questions with their answers, prepared for Class 12 CHSE Biology students, covering the chapters "Molecular Basis of Inheritance" and "Evolution."

Chapter: Molecular Basis of Inheritance & Evolution

Multiple Choice Questions (MCQ)

1. Who proved that DNA is the genetic material?

a) Griffith

b) Hershey and Chase

c) Meselson and Stahl

d) Watson and Crick

Answer: b) Hershey and Chase

 

2. The backbone of a DNA molecule is composed of:

a) Nitrogenous bases and phosphate groups

b) Sugar and nitrogenous bases

c) Sugar and phosphate groups

d) Nitrogenous bases only

Answer: c) Sugar and phosphate groups

 

3. In a DNA molecule, adenine always pairs with thymine, and guanine always pairs with cytosine. This rule is known as:

a) Mendel's rule

b) Chargaff's rule

c) Watson-Crick rule

d) Sutton-Boveri rule

Answer: b) Chargaff's rule

 

4. The packaging of DNA in eukaryotes involves positively charged proteins called:

a) Histones

b) Non-histone chromosomal proteins

c) Nucleosomes

d) Chromatin

Answer: a) Histones

 

5. The process of copying genetic information from one strand of DNA into RNA is called:

a) Replication

b) Translation

c) Transcription

d) Translocation

Answer: c) Transcription

 

6. Which enzyme is primarily responsible for DNA replication in E. coli?

a) DNA ligase

b) RNA polymerase

c) DNA polymerase III

d) Helicase

Answer: c) DNA polymerase III

 

7. How many codons code for amino acids in the genetic code?

a) 61

b) 64

c) 20

d) 3

Answer: a) 61

 

8. The phenomenon of "wobble hypothesis" is related to:

a) DNA replication

b) Transcription

c) Translation

d) DNA repair

Answer: c) Translation

 

9. The operon model for gene regulation was proposed by:

a) Watson and Crick

b) Jacob and Monod

c) Hershey and Chase

d) Meselson and Stahl

Answer: b) Jacob and Monod

 

10. Which of the following is NOT a component of the Lac Operon?

a) Regulator gene

b) Operator gene

c) Promoter gene

d) Enhancer gene

Answer: d) Enhancer gene

 

11. The Miller-Urey experiment demonstrated the synthesis of:

a) DNA

b) RNA

c) Amino acids

d) Proteins

Answer: c) Amino acids

 

12. The presence of homologous structures like the forelimbs of humans, cheetahs, bats, and whales is an example of:

a) Divergent evolution

b) Convergent evolution

c) Adaptive radiation

d) Analogous organs

Answer: a) Divergent evolution

 

13. Industrial melanism in peppered moths (Biston betularia) is an example of:

a) Genetic drift

b) Mutation

c) Natural selection

d) Gene flow

Answer: c) Natural selection

 

14. Which principle states that allele frequencies in a population remain constant from generation to generation in the absence of evolutionary influences?

a) Lamarck's Principle

b) Darwin's Theory

c) Hardy-Weinberg Principle

d) Modern Synthetic Theory

Answer: c) Hardy-Weinberg Principle

 

15. The earliest known human ancestor to walk upright (bipedalism) was likely:

a) Homo habilis

b) Homo erectus

c) Australopithecus

d) Homo sapiensAnswer: c) Australopithecus

Short Answer Questions (2-3 marks)

Briefly describe Griffith's transforming principle experiment.

Answer: Griffith worked with Streptococcus pneumoniae and observed that heat-killed S-strain bacteria (virulent) when mixed with live R-strain bacteria (non-virulent) could transform the R-strain into virulent S-strain, leading to the death of the mice. He concluded that some "transforming principle" from the heat-killed S-strain transferred to the R-strain, making it virulent.

2. Name the two types of nucleic acids. What is the main sugar present in each?

Answer: * Deoxyribonucleic Acid (DNA): Contains deoxyribose sugar. * Ribonucleic Acid (RNA): Contains ribose sugar.

3. Draw a simplified diagram of a nucleosome.

Answer: (Diagram not possible in text, but should show DNA wrapped around a core of histone octamer, linked by H1 histone).

4. What is the central dogma of molecular biology?

Answer: The central dogma of molecular biology states that genetic information flows from DNA to RNA (through transcription) and then from RNA to protein (through translation). In some cases, information can flow from RNA back to DNA (reverse transcription).

5. Name the three types of RNA and state their primary functions.

Answer: * mRNA (messenger RNA): Carries genetic information from DNA to the ribosomes for protein synthesis. * tRNA (transfer RNA): Carries specific amino acids to the ribosome during translation. * rRNA (ribosomal RNA): A structural and catalytic component of ribosomes, essential for protein synthesis.

6. What is a codon? How many codons specify amino acids?

Answer: A codon is a triplet sequence of nucleotides on mRNA that specifies a particular amino acid or signals termination of protein synthesis. Out of 64 possible codons, 61 codons specify amino acids (the other 3 are stop codons).

7. Briefly explain the concept of redundancy (degeneracy) in the genetic code.

Answer: Redundancy (or degeneracy) of the genetic code means that more than one codon can specify the same amino acid. For example, both UUU and UUC code for Phenylalanine. This provides a buffer against some point mutations.

