A2.3 Viruses (HL Only) - IB Biology 2025 PDF

First Exams 2025 A2.3 Viruses Theme: Unity and Diversity Level of Organisation: Cells Additional HL Content HL Content Only From IB Guiding Questions the IB How can viruses exist with so few genes?...

First Exams 2025 A2.3 Viruses Theme: Unity and Diversity Level of Organisation: Cells Additional HL Content HL Content Only From IB Guiding Questions the IB How can viruses exist with so few genes? In what ways do viruses vary? HL Content Only From Additional HL Content - A2.3 Viruses the IB A2.3.1: Structural features common to viruses A2.3.2: Diversity of structure in viruses A2.3.3: Lytic cycle of a virus A2.3.4: Lysogenic cycle of a virus A2.3.5: Evidence for several origins of viruses from other organisms A2.3.6: Rapid evolution in viruses HL Content Only HL Only Key Terms Virus Lysogenic Cycle Antibodies Capsid Prophage Antigens Bacteriophage Lysogen Immunity Lytic Cycle Parasite Mutation Lysis Obligate Parasite Antigenic drift Metabolism Convergent Antigenic shift Evolution HL Content Only From A2.3.1: Structural features common to the IB viruses Relatively few features are shared by all viruses: small, fixed size; nucleic acid (DNA or RNA) as genetic material; a capsid made of protein; no cytoplasm; and few or no enzymes. HL Content Only Viruses A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, including animals, plants and bacteria. HL Content Only Virus Structure ❓ List three features common to all viruses. HL Content Only Common Features of Viruses Viruses are a very diverse group, but all viruses have the following features in common: Small fixed size, typically 20 to 200 nm in diameter [Bacteria are 2000 to 3000 nm] Genetic material in the form of DNA or RNA A protective protein coat, known as a capsid, surrounding the genetic material No cytoplasm Few or no enzymes Virus do not have cells, and most scientists do not consider them to be alive. HL Content Only From A2.3.2: Diversity of structure in viruses the IB Students should understand that viruses are highly diverse in their shape and structure. Genetic material may be RNA or DNA, which can be either single- or double-stranded. Some viruses are enveloped in host cell membrane and others are not enveloped. Virus examples include bacteriophage lambda, coronaviruses and HIV. HL Content Only Virus Diversity Viruses are highly diverse in their shape and structure. Differences between viruses include: Shape (See next slide) Genetic material, which can be DNA or RNA ○ If DNA is the genetic material, then it can be single-stranded or double-stranded. Some viruses are enveloped in host cell membrane, while others are not. Viruses include HIV, coronavirus and bacteriophage lambda. HL Content Only Virus Shapes Most viruses can be classified as having an icosahedral shape (a 3D-shape with 20 faces) or a helical shape. However some viruses, such as bacteriophages, have a more complex shape. Icosahedral Virus Shape Helical Virus Shape Bacteriophage virus HL Content Only From A2.3.3: Lytic cycle of a virus the IB Students should appreciate that viruses rely on a host cell for energy supply, nutrition, protein synthesis and other life functions. Use bacteriophage lambda as an example of the phases in a lytic cycle. HL Content Only Bacteriophage Lambda Viruses such as the bacteriophage lambda, are not alive, and do not have metabolism. They depend on their host cell for: Energy supply Nutrition Protein synthesis All other life functions The bacteriophage lambda infects Escherichia coli Bacteriophage Lamda bacterial cells, by the means of two different cycles. HL Content Only Lytic Cycle of the Lambda Bacteriophage The genetic material of bacteriophage lambda is a double stranded DNA molecule. HL Content Only The Lytic Cycle Read the linked article on the lytic cycle. Watch the video at the end of the article. ❓ Describe the lytic cycle of the lambda bacteriophage. Lytic Cycle HL Content Only Stages of the Lytic Cycle Attachment: The bacteriophage lambda attaches to receptors on an E. coli cell. The capsid remains outside the bacterial cell. Penetration: The bacteriophage lambda injects its DNA into the E. coli cell. Phage DNA Replication: Endonuclease enzymes produced by the virus degrade the E. coli chromosomes, allowing the bacteriophage to hijack the cells’ transcription and translation metabolism. The host E. coli cell synthesises many copies of the phage’s DNA and capsid. HL Content Only Stages