CBSE Class 12 Biology Notes Chapter 6 Molecular Basis of Inheritance

Mutations and Genetic Code

Mutations involve changes in the genetic sequence, and one common type is a point mutation , where a single base pair is altered. A well-known example is sickle cell anemia , caused by a mutation in the gene coding for the 𝛽-globin chain of hemoglobin. In this case, the amino acid glutamate is replaced by valine , leading to the sickling of red blood cells. Another type of mutation is the frameshift mutation , which occurs when there is a loss or gain of one or two base pairs in the DNA sequence. This changes the reading frame from the point of mutation, altering the entire sequence of amino acids downstream. Frameshift mutations can have significant impacts on the resulting protein, often leading to nonfunctional proteins or severe genetic disorders.

Translation

Translation is the process through which amino acids are linked together to form proteins, facilitated by peptide bonds . During this process, all three types of RNA messenger RNA (mRNA) , transfer RNA (tRNA) , and ribosomal RNA (rRNA) play distinct roles. The first step in translation is the aminoacylation of tRNA , where tRNA molecules are charged with their respective amino acids. Ribosomes act as the site of protein synthesis, functioning as catalysts for the formation of peptide bonds. Translation proceeds in the 5’ to 3’ direction, and the large subunit of the ribosome contains two sites that accommodate tRNAs carrying amino acids, bringing them close enough together to facilitate peptide bond formation.

Regulation of Gene Expression

Gene expression, the process by which genes are converted into polypeptides, can be regulated at various levels in eukaryotes, including:

1. During the formation of the primary transcript (transcription).
2. During the processing or splicing of the mRNA.
3. When transporting mRNA from the nucleus to the cytosol.
4. During protein synthesis (translation).

Gene expression is influenced by environmental, physiological, and metabolic conditions. In the development and differentiation of an embryo, the coordinated regulation and expression of multiple genes are crucial. In prokaryotes, gene expression primarily occurs at the initiation of transcription. The activity of RNA polymerase at the start site is controlled by regulatory proteins that can act as repressors or activators. The accessibility of the promoter region is influenced by an operator sequence located adjacent to it, which binds specific proteins, typically repressors. This regulatory mechanism ensures that gene expression can be finely tuned according to the cell’s needs.

Benefits of CBSE Class 12 Biology Notes Chapter 6 Molecular Basis of Inheritance

• Structured Learning : The notes provide a well-organized structure that aligns with the CBSE syllabus, helping students to systematically cover all important topics and concepts needed for the exam.
• Focused Content : They highlight key concepts such as DNA structure, replication, transcription, translation and gene regulation, allowing students to concentrate their study efforts on the most relevant material.
• Conceptual Clarity : By simplifying complex ideas into understandable terms, the notes enhance students conceptual clarity, which is important for answering application-based questions in exams.
• Quick Reference : The notes are a quick reference guide during last-minute revisions, making it easier for students to recall important facts and processes.
• Practice Preparation : The notes prepare students for various types of questions, including theoretical, application-based and diagrammatic questions, ensuring they are well-equipped for different exam formats.
• Confidence Boost : By providing a detailed overview of the chapter, the notes help boost students confidence in their knowledge and preparedness for the exam.
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