Bihar Board - Class 12 Biology - Chapter 6: Molecular Basis of Inheritance Long Answer Question

Q.1.What is transformation? Describe Griffith’s experiment to show transformation? What did he prove from his experiment?

Ans: Transformation means a change in the genetic makeup of an individual. Fredrick Griffith conducted a series of experiments on streptococcus pneumoniae. He observed two strains of this bacterium –one forming smooth colonies with capsule (s-type) & the other forming rough colonies without capsule (R-type).

1. When live s-type cells are injected into mice, they produce pneumonia & mice die.

2. When liveR-type cells are injected into mice, the disease was not produced and did not appear.

3. When heat-killed S-type cells were injected into mice, the disease did not appear.

4. When heat-killed S-type cells were mixed with live R-cells & injected into mice, the mice died.

He concluded that R-strain bacteria had somehow been transformed by heat-killed S-strain bacteria which must be due to the transfer of genetic material.

Q.2. Two claimant fathers filed a case against a lady claiming to be the father of her only daughter. How could this case be settled by identifying the real biological father?

Ans: This case to identify the real biological father could have settled lay DNA – fingerprinting technique. In this technique:-

1. First of all, the DNA of the two claimants who have to be tested is isolated.

2. Isolated DNA is then digested with a suitable restriction enzyme & digest is subjected to gel electrophoresis.

3. The fragments of ds DNA are denatured to produce ss DNA by alkali treatment.

4. The electrophoresed DNA is then transferred from getting into a nitrocellulose filter paper where it is fixed.

5. A known sequence of DNA is prepared called probe – DNA & is labeled with radioactive isotope 32p & then the probe is added to nitrocellulose paper.

6. The nitrocellulose paper is photographed on X-ray film through autoradiography. The film is analyzed to determine the presence of hybrid nucleic acid.

Then, the DNA fingerprints of the two claimants are compared with the DNA fingerprint of the lady & her daughter, whosoever matches with each other would be declared as the biological father of her daughter.

Q.3.Describe the continuous & discontinuous Synthesis of DNA?

Ans: Synthesis of a new strand of DNA takes place to lay the addition of fresh nucleotides to the 3 – OH group of the last nucleotide of the primer. This synthesis takes place in 5 direction enzymes that catalyze this DNA – polymerase synthesis of a strand called leading strands is continuous. The replication of the second strand of the DNA molecule is discontinuous on a strand called a lagging strand.

Primase initiates primer synthesis on the strand near the fork. The RNA – primer thus formed provides free for replication of single-stranded regions on the lagging strand; the new complementary strand is formed in small fragments of DNA called Okazaki fragments. It is called discontinuous because it has to be initiated several times & every time an Okazaki fragment is produced.

Q.4.What is an operon? Describe the major steps involved in an operon?

Ans: An operon is a group of controllers & structural genes which control the catabolism of the cell genetically lactose operon/lac operon.

1. When inducer or lactose is absent:-

The lac regulator gene synthesizes a repressor protein by transcription & translation. This repressor protein binds with the operator site of lac operon & blocks RNA polymerase. Thus, RNA polymerase is unable to transcribe mRNA & structural genes are unable to translate enzyme B- galactosidase.

2. When inducer or lactose is present:

The lac regulator gene transcribes mRNA & synthesizes active lac repressor protein & at the same time lactose is converted into isomer allolactose. Allolactose binds to an active lac repressor due to which it is converted to an inactive repressor. This inactive repressor is released from the operator site of lac operon & RNA polymerase binds to promoter & starts to transcribe mRNA & forms β-galactosidase which converts lactose into glucose vs galactose.

Thus, the presence of lactose determines whether or not lac. The repressor is bound to the operator & genes are expressed on not.

Q.5. What are special characters of genetic code?
Answer: Genetic Code

• Amino acids can be defined as the sequence of bases present within the mRNA and are responsible for coding for a specific amino acid.

• Each of the codes is composed of three nucleotides and are termed as a triplet. Apart from the codons of some protozoans and mitochondria, codons are almost universal.

• More than one triplet can code for one particular type of amino acid and thus the code is said to have degenerated.

• In total, there are 64 codons, 61 of which code for amino acids.

• There are three different stop codons, named - UAA, UAG, UGA that would not code for any amino acids.

• Along with being a starting codon, AUG also codes for methionine.

Q. 6. Explain the Double Helix Model Given for the DNA Structure.

Ans. 

• In 1953, Watson and Crick suggested the DNA double-helix structure.

• The ratio of adenine and thymine to that of guanine and cytosine is one and tends to remain constant. This was stated by Ervin Chargaff.

• DNA is formed by two polynucleotide chains, where the backbone is sugar-phosphate and bases are present within.

• There exists opposite polarity between the two chains, i.e. and the one with 3'-5' polarity other with 5'-3' polarity.

• The base pair (bp) is created by the formation of hydrogen bonding amongst the nitrogen bases that present on two of the polypeptide chains.

• To make a base pair, a purine base of one nucleotide chain is often connected to a pyrimidine base of another nucleotide chain or vice versa.

• Two hydrogen bonds (A=T) pair Adenine with Thymine (or Uracil in RNA) while three hydrogen bonds pair Guanine with Cytosine (G).

Q.7.Who performed the blender experiment? What does this experiment prove? Describe the steps followed in this experiment?

Ans: The proof for DNA as the genetic material came from the experiments of Hershey & chase who worked with bacteriophages.

The bacteriophage on infection injects only the DNA into the bacterial cell & not the protein coat.

Bacterial cells treat the viral DNA as their own & subsequently manufacture more virus particles.

They grew some viruses on a medium that contained radioactive Phosphorus & some others on a medium that contained radioactive sulfur. Viruses grown in the presence of radioactive phosphorus contained radioactive DNA but not proteins because DNA contains phosphorus. Similarly, viruses grown on radioactive sulfur contain radioactive protein because DNA does not contain sulfur.

Radioactive phages are allowed to infect E. coli bacteria & soon after infection the cultures were gently agitated in a blender to separate the adhering protein coat of the virus from the bacterial cells. It was found that when a phage containing radioactive DNA was used to infect the bacteria its radioactivity was found in bacterial cells indicating that DNA has been injected into the bacterial cells. So, the DNA is the genetic material & not proteins.