Chapter 26 Flashcards
memorise
What are viruses composed of?
Viruses are composed of genetic material (DNA or RNA or both), a protein coat called a capsid that protects the genetic material, and sometimes an envelope derived from the host cell membrane. The envelope contains host cell phospholipids, membrane proteins, and viral proteins.
Why are viruses considered non-living?
Viruses are considered non-living because they cannot reproduce or metabolize independently. They lack essential cellular components such as ribosomes, cytoplasm, and cellular organelles. They exist in a state between life forms and chemicals, leading a “borrowed” life by depending entirely on host cells for reproduction and metabolism.
What is the size range of viruses?
Viruses are extremely small, even smaller than prokaryotic cells. The smallest viruses can be as tiny as 20 nm, smaller than a ribosome, while the largest viruses can be up to 1500 nm and visible under light microscopy.
Who first demonstrated that tobacco mosaic disease could be transmitted through plant sap?
Adolf Mayer in 1883. He studied tobacco mosaic disease (TMD) in tobacco plants, noting symptoms like stunted growth and mottled leaf coloration. Mayer demonstrated that the disease could be transmitted through plant sap, hypothesizing that it was caused by small, invisible bacteria.
What did Dmitri Ivanowsky discover about the infectious agent of tobacco mosaic disease?
Dmitri Ivanowsky, in 1890, filtered sap through bacteria-excluding filters and observed that the filtered sap still caused infection, ruling out typical bacteria. He suggested that the infectious agent was either a very small bacterium or a toxin secreted by bacteria.
Who coined the term “virus” and what was his contribution?
Martinus Beijerinck in 1898. He conducted experiments showing that the infectious agent of tobacco mosaic disease replicated only in host plants and could not be grown in nutrient-rich lab media. He proposed that the agent was a replicating particle far smaller and simpler than bacteria, coining the term “virus.”
What are the types of viral genomes?
Viral genomes can be double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), double-stranded RNA (dsRNA), or single-stranded RNA (ssRNA). Examples include:
dsDNA: Adenovirus (respiratory illness), Herpesvirus (cold sores, shingles)
ssDNA: Parvovirus (mild rash)
dsRNA: Reovirus (rotavirus; diarrhea)
ssRNA (direct mRNA): Picornavirus (common cold, polio), Flavivirus (Zika, yellow fever), Coronavirus (SARS, MERS)
ssRNA (template for mRNA synthesis): Influenza, Ebola
ssRNA (template for DNA synthesis, retroviruses): HIV (AIDS)
What is the function of the protein coat (capsid) in viruses?
The protein coat, or capsid, encloses the viral genome, protecting it from environmental factors and aiding in the infection of host cells. It is composed of protein subunits called capsomeres, which provide structural support and protect the nucleic acid.
Flashcard 9
What are bacteriophages and their significance?
Bacteriophages, or phages, are viruses that infect bacteria. They are significant in molecular biology and genetics research, having been used to demonstrate that DNA is the genetic material. Phages have complex structures, including an elongated icosahedral head and a protein tail used to inject DNA into bacterial cells.
What is the role of a viral envelope?
A viral envelope surrounds the capsids of some viruses, mainly those infecting animals. It is derived from the host cell membrane and contains host cell phospholipids, membrane proteins, viral proteins, and glycoproteins. The envelope aids in host infection by enabling the virus to fuse with the host cell membrane and helps the virus evade the host’s immune system.
How do viruses replicate?
Viruses replicate by hijacking the host cell’s machinery. The process involves:
Attachment and entry: Viruses bind to host cells using specific surface proteins that recognize and attach to receptor molecules on the host’s surface.
Reprogramming the host cell: The viral genome hijacks the host’s cellular machinery to replicate viral nucleic acids and synthesize viral proteins.
Assembly: Capsomeres and viral genomes self-assemble into complete viruses within the host cell.
Exit: Newly assembled viruses exit the host cell, often causing cellular damage and death, leading to symptoms of viral infections.
What is the lytic cycle in phages?
The lytic cycle involves the destruction of the host cell. After the phage infects a bacterium, it hijacks the host cell’s machinery to produce phage particles. Once production is complete, the host bacterium bursts (lyses), releasing newly made phages that can infect more bacterial cells.
What is the lysogenic cycle in phages?
The lysogenic cycle allows the phage to coexist with the host bacterium. The phage genome integrates into the bacterial chromosome without destroying the host cell. This cycle allows the virus to replicate along with the host’s genome during cell division, spreading the phage genome without harming the host.
What are temperate phages?
Temperate phages are phages that can undergo both the lytic and lysogenic cycles. An example is the lambda phage, which can alternate between these cycles. In the lysogenic cycle, the phage integrates into a specific site on the bacterial chromosome, becoming a prophage.
How do bacteria defend against phages?
Bacteria have evolved several defense mechanisms against phages, including:
Mutation of surface receptors: Bacterial mutants produce surface proteins that phages cannot recognize, preventing attachment and infection.
Restriction enzymes: These enzymes cut up invading phage DNA, limiting its ability to replicate within the host cell.
CRISPR-Cas system: This adaptive immune system stores DNA from previous phage infections and uses it to recognize and cut phage DNA during subsequent infections.
