Science of Inheritance: From Chromosomes to DNA Profiling

The study of inheritance represents a cornerstone in biological sciences, delving into the mechanisms underlying the transmission of traits across generations. From the seminal work of Mendel to contemporary breakthroughs in molecular genetics, our comprehension of inheritance has undergone significant evolution.

Chromosomal Basis of Inheritance

In 1900, three Scientists (de Vries, Correns and von Tschermak) independently rediscovered Mendel’s results on the inheritance of characters.

• Chromosomes, as well as genes, occur in pairs.
• The two alleles of a gene pair are located on homologous sites on homologous chromosomes.
• Pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried.

Molecular Basis of Inheritance

Role of DNA: DNA, or deoxyribonucleic acid, is the central information storage system of most animals and plants, and even some viruses.

• Classification: The name comes from its structure, which is a sugar and phosphate backbone which have bases sticking out from it—so-called bases.

• • It’s a polymer of four bases – A, C, T, and G.
  • DNA is organised structurally into chromosomes and then wound around nucleosomes as part of those chromosomes. 
  • The two chains have antiparallel polarity. 
  • It means if one chain has the polarity 5′ to 3′, the other has 3′ to 5′.
• Double Helix model: In 1953 James Watson and Francis Crick, based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, proposed a very simple but famous Double Helix model for the structure of DNA. 
  • Hallmarks: Base pairing between the two strands of polynucleotide chains.

Ribonucleic acid (RNA)

• About: Like DNA, each RNA strand has the same basic structure, composed of nitrogenous bases covalently bound to a sugar-phosphate backbone. 
  • However, unlike DNA, RNA is usually a single-stranded molecule.
  • Sugar in RNA is ribose instead of deoxyribose (ribose contains one more hydroxyl group on the second carbon), which accounts for the molecule’s name.

• RNA consists of four nitrogenous bases: adenine, cytosine, uracil, and guanine.
• Pairing: Uracil is a pyrimidine that is structurally similar to the thymine, another pyrimidine that is found in DNA. 
  • Like thymine, uracil can base-pair with adenine.
• RNA World Hypothesis: RNA stored both genetic information and catalysed the chemical reactions in primitive cells. 

• Only later in evolutionary time did DNA take over as the genetic material and proteins became the major catalyst and structural component of cells.

Why is DNA widely acceptable as a better Genetic material?

• The presence of free-OH at the 2nd position at every nucleotide in RNA is a reactive group and makes RNA labile and easily degradable. 
• DNA chemically is less reactive and structurally more stable when compared to RNA. Therefore, among the two nucleic acids, DNA is a better genetic material.

DNA Profiling or DNA Fingerprinting

About: DNA profiling is the process where a specific DNA pattern, called a profile, is obtained from a person or sample of bodily tissue.

• It is a forensic technique in criminal investigations, comparing criminal suspects’ profiles to DNA evidence so as to assess the likelihood of their involvement in the crime.
• Application: It is used in parentage testing, to establish immigration eligibility, and in genealogical and medical research.

Microsatellite DNA

Short segment of DNA, usually one to six or more base pairs in length, that is repeated multiple times in succession at a particular genomic location. 

• These DNA sequences are typically non-coding. 
• The number of repeated segments within a microsatellite sequence often varies among people, which makes them useful as polymorphic markers for studying inheritance patterns in families or for creating a DNA fingerprint from crime scene samples. [UPSC 2023]

Conclusion

When traits are passed down through generations, researchers have uncovered a solid framework by studying chromosomes. The ongoing discoveries in molecular genetics have improved our understanding of Inheritance, highlighting DNA’s pivotal role as the keeper of genetic data, supported by RNA’s diverse functions.