The protein coat that surrounds the nucleic acid of a virus.

What is the viral envelope and where does it come from?

A lipid bilayer, with viral glycoproteins, that surrounds the capsid of some viruses; it is taken from the host cell membrane.

Describe the structure of TMV.

A rod-shaped virus with a single strand of RNA inside a helical protein capsid, with no envelope.

Describe the structure of HIV.

A roughly spherical enveloped virus with two strands of RNA and reverse transcriptase inside a capsid; glycoproteins in the envelope bind to CD4.

What type of virus is lambda phage and what is its structure?

A bacteriophage with a head containing double-stranded DNA and a tail with tail fibres; it has no envelope.

What are the main steps of the lytic cycle?

Attachment, injection of DNA, replication and making of new phage components by the host cell, assembly, and lysis to release new phages.

A state in which viral DNA is integrated into the host genome and replicated with it, without making new viruses.

What triggers a latent phage to enter the lytic cycle?

A change in conditions such as ultraviolet light, which makes the DNA leave the chromosome.

What does HIV bind to on a helper T cell?

The CD4 receptor, through a glycoprotein in its envelope.

What is the role of reverse transcriptase in HIV?

It makes a DNA copy of the viral RNA, which is then inserted into the host cell DNA.

Why do HIV-infected people develop AIDS?

HIV destroys helper T cells, which weakens the immune response and allows opportunistic infections.

How does M. tuberculosis survive inside macrophages?

Its waxy mycolic acid cell wall resists digestion by lysosomal enzymes.

A nodule of macrophages and lymphocytes that walls off TB bacteria in the lung, in which they may remain dormant.

Name the main symptoms of active tuberculosis.

Persistent cough with blood in the sputum, weight loss, fever and night sweats.

Bacteria, viruses and infectionEdexcel International A Level Biology: Flashcards

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Name three structural features of a bacterium that a virus lacks.

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Name three structural features of a bacterium that a virus lacks.: Cell wall, plasma membrane/cytoplasm and ribosomes (also a cell with metabolism).
What is a capsid?: The protein coat that surrounds the nucleic acid of a virus.
What is the viral envelope and where does it come from?: A lipid bilayer, with viral glycoproteins, that surrounds the capsid of some viruses; it is taken from the host cell membrane.
Describe the structure of TMV.: A rod-shaped virus with a single strand of RNA inside a helical protein capsid, with no envelope.
Describe the structure of HIV.: A roughly spherical enveloped virus with two strands of RNA and reverse transcriptase inside a capsid; glycoproteins in the envelope bind to CD4.
What type of virus is lambda phage and what is its structure?: A bacteriophage with a head containing double-stranded DNA and a tail with tail fibres; it has no envelope.
What are the main steps of the lytic cycle?: Attachment, injection of DNA, replication and making of new phage components by the host cell, assembly, and lysis to release new phages.
What is latency?: A state in which viral DNA is integrated into the host genome and replicated with it, without making new viruses.
What triggers a latent phage to enter the lytic cycle?: A change in conditions such as ultraviolet light, which makes the DNA leave the chromosome.
What does HIV bind to on a helper T cell?: The CD4 receptor, through a glycoprotein in its envelope.
What is the role of reverse transcriptase in HIV?: It makes a DNA copy of the viral RNA, which is then inserted into the host cell DNA.
Why do HIV-infected people develop AIDS?: HIV destroys helper T cells, which weakens the immune response and allows opportunistic infections.
How does M. tuberculosis survive inside macrophages?: Its waxy mycolic acid cell wall resists digestion by lysosomal enzymes.
What is a tubercle?: A nodule of macrophages and lymphocytes that walls off TB bacteria in the lung, in which they may remain dormant.
Name the main symptoms of active tuberculosis.: Persistent cough with blood in the sputum, weight loss, fever and night sweats.

Exam questions on Bacteria, viruses and infection

A microbiologist is comparing the structure of three infectious agents: the bacterium Escherichia coli, tobacco mosaic virus (TMV) and human immunodeficiency virus (HIV). TMV is a rod-shaped virus with a single strand of RNA surrounded by a protein capsid. HIV has two strands of RNA inside a capsid, which is surrounded by a lipid envelope containing glycoproteins.
Compare the structure of E. coli with the structure of TMV, giving two differences.2 marks
Bacteriophage lambda is a virus with a head, containing double-stranded DNA, and a tail with tail fibres. It infects the bacterium E. coli. In some infected bacteria the viral DNA is inserted into the bacterial chromosome and is copied each time the bacterium divides, without new phages being made. When these bacteria are later exposed to ultraviolet light, they burst open and release hundreds of new phages.
Explain why exposing the bacteria to ultraviolet light leads to the release of new phages.2 marks
A man has had a persistent cough, with blood in his sputum, together with weight loss, fever and night sweats for six weeks. A sample of his sputum grew Mycobacterium tuberculosis, a bacterium with a waxy cell wall that contains mycolic acid. The bacteria were inhaled in droplets and reached the alveoli of the lungs, where they were engulfed by macrophages.
Explain how Mycobacterium tuberculosis infects cells in the lungs and avoids destruction.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).