The Discovery of Viruses
Adolf Mayer, a German scientist, saw that he could transmit the disease from plant to plant by rubbing sap from diseased leaves onto healthy plants. The disease was caused by small bacteria that could not be seen with the microscope. Dimitri Ivanowsky, then passed sap from infected tobacco leaves through a filter designed to remove bacteria. In the end sap still produced mosaic disease.
Structure of Virues
The tiniest viruses are only 20 nm in diameter—smaller than a ribosome. Millions could easily fit on a pinhead. Even the largest viruses are barely visible in the light microscope. Viruses are infectious particles consisting of nucleic acid enclosed in a protein coat.
Viral Genomes
Genomes may consist of double–stranded DNA, single–stranded DNA, double–stranded RNA, or single–stranded RNA. The genome is a single linear or circular molecule of nucleic acid. The smallest viruses have 4 genes but the largest have several 100.
Capsids and Envelopes
The capsid may be rod–shaped, polyhedral, or more complex in shape (T4). Capsids are built from a large number of protein subunits called capsomeres, but the number of different kinds of proteins is usually small.
Reproductive Cycles of Phages
Viruses are obligate intracellular parasites: They can reproduce only within a host cell. An isolated virus is unable to reproduce or do anything else except infect an appropriate host cell. Viruses lack metabolic enzymes, ribosomes. Each type of virus can infect only a limited range of host cells (host range) Viruses identify their host cells by a “lock–and–key” fit between proteins on the outside of the virus and specific receptor molecules on the surface of cells.
The Lytic Cycle
A phage reproductive cycle that culminates in death of the host cell is known as alytic cycle. The term refers to the last stage of infection, during which the bacterium lyses (breaks open) and releases the phages that were produced within the cell. Each of these phages can then infect a healthy cell, and a few successive lytic cycles can destroy an entire bacterial population in just a few hours. A phage that reproduces only by a lytic cycle is a virulent phage.
The Lysogenic Cycle
Thelysogenic cycle replicates the phage genome without destroying the host. Phages capable of using both modes of reproducing within a bacterium are called temperate phages.
Viral Envelopes
An animal virus equipped with an outer membrane, or viral envelope, uses it to enter the host cell. Protruding from the outer surface of this envelope are viral glycoproteins that bind to specific receptor molecules on the surface of a host cell. Viral Diseases in Plants
The link between a viral infection and the symptoms it produces is often obscure. Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes. Some viruses cause infected cells to produce toxins that lead to disease symptoms, and some have molecular components that are toxic, such as envelope proteins. How much damage a virus causes depends partly on the ability of the infected tissue to regenerate by cell division. People usually recover completely from colds because the epithelium of the respiratory tract, which the viruses infect, can efficiently repair itself. In contrast, damage inflicted by poliovirus to mature nerve cells is permanent, because these cells do not divide and usually cannot be replaced. Many of the temporary symptoms associated with viral infections, such as fever and aches, actually result from the body′s own efforts at defending itself against infection.
The Bacterial Genome and its Replication
The main component of the genome in most bacteria is one double–stranded, circular DNA molecule that is associated with a small amount of protein. Although we will refer to this structure as the bacterial chromosome, it is very different from eukaryotic chromosomes, which have linear DNA molecules associated with a large amount of protein. In E. coli, the chromosomal DNA consists of about 4.6 million nucleotide pairs, representing about 4,400 genes. This is 100 times more DNA than is found in a typical virus, but only about one–thousandth as much DNA as in an average human cell. Still, this is a lot of DNA to be packaged in such a small container.
Conjugation and Plasmids
Sometimes referred to as bacterial “sex,”conjugation is the direct transfer of genetic material between two bacterial cells that are temporarily joined. The DNA transfer is one–way: One cell donates DNA, and its “mate” receives the DNA. The donor, sometimes called the “male,” uses appendages called sex pili (singular, sex pilus) to attach to the recipient, sometimes called the “female”
Transposition of Genetic Elements
The DNA of a single cell can also undergo recombination owing to movement of so–called transposable genetic elements , or simplytransposable elements, within the cell′s genome. Unlike a plasmid or prophage, transposable elements never exist independently but are always part of chromosomal or plasmid DNA. During the movement of these elements, called transposition, the transposable element moves from one site in a cell′s DNA to another site—a target site—by a type of recombination process. In a bacterial cell, a transposable element may move within the chromosome, from a plasmid to the chromosome (or vice versa), or from one plasmid to another.
