CBSE Class 12 Biology (2026–27)
Chapter 5: Molecular Basis of Inheritance
20 Important Questions and Answers
The chapter covers DNA structure, genetic material, replication, transcription, translation, genetic code, gene regulation, Human Genome Project, and DNA fingerprinting as prescribed in the CBSE syllabus.
Q1. What is a nucleotide? Mention its components.
Answer:
A nucleotide is the basic structural and functional unit of nucleic acids such as DNA and RNA. Each nucleotide consists of three components: a nitrogenous base, a pentose sugar, and a phosphate group. The nitrogenous bases are of two types—purines (adenine and guanine) and pyrimidines (cytosine, thymine in DNA, and uracil in RNA). In DNA, the sugar is deoxyribose, whereas in RNA it is ribose. Nucleotides join together through phosphodiester bonds to form long polynucleotide chains. The sequence of nucleotides stores genetic information and determines the characteristics of an organism.
Q2. State the important features of Watson and Crick’s model of DNA.
Answer:
Watson and Crick proposed the double-helical model of DNA in 1953. According to this model, DNA consists of two polynucleotide strands coiled around a common axis. The strands run in opposite directions, making them antiparallel. Nitrogenous bases face inward and pair specifically through hydrogen bonds: adenine pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds. The sugar-phosphate backbone lies on the outside. One complete turn of the helix measures 3.4 nm and contains 10 base pairs. The diameter of the DNA helix is approximately 2 nm. These features ensure stability and accurate replication of genetic material.
Q3. Explain Chargaff’s rule.
Answer:
Chargaff’s rule states that in a double-stranded DNA molecule, the amount of adenine equals thymine and the amount of guanine equals cytosine. Thus, A = T and G = C. This means the total purines equal the total pyrimidines. Chargaff arrived at this conclusion after analyzing DNA from different organisms. The rule provided strong evidence for complementary base pairing in DNA and later helped Watson and Crick formulate the double-helix model. Although the proportion of A+T and G+C may vary among species, the equality of A with T and G with C remains constant in double-stranded DNA molecules.
Q4. Why is DNA considered the genetic material?
Answer:
DNA is considered the genetic material because it stores, replicates, and transmits hereditary information from one generation to another. Experimental evidence from Griffith, Avery-MacLeod-McCarty, and Hershey-Chase established DNA as the genetic material. DNA can replicate accurately due to complementary base pairing. It is chemically stable and can undergo mutations, producing variations essential for evolution. DNA also directs protein synthesis through transcription and translation, thereby controlling cellular activities. These characteristics make DNA suitable for storing and expressing genetic information in almost all living organisms, except some viruses where RNA acts as the genetic material.
Q5. Describe Griffith’s transformation experiment.
Answer:
Frederick Griffith conducted experiments on Streptococcus pneumoniae bacteria in 1928. He used virulent smooth (S) strains and non-virulent rough (R) strains. When live S bacteria were injected into mice, the mice died. Live R bacteria did not cause disease. Heat-killed S bacteria also failed to kill mice. However, when live R bacteria were mixed with heat-killed S bacteria and injected, the mice died. Griffith concluded that some “transforming principle” from dead S bacteria transformed the harmless R bacteria into virulent forms. This experiment provided the first evidence that genetic information could be transferred from one organism to another.
Q6. What was concluded from the Hershey and Chase experiment?
Answer:
Hershey and Chase used bacteriophages to determine whether DNA or protein acts as genetic material. They labeled DNA with radioactive phosphorus (³²P) and proteins with radioactive sulfur (³⁵S). After allowing the phages to infect bacteria, they separated the viral coats from bacterial cells using a blender. Radioactivity from ³²P was found inside the bacterial cells, whereas ³⁵S remained outside in the phage coats. This demonstrated that DNA entered the bacterial cells and directed the formation of new phages. Therefore, Hershey and Chase concluded that DNA, and not protein, is the genetic material responsible for inheritance.
Q7. What is semiconservative DNA replication?
Answer:
Semiconservative replication is the process by which DNA duplicates itself before cell division. In this mechanism, each parental DNA strand acts as a template for the synthesis of a new complementary strand. As a result, each newly formed DNA molecule contains one old parental strand and one newly synthesized strand. This model was proposed by Watson and Crick and experimentally proved by Meselson and Stahl using nitrogen isotopes. Semiconservative replication ensures accurate transmission of genetic information and minimizes errors during DNA duplication. It is the universally accepted mechanism of DNA replication in living organisms.
Q8. Explain the Meselson and Stahl experiment.
Answer:
Meselson and Stahl demonstrated semiconservative DNA replication in E. coli. They first grew bacteria in a medium containing heavy nitrogen (¹⁵N) and then transferred them to a medium containing light nitrogen (¹⁴N). DNA samples were analyzed using density-gradient centrifugation. After one generation, DNA showed an intermediate density band, indicating hybrid DNA. After two generations, one hybrid and one light DNA band appeared. These results ruled out conservative and dispersive replication models and confirmed that DNA replication is semiconservative. Thus, each daughter DNA molecule contains one parental strand and one newly synthesized strand.
Q9. Define transcription.
Answer:
Transcription is the process of synthesizing RNA from a DNA template. During transcription, RNA polymerase binds to the promoter region of DNA and uses one strand, called the template strand, to produce a complementary RNA molecule. The RNA strand is synthesized in the 5′ to 3′ direction. The resulting RNA may be mRNA, tRNA, or rRNA depending on the gene being transcribed. In eukaryotes, the primary transcript undergoes processing such as capping, tailing, and splicing before becoming mature mRNA. Transcription is the first step in gene expression and protein synthesis.
Q10. What is the Central Dogma of molecular biology?
