NCERT Class 12 Biology: Chapter –6 Molecular Basis of Inheritance Part 11

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Question 5:

Define an operon. Giving an example, explain an Inducible operon.


1. The Operon model was given by François Jacob and Jacques Monad in 1961.

2. Operon is a cluster of gene which have related functions and are involved in the catabolism or degradation of lactose.

3. There are two types of operons, Inducer operon also called Lac Operon (lactose) and Trp Operon also called Repressor operon (Tryptophan operon).

Inducible Operator System:

1. It is an operon also known as Lac Operon found in bacteria E. coli.

2. When E. coli feeds on something it prefers Glucose over lactose.

3. When Glucose is fully taken up by the E. coli, it starts using Lactose.

4. When intake of lactose is started by E. coli, Lac operon gets activated which proves that lactose is an inducer for the Lac Operon.

Structure of Lac Operon:

Structure of Lac Operon

Structure of Lac Operon

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The given diagram shows the working of lac operon. In the absence of an inducer the repressor binds to the operator region and prevents transcription.

In the presence of an inducer, repressor becomes inactive. This allows transcription in the operator region which results in release of mRNA subsequently, mRNA promotes translation and protein synthesis is accomplished.

Question 6:

‘There is a paternity dispute for a child’. Which technique can solve the problem? Discuss the principle involved.


1. To solve the dispute DNA fingerprinting should be used.

2. In DNA fingerprinting a pattern of bands is formed due to small stretch of DNA sequence which is repeated numerous times called as Variable Number of Tandem Repeats (VNTR’S), also known as Mini Satellites. These VNTR’s are specific for each individual.

Procedure of DNA Fingerprinting:

Procedure of DNA Fingerprinting

Procedure of DNA Fingerprinting

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1. For DNA fingerprinting, blood sample of the respective person is taken. Now the DNA is extracted from the blood sample.

2. This DNA is cut into smaller fragments by an enzyme called restriction endonuclease enzyme.

3. These DNA fragments are then separated by the process of Gel Electrophoresis. In which electric field is applied to separate the DNA fragments based on size. The shorter ones move faster, then medium and longer ones are separated.

4. The DNA band pattern now formed by Gel electrophoresis is transferred to the Nylon Sheath, this process is also called Southern Bloating.

5. The band is now mixed with radio isotopic solution and is exposed to X-ray radiation.

6. After exposure to X-ray radiation coloured band patterns or VNTR’S are seen which are specific for everyone.

By the method of DNA fingerprinting the paternity dispute can be solved as the VNTR for the father would be specific for an individual.

Principle Involved:

The principle involved behind DNA fingerprinting is the Variable Number of Tandem Repeats (VNTR’S), also known as Mini Satellites. These are a pattern of band formed due to small stretch of DNA sequence which is repeated numerous times. These VNTR’s are specific for each individual.