Class 12 Biology Chapter 6 Molecular Basis of inheritance
Chapter 6

Molecular Basis of inheritance


Q1. Group the following as nitrogeneous bases and nucleosides:
Adenine, Cytidine, Thymine, Guanosine, Uracil and Cytosine
Answer: Adenine, thymine, Uracil and cytosine are the nitrogenous bases.
Cytidine and guanosine are the nucleosides.


Q2. If a double stranded DNA has 20 percent of cytosine, calculate the percent of adenine in the DNA.
Answer: According to Chargaff’s rule-
The purines are always equal to Pyrimidines, i.e A=T and G=C.
If a double stranded has 20 % of cytosine then it has 20 % guanine. Thus, the remaining 60 % is present in A+T. As, the number of adenine is equal to thymine. Therefore, 30 % of adenine is present in DNA.

Q3. If the sequence of one strand of DNA is written as follows:
5’-ATGCATGCATGCATGCATGCATGCATGC-3’
Write down the sequence of complementary strand in 5’--->3’.
Answer: The DNA strands are always complementary to one another with respect to their base pairs.
If the sequence of one strand is  
5’-ATGCATGCATGCATGCATGCATGCATGC-3’
Then, the complementary strand is
3’-TACGTACGTACGTACGTACGTACGTACG-5’
Therefore, the sequence of complementary strand in 5 ---> 3’
5’-GCATGCATGCATGCATGCATGCATGCAT-3’



Q4. If the sequence of the coding strand in a transcription unit is written as follows:
5’-ATGCATGCATGCATGCATGCATGCATGC-3’
Write down the sequence of mRNA.
Answer:  If the sequence of the coding strand in a transcription unit is
5’-ATGCATGCATGCATGCATGCATGCATGC-3’
Then, the complementary strand is
3’-TACGTACGTACGTACGTACGTACGTACG-5’
However, in RNA, thymine is substituted by Uracil.
Therefore, the sequence of mRNA will be
5’-AUGCAUGCAUGCAUGCAUGCAUGCAUGC-3’



Q5. Which property of DNA double helix led Watson and Crick to hypothesis semi-conservative mode of DNA replication? Explain:
Answer: Watson and crick found that the polynucleotide DNA chain is complementary to each other with respect to the nitrogeneous base. That means each strand of DNA acts as a template strand for the formation of new strand. Thus, the DNA molecule formed would be identical to the parental DNA strand. Thus, one parental DNA strand is conserved in each DNA molecule, which is said to be semi-conservative mode of DNA replication. This type of property of DNA double helix led Watson and Crick to hypothesis semi-conservative mode of DNA replication.



Q6. Depending upon the chemical nature of the template (DNA or RNA) and the nature of nucleic acids synthesized from it (DNA or RNA), list the types of nucleic acid polymerases.
Answer: There are two kinds of nucleic acid polymerases-
    1) DNA-dependent DNA polymerases
    2) DNA-dependent RNA polymerases 
DNA-dependent DNA polymerases synthesize a new DNA strand with the help of DNA template. On the other hand DNA-dependent RNA polymerases synthesize RNA with the help of DNA template.



Q7. How did Hershey and Chase differentiate between DNA and protein in their experiment while proving that DNA is the genetic material?
Answer: Hershey and Chase performed his experiment with virus that infects bacteria known as bacteriophages to prove that DNA is the genetic material. He grew the viruses on the medium containing radioactive phosphorous to recognize DNA and some were grew on medium containing radioactive sulfur to recognize the presence of protein. Then these bacteriophages were allowed to attach E.coli. After infection, the viral coats were eliminated from the bacteria by blending and then they undergo centrifugation for separating the virus particles from bacteria. The bacteria that were infected by radioactive viruses had radioactive DNA, which means that DNA was the material that passed from the virus to the bacteria.
 