Answer:
The Central Dogma explains the flow of genetic information in living organisms. It states that genetic information passes from DNA to RNA and then to proteins. DNA undergoes replication to produce identical DNA molecules. Through transcription, DNA forms messenger RNA (mRNA). During translation, the mRNA sequence is decoded by ribosomes to synthesize proteins. Proteins then perform structural and functional roles within the cell. Proposed by Francis Crick, the Central Dogma forms the basis of molecular genetics and explains how genes control the characteristics and activities of organisms.
Q11. What are the important features of the genetic code?
Answer:
The genetic code is a set of codons that specify amino acids during protein synthesis. It is triplet in nature, meaning each codon consists of three nucleotides. It is universal, as most organisms use the same codons. The code is degenerate because multiple codons can specify the same amino acid. It is non-overlapping and comma-less, ensuring continuous reading of codons. AUG serves as the initiation codon and codes for methionine, while UAA, UAG, and UGA are termination codons. These properties ensure accurate translation of genetic information into proteins.
Q12. What is translation?
Answer:
Translation is the process of synthesizing proteins from the information present in mRNA. It occurs on ribosomes in the cytoplasm. During translation, transfer RNA (tRNA) molecules bring specific amino acids corresponding to the codons on mRNA. The ribosome reads the codons sequentially and joins amino acids through peptide bonds to form a polypeptide chain. Translation consists of initiation, elongation, and termination stages. The process continues until a stop codon is reached. Translation is essential because proteins are responsible for structural, enzymatic, and regulatory functions in living organisms.
Q13. Why is tRNA called an adaptor molecule?
Answer:
Transfer RNA (tRNA) is called an adaptor molecule because it acts as a link between the nucleotide language of mRNA and the amino acid language of proteins. Each tRNA carries a specific amino acid at its 3′ end and possesses an anticodon that pairs with the corresponding codon on mRNA. During translation, tRNA recognizes the codon and brings the correct amino acid to the ribosome. This ensures the proper sequence of amino acids in the growing polypeptide chain. Thus, tRNA translates genetic information into protein structure accurately.
Q14. What is the lac operon?
Answer:
The lac operon is a gene regulation system found in E. coli. It was proposed by Jacob and Monod. The operon contains structural genes responsible for lactose metabolism, along with regulatory elements such as promoter and operator. In the absence of lactose, a repressor protein binds to the operator and prevents transcription. When lactose is present, it acts as an inducer by binding to the repressor, making it inactive. This allows RNA polymerase to transcribe the structural genes. The lac operon is an example of inducible gene regulation in prokaryotes.
Q15. Differentiate between exons and introns.
Answer:
Exons are coding sequences of genes that remain in mature mRNA after RNA processing and participate in protein synthesis. Introns are non-coding sequences present between exons. During RNA processing in eukaryotes, introns are removed by splicing, while exons are joined together to form functional mRNA. Exons contain genetic information required for protein formation, whereas introns do not directly code for proteins. The removal of introns and joining of exons ensure that only useful genetic information is translated into proteins. This process contributes to efficient gene expression in eukaryotic cells.
Q16. What are the objectives of the Human Genome Project?
Answer:
The Human Genome Project (HGP) was an international scientific initiative aimed at mapping and sequencing the entire human genome. Its objectives included identifying all human genes, determining the sequence of approximately three billion base pairs, storing genetic information in databases, improving data-analysis tools, and developing technologies for genome research. The project also sought to address ethical, legal, and social issues related to genetic information. The successful completion of HGP has greatly enhanced our understanding of human genetics, inherited diseases, evolution, and biotechnology applications.
Q17. Mention four salient features of the human genome.
Answer:
The Human Genome Project revealed several important features of the human genome. First, the human genome contains about 3.1 billion nucleotide base pairs. Second, humans possess approximately 20,000–30,000 genes. Third, less than 2% of the genome codes for proteins, while a large portion consists of non-coding DNA. Fourth, all humans share about 99.9% similarity in their DNA sequence, and the remaining 0.1% accounts for genetic variation among individuals. These findings have provided valuable insights into human evolution, disease susceptibility, and genetic diversity.
Q18. What is DNA fingerprinting?
Answer:
DNA fingerprinting is a molecular technique used to identify individuals based on unique DNA patterns. It relies on highly variable repetitive DNA sequences called Variable Number Tandem Repeats (VNTRs). Since the pattern of VNTRs differs among individuals, DNA fingerprinting can distinguish one person from another, except identical twins. The technique involves isolation of DNA, amplification or analysis of specific regions, and comparison of banding patterns. DNA fingerprinting has important applications in forensic science, paternity testing, identification of missing persons, and wildlife conservation studies.
Q19. What is RNA world hypothesis?
Answer:
The RNA world hypothesis suggests that RNA was the first genetic material on Earth before the evolution of DNA and proteins. Scientists proposed this idea because RNA can both store genetic information and act as a catalyst. Certain RNA molecules called ribozymes possess enzymatic activity and can catalyze biochemical reactions. DNA is more stable than RNA and may have evolved later as the primary genetic material. Proteins subsequently evolved as specialized catalysts. The RNA world hypothesis helps explain the origin of life and the evolution of genetic systems in early organisms.
Q20. Why is DNA more stable than RNA?
Answer:
DNA is more stable than RNA for several reasons. DNA contains deoxyribose sugar, which lacks the reactive hydroxyl group present in ribose sugar of RNA. This makes DNA less susceptible to hydrolysis. DNA is usually double-stranded, providing additional stability through complementary base pairing and hydrogen bonding. Furthermore, DNA contains thymine instead of uracil, reducing mutation-related errors. Because of its stability, DNA is better suited for long-term storage and transmission of genetic information. RNA, being less stable, is mainly involved in temporary functions such as protein synthesis and regulation.