of the Lytic Cycle Phage Assembly: The E coli cell’s metabolism is used to assemble phage DNA and capsids into many new bacteriophage lambdas. Host cell Lysis: Enzymes are produced, which damages the E. coli cell wall. The cell lyses (bursts) releasing many bacteriophage lambda, which can attach to and infect other E. coli cells. Note: During the lytic cycle, the bacteriophage lambda DNA remains separate from the host cell’s DNA. The DNA from the bacteriophage lambda takes control of the cell’s metabolism. HL Content Only From A2.3.4: Lysogenic cycle of a virus the IB Use bacteriophage lambda as an example. HL Content Only Lysogenic Cycle of Viruses The bacteriophage lambda can also follow the lysogenic cycle. HL Content Only Lysogenic Cycle of Lambda Bacteriophage Read the linked article on the lysogenic cycle. The article begins with a review of the lytic cycle. ❓ Describe the lysogenic cycle of the lambda bacteriophage. Lysogenic Cycle HL Content Only Lysogenic Cycle of Lambda Bacteriophage Attachment: The bacteriophage lambda attaches to receptors on an E. coli cell. The capsid remains outside the bacterial cell. Penetration: The bacteriophage lambda injects its DNA into the E. coli cell. Prophage formation: The phage DNA is incorporated into the E. coli chromosome, to form a prophage. The infected bacterium is known as a lysogen. HL Content Only Lysogenic Cycle of Lambda Bacteriophage E. coli reproduction: The infected E. coli lysogen (infected cell) reproduces. When the E. coli reproduces, the lambda bacteriophage DNA is replicated along with the bacterial chromosome. All offspring of the infected bacteria will contain the lambda bacteriophage DNA. Induction: An E. coli lysogen is triggered to enter the lytic cycle. The prophage (lambda bacteriophage DNA) is excised from the bacterial chromosome. Phage DNA Replication begins, leading to lysis and the release of many copies of the lambda bacteriophage to infect other E. coli cells. HL Content Only From A2.3.5: Evidence for several origins of viruses the IB from other organisms The diversity of viruses suggests several possible origins. Viruses share an extreme form of obligate parasitism as a mode of existence, so the structural features that they have in common could be regarded as convergent evolution. The genetic code is shared between viruses and living organisms. HL Content Only Origins of Viruses All viruses are obligate parasites, that require host cells for replication. Viruses use the same genetic code as all organisms. There are several competing hypotheses on the origin of viruses. However, it is possible that all viruses do not share a common ancestor, and viruses could have developed in different ways. The similarities between viruses could result from convergent evolution, as similar adaptations are required for being obligate parasites. HL Content Only Origins of Viruses Read the linked article about hypotheses on the origin of viruses. Watch the video. ❓ Outline the evidence for and against the three hypotheses for the origin of viruses. HL Content Only Virus First Hypothesis Virus First Hypothesis The Virus First Hypothesis proposes that viruses existed before cells, as they are much simpler than cells. The ancestors of modern viruses could have provided the raw material for the first cells. Strength: Virus genomes have genes that are not present in cells. Weakness: All modern viruses can only replicate using cells, suggesting that viruses could not have existed before cells. HL Content Only Escape Hypothesis Escape hypothesis The Escape Hypothesis suggests viruses evolved from sections of DNA or RNA that escaped from cells. Strengths: Modern bacterial cells exchange genetic material, suggesting a possible escape mechanism for genetic material. The hypothesis would explain the diversity of viruses if genetic material escaped many times. Weaknesses: Most of the genes and proteins found in viruses are not found in cells. HL Content Only Regressive Hypothesis The Regressive Hypothesis suggests viruses were once small cells that parasitized larger cells. The genes not required for their parasitism have been lost over time. Strengths: The existence of giant viruses which have similar genetic material to Regressive hypothesis parasitic bacteria. Weaknesses: The smallest cellular parasites do not resemble viruses in any way.  