8. What is a cistron?

Answer: A cistron is a segment of DNA that codes for a polypeptide. It is essentially a synonym for a gene in terms of its functional definition.

9. What is DNA fingerprinting? Mention one of its applications.

Answer: DNA fingerprinting (or DNA profiling) is a molecular technique used to identify individuals based on their unique DNA profiles. It utilizes repetitive DNA sequences called VNTRs (Variable Number Tandem Repeats). Application: Forensic science (identifying criminals, paternity testing), personal identification.

10. What is the Miller-Urey experiment, and what did it demonstrate?

Answer: The Miller-Urey experiment (1953) was a classic experiment that simulated early Earth conditions (high temperature, volcanic eruptions, reducing atmosphere of methane, ammonia, hydrogen, water vapor) and demonstrated the abiotic synthesis of organic molecules, specifically amino acids, from inorganic precursors. This provided strong evidence for the chemical evolution of life.

11. Differentiate between homologous and analogous organs with one example each.

Answer: * Homologous Organs: Organs that have similar basic structural plans and embryonic origin, but may perform different functions. They indicate divergent evolution and common ancestry. Example: Forelimbs of humans, whales, bats, and cheetahs. * Analogous Organs: Organs that have different basic structural plans and embryonic origins but perform similar functions. They indicate convergent evolution. Example: Wings of insects and wings of birds.

12. State any two main points of Darwin's Theory of Natural Selection.

Answer: * Overproduction/Prodigality of Production: Organisms produce more offspring than can survive. * Struggle for Existence: Competition among individuals for limited resources. * Variation: Individuals within a population exhibit variations. * Survival of the Fittest/Natural Selection: Individuals with advantageous variations are better adapted, survive, and reproduce more successfully. * Inheritance of Variations: Favorable variations are passed on to offspring.

13. What is industrial melanism? How is it an example of natural selection?

Answer: Industrial melanism is a phenomenon observed in peppered moths (Biston betularia) in industrial areas. Before industrialization, light-colored moths were dominant as they blended with lichen-covered trees. After industrial pollution darkened tree trunks (due to soot), dark-colored moths became better camouflaged and survived preferentially, increasing their numbers. This demonstrates natural selection favoring adaptive traits in a changing environment.

14. Define genetic drift. What is the founder effect?

Answer: Genetic drift is a random change in allele frequencies in a population from one generation to the next, particularly noticeable in small populations. The founder effect is a type of genetic drift that occurs when a new population is established by a very small number of individuals (founders) from a larger population. The gene pool of the new population may then be significantly different from the original population due to chance.

15. What is adaptive radiation? Give an example.

Answer: Adaptive radiation is the evolutionary process by which a single ancestral species diversifies into a multitude of new species, each adapted to occupy a different ecological niche. This often occurs when a species colonizes a new environment with abundant resources or when a new trait allows exploitation of new niches. Example: Darwin's finches on the Galapagos Islands (diversified from a common ancestor to different beak types for different food sources).

Long Answer Questions (5-6 marks)

1. Describe the structure of a DNA molecule as proposed by Watson and Crick. Explain the process of DNA replication in a semi-conservative manner.

Answer:

Structure of DNA (Watson and Crick Model): In 1953, James Watson and Francis Crick proposed the double helical structure of DNA based on X-ray diffraction data by Rosalind Franklin and Maurice Wilkins, and Chargaff's rules.

Key features of the DNA double helix:

Double Helix:

DNA consists of two polynucleotide chains coiled around a common axis, forming a right-handed double helix.

Antiparallel Polarity:

The two chains run in opposite directions; if one strand has a 5' to 3' polarity, the other has a 3' to 5' polarity.

Sugar-Phosphate Backbone:

The backbone of each strand is formed by alternating deoxyribose sugar and phosphate groups, linked by phosphodiester bonds. The nitrogenous bases project inwards.

Base Pairing:

The two strands are held together by hydrogen bonds between the nitrogenous bases. Adenine (A) always pairs with Thymine (T) with two hydrogen bonds (A=T), and Guanine (G) always pairs with Cytosine (C) with three hydrogen bonds (G≡C). This specific pairing is called

complementary base pairing

.

Uniform Diameter:

The pairing of a purine (A or G) with a pyrimidine (T or C) ensures a uniform distance between the two strands, giving the helix a constant diameter (approx. 2 nm).

Turns and Pitch:

Each turn of the helix is approximately 3.4 nm and contains roughly 10 base pairs.

Grooves:

The helix has a major groove and a minor groove.

DNA Replication (Semi-Conservative Model): DNA replication is the process by which a DNA molecule makes an exact copy of itself. The semi-conservative model, proposed by Watson and Crick and experimentally proven by Meselson and Stahl, states that each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

The process involves several steps and enzymes:

Initiation:

Replication begins at specific sites called origins of replication.

Helicase enzyme unwinds the double helix by breaking the hydrogen bonds between base pairs, creating a Y-shaped structure called the replication fork.

