Bacillus cereus

Spirochetes

Spiral-shaped bacteria

E.g. Leptospira

Bacteria Cell Wall Structure

Almost all bacteria have a cell wall surrounding their

cell membrane.

The cell wall is for support and protection.

There are two types of bacterial cell walls. They

both contain peptidoglycan (sugar layer) but one

contains significantly less peptidoglycan and has an

outer membrane instead.

Bacteria Cell Wall Structure

Gram positive bacteria have a large layer of

peptidoglycan in their cell walls.

Gram negative bacteria have a smaller layer of

peptidoglycan in their cell walls and have an extra

cell membrane layer.

Bacteria Motility

Some prokaryotes are motile while others are not.

For motility, some bacteria use flagella to propel

themselves toward food, light, or oxygen.

Others which are spiral-shaped can use a

corkscrew motion to move.

Some bacteria also secrete a mucous that they can

glide on.

Bacteria Nutrition

Nutritional Mode Definition

Heterotroph

Organism that requires a source of organic

materials from the environment, or through

ingestion of other organisms.

Photoautotroph

Organism that is able to use light as an energy

source and convert carbon dioxide into

carbon compounds.

Photoheterotroph

Organism that is able to use light as an energy

source but requires organic material as a

source of carbon.

ChemoheterotrophOrganism that uses inorganic energy sources

such as sulfur.

Bacteria Reproduction

Bacteria reproduce through binary fission

Bacteria Reproduction

Binary fission is considered asexual reproduction

that generates clones of the original cell.

However, genetic recombination is still possible

through the following processes:

Transformation – Fragments of free DNA from the

environment is taken in by the bacterium

Transduction – Viral infections can result in transfer of

genetic information from one bacteria to the next

through the viral chromosome

Conjugation – Bacteria are joined together and

genetic information in the form of circular DNA (a

plasmid) is transferred

The Doman Archaea

The domain archaea consists of prokaryotes which different from other living things.

Little is known about archaea due to the fact that many inhabit extreme environments.

Like bacteria, they have a cell wall around their cell membranes. However, the cell walls of archaea usually lack peptidoglycan. These cell walls are more resistant to physical and chemical disruptions.

Examples of Archaea Groups

Sample types Key Features

Methanogens

• Live in low-oxygen environments

• Generate energy by converting chemical

compounds into methane gas

Halophiles

• Salt-loving organisms that live in high salt

environments

• Get energy from organic molecules

• Some able to use light

Extreme

Thermophiles

• Hot-loving organisms that can live in extremely hot

environments

• Optimal range is between 70°C to 95°C

Psychrophiles

• Cold-loving organisms that can live in extremely

cold environments

• Optimal range is between -10°C to -20°C