Insertion Sequences
The simplest transposable elements, calledinsertion sequences, exist only in bacteria. An insertion sequence contains a single gene, which codes for transposase, an enzyme that catalyzes movement of the insertion sequence from one site to another within the genome. The transposase gene is bracketed by a pair of noncoding DNA sequences about 20 to 40 nucleotides long. These sequences are called inverted repeats because the base sequence at one end of the insertion sequence is repeated upside down and backward (inverted) at the other end
Adolf Mayer, a German scientist, saw that he could transmit the disease from plant to plant by rubbing sap from diseased leaves onto healthy plants. The disease was caused by small bacteria that could not be seen with the microscope. Dimitri Ivanowsky, then passed sap from infected tobacco leaves through a filter designed to remove bacteria. In the end sap still produced mosaic disease.
Structure of Virues
The tiniest viruses are only 20 nm in diameter—smaller than a ribosome. Millions could easily fit on a pinhead. Even the largest viruses are barely visible in the light microscope. Viruses are infectious particles consisting of nucleic acid enclosed in a protein coat.
Viral Genomes
Genomes may consist of double–stranded DNA, single–stranded DNA, double–stranded RNA, or single–stranded RNA. The genome is a single linear or circular molecule of nucleic acid. The smallest viruses have 4 genes but the largest have several 100.
Capsids and Envelopes
The capsid may be rod–shaped, polyhedral, or more complex in shape (T4). Capsids are built from a large number of protein subunits called capsomeres, but the number of different kinds of proteins is usually small.
Reproductive Cycles of Phages
Viruses are obligate intracellular parasites: They can reproduce only within a host cell. An isolated virus is unable to reproduce or do anything else except infect an appropriate host cell. Viruses lack metabolic enzymes, ribosomes. Each type of virus can infect only a limited range of host cells (host range) Viruses identify their host cells by a “lock–and–key” fit between proteins on the outside of the virus and specific receptor molecules on the surface of cells.
The Lytic Cycle
A phage reproductive cycle that culminates in death of the host cell is known as a
The Lysogenic Cycle
The
Viral Envelopes
An animal virus equipped with an outer membrane, or viral envelope, uses it to enter the host cell. Protruding from the outer surface of this envelope are viral glycoproteins that bind to specific receptor molecules on the surface of a host cell. Viral Diseases in Plants
The link between a viral infection and the symptoms it produces is often obscure. Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes. Some viruses cause infected cells to produce toxins that lead to disease symptoms, and some have molecular components that are toxic, such as envelope proteins. How much damage a virus causes depends partly on the ability of the infected tissue to regenerate by cell division. People usually recover completely from colds because the epithelium of the respiratory tract, which the viruses infect, can efficiently repair itself. In contrast, damage inflicted by poliovirus to mature nerve cells is permanent, because these cells do not divide and usually cannot be replaced. Many of the temporary symptoms associated with viral infections, such as fever and aches, actually result from the body′s own efforts at defending itself against infection.
The Bacterial Genome and its Replication
The main component of the genome in most bacteria is one double–stranded, circular DNA molecule that is associated with a small amount of protein. Although we will refer to this structure as the bacterial chromosome, it is very different from eukaryotic chromosomes, which have linear DNA molecules associated with a large amount of protein. In E. coli, the chromosomal DNA consists of about 4.6 million nucleotide pairs, representing about 4,400 genes. This is 100 times more DNA than is found in a typical virus, but only about one–thousandth as much DNA as in an average human cell. Still, this is a lot of DNA to be packaged in such a small container.
Conjugation and Plasmids
Sometimes referred to as bacterial “sex,”
Transposition of Genetic Elements
The DNA of a single cell can also undergo recombination owing to movement of so–called transposable genetic elements , or simply
Insertion Sequences
The simplest transposable elements, called
