Molecular Basis of Inheritance Class 12 Notes

What is DNA?

DNA- Deoxyribonucleic Acid is considered as the molecule of inheritance as it carries genetic information in all living organisms. It is a long polymer chain of deoxyribonucleotides. Its length depends on the number of nucleotide base pair present in it.

Watson and Crick were the first scientists who proposed a double helical model for DNA, based on X-ray crystallography of the molecule. Each strand of DNA is a polymer of nucleotides, every nucleotide consists of a deoxyribose sugar, a nitrogen base and a phosphate.

According to the central dogma of molecular biology, the genetic information flows from DNA to RNA to Protein.

The complete DNA structure looks like a twisted ladder. The two strands of DNA are held together by weak hydrogen bonds between the nitrogen bases. A purine base, always pairs with a pyrimidine base, i.e., adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C).

What is a Gene?

A gene is the functional unit of inheritance. In all eukaryotic organisms, DNA consists of both coding and non-coding sequences of nucleotides. The coding sequences are defined as Exons and non-coding sequences are defined as introns. These Exons appear in the matured RNA but the introns do not appear.

RNA- Ribonucleic Acid. It is a single strand of nucleic acid present in all living cells and acts as a messenger carrying instructions from DNA for controlling the protein synthesis. The three primary types of RNA molecules are

  • Messenger RNA, (mRNA)
  • Transfer RNA(tRNA)
  • Ribosomal RNA (rRNA).

Few Important Questions

  • What is Polymorphism?
  • What is DNA fingerprinting?
  • List out the functions of Promoter?
  • Differentiate between mRNA and tRNA?
  • List out the goals of the Human Genome Project?

Learn more about Heredity, Law of Inheritance, chromosomes, and genes from the topics given below:

Practise This Question

Assertion (A): A group of apes split into two smaller groups - one evolved into the modern gorillas, and the other into a chimpanzee - human ancestor.
Reason (R):  Bipedal locomotion is advantageous because it frees forelimbs for more actions.