From A2.3.6: Rapid evolution in viruses the IB Include reasons for very rapid rates of evolution in some viruses. Use two examples of rapid evolution: evolution of influenza viruses and of HIV. Consider the consequences for treating diseases caused by rapidly evolving viruses. HL Content Only Viruses and Antigens Antigens are any substance that causes the immune system to produce antibodies. Parts of the capsid (protein coat) of viruses acts as antigens, and stimulate the immune system to produce antibodies. Spike Proteins of Covid-19 If the genetic material of the virus mutates, this can cause the shape of the The spike proteins on the Covid-19 protein coat, including the antigens, to virus are an example of antigens. change. HL Content Only Rapid Evolution of Viruses Some viruses have an unstable genome and mutate very rapidly for the following reasons: Viruses have a very high replication rate, increasing the chance of a random mutation occurring. Viruses do not have a proofreading mechanism during replication, making it more likely that a mutation happens. This is particularly true of RNA viruses such as the influenza virus and HIV. Immune systems select against viruses that have not mutated as they recognise the antigens on the capsid surface. Immune systems select for mutated versions of the virus, as they do not recognise the antigens on the capsid surface. HL Content Only Rapid Evolution of Flu The influenza virus (flu virus) is an RNA virus that can mutate through antigenic drift and antigenic shift. Both processes involve mutations which cause a shape change to the HA and NA surface proteins of the influenza virus. HL Content Only Antigenic Shift and Antigenic Drift Read the linked article on antigenic drift and antigenic shift. ❓ Describe the processes of antigenic shift and antigenic drift. Antigenic shift and antigenic drift HL Content Only Antigenic Drift Antigenic drift is a gradual process. The influenza virus has a high replication rate resulting in mutations. This leads to the gradual accumulation of mutations in the genes coding for the HA and NA surface proteins (antigens). The mutations cause the shape of the HA and NA antigens to change over time, resulting in new strains of the influenza virus which the immune system no longer recognises. A person who is immune to the original virus will not be immune to the mutated virus, due to the different-shaped antigens. HL Content Only Antigenic Shift Antigenic shift is an abrupt major change to the HA and NA surface proteins of the influenza virus. Antigenic shift occurs when an organism, such as a pig, is infected with two different strains of the influenza virus. When the viruses are being synthesised by cells (for example, the pig’s cells), the genetic material from the two virus strains can recombine, resulting in novel combinations of the HA and NA genes. The resulting virus produces combinations of HA and NA surface proteins (antigens) not recognised by the immune system, and may result in a pandemic. HL Content Only Rapid Evolution of HIV The Human Immunodeficiency Virus (HIV) is also capable of mutating rapidly. HL Content Only Consequences of Rapid Evolution of Viruses Rapidly evolving viruses can quickly develop resistance to existing drugs and treatment, requiring more research and development of new drugs. The mutations often produce novel subtypes of the virus, which are no longer recognised by the immune system. The novel viruses will also impact the effectiveness of current vaccines, which may no longer confer immunity. If the rapidly evolving viruses are not contained, it can result in epidemics or pandemics. HL Content Only HL Only Key Terms Virus Lysogenic Cycle Antibodies Capsid Prophage Antigens Bacteriophage Lysogen Immunity Lytic Cycle Parasite Mutation Lysis Obligate Parasite Antigenic drift Metabolism Convergent Antigenic shift Evolution HL and SL Content From A2.3 Viruses - IB Linking Questions the IB What mechanisms contribute to convergent evolution? To what extent is the natural history of life characterized by increasing complexity or simplicity? All 40 Topics Now Available For SL and HL Also Available: FULL COURSE MEGABUNDLE Bundles for Themes A, B, C, D  See My Other Resources IBDP Biology IB MYP Science First Exams 2025 - New Course General Resources Criterion A Resources Final Exams 2024 - Current/Old Criteria B and C Resources Course Criterion D Resources For more quality IB MYP Science and IB Diploma Biology resources, visit my TpT store.  Thank You! 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