Q8. Differentiate between the followings:

a) Repetitive DNA and Satellite DNA
Answer
Repetitive DNA
    • Repetitive DNA is the small segments of DNA sequence which are repeated several times.
Satellite DNA
    • Satellite DNA is the DNA sequences which have highly repetitive DNA.

b) mRNA and tRNA
Answer: mRNA
    • mRNA provides the template for the process of transcription.
    • It is a linear molecule
tRNA
    • tRNA brings amino acid to read the genetic code present in the mRNA.
    • It is clover leaf shape

c) Template strand and Coding strand
Answer: Template strand
    • Template strand used for synthesize mRNA during the process of transcription.
    • It has polarity 3’ --> 5’ direction.
Coding strand
    • The strand that has same base as RNA where thymine is replaced by Uracil during transcription.
    • It has polarity 5’ --> 3’ direction.


Q9. List two essential roles of ribosome during translation.
Answer: The two essential roles of ribosomes during translation are-
    • Ribosome is the site of protein synthesis.
    • Ribosome acts as a catalyst for peptide bond formation.


Q10. In the medium where E. coli was growing, lactose was added, which induced the lac operon. Then, why does lac operon shut down some time after addition of lactose in the medium?
Answer: Lac operon is a DNA segment which is composed of three structural genes such as an operator gene, a promoter and a regulator gene. Lactose controls switching on and off the operon, thus, it acts as an inducer. The repressor is inactivated when binds to inducer. This allows the RNA polymerase to attach to the promoter region and transcription proceeds.

When the level of inducer decreases due to the metabolized by enzymes. The repressor attached to the operator and stops the RNA polymerase to undergo transcription. This kind of regulation is called negative regulation.
 


Q11. Explain (in one or two lines) the function of the followings:

a) Promoter
Answer: Promoter is the DNA region helps in initiating the transcription process. It also provide binding site for RNA polymerase.

b) tRNA
Answer: tRNA helps in bringing specific amino acid to mRNA during translation process.

c) Exons
Answer: Exons are the coding sequence present in DNA that transcribe for protein synthesis.


Q12. Why the Human Genome project is called a mega project?
Answer: Human genome project is called a mega project because it purposes is to sequence each base pair presents in the human genome. It helps in recognizing all genes in the Human DNA. It determines 3 billion chemical base pairs sequences that make up human DNA.


Q13. What is DNA fingerprinting? Mention its application.
Answer: DNA fingerprinting is the method used to recognize and analyze the variations present in different individuals at the DNA level. It aims is to find the genetic differences between the individuals of a population.
Application:
    • It helps in determining population and genetic diversities.
    • It is used in the field of forensic science to recognize potential crime suspects.
     It is used in the field of evolutionary biology to find out the organism evolutionary history.



Q14. Briefly describe the following:

a) transcription
Answer: Transcription is the process of formation of RNA from template DNA. The events that take place during the process of transcription are-
        I. Initiation
        II. Elongation
        III. Termination
In initiation, RNA polymerase attach to promoter and starts transcription. RNA polymerize utilizes nucleoside triphosphate as substrate and polymerizes in a template to form mRNA depending on the complementary bases present on the template DNA. This elongation continues until RNA polymerase reaches terminator region. In termination, RNA polymerase reaches the terminator region where transcription stops.


b) Polymorphism
Answer: Polymorphism is a kind of genetic variation in which separate nucleotide sequence can present at a particular site in a DNA. It occurs due to mutation in somatic cell or in germ cell. The germ cell can be passes to next generation which brings variation and polymorphism.


c) Translation
Answer: Translation is the process of amino acid polymerization for the formation of protein. The three steps are involve in the process of transcription-
        I. Initiation
        II. Elongation
        III. Termination
In initiation, amino acid is activated in the presence of ATP which is then attached to tRNA. This tRNA is known as charged tRNA. The initiator tRNA recognize the start codon present in mRNA. In this process small ribosomal subunit binds to the mRNA followed by the large subunit. In elongation process, the ribosome move from codon to codon along with the newly synthesize polypeptide chain. In termination, the ribosomes reach the stop codon where it terminates the translation, the polypeptide chain released and the ribosomal subunits get detached.


d) Bioinformatics:
Answer: Bioinformatics is the application of computational techniques used in the field of molecular biology. Human genome project was associated with the development of a bioinformatics. Bioinformatics stores the biological databases which contains information of biology.













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