6 Most SAQ’s of Molecular Basis of Inheritance Chapter in Inter 2nd Year Botany (TS/AP)

4 Marks

SAQ-1 : Define transformation in Griffith’s experiment. Discuss how it helps in the identification of DNA as genetic material.

Introduction

Frederick Griffith’s transformation experiments, conducted in 1928 with Streptococcus pneumoniae, were crucial in identifying DNA as the carrier of genetic information. His discovery of ‘transformation’ significantly advanced our understanding of genetic material.

Definition of Transformation

Transformation is a process where a cell genetically alters by directly uptaking and incorporating genetic material from its surroundings.

Details of Griffith’s Experiment

1. Griffith worked with two strains of Streptococcus pneumoniae: ‘S-type’ (smooth, virulent, capsule-forming) and ‘R-type’ (rough, non-virulent, without a capsule).
2. Mice infected with S-type bacteria died of pneumonia, whereas those with R-type bacteria survived.
3. Intriguingly, mice injected with a mixture of heat-killed S-type bacteria and live R-type bacteria also died, and live S-type bacteria were recovered from their bodies.

Summary

1. Griffith’s conclusion was that R-type bacteria were ‘transformed’ into virulent S-type bacteria by a ‘transforming principle’ from the heat-killed S-type bacteria.
2. Although Griffith did not identify this ‘transforming principle’, subsequent experiments by Avery, McLeod, and McCarty used his findings to demonstrate that DNA is responsible for transformation, thus establishing DNA as the genetic material.
3. Griffith’s discovery of bacterial transformation thus laid the foundation for the recognition of DNA as the molecule of heredity.

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SAQ-2 : How many types of RNA polymerases exist in cells? Write their names and functions.

Introduction

RNA polymerases are crucial enzymes in transcription, the process of creating a complementary RNA sequence from a DNA template. In eukaryotic cells, there are three primary types of RNA polymerases, each with unique roles in RNA synthesis.

Types of RNA Polymerases and Their Functions

1. RNA Polymerase I
   • Function: Primarily involved in transcribing ribosomal RNAs (rRNAs).
   • Specifics: Transcribes 28S, 18S, and 5.8S rRNAs, key components of the ribosome, essential for protein synthesis.
2. RNA Polymerase II
   • Function: Responsible for transcribing the precursor of messenger RNA (mRNA), known as heterogeneous nuclear RNA (hnRNA).
   • Specifics: Facilitates the first step in gene expression, leading to protein production based on DNA information.
3. RNA Polymerase III
   • Function: This enzyme transcribes transfer RNA (tRNA), 5S ribosomal RNA (5SrRNA), and small nuclear RNAs (snRNAs).
   • Specifics: tRNA is crucial in protein synthesis as an adapter molecule. 5SrRNA is part of the ribosomal complex, and snRNAs contribute to processes like pre-mRNA splicing.

Summary

The three RNA polymerases – I, II, and III – play significant and specific roles in cell function. They are responsible for transcribing different types of RNA, underpinning key cellular processes such as protein synthesis and RNA splicing, which are fundamental to life.

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SAQ-3 : Draw the schematic/diagrammatic presentation of the lac operon.

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SAQ-4 : What are the differences between DNA and RNA

Introduction

DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are essential nucleic acids in living organisms, crucial for genetic information storage, transmission, and expression. Despite similarities, they have key differences.

Comparison of DNA and RNA

DNA

1. Structure: Double-stranded helix, providing stability.
2. Stability: Stable under alkaline conditions.
3. Sugar Component: Contains deoxyribose sugar.
4. Size: Can consist of millions of nucleotides; the human genome has over 3 billion.
5. Self-Replication: Capable of self-replicating, crucial for cell division and inheritance.
6. Role: Primary genetic material, carrying instructions for development and functioning.
7. Protein Synthesis: Does not directly participate but provides the blueprint.
8. Variety: One type in organisms.
9. Base Pairing: Adenine (A) pairs with Thymine (T), Guanine (G) pairs with Cytosine (C).

