NEET Practice Questions (MCQs) with Answers & Solutions

The paragraph about Wien's law states: 'Light from the moon is found to have a maximum intensity near the wavelength $14 \mu m$. By Wien’s law, the surface of the moon is estimated to have a temperature of 200 K.'

The text explains: 'Internal energy is thus, the sum of molecular kinetic and potential energies in the frame of reference relative to which the centre of mass of the system is at rest. Thus, it includes only the (disordered) energy associated with the random motion of molecules of the system.' It further adds: 'The important thing about internal energy is that it depends only on the state of the system, not on how that state was achieved. Internal energy U of a system is an example of a thermodynamic ‘state variable’'.

Restriction endonucleases belong to the class of nucleases called endonucleases, which make cuts at specific positions within the DNA. Exonucleases remove nucleotides from the ends, and ligases join DNA fragments.

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 comes from Escherichia coli RY 13. In EcoRI, the letter ‘R’ is derived from the name of the strain.

Each restriction endonuclease recognises a specific palindromic nucleotide sequences in the DNA. A palindromic sequence reads the same on the two strands when orientation of reading is kept the same.

The first restriction endonuclease– Hind II, whose functioning depended on a specific DNA nucleotide sequence was isolated and characterised five years after the initial discovery of enzymes restricting bacteriophage growth. It always cut DNA molecules at a particular point by recognising a specific sequence of six base pairs.

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, called sticky ends. These are named so because they form hydrogen bonds with their complementary cut counterparts. This stickiness of the ends facilitates the action of the enzyme DNA ligase.

When cut by the same restriction enzyme, the resultant DNA fragments have the same kind of ‘sticky-ends’ and, these can be joined together (end-to-end) using DNA ligases. You may have realised that normally, unless one cuts the vector and the source DNA with the same restriction enzyme, the recombinant vector molecule cannot be created.

DNA is a negatively charged molecule, hence it moves towards the positive electrode (anode). The DNA fragments separate (resolve) according to their size through sieving effect provided by the agarose gel.

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.