Botany 103 M.Sc. I Chapter VI Transposons

 

Chapter VI   Transposons

                    Prof.Dr.Sadhana Nikam

                                 M.Sc. B.Ed. PGDDE,Ph.D

                                 Vice Principal RJ College(CSN)

Define Transposons ? Describe types and importance of transposons

(15 marks)
Def : The sequences of DNA that can move or transpose themselves to new positions within the genome of a single cell are called Transposons.
     Transposons is a class of genetic elements that can jump to different locations within a genome. Transposons are called jumping genes.Transposons were first discovered in corn (Zea mays- Maize) in 1948 by American scientist Barbara McClintock whose work won her noble prize for physiology or medicine in 1983.

       About 50% of total genome of Maize consist of transposons. In primates including humans some sequences of  repetitive DNA are present. This common form of transposon in humans is called  ‘Alu ‘ sequence. It is approximately  300 bases long.
Types of Transposons :
On the basis of their mechanism, there are three basic types of Transposons are as follows.
A. class II Transposons
B. class III Transposons OR Miniature inverted repeat transposable elements (MITEs).
C. Retrotransposons (class I transposons)
A. Class II Transposons :
i) These are segments of DNA that move from One place to another via cut and paste mechanism.
ii) Most of these elements encode an enzyme called transposase which break the ends of the Transposon from its initial location of genome.
iii) Transposed also break target sites and to this site elements can be inserted.
Once the transposes is ligated or bound in new position the gaps left in DNA sequence filled through the synthesis of nucleotides.
iv)Class II Transposons range in length from 1000 to as many as 40, 000 base pairs.
B.Class III  Transposons OR Miniature Inverted Repeat Transposable Elements (MITEs) :
i.They have short length about 400 to 600 base pairs.
ii.They can be stretch about 15 base pairs at each end of element in an inverted fashion as a mirror sequences.
iii.Thousands of MITEs have been identified in the genomes of Oryza sativa, Caenorhabditis elegant, a type of nematode and other organism.
iv.Most insertion of these elements occur in euchromatin which contain more active genes.
v.Some microRNAs are derived from Mites play important role in DNA interference.
C.Retrotransposons :
a. RNA transposons :                                          i. It represent a highly unique group of transposable elements.
ii.It form large portions of the genomes of many Eukaryotes.
iii.Retrotransposons function by a copy and paste mechanism.
iv.They leave the original copy and generate a second copy.
v.This process results in the insertion of repetitive Sequences of DNA throughout the genome.
vi.The mechanism is responsible for vast spread of transposable elements in many higher organism.
vii.The first step in retrotransposition occurs when Transposable DNA is copied into RNA  by transcription & then from RNA back to DNA by reverse transcription.
viii.The RNA segment then jumps to another location in the genome.
When Retrotransposons inserted in to the genome at the new site the RNA must be copied  back into DNA by an enzyme Reverse transcriptase.                                    ix. The DNA copy is then inserted in the new position by Copy and  Paste.
x.There are several different types of retrotransposons as Long Interspersed Nuclear Elements (LINEs) and Short Interspersed Nuclear Elements (SINEs)
xi.About 20 percent of the human genome is made up of Lines.

b. DNA transposons : 

i. They do not involve RNA  intermediate.  ii. The enzyme transposes make a staggered cut at the target site producing sticky ends.                                                          iii. The transposon cut out the transposon and ligates  in new position by Cut and Paste.

 Importance of Transposons :

1. Transposons carried resistance for antibiotic Ampicilin.
2. In bacteria Transposons can jump from Chromosomal DNA to plasmid DNA and back allowing for transfer and permanent addition of genes which coding for antibiotic resistance. 
3. Multi antibiotic resistant bacterial strains can be generated.
4. Transposons act as Selfish DNA as they copy and amplify themselves within genomes.
5. In rare cases Transposons associated with genetic mutations or chromosomal rearrangement that cause disease in humans.
6. Disease arise by insertion of Transposons into particular regions of genes involved in regulating Gene activity.
7. Insertion of Transposons near promotor regions used to initiate gene transcription.
8. Synthesis of DNA from DNA can lead to overactivity of genes which can give rise Cancer.
9.In other cases Class II elements cut out the genome which cause mutation that interfere gene regulation which cause Cell dysfunction.
10.There are several diseases such as Haemophilia and Duchenne muscular dystrophy associated with repetitive DNA arising from retrotransposons.
11. As Transposons carry signals for initiation of RNA synthesis they activate dormant genes.
12. Insertion of Transposons may produce deletions, inversions, chromosomal fusion and even more complicated rearrangement. 
13. Transposons code for the enzymes which bring about insertion of an identical copy of themselves into new DNA site.
14. Transposons are also essential for both Recombination &replication process which generate 2 daughter copies of original transposable elements.