Viruses — IB Diploma Biology HL
Features shared by all viruses, their diversity, the lytic and lysogenic cycles of lambda phage, viral origins and rapid evolution of influenza and HIV.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Viruses share relatively few features: a small, fixed size, nucleic acid (DNA or RNA) as genetic material, a capsid made of protein, no cytoplasm, and few or no enzymes. Otherwise they are highly diverse: their genetic material may be RNA or DNA, single- or double-stranded, and some are enveloped in host cell membrane while others are not.
In the lytic cycle, bacteriophage lambda attaches to an E. coli cell, injects its DNA and uses the host's energy supply, nucleotides, amino acids and ribosomes to make new viruses, which are released when the cell bursts. In the lysogenic cycle, the viral DNA integrates into the host chromosome as a prophage and is copied each time the bacterium divides.
The diversity of viruses suggests several origins, and their shared features may be convergent evolution resulting from obligate parasitism. Some viruses, including influenza and HIV, evolve very rapidly because they have high mutation rates, short generation times and huge numbers of offspring. As a result vaccines must be updated and antiviral drugs can quickly lose effectiveness.
Exam questions
7 exam-style questions (23 marks), each with its mark scheme.
Answer the questionsFlashcards
13 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of cells (unity and diversity) on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
What is the protein coat of a virus called?
Show answer
The capsid.
What is a prophage?
Show answer
Viral DNA integrated into the host bacterium's chromosome.
Which host does bacteriophage lambda infect?
Show answer
The bacterium E. coli.
Give two reasons why HIV evolves rapidly.
Show answer
Reverse transcriptase does not proofread, so the mutation rate is high; and huge numbers of virus particles are produced with a short generation time.
Frequently asked questions
What do all viruses have in common?
All viruses have a small, fixed size, nucleic acid (either DNA or RNA) as their genetic material, a capsid made of protein, no cytoplasm, and few or no enzymes. Beyond these shared features they are highly diverse in shape, in the type of nucleic acid they carry and in whether they have an envelope.
What is the difference between the lytic and lysogenic cycle?
In the lytic cycle a virus uses the host cell to make many new viruses quickly and then bursts the cell. In the lysogenic cycle the viral DNA integrates into the host chromosome as a prophage, so the host survives and copies the viral DNA each time it divides, until induction triggers the lytic cycle.
Why does the flu vaccine change every year?
The flu vaccine changes every year because influenza viruses evolve rapidly. Their RNA genome mutates often, gradually changing the surface antigens haemagglutinin and neuraminidase (antigenic drift), so antibodies made against earlier strains no longer bind well. The vaccine is reformulated each year to match the strains expected to circulate.
Why is HIV so hard to treat?
HIV is hard to treat because it evolves rapidly. Its reverse transcriptase makes many copying errors and huge numbers of virus particles are made every day, so drug-resistant variants arise and are selected. Patients therefore take combinations of drugs with different targets, and the variable surface proteins make a vaccine hard to develop.
Written and checked against the IB Biology HL specification · Updated October 2026