Biology Class 12 NCERT Solutions: Chapter 6 Molecular Basis of Inheritance Part 3

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Q.9 List two essential roles of ribosome during translation.

Answer:

  • The important functions of ribosome during translation are as follows.

  • Ribosome acts as the site where protein synthesis takes place from individual amino acids. It is made up of two subunits.

  • The smaller subunit comes in contact with mRNA and forms a protein synthesizing complex whereas the larger subunit acts as an amino acid binding site.

  • Ribosome acts as a catalyst for forming peptide bond. For example, r-RNA in bacteria acts as a ribozyme.

Q.10 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 segment of DNA that is made up of three adjacent structural genes, namely, an operator gene, a promoter gene, and a regulator gene.

  • It works in a coordinated manner to metabolize lactose into glucose and galactose. In lac operon, lactose acts as an inducer. It binds to the repressor and inactivates it.

  • Once the lactose binds to the repressor, RNA polymerase binds to the promoter region. Hence, three structural genes express their product and respective enzymes are produced.

  • These enzymes act on lactose so that lactose is metabolized into glucose and galactose. After sometime, when the level of inducer decreases as it is completely metabolized by enzymes, it causes synthesis of the repressor from regulator gene.

  • The repressor binds to the operator gene and prevents RNA polymerase from transcribing the operon.

  • Hence, the transcription is stopped. This type of regulation is known as negative regulation.

Image of Lac Operon

Image of Lac Operon

Q.11 Explain (in one or two lines) the function of the followings:

(a) Promoter

(b) tRNA

(c) Exons

Answer:

(a) Promoter

  • Promoter is a region of DNA that helps in initiating the process of transcription. It serves as the binding site for RNA polymerase.

(b) tRNA

  • tRNA or transfer RNA is a small RNA that reads the genetic code present on mRNA. It carries specific amino acid to mRNA on ribosome during translation of proteins.

(c) Exons

  • Exons are coding sequences of DNA in eukaryotes that transcribe for proteins.

Q.12 Why is the Human Genome project called a mega project?

Answer:

  • Human genome project was considered to be a mega project because it had a specific goal to sequence every base pair present in the human genome.

  • It took around years for its completion and got accomplished in year 2006. It was a large scale project, which aimed at developing new technology and generating new information in the field of genomic studies.

  • As a result of it, several new areas and avenues have opened up in the field of genetics, biotechnology, and medical sciences. It provided clues regarding the understanding of human biology.

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