Genes: Definition, History, Structure, Types, and Function

Genes: You have beautiful hair! “Everyone in my family is blessed with beautiful hair; it’s in my genes.” How often have you come across people saying like this when referring to a familial trait? Well! it’s very true. Genes do regulate aspects like hair colour, texture along with many other characteristics before and after birth. Before birth, genes have the blueprint that leads to the formation of the entire body from a single cell.

After delivery, genes regulate every single trait and function. Your facial features, the colour of your eyes, height, and many other things resemble your parents because of genes.  Did you know, every human has around 20,000 to 25,000 genes and 3.2 billion base pairs, and this entire sequence of bases is referred to as the genome. Read further to know more about genes, their types, and function.

Define Genes

Danish botanist Wilhelm Johannsen who coined the word “gene”, describes it as the fundamental, physical, and functional units of heredity. Gene is a functioning segment of DNA that codes for a polypeptide chain.

History of Genes

Genetic research began with Gregor Mendel, the “Father of Genetics”, long before discovering DNA. He never used the term gene, but he talked about a factor responsible for the inheritance in his paper published in 1866. Later in 1909, Johannsen coined the term “gene” and described it as a fundamental physical and functional unit of heredity.

Structure of Genes

Gene is a functional part of the DNA thread which means the chemical structure of DNA is similar to that of DNA. Genes are made up of sugar, phosphates, and nitrogenous bases. The complementary pairing involves H-bond between a purine and a pyrimidine. Bases on antiparallel strands are linked by H- bonds where Adenine (A) forms two H- bonds with Thymine (T), and Guanine (G) forms three H-bonds with Cytosine (C). DNA is a stretch of a polynucleotide chain, whereas genes are the functional part of DNA. Genes are scattered throughout the DNA.
In prokaryotes, genes are arranged one after the other, whereas, in eukaryotes, genes are interrupted by a non-functional sequence. The functional sequence that contains information for the polypeptide chain formation is called exon, whereas non-coding parts are called introns.

Fig: Exons, Introns, and Genes on a Chromosome

Additionally, a gene has a regulatory sequence like promoter, terminator, enhancers, etc. The promoter is located towards the 5‘-end (upstream) of the structural gene, whereas the terminator is present towards the 3‘ end(downstream). Promoter provides space for RNA polymerase to bind and initiate gene expression, also called transcription, whereas the terminator sequence is where the transcription ends.

Fig: Gene with its regulatory elements

Types of Genes

There are various types of genes found in bacteria, viruses, plants, and animals. Some important types of genes are discussed below:

Housekeeping Genes

Housekeeping genes are also called constitutive genes as they are required for the maintenance of basic cellular function. They are expressed in every cell of an organism under normal conditions. So far, 3408 housekeeping genes have been reported in humans. Example: PGK1 (genes for Phosphoglycerate kinase 1) and genes of ATPase enzyme.

Non-constitutive Genes

They are also called luxury genes or specialist genes as they do not continuously express in every cell. They can be switched on or off based on the cellular requirement. These genes can be further classified into two:

Inducible genes: These genes switch on only in the presence of the inducer. Example: Lac operon is induced in the presence of lactose acting as an inducer.

Repressible genes: They remain switched on until repressed by a chemical or repressor. Example: trp genes are repressed by tryptophan.

Structural Genes (Cistrons)

They are protein-coding genes. In eukaryotes, these genes contain intervening sequences or introns in between exons sequences. These genes are controlled by regulators, enhancers, operators, and promoters present upstream to the gene. A gene can be divided into smaller units of function, i.e., mutation and recombination.

Muton: It is the smallest part of a cistron or chromosome, which undergoes mutation.

Recon: It is the smallest segment capable of undergoing recombination. This segment participates in recombination events during meiosis.

Cistronic genes can be monocistronic or polycistronic.

Monocistronic genes are defined as genes that have their own regulatory sequence and code for only one polypeptide chain. Monocistronic genes are found in eukaryotes. For example- All eukaryotic structural genes

Polycistronic genes are those where one operator controls the functionality of multiple genes. As a result, codes for multiple polypeptide chains. Polycistronic genes are found in prokaryotes. Polycistronic genes are involved in the interrelated function and together form an operon system. Example- Lac operon – One regulatory system controls three genes (z, y, and a). The z gene codes for beta-galactosidase (β-gal), the y gene codes for permease, and agene encodes for the transacetylase. All three gene products help in the metabolism of lactose.

