From one generation to the next generation, certain substance or materials gets transferred and help the offspring to express characteristics. This material which gets transferred from parental generation to the offspring is called genetic material. In living organisms, DNA(Deoxyribonucleic acid) and RNA(Ribonucleic acid) are the main genetic materials. Hence, a genetic material must be able to :
1) Divide(replicate) and be inherited to the offspring.
2) Carry and process all the information required for the function of a cell.
3) Change due to mutation.
The chemical basis of life is called a gene. It is also the unit of heredity that is transmitted from one generation to the other. Therefore, a gene is a part or segment of the DNA, which encodes the character of an organism. The term 'gene' was derived from the greek word 'genesis' which gives the meaning 'to be born' and was coined by a Danish Geneticist- Wilhelm Johannsen in 1909.
Function of Genes:
Gene is a complex structure which performs more than one function. Some of its important features are as follows:
1. All genes found in the DNA of an organism which is found on their chromosomes. There are 23 pairs of chromosomes in human. Hence there are nearly 3 billion base pairs of DNA in the chromosomes.
2. Each gene is present in a specific position called a gene locus.
3. Gene is present in the DNA. DNA is capable of duplication or multiplication. This is called the process of replication.
4. Each gene has a contrasting pair of characters, called alleles.
5. Genes are capable of undergoing changes and modification due to various terminologies like mutation or recombination.
6. The physical and genetic components of an organism are determined by genes. These components are transferred from parents to their offsprings.
Molecular Structure of a Gene:
As stated above,Gene is a complex structure which performs more than one function. A gene determines a character. It means a gene is responsible for a character. Chemically, a gene is formed of DNA, hence it may have more than one functional units. These findings and the foundation of DNA's structure and function, Seymour Benzer, in 1962, coined the terms cistron, muton, and recon.
1-Cistron: (Gene as a unit of function) The functional unit of the gene is called the cistron.It is also called the genetic unit of DNA molecule(functionally) because this part of DNA directs the synthesis of a single polypeptide chain, which controls a single physiological function. Production of a specific protein is done by a cistron. Hence, to produce one polypeptide or one function, separate cistron will be present.
2-Muton:(Gene as a unit of mutation) The smallest part of DNA which can undergo mutation(change), is called a muton. In smaller terms, muton represents a change in a pair of nucleotides. Any change in the base of a genetic code will modify the message carried by the codon.
3-Recon;(Gene as a unit of recombination) The smallest unit of DNA which can undergo recombination and crossing over, is called a recon. A recon may be small as one nucleotide pair of DNA.
A gene consists of only one cistron. A cistron consists of several recons, and a recon consists of several mutons. Therefore, muton is the smallest unit of a cistron.
The flow of information takes place from DNA to RNA(esp. mRNA) to proteins. This relation of flow of information is known as central dogma. Thus, it can be said that genetic information flows from nucleic acids to proteins. The flow of this information is unidirectional (i:e. in one direction). The concept of central dogma was advanced by Crick in 1958.
Therefore, central dogma is defined as "the unidirectional flow of genetic information which occurs from DNA to proteins through RNA".
Some viruses, commonly known as retroviruses, contain RNA as the genetic material. Hence, reverse transcription occurs in such cases (i.e. formation of DNA from RNA). For this certain enzyme called reverse transcriptase plays an important role. Therefore, information flows from RNA to DNA and from DNA to mRNA and finally into protein. Such flow of information is called reverse central dogma. This was reported by Temin in 1970 and Baltimore in 1970 in RSV(Raus's Sarcoma virus, a kind of retrovirus).
This phenomenon is known as reverse transcription or teminism,
Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.
Mehta, Krishna Ram. Principle of biology. 2nd edition. Kathmandu: Asmita, 2068,2069.
Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.
© 2019-20 Kullabs. All Rights Reserved.