Keywords: IB Biology Topic A2.3, Viruses, Viral Structure, Bacteriophage, HIV, SARS-CoV-2, Lytic Cycle, Lysogenic Cycle, Viral Evolution, Non-living entities.
Welcome to the biological 'edge-case': Topic A2.3 Viruses. In the new IB Biology syllabus, viruses are no longer just a footnote in the cell unit; they are explored as unique biological entities that challenge our definition of life. Because they lack a metabolism and cannot reproduce without a host, they occupy the gray area between living and non-living. To master this unit, you must understand the 'Bio-Logic' of viral efficiency: they are essentially genetic hijackers.
The IBO has introduced specific case studies that you must know in detail, including the bacteriophage Lambda, the HIV virus, and the SARS-CoV-2 virus. You are expected to distinguish between their structures—whether they have an envelope or a capsid, and whether their genome is DNA or RNA. In Paper 1A (MCQs), the most frequent traps involve the viral replication cycles (Lytic vs. Lysogenic) and the extreme genetic diversity resulting from rapid mutation rates in RNA viruses.
Before we dive into the mechanics, remember the core paradox: Viruses are technically 'non-living,' yet they evolve by the same rules of natural selection as humans do. They are the ultimate minimalists. If you think of a virus as a 'message in an envelope,' the whole unit becomes a study of how that message gets delivered and translated by an unsuspecting cell.
Despite their diversity, all viruses share two fundamental parts: a genetic core (DNA or RNA) and a protein coat called a capsid. Some also steal a piece of the host's membrane to form an outer envelope.
Take a look at the question below:
The Bio-Logic: Many viruses, like the bacteriophage, do not have an envelope (Option A). Viral genomes can be DNA or RNA, single or double-stranded (Option B). Viruses never have cytoplasm or ribosomes (Option D)—that is why they are obligate parasites. However, every virus must have a capsid (Option C) to protect its genome while moving between hosts.
This is the 'bread and butter' of viral replication questions. The lytic cycle is a 'quick kill,' while the lysogenic cycle is a 'long-term undercover mission.'
Take a look at the question below:
The Approach: In the lytic cycle, the cell is turned into a virus factory and destroyed. In the lysogenic cycle, the virus plays the long game. It hides its DNA inside the host's DNA (Option C). Every time the cell divides, the viral "prophage" is copied along with it, staying silent until a stressor triggers it to enter the lytic cycle.
The new curriculum requires you to understand the unique challenges posed by specific viruses. HIV is a retrovirus (RNA to DNA), while SARS-CoV-2 is a positive-sense RNA virus.
Take a look at the two questions below:
The Bio-Logic for Question A: HIV is a "Retro" virus because it works backward. It uses reverse transcriptase (Option B) to turn its RNA into DNA so it can hide in the host's nucleus. The Bio-Logic for Question B: DNA replication is checked for errors by enzymes. RNA replication is much "sloppier." Because RNA polymerase doesn't proofread (Option B), mistakes (mutations) happen constantly. This is why we need new vaccines for RNA viruses so frequently—they change their "disguise" very quickly.
How did viruses begin? The IBO wants you to be familiar with the three main theories of viral evolution.
The Logic: "Regressive" implies moving backward from complex to simple. The Regressive Hypothesis (Option B) suggests that if you don't use a feature (like a ribosome) because your host provides it, you eventually lose the genes for it to save energy.
When you are asked to compare a virus to a prokaryote or eukaryote, use this Bio-Logic checklist:
Final Summary: Topic A2.3 is a study in biological efficiency. Viruses are the ultimate 'hackers' of the natural world. By understanding their structures, their replication cycles, and their origins, you gain a deeper appreciation for the definition of life itself. Master the differences between HIV, Phage, and SARS-CoV-2, and you will find this topic to be one of the most interesting and scorable sections of the exam.
Click the black box to reveal the answers!