Basic Concepts of Genetics


Genetics deals with the origin of genetic variation and expression of traits resulting from these variations. It is the study of heredity, the process by which characters are passed on from parents to their offspring are identical. They differ from each other and also from their parents in some respects.

Genetics tries to explain the mechanism and the basis for similarities and differences between related individuals. These two aspects are respectively called Heredity and Variation. Heredity is also called Inheritance. It is the process of transmission of characters in living beings from parents to offspring.

Concepts of Genetics:

1. Centre of Heredity in a Cell: In eukaryotic cells, the nucleus contains genetic material. In prokaryotic cells, the genetic material lies uncovered, embedded in the cytoplasm in the nucleoid region, as in bacteria. In viruses, the genetic material lies in the viral head formed of a protein coat.

2. Genetic Material: In eukaryotic and prokaryotic cells, DNA is the genetic material. Hereditary information is stored in it in the specific sequence of nucleotides. In viruses, the hereditary material may be DNA or RNA.

3. DNA: DNA is a double-stranded helix, formed of two antiparallel spirally twisted polynucleotide strands. It is a macromolecule, a polymer of thousands of nucleotides arranged in a specific sequence. DNA is organised into functional units called Genes. In viruses, DNA may be single or double-stranded.

In viruses and prokaryotes, DNA is naked but in eukaryotes, DNA is associated with basic proteins called histones and forms a long nucleoprotein fibre. The chromatin fibre remains extended or unwounded in non-dividing cells. In dividing nuclei, the chromatin condenses in the form of darkly stained called chromosomes.

4. Gene: Gene is the functional unit of heredity. In chemical terms, a gene is a linear array of nucleotides or a segment of DNA which is capable of undergoing replication, mutation, transcription and translation.

5. Chromosomes: In eukaryotes, the chromosomes are linear DNA molecules complex with protein. The nucleoprotein fibre, composing it, is highly folded, coiled and recoiled forming a super solenoid structure. It is visible only in dividing cells when chromatin condenses to form solid darkly stained rods or filaments.

A chromosome is differentiated into regions, some of which do not contain coding information and some of these regions contain clusters of similar genes. In prokaryotes (bacteria), the chromosome is a long circular double-helical DNA molecule, while in viruses, it may be in the form of linear or circular DNA or RNA. It may be single or double-stranded.

6. Mitosis and Meiosis: Genetic material (DNA) replicates in interphase. During mitosis, it divides equally into two daughter chromosomes. These are equally distributed to the two daughter cells.

In meiosis, four daughter cells are formed from each parent cell. Each daughter cell receives half the number of chromosomes of the parental cell. The haploid daughter cells form gametes for sexual reproduction. The male gametes are called sperm and female gametes are called ova or eggs. The fusion of a male and a female gamete is called fertilisation.

7. Genetic Code: The sequence of nucleotides in a segment of DNA is translated into a sequence of amino acids in the polypeptide chain synthesized during protein synthesis. This information of amino acid sequencing is encoded in the form of a genetic code. A sequence of three nitrogenous bases codes for one amino acid of the polypeptide chain and is called a codon.

Expression of Genetic Code: The coded information in DNA is transferred into messenger RNA (mRNA) during a process called transcription. mRNA comes out of the nucleus into the cytoplasm where in association with ribosomes it is translated into a polypeptide chain.