RNA

1. Structure: Usually single-stranded, less structurally stable than DNA.
2. Stability: Less stable under alkaline conditions.
3. Sugar Component: Contains ribose sugar.
4. Size: Typically shorter than DNA, often a few thousand nucleotides or less.
5. Self-Replication: Does not self-replicate; transcribed from DNA.
6. Role: Crucial in gene expression and protein synthesis.
7. Protein Synthesis: Directly participates, with various RNA types involved.
8. Variety: Three primary types: mRNA, tRNA, and rRNA.
9. Base Pairing: Adenine (A) pairs with Uracil (U), Guanine (G) pairs with Cytosine (C).

Summary

DNA and RNA are chemically similar yet functionally distinct, each suited to their specific roles in genetics and biochemistry. DNA acts as a permanent genetic store, while RNA dynamically converts genetic information into proteins, facilitating life’s functions.

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SAQ-5 : Write the important features of Genetic code?

Introduction

The genetic code is a set of rules dictating how DNA’s four-letter code is translated into the 20-letter code of proteins, comprised of amino acids. This code is almost universally the same across all organisms, highlighting its crucial role in biology.

Key Features

1. Triplet Code: The genetic code is read in triplets of nucleotides, known as codons. Each triplet corresponds to a specific amino acid or a start/stop signal for protein synthesis.
2. Specific and Unambiguous: Each codon uniquely codes for a single amino acid. For instance, the codon UUU always codes for phenylalanine, ensuring the code’s specificity and lack of ambiguity.
3. Redundancy and Degeneracy: The code is degenerate, meaning one amino acid can be coded by multiple codons, such as leucine being coded by six different codons.
4. Non-overlapping: The genetic code is read from a fixed starting point, three nucleotides at a time, in a non-overlapping and sequential manner.
5. Universal: The genetic code is nearly universal, signifying its consistency across various organisms, supporting the theory of common descent.
6. Punctuation: Specific ‘start’ (AUG) and ‘stop’ (UAA, UAG, UGA) codons exist in the genetic code. AUG also codes for methionine, while stop codons signal the end of protein synthesis.
7. Continuous: The reading of the genetic code is continuous, with codons read in order without interruptions or breaks.

Summary

The genetic code is a fundamental and universal set of instructions for translating DNA and RNA sequences into amino acid sequences of proteins. Its consistency across diverse life forms underscores its vital importance in the study of biology and genetics.

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SAQ-6 : Write briefly on nucleosomes.

Introduction

Nucleosomes are essential structural units of eukaryotic chromatin, integral in DNA organization, replication, and gene expression within the cell nucleus.

Key Details

1. Composition: A nucleosome consists of DNA wound around a core of eight histone proteins, forming a histone octamer. The histones include two each of H2A, H2B, H3, and H4.
2. DNA Wrapping: About 146 to 147 base pairs of DNA are wrapped in 1.65 negative superhelical turns around the histone octamer, with a left-handed coil.
3. Charge Interaction: Histones, being positively charged due to amino acids like lysine and arginine, interact with the negatively charged phosphate groups of DNA, facilitating the wrapping of DNA around the histone core.
4. Structural Significance: Nucleosomes are the primary components of chromatin, enabling efficient DNA packaging within the nucleus. Chromatin can be further compacted into higher-order structures and ultimately into chromosomes.
5. Beads-on-a-String: Electron microscopy reveals a beads-on-a-string appearance of nucleosomes, especially noticeable during interphase when DNA is less compact.
6. Role in Gene Regulation: Nucleosomes are pivotal in gene regulation. The placement of nucleosomes along DNA affects the accessibility of transcription machinery, thus influencing gene expression.

Summary

Nucleosomes are vital for chromatin structure, playing a key role in compacting DNA and regulating gene activity. Their structure allows for the necessary dynamic changes in DNA replication, repair, recombination, and transcription processes.