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Biotechnology: Principles and Processes — BIOLOGY STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYBiotechnology: Principles and Processes4 Marks
Question
Explain- Restriction enzymes.
✓

Answer

Empty → In the year 1963, the two enzymes responsible for restricting the growth of bacteriophage in Escherichia coli were isolated.
→ One of these added methyl groups to DNA, while the other cut DNA.
→ The later was called restriction endonuclease.
→ The first restriction endonuclease Hind - II, whose functioning depended on a specific DNA nucleotide sequence.
→ Hind -II was isolated and characterised five years later.
→ It was found that Hind II always cut DNA molecules at a particular point by recognising a specific sequence of six base pairs.
→ This specific base sequence is known as the recognition sequence for Hind II.
→ Besides Hind II, today we know more than 900 restriction enzymes that have been isolated from over 230 strains of bacteria each of which recognise different recognition sequences.
Nomenclature :
→ The convention for naming these enzymes is the first letter of the name comes from the genus and the second two letters come from the species of the prokaryotic cell from which they were isolated.
e.g., EcoRI
→ It comes from Escherichia coli RY 13.
→ In EcoRI, the letter 'R' is derived from the name of strain.
→ Roman numbers following the names indicate the order in which the enzymes were isolated from that strain of bacteria.
TYPES :
→ Restriction enzymes belong to a larger class of enzymes called nucleases. These are of two kinds.
(a) Exonucleases
(b) Endonucleases.
(1) Exonucleases remove nucleotides from the ends of the DNA.
(2) Endonucleases make cuts at specific positions within the DNA.
→ Each restriction endonuclease functions by 'inspecting' the length of a DNA sequence.
→ Once it finds its specific recognition sequence, it will bind to the DNA and cut each of the two strands of the double helix at specific points in their sugar -phosphate backbones.
PALINDROMIC SEQUENCE :
→ Each restriction endonuclease recognises a specific palindromic nucleotide sequences in the DNA.
→ The palindrome in DNA is a sequence of base pairs that reads same on the two strands when orientation of reading is kept the same. For example, the following sequences reads the same on the two strands in 5'→3' This is also true if read in the 3' → 5' direction. 5' direction.
5' ____ GAATTC _____ 3'
3' ____ CTTAAG _____ 5'
Mechanism of action :
→ Restriction enzymes cut the strand of DNA a little away from the centre of the palindrome sites, but between the same two bases on the opposite strands.
→ This leaves single stranded portions at the ends.
→ There are overhanging stretches called sticky ends on each strand.
Image
→ When cut by the same restriction enzyme, the resultant DNA fragments have the same kind of 'sticky-ends' and, these can be joined together using DNA ligases.
→ Unless one cuts the vector and the source DNA with the same restriction enzyme, the recombinant vector molecule cannot be created.
→ Restriction endonucleases are used in genetic engineering to form 'recombinant' molecules of DNA, which are composed of DNA from different sources/genomes.

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