Genetics is a discipline of biology, is the science of genes, heredity, and variation in living organisms. The word is derived from Ancient Greek word “genetikos”, (genitive) and that from genesis meaning origin.

Genetics deals with the

• molecular structure and function of genes;
• Gene behavior in context of a cell or organism;
• Patterns of inheritance from parent to offspring and
• Gene distribution, variation and change in populations.

Gene is the genetic unit of function. A gene may encode a polypeptide or a molecule of non-translated RNA (e.g. ribosomal RNA, transfer RNA, or a regulatory RNA). As per the Mendel’s observation, gene is an unit of inheritance.

Given that genes are universal to living organisms, genetics can be applied to the study of all living systems, from viruses and bacteria, through plants and domestic animals, to humans.

The modern science of genetics seeks to understand the process of inheritance, began with the work of Gregor Mendel in the mid-19th century. Although he did not know the physical basis for heredity, Mendel observed that organisms inherit traits via separate units of inheritance, which are now called genes.

• Genes correspond to regions within DNA, a molecule composed of a chain of four different types of nucleotides—the sequence of these nucleotides is the genetic information organisms inherit.
• DNA naturally occurs in a double stranded form, with nucleotides on each strand complementary to each other. Each strand can act as a template for creating a new partner strand. This is the physical method for making copies of genes that can be inherited.
• The sequence of nucleotides in a gene is translated by cells to produce a chain of amino acids, creating proteins—the order of amino acids in a protein corresponds to the order of nucleotides in the gene. This relationship between nucleotide sequence and amino acid sequence is known as the genetic code.
• Proteins carry out almost all the functions needed for cells to live. A change to the DNA (refers mutation) in a gene can change a protein's amino acids, changing its shape and function. This can have a dramatic effect in the cell and on the organism as a whole.

Nucleic Acids – Over view

Two nucleic acids, universally present in all the living organisms are Deoxyribo nucleic acid and Ribonucleic acid.

• Deoxyribonucleic acid (DNA) contains the 2’deoxyribonucleosides of adenine, guanine, cytosine, and thymine. Ribonucleic acid (RNA) is composed of the ribonucleosides of adenine, guanine, cytosine, and uracil (instead of thymine).
• In both DNA and RNA, nucleosides are joined by phosphate groups (phosphodiester bonds) to form long polynucleotide chains. The differences in chemical composition between the chains reside in their sugar and pyrimidine bases: DNA has deoxyribose and thymine; RNA has ribose and uracil in place of thymine.

DNA

• Deoxyribonucleic acids are very large molecules, usually composed of two polynucleotide chains coiled together to form a double helix (2.0 nm in diameter).
• Each chain contains purine and pyrimidine deoxyribonucleosides joined by phosphodiester bridges. That is, two adjacent deoxyribose sugars are connected by a phosphoric acid molecule esterified to a 3’-hydroxyl of one sugar and a 5’-hydroxyl of the other.
• Purine and pyrimidine bases are attached to the 1’-carbon of the deoxyribose sugars and extend toward the middle of the cylinder formed by the two chains. They are mounded on top of each other in the center, one base pair every 0.34 nm.
• The purine adenine (A) is always paired with the pyrimidine thymine (T) by two hydrogen bonds. The purine guanine (G) pairs with cytosine (C) by three hydrogen bonds. This AT and GC base pairing means that the two strands in a DNA double helix are complementary.
• The two polynucleotide strands fit together much like the pieces in a jigsaw puzzle because of complementary base pairing.
• The B form of DNA (probably the most common form in cells), shows that the two strands are not positioned directly opposite one another in the helical cylinder. Therefore, when the strands twist about one another, a wide major groove and narrower minor groove are formed by the backbone.
• Each base pair rotates 36° around the cylinder with respect to adjacent pairs so that there are 10 base
• pairs per turn of the helical spiral. Each turn of the helix has a vertical length of 3.4 nm.
• The helix is right-handed—that is, the chains turn counterclockwise as they approach a viewer looking down the longitudinal axis.
• The two backbones are antiparallel or run in opposite directions with respect to the orientation of their sugars.
• One end of each strand has an exposed 5’ hydroxyl group, often with phosphates attached, whereas the other end has a free 3′-hydroxyl group. If the end of a double helix is examined, the 5′ end of one strand and the 3′ end of the other are visible. In a given direction one strand is oriented 5′ to 3′ and the other, 3′ to 5′.

RNA

• Besides differing chemically from DNA, ribonucleic acid is usually single stranded rather than double stranded like most DNA.
• An RNA strand can coil back on itself to form a hairpin-shaped structure with complementary base pairing and helical organization.
• Cells contain three different types of RNA—messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA) —that differ from one another in function and structure.
• Among the three RNAs, rRNA form complex structure with proteins and designated as ribosome.
• All the three RNA together involved in protein synthesis through transcription and translation.