# Molecular Basis of Inheritance, Class 12 Biology | SwaVid Molecular Basis of Inheritance, NCERT Class 12 Biology: notes with key ideas, key terms, common mistakes and 3 quick checks with worked answers. Canonical: https://www.swavid.com/cbse/class-12/biology/chapter/molecular-basis-of-inheritance Source: https://www.swavid.com/cbse/class-12/biology/chapter/molecular-basis-of-inheritance # Molecular Basis of Inheritance ## Key terms ## Key ideas ## Common mistakes ## Quick checks ## Keep going ## Questions parents ask ## Class 12 Biology on SwaVid ### Structure of polynucleotide chains ### Packaging of DNA helix ### Search for genetic material ### Replication of DNA ### Transcription and RNA processing ### Genetic code and translation ### Regulation and genomics ### Next chapter ### Related chapter ### Related chapter ### All chapters SwaVid › CBSE › Class 12 › Biology › Molecular Basis of Inheritance Notes written by SwaVid from the NCERT textbook. Class 12 Biology, Chapter 5. Key ideas, formulas and common mistakes. CBSE prints 20 marks for Unit VII, not for this chapter alone. Short answer. This chapter explains the chemical structure, replication, and expression of genetic material, mainly DNA and RNA. It describes how genetic information flows from nucleic acids to proteins through transcription and translation, and how these processes are regulated. Source: NCERT Biology (Reprint 2026-27) , Chapter 5, Molecular Basis of Inheritance A nucleotide has a nitrogenous base, a pentose sugar, and a phosphate group linked by glycosidic and phosphodiester bonds. DNA is a double-stranded antiparallel helix where adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three hydrogen bonds. In eukaryotes, negatively charged DNA wraps around a positively charged octamer of basic histone proteins to form a nucleosome containing about 200 base pairs. Nucleosomes repeat to form chromatin, which condenses into chromosomes during cell division. Griffith discovered the transforming principle in Streptococcus pneumoniae, which Avery, MacLeod, and McCarty identified as DNA. Hershey and Chase proved conclusively that DNA is the genetic material using bacteriophages labelled with radioactive phosphorus (³²P) and sulfur (³⁵S). Meselson and Stahl proved that DNA replicates semiconservatively using heavy nitrogen (¹⁵N). DNA-dependent DNA polymerase synthesises a continuous leading strand and a discontinuous lagging strand made of Okazaki fragments joined by DNA ligase in the 5' to 3' direction. Transcription copies genetic information from a template strand of DNA into RNA using RNA polymerase. In eukaryotes, primary transcripts (hnRNA) undergo splicing to remove introns, 5'-capping with methylguanosine triphosphate, and 3'-polyadenylation. The genetic code is a triplet, degenerate, unambiguous, and nearly universal system where 61 codons code for amino acids and 3 codons act as stop signals. Translation is the synthesis of a polypeptide chain at the ribosome guided by mRNA codons and matched by tRNA anticodons. The lac operon in Escherichia coli regulates lactose metabolism through a repressor protein that binds an operator until an inducer inactivates it. The Human Genome Project sequenced the complete human genome, identifying 3164.7 million base pairs and an estimated 30,000 genes. Try each question first. Then tap it to see the answer. According to Chargaff's rule, % Cytosine = % Guanine = 20% Total G + C = 20% + 20% = 40% Remaining A + T = 100% - 40% = 60% Since % A = % T, Adenine = 60% / 2 = 30% 30% Distance between adjacent base pairs = 0.34 nm = 0.34 × 10⁻⁶ mm Number of base pairs = Total length / Distance per base pair Number of base pairs = (1.36 mm) / (0.34 × 10⁻⁶ mm) = 4 × 10⁶ 4 × 10⁶ base pairs The mRNA sequence is identical to the coding strand except that thymine (T) is replaced by uracil (U) Polarity remains unchanged at 5' to 3' Substitute each T with U in 5'-ATGCATGC-3' 5'-AUGCAUGC-3' Evolution . Principles of Inheritance and Variation . Biotechnology: Principles and Processes . The 13 chapters of Class 12 Biology . CBSE does not give marks to one chapter. It prints 20 marks for Unit VII, Genetics and Evolution, which includes Molecular Basis of Inheritance. No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed. Evolution. SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works. - Ch 5 NCERT book - 20 marks, Unit VII - 3 quick checks - Writing the mRNA transcript with thymine instead of uracil . Remember that RNA synthesises uracil opposite to adenine on the DNA template, never thymine. - Identifying the coding strand as the template for transcription . Check the polarity carefully; RNA polymerase uses the 3' to 5' DNA strand as the template, making the mRNA identical in sequence to the 5' to 3' coding strand except for U in place of T. - Confusing degenerate codons with ambiguous codons . Degenerate means multiple codons can code for the same single amino acid; unambiguous means one specific codon codes for only one amino acid. ## Key Links - [CBSE](https://www.swavid.com/cbse) - [Class 12](https://www.swavid.com/cbse/class-12) - [Biology](https://www.swavid.com/cbse/class-12/biology/chapters) - [Start free →](https://www.swavid.com/signup) - [All 13 chapters](https://www.swavid.com/cbse/class-12/biology/chapters) - [NCERT Biology (Reprint 2026-27)](https://ncert.nic.in/textbook.php) - [Open notes](https://www.swavid.com/cbse/class-12/biology/chapter/evolution) - [Open notes](https://www.swavid.com/cbse/class-12/biology/chapter/principles-of-inheritance-and-variation) - [Open notes](https://www.swavid.com/cbse/class-12/biology/chapter/biotechnology-principles-and-processes) - [Class 12 Biology](https://www.swavid.com/cbse/class-12/biology/chapters) - [Start free →](https://www.swavid.com/signup)