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Q14:
What is point mutation? Give one example.
Solution :
1. Definition of Point Mutation
A point mutation is a category of genetic mutation characterized by the substitution, insertion, or deletion of a single nucleotide base pair within a DNA sequence. This alteration occurs at a specific, discrete locus on the chromosome [Per the molecular definition of genetic variation].
2. Mechanisms of Point Mutation
Point mutations generally manifest through the following biochemical processes:
- Transitions: A purine is replaced by another purine ($A \leftrightarrow G$) or a pyrimidine by another pyrimidine ($C \leftrightarrow T$).
- Transversions: A purine is replaced by a pyrimidine, or vice versa (e.g., $A \rightarrow C$ or $G \rightarrow T$).
- Frameshift Consequences: Insertions or deletions of a single base pair disrupt the triplet reading frame of the mRNA during translation [Per the Central Dogma of Molecular Biology].
3. Exemplar: Sickle Cell Anemia
The most clinically significant example of a point mutation is the substitution mutation causing Sickle Cell Anemia in humans.
| Feature | Normal Hemoglobin ($Hb^A$) | Sickle Cell Hemoglobin ($Hb^S$) |
|---|---|---|
| DNA Sequence (Codon) | $GAG$ | $GTG$ |
| mRNA Codon | $GAG$ | $GUG$ |
| Amino Acid | Glutamic Acid | Valine |
In this instance, a single base pair substitution ($A \rightarrow T$) in the $\beta$-globin gene changes the sixth codon from $GAG$ to $GTG$. This results in the replacement of negatively charged Glutamic acid with hydrophobic Valine [Per the structural biology of proteins], causing the hemoglobin molecules to polymerize under low oxygen tension.
4. Visual Representation of Point Mutation
Final Solution: A point mutation is a mutation resulting from a change in a single base pair of DNA. A classical example is the substitution of the 6th codon of the $\beta$-globin gene from $GAG$ to $GTG$, which leads to the synthesis of abnormal hemoglobin responsible for Sickle Cell Anemia.
More Questions from Class 12 Biology Principles of Inheritance and Variation EXERCISES
- Q1: Mention the advantages of selecting pea plant for experiment by Mendel.
- Q1: Mention the advantages of selecting pea plant for experiment by Mendel.
- Q2(a): Differentiate between the following – (a) Dominance and Recessive
- Q2(a): Differentiate between the following – (a) Dominance and Recessive
- Q2(b): Differentiate between the following – (b) Homozygous and Heterozygous
- Q2(b): Differentiate between the following – (b) Homozygous and Heterozygous
- Q2(c): Differentiate between the following – (c) Monohybrid and Dihybrid.
- Q2(c): Differentiate between the following – (c) Monohybrid and Dihybrid.
- Q3: A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
- Q3: A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
- Q4: Explain the Law of Dominance using a monohybrid cross.
- Q4: Explain the Law of Dominance using a monohybrid cross.
- Q5: Define and design a test-cross.
- Q5: Define and design a test-cross.
- Q6: Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
- Q6: Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
- Q7(a): When a cross in made between tall plant with yellow seeds (\(TtYy\)) and tall plant with green seed (\(Ttyy\)), what proportions of phenotype in the offspring could be expected to be (a) tall and green.
- Q7(a): When a cross in made between tall plant with yellow seeds ((TtYy)) and tall plant with green seed ((Ttyy)), what proportions of phenotype in the offspring could be expected to be (a) tall and green.
- Q7(b): When a cross in made between tall plant with yellow seeds (\(TtYy\)) and tall plant with green seed (\(Ttyy\)), what proportions of phenotype in the offspring could be expected to be (b) dwarf and green.
- Q7(b): When a cross in made between tall plant with yellow seeds ((TtYy)) and tall plant with green seed ((Ttyy)), what proportions of phenotype in the offspring could be expected to be (b) dwarf and green.
- Q8: Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in \(F_{1}\) generation for a dibybrid cross?
- Q8: Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in (F_{1}) generation for a dibybrid cross?
- Q9: Briefly mention the contribution of T.H. Morgan in genetics.
- Q9: Briefly mention the contribution of T.H. Morgan in genetics.
- Q10: What is pedigree analysis? Suggest how such an analysis, can be useful.
- Q10: What is pedigree analysis? Suggest how such an analysis, can be useful.
- Q11: How is sex determined in human beings?
- Q11: How is sex determined in human beings?
- Q12: A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
- Q12: A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
- Q13(a): Explain the following terms with example (a) Co-dominance
- Q13(a): Explain the following terms with example (a) Co-dominance
- Q13(b): Explain the following terms with example (b) Incomplete dominance
- Q13(b): Explain the following terms with example (b) Incomplete dominance
- Q14: What is point mutation? Give one example.
- Q15: Who had proposed the chromosomal theory of the inheritance?
- Q15: Who had proposed the chromosomal theory of the inheritance?
- Q16: Mention any two autosomal genetic disorders with their symptoms.
- Q16: Mention any two autosomal genetic disorders with their symptoms.
CBSE Solutions for Class 12 Biology Principles of Inheritance and Variation
Chapters in CBSE - Class 12 Biology
- Reproduction in Organisms
- Human Reproduction
- Reproductive Health
- Molecular Basis of Inheritance
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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