Prokaryotes and viruses: Class 11 Biology

Key points

Prokaryotes are organisms whose cells have no true nucleus and no membrane bound organelles. They include the domains Bacteria and Archaea. They are the oldest forms of life and live almost everywhere: soil, water, air, and inside other organisms.

A bacterial cell is usually 1 to 10 µm long. Its parts are a cell wall made of peptidoglycan, a plasma membrane, cytoplasm with 70S ribosomes, and a single circular chromosome in the nucleoid. Many also have small circular DNA rings called plasmids, a slime capsule outside the wall, flagella for movement and pili for attachment.

Shapes of bacteria:

• Cocci: spherical, such as Streptococcus (chains) and Staphylococcus (clusters).
• Bacilli: rod shaped, such as Bacillus anthracis and Escherichia coli.
• Spirilla: spiral or twisted, such as Spirillum.
• Vibrios: comma shaped, such as Vibrio cholerae, which causes cholera.

Gram staining separates bacteria into two groups. Gram positive bacteria have a thick peptidoglycan wall and stain purple. Gram negative bacteria have a thin wall with an outer membrane and stain pink or red. The difference matters because antibiotics act differently on the two types.

Bacteria differ in nutrition. Autotrophs make their own food, either by photosynthesis (photoautotrophs such as cyanobacteria) or by oxidising chemicals (chemoautotrophs). Heterotrophs take food from others. Saprotrophs feed on dead matter, and parasites feed on a living host. For oxygen, bacteria may be aerobes, anaerobes or facultative.

Bacteria reproduce asexually by binary fission, in which the DNA is copied and the cell divides into two. In good conditions this can happen every 20 minutes or so. Genetic variation arises by mutation and by gene transfer through conjugation, transformation and transduction. Some bacteria form endospores, which are tough resting structures that survive heat and dryness.

Bacteria are useful in many ways. Decomposers recycle nutrients. Nitrogen fixing bacteria such as Rhizobium in root nodules add nitrates to soil. Lactobacillus makes yogurt and cheese. Bacteria are used in genetic engineering to make insulin and in sewage treatment. Harmful bacteria cause diseases such as tuberculosis, typhoid, cholera and tetanus, and spoil food.

Cyanobacteria were once called blue green algae. They are photosynthetic prokaryotes with chlorophyll a. Some, like Nostoc, fix atmospheric nitrogen in special cells called heterocysts. They were among the first organisms to release oxygen into the atmosphere.

Viruses are tiny, non-cellular infectious particles, about 20 to 300 nm. They have no cytoplasm, ribosomes or metabolism, and they can multiply only inside a living host cell. A virus has a core of nucleic acid, either DNA or RNA but never both, covered by a protein coat called the capsid. Some have an outer envelope taken from the host membrane. Because viruses show features of both living and non-living things, their status is debated.

Multiplication of a bacteriophage (lytic cycle):

• Attachment: the phage sticks to the bacterial wall by its tail fibres.
• Penetration: the phage injects its DNA into the cell and the empty capsid stays outside.
• Biosynthesis: the host machinery makes viral DNA and proteins.
• Assembly of new phages, then lysis: the cell bursts and releases new viruses.
• In the lysogenic cycle, the viral DNA joins the host chromosome and is copied with it without killing the cell until it later turns lytic.

Viruses cause human diseases such as influenza, polio, hepatitis, dengue, measles and AIDS. HIV is a retrovirus. Its RNA is changed to DNA by reverse transcriptase, and it destroys helper T cells, so the immune system weakens. Antibiotics do not work on viruses. Vaccines and antiviral drugs are used, and good hygiene limits spread.

Study tip: Draw a bacterial cell and a bacteriophage and label every part. Make a table of bacteria against viruses with six points of difference, as this is a favourite question. Remember that viruses are not killed by antibiotics, and give a correct example for each disease.

Definitions and flashcards

Say the meaning first, then open the card to check.

Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Common mistakes

• Writing that bacteria have no DNA. They have DNA, but not in a nucleus.
• Calling a virus a cell, or saying a virus can reproduce outside a host.
• Saying antibiotics kill viruses, which leads to wrong answers about treating flu.
• Writing that a virus has both DNA and RNA. It has only one kind.
• Mixing up the lytic and lysogenic cycles. Only the lytic cycle bursts the host cell at once.

Practise this chapter

Practice questions and timed test Printable worksheet