NCERT Solutions for Class 12 Biology Chapter 5: Chapter 5 of Class 12 Biology titled Molecular Basis of Inheritance explains how genetic information is passed from one generation to the next. It starts by introducing DNA as the genetic material in living organisms.
The structure of DNA is described as a double helix, made up of units called nucleotides, which consist of a sugar, phosphate group, and nitrogenous bases (adenine, thymine, cytosine, and guanine). The chapter then explains how DNA replicates, or makes a copy of itself, to ensure genetic information is passed on. It also covers the processes of transcription (where DNA is used to make RNA) and translation (where RNA is used to make proteins). The chapter further explains how mutations in DNA can affect traits and lead to variation in organisms. Overall this chapter helps students understand how DNA works to control heredity and the functioning of cells.Structure of DNA : DNA is made up of two strands coiled into a double helix. Each strand is composed of nucleotides, which include a sugar (deoxyribose), phosphate group, and nitrogenous bases (adenine, thymine, cytosine, and guanine). The base pairing rule (A pairs with T, C pairs with G) is essential for DNA replication.
DNA Replication : DNA replication is the process by which a cell makes an identical copy of its DNA before cell division. This ensures that genetic information is accurately passed on to the next generation of cells.
Transcription and Translation :
Genetic Code : The chapter introduces the concept of the genetic code, which is a set of rules that dictate how the sequence of nucleotides in DNA determines the sequence of amino acids in proteins. The genetic code is universal, meaning it works the same in almost all organisms.
Mutation : Mutations are changes in the DNA sequence. They can occur naturally or due to environmental factors. Mutations can lead to variations in organisms and sometimes cause diseases.
NCERT Solutions for Class 12 Biology Chapter 5 PDF
1. Group the following as nitrogenous bases and nucleosides:
Solution:
Nitrogenous Base | Nucleosides |
---|---|
Adenine | Cytidine |
Thymine | Guanosine |
Uracil | |
Cytosine |
2. If a double-stranded DNA has 20 percent of cytosine, calculate the percent of adenine in the DNA.
Solution:
As per Chargaff’s rule, DNA molecules must have an equal ratio of purines (adenine and guanine) and pyrimidines (cytosine and thymine). This means that the number of adenine molecules is equivalent to the number of thymine molecules. Also, the percentage of guanine equals the percentage of cytosine.3. If the sequence of one strand of DNA is written as follows:
Write down the sequence of the complementary strand in a 5’→3′ direction.
Solution:
The complementary sequence of the strand is obtained by replacing each base with its pair:4. If the sequence of the coding strand in a transcription unit is written as follows:
Write down the sequence of mRNA.
Solution:
In transcription, thymine (T) is replaced with uracil (U) in RNA. Therefore, the mRNA sequence corresponding to the coding strand is:5. Which property of the DNA double helix led Watson and Crick to hypothesize a semi-conservative mode of DNA replication? Explain.
Solution:
Watson and Crick proposed the semi-conservative model of DNA replication based on the observation that the two strands of DNA are complementary and anti-parallel. This means that each strand can serve as a template for the synthesis of a new complementary strand. Thus, during replication, each of the two strands would separate, and each would form a new complementary strand, resulting in two DNA molecules, each consisting of one old strand and one newly synthesized strand.6. 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.
Solution:
The types of nucleic acid polymerases are:7. How did Hershey and Chase differentiate between DNA and protein in their experiment while proving that DNA is the genetic material?
Solution:
Hershey and Chase used radioactive isotopes to label DNA and protein separately. They used phosphorus-32 (32P) to label DNA and sulfur-35 (35S) to label proteins. After the bacteriophages infected E. coli bacteria, they found that the radioactive phosphorus entered the bacteria, indicating that DNA, not protein, was transferred and thus carried the genetic material.8. Differentiate between the following:
(a) Repetitive DNA and Satellite DNA
(b) mRNA and tRNA
(c) Template Strand and Coding Strand
9. List two essential roles of the ribosome during translation.
Solution:
10. In the medium where E. coli was growing, lactose was added, which induced the lac operon. Why does the lac operon shut down sometime after the addition of lactose in the medium?
Solution:
The lac operon is activated when lactose binds to the repressor, inactivating it, allowing RNA polymerase to transcribe the operon. However, once lactose is metabolized and its concentration decreases, the repressor is synthesized again, binds to the operator region, and prevents RNA polymerase from transcribing the operon, thereby shutting it down.11. Explain (in one or two lines) the function of the following:
12. Why is the Human Genome Project called a mega project?
Solution:
The Human Genome Project is called a mega project because it aimed to map and sequence all the genes in the human genome, which took over 13 years to complete. The project had a massive scale and scope, contributing to advancements in genetics, biotechnology, and medicine.13. What is DNA fingerprinting? Mention its application.
Solution:
DNA fingerprinting is a technique used to identify and analyze variations in DNA sequences. Applications include:14. Briefly describe the following:
Clear Understanding of Concepts
Enhanced Problem-Solving Skills
Improved Exam Preparation
Better Retention
Boosts Confidence
Time Management
Preparation for Competitive Exams