Understanding Viruses, Bacteria, and Cellular Life
DNA Replication and Bacteriophages
DNA Replication
Before each cell division, genetic material undergoes a process called DNA replication. Replication is said to be semiconservative and bi-directional:
- Semiconservative: Each strand serves as a template to form a new one.
- Bi-directional: Replication proceeds in two directions.
Question: The accompanying drawing shows a range… what is the role of components marked as 1, 2, 3, and 4?
Answer: (1) SSB protein, (2) DNA polymerase, (3) Okazaki fragments, (4) Helicase.
Question: Explain the reason why DNA replication is continuous in one strand and discontinuous in the other.
Answer: DNA replication is continuous in one strand and discontinuous in the other depending on the direction of the strand (5′ to 3′ or 3′ to 5′). Okazaki fragments are involved in the discontinuous strand.
Bacteriophages
Bacteriophages are viruses that infect bacteria. Besides bacteriophages, what other types of cells can viruses infect?
Answer: Retroviruses (RNA) and adenoviruses (DNA) can infect other types of cells based on the type of genetic material or protein capsule they possess.
Question: How do you recognize the virus that infects a certain type of cell? Explain how viruses make use of the bacterial metabolism.
Answer: Viruses are recognized by their capsid, which recognizes receptors on the cell membrane of the host cell. Viruses lack enzymes and metabolic precursors necessary for their own replication, so they must obtain them from the host cell they infect.
Question: Discuss the steps of the lytic cycle of a bacteriophage that leads to the death of the infected bacteria.
Answer:
- Fixing Phase: Bacteriophages attach to the bacteria through the tips of their tail fibers.
- Penetration Phase: The bacteriophage pierces the bacterial cell wall using lysozyme located in the basal plate. The tail sheath contracts, injecting the viral DNA into the bacterial cytoplasm.
- Eclipse Phase: This is the period of greatest metabolic activity induced by the viral DNA. The viral DNA undergoes replication using the bacterium’s enzyme complexes.
- Lysis Phase: The enzyme endolysine is produced, causing lysis of the bacterial cell. New virions are released and can infect other bacteria.
Classification of Organisms Based on Energy and Carbon Sources
Prokaryotes and Other Organisms
Living things can be classified according to the energy source and carbon source they use. Using a table, describe the four main groups of organisms based on these criteria:
| Group | Energy Source | Carbon Source |
|---|---|---|
| Photoautotrophs | Light | CO2 |
| Photoheterotrophs | Light | Organic compounds |
| Chemoautotrophs | Chemical energy from reduced inorganic compounds | CO2 |
| Chemoheterotrophs | Organic compounds | Organic compounds |
Question: Is it fair to classify viruses as prokaryotes?
Answer: No, because viruses lack a cellular structure, cytoplasm, and the enzymes necessary for metabolism.
Question: What sources of energy and carbon do viruses use?
Answer: Viruses do not directly use energy sources like light or CO2. They rely on the host cell’s metabolic machinery for energy and building blocks. They utilize the host cell’s resources during their life cycle, which can develop in two forms: the lytic cycle (bacteriophage) and the lysogenic cycle (attenuated virus).
Viruses and Human Diseases
Viral Infections
Certain viruses that affect people produce antigens that generate immunity to humans.
Question: What types of molecules are antigens? Discuss why the presence of antigens can prevent reinfection.
Answer: Antigens are proteins. The presence of antigens can prevent reinfection because the immune system recognizes them and mounts a faster and stronger response upon subsequent exposure to the same virus.
Question: Explain the stages of a viral infection.
Answer: The stages of a viral infection are similar to those described for the lytic cycle of a bacteriophage:
- Fixing Phase: The virus attaches to the host cell.
- Entry Phase: The virus enters the host cell.
- Eclipse Phase: The virus replicates its genetic material and synthesizes viral proteins.
- Lysis Phase: New virions are released from the host cell.
Bacterial Cell Structure
A large number of diseases are caused by bacteria, as shown in the diagram below.
Question: Identify the labeled components of the bacterial cell in the scheme.
Answer: (1): Flagellum, (2): Plasma membrane, (3) Cell wall, (4 ): Capsule, (5): Ribosomes, (6): Bacterial DNA.
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