ROUTERA

Chapter 6 Molecular basis of Inheritance

Class 12th Biology Chapter Case Study

Case Study 1: Structure of DNA

DNA, or deoxyribonucleic acid, is the genetic material in all living organisms. The structure of DNA was discovered by James Watson and Francis Crick in 1953. It is composed of two long strands forming a double helix. These strands are made up of nucleotides, each consisting of a sugar, a phosphate group, and a nitrogenous base. The bases pair specifically: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). This base pairing is essential for DNA replication and transmission of genetic information.

Questions:

  1. The two strands of DNA in the double helix are held together by:

    • A) Hydrogen bonds between nitrogenous bases
    • B) Covalent bonds between sugar molecules
    • C) Ionic bonds between phosphate groups
    • D) Hydrophobic interactions between the sugar-phosphate backbone

    Answer: A) Hydrogen bonds between nitrogenous bases

  2. In the DNA structure, the sugar and phosphate groups form the:

    • A) Rungs of the ladder
    • B) Hydrogen bonds
    • C) Backbone of the helix
    • D) Nitrogenous bases

    Answer: C) Backbone of the helix

  3. Which of the following pairs of nitrogenous bases forms complementary base pairs in DNA?

    • A) Adenine (A) – Cytosine (C)
    • B) Adenine (A) – Thymine (T)
    • C) Guanine (G) – Thymine (T)
    • D) Cytosine (C) – Thymine (T)

    Answer: B) Adenine (A) – Thymine (T)

  4. The discovery of the double-helix structure of DNA by Watson and Crick was based on:

    • A) X-ray diffraction data from Maurice Wilkins and Rosalind Franklin
    • B) Chemical analysis of nucleotides
    • C) The work on gene expression by Francis Crick
    • D) Microscopic observation of chromosomes

    Answer: A) X-ray diffraction data from Maurice Wilkins and Rosalind Franklin

Case Study 2: DNA Replication

DNA replication is a semi-conservative process that ensures the accurate copying of genetic information. During replication, the double helix unwinds, and the two strands of DNA serve as templates for the synthesis of new complementary strands. The enzyme DNA helicase unwinds the double helix, and DNA polymerase adds new nucleotides to form the new strand. DNA replication is essential for cell division and the transmission of genetic material.

Questions:

  1. Which enzyme is responsible for unwinding the DNA double helix during replication?

    • A) DNA polymerase
    • B) RNA polymerase
    • C) DNA helicase
    • D) Ligase

    Answer: C) DNA helicase

  2. During DNA replication, the new strand is synthesized in the:

    • A) 3' to 5' direction
    • B) 5' to 3' direction
    • C) Both directions
    • D) Random direction

    Answer: B) 5' to 3' direction

  3. The enzyme responsible for adding new nucleotides during DNA replication is:

    • A) DNA ligase
    • B) DNA polymerase
    • C) RNA polymerase
    • D) Helicase

    Answer: B) DNA polymerase

  4. The process of DNA replication is called semi-conservative because:

    • A) Both new strands are synthesized from RNA templates
    • B) Each new DNA molecule consists of one original strand and one newly synthesized strand
    • C) Replication occurs only in certain regions of the genome
    • D) DNA molecules are split into two halves before replication

    Answer: B) Each new DNA molecule consists of one original strand and one newly synthesized strand

Case Study 3: Transcription in Eukaryotes

Transcription is the process by which an RNA molecule is synthesized from a DNA template. In eukaryotes, transcription occurs in the nucleus. The enzyme RNA polymerase binds to the promoter region of a gene and synthesizes a complementary RNA strand in the 5' to 3' direction. The RNA transcript undergoes processing, including the addition of a 5' cap, poly-A tail, and splicing of introns, before being transported to the cytoplasm for translation.

Questions:

  1. In eukaryotic cells, transcription occurs in the:

    • A) Cytoplasm
    • B) Mitochondria
    • C) Nucleus
    • D) Ribosome

    Answer: C) Nucleus

  2. Which enzyme is responsible for synthesizing the RNA molecule during transcription?

    • A) RNA polymerase
    • B) DNA polymerase
    • C) Ligase
    • D) Ribosome

    Answer: A) RNA polymerase

  3. Which of the following modifications occurs to the RNA molecule after transcription in eukaryotes?

    • A) Addition of a 5' cap
    • B) Removal of exons
    • C) Addition of introns
    • D) Conversion into a protein

    Answer: A) Addition of a 5' cap

  4. The process of splicing involves:

    • A) The removal of exons and joining of introns
    • B) The joining of exons and removal of introns
    • C) The addition of a 5' cap
    • D) The synthesis of the RNA molecule

    Answer: B) The joining of exons and removal of introns

Case Study 4: Translation and the Genetic Code

Translation is the process by which an RNA molecule is decoded to produce a specific polypeptide (protein). The messenger RNA (mRNA) carries the genetic information from the DNA to the ribosome, where transfer RNA (tRNA) molecules bring amino acids. The ribosome reads the mRNA in sets of three nucleotides called codons, each of which specifies an amino acid. The genetic code is universal, meaning it is the same in almost all organisms.

Questions:

  1. During translation, the mRNA is read in sets of three nucleotides called:

    • A) Codons
    • B) Anticodons
    • C) Exons
    • D) Intron pairs

    Answer: A) Codons

  2. What is the role of tRNA in translation?

    • A) To provide energy for protein synthesis
    • B) To carry amino acids to the ribosome
    • C) To decode the mRNA sequence
    • D) To synthesize the mRNA molecule

    Answer: B) To carry amino acids to the ribosome

  3. The genetic code is said to be degenerate because:

    • A) Multiple codons can specify the same amino acid
    • B) Each codon specifies more than one amino acid
    • C) There are 64 codons for only 20 amino acids
    • D) All codons specify the same amino acid

    Answer: A) Multiple codons can specify the same amino acid

  4. The process of translation occurs in the:

    • A) Nucleus
    • B) Cytoplasm
    • C) Mitochondria
    • D) Endoplasmic reticulum

    Answer: B) Cytoplasm

Case Study 5: Mutations and Their Effects

A mutation is a change in the nucleotide sequence of DNA. Mutations can occur naturally or be induced by environmental factors such as UV radiation. Mutations can have various effects on the organism, including no effect, a beneficial effect, or a harmful effect. Mutations in the coding regions of genes can lead to changes in the protein produced, potentially causing diseases or altering the organism’s traits.

Questions:

  1. A mutation that changes a single nucleotide in the DNA sequence is known as:

    • A) Frameshift mutation
    • B) Point mutation
    • C) Inversion
    • D) Translocation

    Answer: B) Point mutation

  2. A mutation that causes a shift in the reading frame of the genetic code is called:

    • A) Silent mutation
    • B) Missense mutation
    • C) Nonsense mutation
    • D) Frameshift mutation

    Answer: D) Frameshift mutation

  3. Mutations that occur in somatic cells:

    • A) Are inherited by offspring
    • B) Do not affect the organism’s phenotype
    • C) Can affect the phenotype of the individual but not be passed to the next generation
    • D) Always lead to cancer

    Answer: C) Can affect the phenotype of the individual but not be passed to the next generation

  4. A silent mutation results in:

    • A) A change in the amino acid sequence of the protein
    • B) No change in the protein’s amino acid sequence
    • C) A stop codon being introduced
    • D) A frameshift in the protein

    Answer: B) No change in the protein’s amino acid sequence