CBSE Class 12 Biology Notes Chapter 6 Molecular Basis of Inheritance

The molecular basis of inheritance refers to the study of genes, genetic variations, and how traits are passed down through generations.

• There are two types of nucleic acids present in living organisms.
• These are Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA).
• During the early years, the pattern of factors that regulate inheritance was not clear. 
• After an investigation of over a hundred years, DNA was determined as the genetic material in most living beings.
• RNA also acts as the genetic material in some viruses and mostly as a messenger molecule.
• It can also function as an adapter, structural component, and even a catalyst in some cases.

CBSE Class 12 Biology Notes for Chapter 6 Molecular Basis of Inheritance are given in the article below for easy preparation and understanding of the concepts involved.

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Class 12 Biology Chapter 13 Notes – Organisms and Populations

What is DNA?

• DNA, or deoxyribonucleic acid, is the molecule of inheritance. 
• It carries the genetic information for all living organisms. 
• This information is stored in a long, chain-like molecule made of smaller units called nucleotides.
• The length of DNA depends on the number of nucleotide base pairs it contains. 
• These pairs connect the two strands of DNA together, forming a twisted ladder-like structure.
• Watson and Crick, based on X-ray crystallography of the molecule, proposed the double-helical model of DNA. 
• This model is the foundation of our understanding of DNA structure.
• Each strand of the DNA ladder is made up of a pair of nucleotides. 
• Each nucleotide consists of a sugar molecule, a phosphate group, and a nitrogenous base.
• The two strands of DNA are held together by weak hydrogen bonds.
• These bonds form between specific pairs of nitrogenous bases: adenine (A) with thymine (T), and guanine (G) with cytosine (C).
• The central dogma of molecular biology describes the flow of genetic information.
• This information flows from DNA to RNA to protein.

DNA

Structure of Polynucleotide

• A nucleotide is the building block of DNA and RNA. 
• It has three parts: a nitrogenous base, sugar, and a phosphate group.
• Nitrogenous bases are in the form of Purines (Adenine, Guanine) and Pyrimidines (Cytosine and Thymine). 
• The sugar part consists of pentose sugar (ribose in RNA and deoxyribose in DNA).
• The phosphate group consists of nucleosides and nucleotides.

Nucleotide

What Is a Gene?

• Genes are the functional unit of inheritance and reside within DNA. 
• In eukaryotic organisms, DNA contains both coding and non-coding sequences of nucleotides. 
• Coding sequences, called exons, appear in mature RNA.
• Non-coding sequences, called introns, do not appear in mature RNA.
• Ribonucleic acid (RNA) acts as a messenger molecule, carrying instructions from DNA for protein synthesis. 
• Three main types of RNA exist: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

Packaging DNA Helix

• In prokaryotes, DNA is arranged as a large loop that resides within the nucleoid region. 
• Here negatively charged DNA is tightly bound to positively charged proteins.
• In eukaryotes, DNA arrangement becomes more complex in chromosomes. 
• DNA wound around the core of histone octamers, forming units known as nucleosomes. 
• Histones are positively charged proteins abundant in lysine and arginine, essential amino acids.
• These proteins come in five types: H1, H2A, H2B, H3, and H4. 
• The histone octamer has 2 molecules of 4 histone proteins vital in gene regulation.
• Nucleosomes repeat along the chromatin fiber, each containing about 200 base pairs of DNA. 
• This packaging prevents DNA tangling. 
• The further packaging of chromatin is facilitated by NHC (Non-histone chromosomal proteins).
• Euchromatin, where chromatin is loosely packed and stains lightly, represents transcriptionally active areas.
• Within chromatin, euchromatin represents loosely packed, transcriptionally active regions that stain lighter.
• Conversely, heterochromatin is densely packed, inactive DNA that stains darker.
• Heterochromatin, densely packed and staining dark, represents transcriptionally inactive regions.

Packaging DNA Helix

RNA World

• The RNA world hypothesis proposes that RNA was the first genetic material.
• Evidence suggests that vital life processes, such as metabolism and translation, evolved around RNA. 
• While RNA could act as both genetic material and a catalyst, its high reactivity made it unstable. 
• Hence, DNA evolved from RNA with chemical alterations making it more stable.

Replication

• Watson and Crick's model proposed that DNA replication is semiconservative.
• This theory was later confirmed by Meselson and Stahl's experiment in 1958.
• Taylor et al. demonstrated in another experiment on faba beans (Vicia faba) using radioactive thymidine that DNA replication is semiconservative.
• The enzyme DNA polymerase catalyzes DNA replication.
• It can polymerize only in the 5’→3’ direction. 
• In a strand with 5’→3’ direction, replication is continuous called the leading strand.
• In a strand with 5’→3’ direction, replication is discontinuous called the lagging strand.

Transcription

• Transcription transfers the genetic information from a single DNA segment into RNA. 
• Unlike DNA, RNA uses uracil instead of thymine to pair with adenine. 
• Transcription of DNA involves three regions: the structural gene, promoter, and terminator.
• RNA polymerase catalyzes transcription and drives transcription in the same 5’→3’ direction as DNA replication. 
• The template strand, with a 5’→3’ polarity, acts as a template for RNA formation, known as an antisense strand.
• The coding strand, with a 5’→3’ polarity, is also known as a sense strand. 
• It consists of the structural gene, promoter, terminator, exons, and introns.

Genetic Code

• Genetic codes are triplets (three-nucleotide sequences) in mRNA that specify amino acids during protein synthesis.
• Out of 64 codons, 61 code for amino acids, while the remaining 3 are stop codons, as they do not code for any amino acid.

Genetic code

Translation

• Translation transforms the genetic code from mRNA into proteins. 
• Amino acids, the building blocks of proteins, are joined by peptide bonds. 
• All three RNAs (mRNA, tRNA, rRNA) play different roles in this process.
• Aminoacylation of tRNA is the first stage in this process.
• Ribosomes, the cellular factories for protein production, act as catalysts for peptide bond formation between amino acids. 
• The translation process always proceeds in the 5’→3’ direction along the mRNA strand. 
• The large ribosomal subunit has two binding sites for tRNAs, allowing amino acids to come close enough for bonding.

There are Some important List Of Top Biology Questions On Molecular Basis of Inheritance Asked In CBSE CLASS XII