Single-strand binding proteins (SSBs) bind to the separated strands to prevent them from re-annealing.

Elongation:

RNA primase synthesizes a short RNA primer on both template strands, as DNA polymerase can only add nucleotides to a pre-existing 3'-OH group.

DNA Polymerase III is the main enzyme responsible for synthesizing new DNA strands by adding complementary nucleotides to the template strand. It moves in a 5' to 3' direction.

Leading Strand Synthesis: One template strand (3' to 5' direction) is replicated continuously in the 5' to 3' direction towards the replication fork. This is the leading strand.

Lagging Strand Synthesis: The other template strand (5' to 3' direction) is replicated discontinuously in short segments called Okazaki fragments. Each fragment requires a separate RNA primer. This is the lagging strand.

DNA Polymerase I removes the RNA primers and fills the gaps with DNA nucleotides.

DNA ligase joins the Okazaki fragments on the lagging strand, forming a continuous strand.

Termination:

Replication continues until the entire DNA molecule is copied or until replication forks meet.

Specific termination sequences can signal the end of replication.

Each new DNA double helix formed consists of one old (parental) strand and one newly synthesized strand, thus confirming the semi-conservative nature of replication.

2. Elaborate on the evidences for biological evolution. Discuss Darwinism and the Modern Synthetic Theory of Evolution, highlighting the key differences and contributions of each.

Answer:

Evidences for Biological Evolution: Biological evolution is the process by which life forms on Earth have changed over successive generations, leading to the diversity of life we see today. Several lines of evidence support this concept:

Paleontological Evidence (Fossil Records):

Fossils are preserved remains or traces of organisms from the past.

Fossil records show that different life forms existed at different geological periods.

They reveal a gradual increase in complexity over time (e.g., simple to complex organisms, invertebrates to vertebrates).

They provide evidence of transitional forms (e.g., Archaeopteryx showing characteristics of both reptiles and birds).

Comparative Anatomy and Morphology:

Homologous Organs: Structures with similar basic anatomical structure and embryonic origin but different functions, indicating common ancestry and divergent evolution (e.g., forelimbs of humans, whales, bats, cheetahs).

Analogous Organs: Structures with different anatomical structures and embryonic origins but similar functions, indicating convergent evolution (e.g., wings of insects and birds).

Vestigial Organs: Rudimentary or non-functional organs in an organism that were functional in its ancestors (e.g., appendix, wisdom teeth in humans).

Embryological Evidence (Comparative Embryology):

Comparing the early developmental stages of different vertebrates reveals striking similarities (e.g., presence of gill slits and notochord in all vertebrate embryos, even in terrestrial ones). This suggests common ancestry.

However, Haeckel's "biogenetic law" (ontogeny recapitulates phylogeny) has been oversimplified and is not entirely accurate.

Molecular Evidence:

DNA and Protein Homology: The universality of the genetic code, the presence of similar DNA sequences, and the similarity in amino acid sequences of proteins (e.g., cytochrome c, hemoglobin) across diverse organisms strongly suggest common ancestry. The more similar the molecules, the closer the evolutionary relationship.

Chromosome Structure: Similarities in chromosome number and banding patterns among related species.

Biogeographical Evidence:

The distribution of species across different geographical regions provides insights into their evolutionary history (e.g., unique marsupial fauna in Australia due to continental drift and isolation).

Darwinism (Theory of Natural Selection): Proposed by Charles Darwin, it is based on several key observations and inferences:

Overproduction: Organisms produce more offspring than can survive.

Struggle for Existence: Due to limited resources, individuals compete for survival.

Variation: Individuals within a population exhibit heritable variations.

Survival of the Fittest (Natural Selection): Individuals with advantageous variations are better adapted to their environment, thus have a higher chance of survival and reproduction. Nature "selects" the fittest.

Inheritance of Variations: These favorable variations are passed on to the next generation.

Speciation: Over long periods, accumulation of favorable variations leads to the formation of new species.

Modern Synthetic Theory of Evolution (Neo-Darwinism): The Modern Synthetic Theory integrates Darwinian ideas of natural selection with advancements in genetics, molecular biology, and population biology. It explains the mechanisms of evolution more comprehensively.

Key Contributions/Differences:

Feature	Darwinism (Classical)	Modern Synthetic Theory (Neo-Darwinism)
Source of Variation	Emphasized natural variation, but mechanism unknown	Clearly identifies Mutation and Genetic Recombination as the primary sources of variation.
Mechanism of Evolution	Natural Selection	Natural Selection is the main driving force, but also includes Gene Flow, Genetic Drift, and Isolation.
Inheritance	Lacked understanding of genetics	Incorporates Mendelian genetics (genes and chromosomes) to explain inheritance of variations.
Unit of Evolution	Individual	Population (changes in gene pool)
Speciation	Gradual accumulation of variations	Result of genetic changes, natural selection, and reproductive isolation over time.

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The Modern Synthetic Theory provides a more robust and comprehensive explanation for evolutionary change, incorporating the molecular basis of heredity that was unknown to Darwin. It acknowledges that evolution is a change in allele frequencies in a population over time, driven by various factors.