What is a gene?

Introduction to genes:

The basic unit of the hereditary material is known as a gene or a cistron. It is an ordered sequence of nucleotides. These sequences encode polypeptide chain via mRNA molecule. Both DNA and RNA are known as nucleic acids. Genetics is the study of structure, function, and the regulation of genes. Studying genes helps us to know more about proteins, cellular functions, and disorders associated with them. In humans, gene targets help to study mutations and target them through advanced drug discovery and therapeutics. In bacteria, genes are manipulated to obtain the desired product. The genetic engineering or recombinant DNA technology helps to manipulate the genes. The desired gene can be integrated into a vector to obtain the desired product.

Modification of plant genes improves the quality of the crop and improves the yield. The genes play an important role in the growth and reproduction of an organism. Slight gene mutations, if harmful, lead to drastic changes in the cell and cause genetic disorders. Genetics plays an important role in preventing genetic disorders through prenatal testing, cytogenetic and molecular genetic techniques. Genes form the main basis of inheritance. We all have some traits obtained from our ancestors. The genes get passed on from generation to generation. We look similar to our parents, yet appear different. Helpful mutations in the genes lead to genetic diversity and variation. That is why we all look different from each other. The complete set of chromosomal and extrachromosomal genes of an organism is known as the genome. It consists of the complete genetic composition of an organism. 

The gene is a heritable determinant of a trait showing the property of segregation. With this reference, the way genes pass within the family are studied. The genes in a family may or may not have diverged from each other. Gene family involves a set of genes that descend from a common ancestor. Genes express themselves in several generations. They also interact with each other. Several genes can collaborate with each other and give rise to one phenotypic trait. The genes may be present in the cell in a particular dosage. It is important to know the frequency of a gene. The genes are present in a specific place on a chromosome, described as gene loci. Gene mapping through annotation of DNA sequences is possible based on gene loci information. DNA sequence annotation with regulatory element sites, coding regions, non-coding regions, and mutations accelerates the mapping. Sometimes many copies of a gene may be present in a chromosome, known as gene redundancy. Thus genes are vast and diversified.

Image 1: Basic genetic structure

HUGO gene nomenclature

It is a standard for gene nomenclature decided by the Human Genome Organisation (HUGO) committee. It is a meaningful naming of a gene. Names accompany useful symbols. A gene symbol is a unique abbreviation of the name of a gene. It consists of uppercase letters in italics, letters in Latin and numbers in Arabic. A putative gene name is locus based.

Here are the naming guidelines:

1.     The symbols must be unique and prohibited to use elsewhere.

2.     The gene symbols must have Latin letters and Arabic numbers

3.     Punctuation marks are not allowed in gene symbols.

4.     The gene symbols should not contain any references of species.

5.     The nomenclature of genes must evolve with the latest technology rather than follow age-old rules.

Structure of a gene:

A gene includes regions preceding and following the coding and non-coding regions. The preceding region is known as the leader sequence which is at 5’ position. It is the untranslated region. There is a coding region known as Exon. An exonic region specifies for an amino acid sequence. These exons are interrupted by non-coding regions known as introns. The untranslated trailer sequences follow them. The untranslated trailer sequences are at 3’ end. The spliced RNA molecule consists of only exons as the non-coding regions or introns for splicing out. Although DNA is a double-stranded molecule, only one strand encodes for RNA synthesis. The sense strand runs from 5’ to 3’ direction and encodes specific molecules. The gene has an open reading frame which is an indication of sense strand direction. The extremities of the gene consist of regulatory sequences. Plus the gene also consists of promoters, enhancers, silencers and other regions.

Image 2: Gene structure

Gene Expression- The Central Dogma

Expression of the genes involves the conversion of the genes coded information into the structures present and operating the cells. The genes are expressed to initiate the synthesis of the mRNA molecule and translated into a protein. Other examples of RNA include rRNA and tRNA. The tRNA and rRNA genes remain untranslated. The gene expression also involves a phenotypic manifestation by a process known as gene action. Differential gene expression studies involve gene expression at different levels. They express differently under different experimental conditions. The central dogma is a two-step process describing the gene expression. Francis Crick proposed the central dogma after the discovery of nucleic acids. DNA undergoes a process of transcription to synthesize RNA which further undergoes translation to form proteins. Not all genes express proteins. Thus, not all genes are transcribed to get RNA. Originally through central dogma, it was postulated that the genetic information is transferred only from nucleic acid to nucleic acid and from nucleic acid to protein. Thus, the genetic information gets transferred from DNA to DNA, DNA to RNA and from RNA to protein. The genetic information never transfers from protein to nucleic acid. The cis-trans test determines the functionality of genes. It determines whether the independent mutations occur in a single gene or several genes.

Cis-trans Complementation test

It helps to determine whether the two mutant sites are in the same functional unit or a gene. It is an allelism test and determines whether two different recessive mutations on the opposite chromosome of a diploid complement each other. The same two mutations in a diploid or a partial diploid show a wild-type phenotype. Cis mutations exhibit a wild-type phenotype. There is no genetic complementation when the mutations are in trans. The term cistron indicates gene.

The gene characterization is possible with sequence, transcription, and homology if it does not contribute to a phenotype. A gene is a functional and physical unit of heredity. The gene code is said to be a triplet. There are total 64 codons that code for 20 different amino acids. 

References:

[1] What is a gene?- Genetics Home Reference - NIH

[2] Gene – Wikipedia
[3] HUGO Gene Nomenclature Committee

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