Molecular Basis of Inheritance

The 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 the 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. 

Why is DNA Important in the Molecular Basis of 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 numbers 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 color, hair color, eye color, etc. This explains that DNA is highly responsible for molecular inheritance. In 1952, Alfred Hershey and Martha Chase proved that DNA is the genetic material by performing a bacteriophage experiment. 

Structure of DNA and RNA

Nucleic Acids

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

• Nitrogenous base

• Phosphate group

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1. 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).

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

3. Nitrogenous Base- It is a Nitrogen-containing compound with the 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.

How is a Polynucleotide Formed by Joining Nucleotides?

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

DNA and its polynucleotide chain

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 the 5’ phosphate end and 3’ hydroxyl end. The former is called 5 prime ends and the latter is called 3 prime ends. 

Double Helix Structure of DNA

Watson and Crick proposed this structure for DNA with studies based on X-ray diffraction of DNA in the year 1953. The salient features of the double helix structure of DNA are given below-

• It is composed of 2 polynucleotide chains.

• Sugar and phosphate form the backbone of the helix structure. Nitrogenous bases are the interior parts and are 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. 

• The coiling is right-handed.

• 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 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. 

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Genetic Code

It is the set of rules by which information encoded within the DNA or mRNA sequence is translated into proteins by the living cells. Marshall Nirenberg discovered the genetic code and in 1968 he won the Nobel prize along with Robert W. Holley and Har Gobind Khorana. 

The salient features of the genetic code are:

• The genetic code is applicable universally.

• Each genetic code specifies only one amino acid.

• A codon consists of three adjacent nitrogen bases.

• The synthesis of polypeptides is signaled by initiation codons.

• The termination of the polypeptide chain is signaled by stop codons (UAA, UAG, and UGA).

Human Genome Project

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. The human genome project was launched on 1st October 1990 and it was completed in 2003. 

The goals of this project are given below-

• Storing the information in databases.

• Improving tools for data analysis.

• Determining the sequences of base pairs that make up human DNA.

• Identifying approximately 25000 genes in human DNA.

Conclusion

This is all about the concept of the molecular basis of inheritance and its explanation. Focus on the prime concept and develop your knowledge foundation to proceed with advanced study material regarding this topic.