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1. What is Molecular Basis of Inheritance?

1. What is Molecular Basis of Inheritance?

DNA called the molecule of heredity and RNA are the two components that make up the molecular basis of inheritance. It enables organisms to inherit genetic information from parental genes. Genetic materials are replicated and passed to the progeny cell from the parent cell at each cell division.

The formation of proteins or proteins synthesis starts from DNA. The process of replication helps in making multiple copies of DNA. One strand of DNA is copied to form mRNA and the process is called transcription. Now, during translation, and all types of non-genetic RNAs- rRNA, tRNA and mRNA and then the proteins are formed. This is called Central dogma theory which was formulated by biologist ‘Francis Harry Compton’ where he stated that biological information flows in a unidirectional pattern. DNA → RNA → Protein

Molecular basis of inheritance is the study of genes, hereditary and genetic variations which explains how an offspring looks similar to its maternal or paternal features. DNA, RNA and genetic code are the fundamental parts of the molecular basis of inheritance and are responsible to transmit genes from parents to the offspring. Various traits get inherited from one generation to another generation where variations also come into picture due to recombination. With lots of research and studies, it is found out that the genetic material of most of the organisms is called DNA (DeoxyriboNucleic Acid) and it is responsible for the transfer of traits from one to another. Exceptions are the viruses where RNA is the genetic material.

It’s often complicated to find appropriate notes for this topic; Worry not! Here, we give you a brief understanding of the basics related to molecular inheritance.

If we see a cell under a microscope, we can see a nucleus which is the region containing all genetic materials. The nucleus contains nucleolus and chromatin which are thread-like structures. Chromatin condenses to form chromosomes located with genes. Each chromosome has thousands of genes and each gene points to a particular trait. The number of chromosomes inside each cell of living organisms is fixed, for example, humans have 23 pairs of homologous chromosomes i.e. 46 in number. On the other hand, the number of genes in each chromosome is huge, up to thousands. In varied species, chromosome number may be different. A Gene is made up of a double-stranded structure called DNA; different portions of DNA are responsible for different traits like skin colour, hair colour, eye colour, etc. This explains that DNA is highly responsible for molecular inheritance.

A group of biomolecules contains two kinds of nucleic acids including DNA and RNA that play very important roles in molecular inheritance. DNA is Deoxyribonucleic acid and double-stranded helical structure, ribbon-like wrapped around each other. It is a polynucleotide or polymer (small molecules- many monomers combine to form polymers). Here, the monomer units are deoxyribonucleotides and the length of DNA is determined by the number of nucleotides. RNA is Ribonucleic acid and its structure is similar to DNA structure but consists of a single strand. It is a polymer or polynucleotide, formed by several monomer units called ribonucleotides. RNA is the molecular basis of inheritance in a few viruses. Any Nucleotide Present in DNA or RNA has Three Components:

Pentose Sugar- It is a monosaccharide with 5 carbon atoms. RNA has ribose sugar and DNA has deoxyribose sugar (with the removal of one oxygen molecule).

Phosphate Group- It is an inorganic salt of phosphorus and pentose sugar and the phosphate group forms a backbone of the polynucleotide chain.

Nitrogenous Base- It is a Nitrogen-containing compound with properties of a base.  Two types of nitrogenous bases are purines and pyrimidines. Purines are heterocyclic aromatic organic compounds (atoms from two different elements having alternate double and single bonds) and 9-membered rings with a double ring system. Adenine and Guanine are examples of Purines. Pyrimidines also have a similar structure to purines with 6-membered rings and a single ring system. The examples of pyrimidine are Cytosine, Uracil and Thymine. Depending upon the structure of DNA or RNA, some specific nitrogenous bases are present. Uracil is present only in RNA, not in DNA and Thymine is present only in DNA and not in RNA.

An N-glycosidic bond connects the nitrogenous base with pentose sugar and forms a nucleoside. A phosphate group linked to this nucleoside through phosphoester linkage forms a nucleotide. Multiple nucleotides join together to form a polynucleotide through 3’-5’ phosphodiester bond. This is how nucleic acid chains formed in DNA or RNA.

Phosphate, a pentose sugar and nitrogenous bases including A, T, G or C are present in its polynucleotide chain. The phosphate groups are connected to the sugar molecule of the next nucleotide by phosphodiester bonds, this is how the entire polynucleotide chain is formed. This chain has two free ends which are 5’ phosphate end and 3’ hydroxyl end. The former is called 5 prime ends and the latter is called 3 prime ends.

Watson and Crick proposed this structure for DNA with studies based on X-ray diffraction of DNA. It is composed of 2 polynucleotide chains, the red and the blue ones shown below in the diagram. Sugar and phosphate form the backbone of the helix structure. Nitrogenous bases are the interior parts and paired through hydrogen bonds. It consists of complementary base pairing, Adenine will always pair with Thymine with double bond and Guanine will always pair up with Cytosine with a triple bond. There is a uniform distance between two strands of helix since pairing occurs between one purine and one pyrimidine. The two polynucleotide chains have antiparallel polarity, i.e. 5’ end having a free phosphate group of one strand is paired with the 3’ end with a free hydroxyl group of another complementary strand. The two chains are coiled in a right-handed form in a helix. Width of one turn of a helix called the pitch of the helix is 3.4 nm and there are 10 base pairs in each turn. [Image will be Uploaded Soon]

It is the set of rules by which information encoded within the DNA or mRNA sequence is translated into proteins by the living cells.

It was launched as an international scientific research project for determining the base pairs that make up human DNA. It includes identifying and mapping all the genes of the human genome in terms of physical features and functionalities. It has high importance in the fields of life science, medicine and biotechnology.  

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