Characteristics of viruses

Structure and shape of viruses

A typical virus consists of two parts, an inner core of nucleic acid and protective outer coat of protein. The nucleic acid core may consist of either DNA or RNA, the chemicals that contain coded genetic information. Unlike cells that contain both DNA and RNA, each virus has only one type of nucleic acid. The DNA or RNA enables a virus to reproduce new viruses exactly like itself. In cells the DNA is double stranded and the RNA is single stranded. The outer protein coat makes up 95 percent of the body of the virus. Some of the proteins are enzymes. The arrangement of the protein in the outer coat determines the shape of the virus. Viruses that invade bacteria are called bacteriophages. Some viruses have a membrane that surrounds the outer coat. This membrane consists of proteins, lipids and carbohydrates.

Reproduction

When a virus enters a living host cell, the virus is able to reproduce. It uses the host’s cell parts to reproduce itself. During reproduction many viruses kill the host cell. Such a process is called a lytic cycle that has five main stages: adsorption, entry, replication, assembly, release. The lytic cycle is similar for all viruses though animal and plant viruses differ from bacteriophages in the way they enter cells. The whole animal virus passes through the cell membrane by phagocytes, the same process by which large food particles enter the cell. Once inside the cell, the protein outer coat is destroyed by enzymes. Most plant viruses are injected through cell walls by insects.

Lysogenic cycle

After entering a host cell, some viruses remain inactive for many generations. Then suddenly it may become active and enter a lytic cycle of destruction. Scientists do not yet understand how inactive phages are activated. These phages are known as temperate phages and the inactive cycle they undergo is called a lysogenic cycle. This cycle goes through the stages: attachment, replication, activation.

Transduction

Viruses have the ability to transfer genetic information from one host cell to another. This process is called transduction. Two types of transduction have been identified: general transduction (transfers random fragments of the host’s DNA to the receiving cell; special transduction involves the transfer of the specific genes from one cell to another. Thought transduction, a virus can alter the hereditary code of a cell.

Date: 2014-12-22; view: 947