Pseudogenes: They are also called defunct relatives of functional genes as they are almost similar to functional ones but can not code for functional protein because of mutation. Example: The multiple copies of the GAPDH locus in vertebrate genomes.

Transposons (Jumping Genes): Jumping genes were first discovered by Nobel Prize winner Barbara McClintock(1951)in maize. These genes/DNA segments can jump from one location to another within the genome of a cell. They can even jump from one chromosome to another. These genes do not code for any protein. They are found near the telomeric region and relocate themselves from one chromosome to another. Transposable Elements make up approximately 45% of the human genome.

Processed genes: They are found in eukaryotes. Processed genes lack introns. They are formed due to retroviral reverse transcription.

Overlapping genes: Overlapping genes are two adjacent DNA segments that are partially or entirely overlapped with each other through a shared genomic location. They are mostly found in prokaryotes, eukaryotic organelles, viruses, etc.  They can code for multiple polypeptides by using different open reading frames.

Split genes: They were discovered by Nobel Prize winner Sharp and Robertsin 1993. It is a gene that contains both exons and introns. They are mostly found in eukaryotes.

Regulator gene: These genes turn on or off the transcription of a structural gene. They are present upstream to the coding sequence of cistron or structural gene and produce a repressor mRNA that translates a repressor protein. This repressor protein turns off the gene by binding to the operator.

Operator gene: The operator gene is present upstream to the cistron, and it acts as the switch of a cistron controlled by repressor protein produced by the regulator gene.

Promoter gene: The promoter gene is present upstream of the cistron. It provides the site for the binding of RNA polymerase enzymes and initiates the transcription of the mRNA.

Terminator gene: This gene is present downstream of the cistron, where transcription terminates, and the RNA polymerase enzyme dissociates from DNA.

Silencer gene: As the name suggests, they silence other genes’ activities when the activity of that particular gene is not needed.

Enhancer gene: These genes are responsible for enhancing the activities of other genes related to the production of a particular protein.

Integrated gene: Integrated genes are found in more than one chromosome or at a different locus in a chromosome. All these genes activate simultaneously and produce a particular product.

Epistatic gene: These genes mask the effect of other genes coding for the same phenotype.  Example: Genes responsible for the flower colour in sweet peas.

Hypostatic genes: These are the genes whose expression is altered due to the expression of epistatic genes.

Functions of Genes

Genes perform various functions in the cell.

1. Each gene contains instructions to make proteins. Proteins are the language of the cell. They perform all sorts of functions in the cells, like bodybuilding protein, enzymatic protein, hormonal protein, messenger protein, pigments, etc.
2. Genes decide the appearance of an individual, e.g., height, colour, structure, habits, etc.
3. They help in providing immunity by forming antibodies.
4. They are units of inheritance and carry information from one generation to another.

Summary

Genes are the hereditary unit. Genes are defined as the functional segment of DNA which codes for the protein. A structural gene has three main parts: A promoter, the structural gene, and a terminator. The promoter is present upstream to the structural gene, and it provides RNA polymerase with a space to bind and begin the process of gene expression. Terminator is present downstream, and it causes dissociation of RNA polymerase from the DNA molecule. There are various kinds of genes like the housekeeping gene, cistron, inducer gene, silencer gene, jumping gene, integrated genes, etc. The main function of the gene is to code for a polypeptide chain which forms a product of cellular use. Genes decide our appearance, cellular activities and carry information from one generation to another.

Frequently asked questions (FAQs) on Genes

Q.1. What is the simple definition of the gene?
Ans: Gene is the functional segment of DNA that codes for a polypeptide chain.

Q.2. What is the main function of genes?
Ans: The main function of the gene is to carry information from one generation to another and continue the existence of life forms. Genes control the production of various proteins like enzymes, hormones, etc., depending upon the cellular needs.

Q.3. What is the difference between DNA and genes?
Ans: DNA is a double helical thread of polynucleotide chain, whereas gene is just a segment of DNA. DNA contains many genes.

Q.4. How many genes are there in a chromosome?
Ans: No. of genes is fixed for a particular chromosome. For example, chromosome 1 of a human likely contains 1960-2100 genes. However, Every chromosome has a different number of genes.

Q.5. What is the difference between gene and allele?
Ans: Gene is a segment of DNA, whereas allele is a variant of the gene. In general, a gene has two alleles; one is dominant, expressed in both homozygous and heterozygous conditions, and the other is recessive, which is expressed only in the homozygous condition. For example, if we consider a gene for height, then ‘T’ represents the dominant allele while ‘t’ represents the recessive allele.

Study Properties of Genetic Material Here

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