Chapter 22 Flashcards
Antibody
A highly specific soluble protein molecule that circulates in the blood and lymph, recognizing and binding to antigens and clearing them from the body.
Capsid aka Coat
The protective layer of protein that surrounds the nucleic acid core of a virus in free form.
Envelope
Outer glycoprotein layer surrounding the capsid of some viruses, derived in part from host cell plasma membrane.
Describe the structural characteristics of viruses.
A virus is simply one or more nucleic acid molecules surrounded by a protein coat or capsid. Some capsids may be enclosed within a membrane or envelope derived from their host cell’s membrane. So a virus is not a cell—it does not have a cytoplasm enclosed by a plasma membrane, as do all known living organisms.
They lack a metabolic system to provide energy for their life cycles and cannot reproduce on their own; instead, they are dependent on the host cells they infect for these functions.
The nucleic acid genome of a virus may be either DNA or RNA and can be composed of either a single strand or a double strand of RNA or DNA. Viral genomes range from just a few genes to over a hundred genes; all viruses have genes that encode at least their coat proteins and the enzymes required for nucleic acid replication. Many viruses also have genes that encode recognition proteins that become implanted in the coat surface. These coat proteins recognize and bind to the host cell, promoting entry of the virus particle or its nucleic acid core into that cell.
Most viruses take one of two basic structural forms, helical or polyhedral.
Without a host cell, viruses cannot carry out their life-sustaining functions or reproduce. They cannot synthesize proteins, because they lack ribosomes and must use the ribosomes of their host cells to translate viral messenger RNA into viral proteins. Viruses cannot generate or store energy in the form of adenosine triphosphate (ATP), but have to derive their energy, and all other metabolic functions, from the host cell. They also parasitize the cell for basic building materials, such as amino acids, nucleotides, and lipids (fats).
Helical Virus
A virus in which the protein subunits of the coat assemble in a rodlike spiral around the genome.
A number of them infect plants.
Polyhedral Virus
A virus in which the coat proteins form triangular units that fit together like the parts of a geodesic sphere.
What 7 characteristics separate living things from nonliving things?
- Display Order: arranged in a highly ordered manner, with the cell being the fundamental unit of life.
- Harness and Utilize Energy: acquire energy from the environment and use it to maintain their highly ordered state.
- Reproduce
- Respond to Stimuli: can make adjustments to their structure, function, and behaviour in response to changes to the external environment.
- Exhibit Homeostasis: regulate their internal environment such that conditions remain relatively constant.
- Growth and Development
- Evolve
Bacteriophage aka Phage
A virus that infects bacteria.
Temperate Bacteriophage
Bacteriophage that may enter an inactive phase (lysogenic cycle) in which the host cell replicates and passes on the bacteriophage DNA for generations before the phage becomes active and kills the host (lytic cycle).
Virulent Bacteriophage
Bacteriophage that kills its host bacterial cells during each cycle of infection.
T-even Bacteriophages
Ex: T2, T4, and T6.
Phages with coats divided into a head and a tail. A double-stranded linear molecule of DNA is packed into the head. The tail, assembled from several different proteins, has recognition proteins at its tip that can bind to the surface of the host cell. Once the tail is attached, it functions as a sort of syringe that injects the DNA genome into the cell.
Lytic Cycle
The whole series of events, from infection of a cell through to the release of progeny phages from the ruptured (or lysed) cell.
Explain the lytic cycle.
- Phage collides randomly with the surface of a cell and the tail attaches to the host cell wall.
- An enzyme present in the viral coat, lysozyme, then digests a hole in the cell wall through which the tail injects the DNA of the phage.
- The proteins of the viral coat remain outside. Throughout its life cycle within the bacterial cell, the phage uses host cell machinery to express its genes. One of the proteins produced early in the infection is an enzyme that breaks down the bacterial chromosome. The phage gene for a DNA polymerase that replicates the phage’s DNA is also expressed early on. Eventually, 100 to 200 new viral DNA molecules are synthesized.
- Later in the infection, the host cell machinery transcribes the phage genes for the viral coat proteins.
- As the head and tail proteins assemble, the replicated viral DNA is packed into the heads.
- When viral assembly is complete, the cell synthesizes a phage-encoded lysozyme that lyses the bacterial cell wall, causing the cell to rupture and releasing viral particles that can infect other cells.
Transduction
In cell signalling, the process of changing a signal into the form necessary to cause the cellular response.
In prokaryotes, the process in which DNA is transferred from donor to recipient bacterial cells by an infecting bacteriophage when the phage randomly incorporates fragments of the host cell’s DNA into the heads of the viral particles they assemble.
Generalized Transduction
Gene transfer that occurs by transduction - Transfer of bacterial genes between bacteria using virulent phages that have incorporated random DNA fragments of the bacterial genome.
Contrast the lytic and the lysogenic cycles of virus infection.
The whole series of events, from infection of a cell through to the release of progeny phages from the ruptured (or lysed) cell.
Cycle in which the DNA of the bacteriophage is integrated into the DNA of the host bacterial cell and may remain for many generations.
Explain the lysogenic cycle.
- Viral chromosome integrates into the host cell’s DNA by recombination (steps 1 through 3).
The DNA of a temperate phage typically inserts at one or possibly a few specific sites in the bacterial chromosome through the action of a phage-encoded enzyme that recognizes certain sequences in the host DNA. Once integrated, the genes are mostly inactive, so no phage components are made.
- While inserted in the host cell DNA, the virus is known as a prophage (pro = before). When the host cell DNA replicates, so does the integrated viral DNA, which is passed on to daughter cells along with the host cell DNA (steps 4 and 5).
- Certain environmental signals, such as ultraviolet irradiation, stimulate this change, causing the prophage to enter the lytic cycle (steps 6 through 9). Genes that were inactive in the prophage are now transcribed. Among the first viral proteins synthesized are enzymes that excise the viral chromosome from the host chromosome. The result is a circular viral chromosome that replicates itself and directs the production of viral DNA and coat proteins. This active stage culminates in the lysis of the host cell and the release of infective viral particles.
Outline the HIV infection cycle.
HIV is a retrovirus that contains two copies of single-stranded RNA. It also carries several molecules of an enzyme, reverse transcriptase in its capsid.
In replication of retroviruses the virus’ genome enters the host cell along with reverse transcriptase, which copies the viral RNA onto a complementary strand of DNA.
A second strand of DNA is then synthesized, using the first strand as a template.
The resulting double-stranded DNA integrates into the host cell’s DNA as a provirus (comparable to the prophage).
It is transcribed by the host cell into mRNA, which is translated to produce viral proteins, including capsid proteins and reverse transcriptase molecules.
New virus particles are released from the cell to infect other cells or be passed to new hosts.
