Jumping genes and transmission of viruses

 Jumping genes: 

   Sequences of DNA that jump from one location in the genome to another location, is called jumping genes.

• Other names:  Transposons, mobile genes
• The process that involves recombination is called transposition.
• The segment of DNA that is involved in transposons, is called transposable element.
• These are found in almost all organisms.

Examples:

Transposons in corn:

Transposons was first discovered in corn by Barbara McClintock in late 1940s. She discovered that genes can move around chromosomes, showing that the genome is more dynamic than previous thoughts.

In Morgan's lab, the studies on genetic linkage in Drosophila showed that genes had been considered to have fixed positions on chromosomes. McClintock use corn as model organism, she observed that genes can "jump" or “transposed” from one position to another. She rearranged chromosome segments with the redistribution of genetic traits. 

Transposons, were later found in all kinds of species, from bacteria to humans. McClintock was awarded Nobel Prize in Physiology or Medicine in 1983 for her discovery.

Transposons in bacteria: 

Transposons may found in bacteria where some transposable elements (e.g., Tn family) carry antibiotic resistant genes. These genes can jump from chromosomal DNA to plasmid DNA back and forth, and create some medically important bacteria. Three types of transposons are found in these:

1. 1S elements
2. Composite transposons
3. Non-composite elements

Transposons in humans:

   50% of all human DNA have transposons. The most common type is alu sequence. Approximately 300 bases are present in this sequence. 

Mechanism: 

Although each type of transposable elements has its own special characteristics, but most can be identified in any one of the following three categories on the basis of their transposition.

Conservative transposition:

  In this mechanism, the transposable elements move from the old site and inserted at a new site. As a result, donor site becomes small and is then attached by ligase enzyme. This is also called cut and paste mechanism. This type of transposes is present in prokaryotes.                                     

Replicative transposition:

    In this mechanism, a copy of transpose element is made before moving. The copied DNA is then attached to the receptor site so there is no change in donor site. This is also called copy and paste mechanism.                                                                                    

 Retro transposition: 

In this mechanism, the transpose DNA element is first transcribed into RNA by enzyme transcriptase, and then it is transcribed into cDNA by enzyme reverse transcriptase and then it moves to the receptor site. Also, there is no change in donor site. This type of transposes is present in eukaryotes.                       

Transmission of viruses:

The process by which viruses spread between host (either to same host species or different species in cases of viruses that can cross species barriers), is called viral transmission. Transmission is the fundamental property of viruses.

Viral infection is transmitted on basis of ability of viruses to overcome multiple barriers and move from cell to cell, tissue to tissue, and person to person and even across species. While there are fundamental differences between types of transmission, it has stated that the ability of viruses to utilize and manipulate cell-cell contact contributes to the success of viral infections.

Transmission of viruses in plants:

  Plant viruses are mostly spread by two mechanisms:

• Horizontal transmission (external source, leaf to leaf)
• Vertical transmission (parent to offsprings, plant to plant)

Plants viruses are transmitted in a number of ways. Some of them are given below:

1. Transmission of viruses by seed:

Seeds provide an efficient means in spreading plant virus diseases. Transmission of viruses through seeds has an important role in the long distance spread of viruses. Infected seeds can be carried away to distant places where they act as primary source of inoculum for spread of a virus.

2. Transmission of viruses by pollen:

Virus may also be transmitted by pollen. Cross pollination by pollen from virus infected plant may infect the seed and seedlings grow from it. In some cases,  it can spread by falling of fertilized flower into mother plant.

3. Transmission by vegetative propagation:

Plants are propagated vegetatively by budding or grafting or by cutting or by use of tubers, corms, bulbs or rhizome. In mother plant, any virus present, will almost always be transmitted to the progeny. Viral transmission can also occur through natural root grafts of adjacent plant. 

4. Transmission through sap:

The transmission of a virus from infected to healthy tissues is a best method fundamental to the study of viral diseases. 

In laboratory, this is generally attained by grinding the leaf of a disease plant, and rubbing the infectious sap on to the leaf of a healthy plant.

Sap can also be transmitted after a strong wind injures the leaves of adjacent diseased and healthy plants. Also, when the plants are wounded artificially, during cultural practices by tools, hands or clothes.

5. Transmission by vectors:

Insect transmission:

Insects that chew or suck plants tissues,  are the most common means for transmission of viruses to new hosts in the field.

Examples: Aphids, leafhoppers, whiteflies, thrips.

Mite transmission:

Some families of mites, are efficient vectors of plant viruses. They are too small in size to be dispersed by wind. They feed and survive only in living tissues. They feed by piercing of plant cells and then sucking the contents.

Nematode transmission:

Viruses are also transmitted in plants through nematodes. They infect a wide range of woody and herbaceous hosts. They are sap-transmissible by mechanical inoculation to herbaceous test plants.

Fungus transmission:

Viral infection can also be spread by fungi. For example, some “olpidium species” transmit many plant viruses.