(a) Jacob and Monod proposed the concept of lac operon.
(c) The regulatory
(i) gene codes for repressor protein, which is synthesised constitutively all the time.
(ii) It binds to the operator and prevents the RNA polymerase from transcribing the operon; so the operon is 'switched off'.
(iii) When lactose is present in the cell, it functions as inducer.
(iv) When it binds to the repressor and inactivates it, the repressor does not bind to the operator.
(v) This allows RNA polymerase access to the promoter and transcription proceeds, i.e. the operon is 'switched on'.

DNA-Fingerprinting
The steps/procedure in DNA-fingerprinting include the following.
(i) Extraction. DNA is extracted from the cells in a high-speed, refrigerated centrifuge. 
(ii) Amplification. Many copies of the extracted DNA are made by polymerase chain reaction.
(iii) Restriction Digestion. DNA is cut into fragments with restriction enzymes into precise reproducible sequences.
(iv) Separation of DNA sequences restriction fragments. The cut DNA fragments are introduced and passed through electrophoresis set-up containing agarose polymer gel; the separated fragments can be visualised by staining them with a dye that shows fluorescence under ultraviolet radiation.
(v) Southern Blotting. The separated DNA sequences are transferred on to a nitrocellulose or nylon membrane.
(vi) Hybridisation. The nylon membrane is immersed in a bath and radioactive probes (DNA segments of known sequence) are added; these probes target a specific nucleotide sequence that is complementary to them.
(vii) Autoradiography. The nylon membrane is pressed on an X-ray film and dark bands develop at the probe sites.
The bands form a characteristic pattern, which varies from individual to individual.

(i) DNA polymorphism, i.e. variation at genetic level, refers to the occurrence of inheritable mutations at a frequency greater than 0.01 in a population.
The probability of such variations is more in the non-coding sequences.
There is a variety of types of polymorphism ranging from single nucleotide change to very large scale changes.
The single nucleotide polymorphisms is used in locating the disease associated sequences of DNA on the chromosome and tracing of human history.
The variable number of tandem repeats show a high degree of polymorphism.
(ii) DNA polymorphism arises due to mutations, which occur more in the non-coding sequences.
Since, they do not have an immediate effect on the reproductive ability of the individual, they keep on accumulating generation after generation and form the basis for polymorphism.

(a) They used \(^{ 15 }{ { NH }_{ 4 }Cl }\) and \(^{ 14 }{ { NH }_{ 4 }Cl }\) to grow E.coli, because the density of DNA of E.coli grown on these media can be distinguished based on their densities.
(b) lsN was incorporated into the nitrogenous bases (adenine, guanine, cytosine and thymine) of DNA.
(c) The \(^{ 15 }{ N }-DNA\) can be distinguished from the \(^{ 14 }{ N }-DNA\) by centrifugation in cesium chloride density gradient; \(^{ 15 }{ N }-DNA\) is heavier than \(^{ 14 }{ N }-DNA\).
(d) They took samples at definite intervals as the cells multiplied and extracted the DNA that remained as double stranded elices.
(e) At the end of 20 minutes, the DNA of E.coli had a hybrid or intermediate density.
At the end of 40 minutes, the DNA extracted from the cells contained equal quantities of hybrid DNA and light DNA.
(f) They concluded that DNA replication is semi-conservative.

1. Alfred Hershey and Martha Chase proved that DNA is the genetic material.
Experiment of Hershey and Chase
1. They made two different preparations of the phage; in one, the DNA was made radioactive with \(^{ 32 }P\) and in the other, the protein coat was made radioactive with \(^{ 35 }{ { S } }\).
2. These two phage preparations were allowed to infect the bacterial cells separately.
3. Soon after infection, the cultures were gently agitated in a blender to separate the adhering protein coats of the virus from the bacterial cells.
4. The culture was also centrifuged to separate the viral coat and the bacterial cells.
5. It was found that when the phage containing radioactive DNA was used to infect the bacteria, its radioactivity was found in the bacterial cells (in the sediment) indicating that the DNA has been injected into the bacterial cell.
6. So, DNA is the genetic material and not proteins.

Alfred Hershey and Martha Chase proved that DNA is the genetic material.
Experiment of Hershey and Chase
1. They made two different preparations of the phage; in one, the DNA was made radioactive with \(^{ 32 }P\) and in the other, the protein coat was made radioactive with \(^{ 35 }{ { S } }\).
2. These two phage preparations were allowed to infect the bacterial cells separately.
3.Soon after infection, the cultures were gently agitated in a blender to separate the adhering protein coats of the virus from the bacterial cells.
4.  The culture was also centrifuged to separate the viral coat and the bacterial cells.
5.  It was found that when the phage containing radioactive DNA was used to infect the bacteria, its radioactivity was found in the bacterial cells (in the sediment) indicating that the DNA has been injected into the bacterial cell.
6. So, DNA is the genetic material and not proteins.

1. It was to show that DNA replication is semi conservative.
2. After completion of replication, each new/daughter molecule of DNA has one parental strand and one newly synthesised strand; this is called semiconservative replication of DNA.
3. Experiment
Mathew Meselson and Franklin Stahl performed the experiment.
4. They grew E.coli. in a medium containing \(^{ 15 }{ { NH }_{ 4 } }Cl,\) until \(^{ 15 }N\) was incorporated in the two strands of newly formed E.coli cells; this 'heavy' DNA can be separated from the normal \(^{ 14 }N-DNA\) by centrifugation in cesium chloride (CsCI) density gradient.
5. Then they transferred the cells into a medium with normal \(^{ 14 }{ { NH }_{ 4 } }Cl\) and took out samples at various time intervals.
6. They extracted the DNA and centrifuged it to measure the densities.
7. The DNA extracted from cells after one generation after transfer from the \(^{ 15 }{ { N } }\) medium to \(^{ 14 }{ { N } }\) medium (i.e. after about 20 minutes), had an intermediate/hybrid density.
8. The DNA extracted after two generations (i.e. after 40 minutes) consisted of equal amounts of 'light' DNA and 'hybrid' DNA.
This proves that after replication, each DNA molecule has one parental strand and one newly synthesised strand, i.e. replication is semiconservative.

1. It was to show that DNA replication is semi conservative.
2. After completion of replication, each new/daughter molecule of DNA has one parental strand and one newly synthesised strand; this is called semiconservative replication of DNA.
Experiment
1. Mathew Meselson and Franklin Stahl performed the experiment.
2. They grew E.coli. in a medium containing \(^{ 15 }{ { NH }_{ 4 } }Cl,\) until \(^{ 15 }N\) was incorporated in the two strands of newly formed E.coli cells; this 'heavy' DNA can be separated from the normal \(^{ 14 }N-DNA\) by centrifugation in cesium chloride (CsCI) density gradient.
3. Then they transferred the cells into a medium with normal \(^{ 14 }{ { NH }_{ 4 } }Cl\) and took out samples at various time intervals.
4. They extracted the DNA and centrifuged it to measure the densities.
5. The DNA extracted from cells after one generation after transfer from the \(^{ 15 }{ { N } }\) medium to \(^{ 14 }{ { N } }\) medium (i.e. after about 20 minutes), had an intermediate/hybrid density.
6. The DNA extracted after two generations (i.e. after 40 minutes) consisted of equal amounts of 'light' DNA and 'hybrid' DNA.
This proves that after replication, each DNA molecule has one parental strand and one newly synthesised strand, i.e. replication is semiconservative.