Organization of Genes - Fine structure & Types

  A gene can be described as a polynucleotide chain, which is a segment of DNA. It is a functional unit controlling a particular trait such as hair colour or eye colour.

  According to Lodish and others, gene is defined as the entire nucleic acid sequence that is necessary for the synthesis of a functional gene product, which be polypeptide or any type of RNA. In addition to structural genes i.e. coding genes it also includes all the control sequences and non coding introns.

  Most prokaryotic genes transcribe polycistronie mRNA and most eukaryotic genes transcribe monocistronic mRNA. Total number of genes on a single chromosome is different in different organisms.

   Bacteriophage virus R17 consists of only three genes, SV40 consists 1 mm long chromosome, of 5-10 genes.
E. coli bacteria have more than 3000 genes on single

Fine Structure of a Gene :

  A gene is present only in one strand of DNA, which is a double stranded helix. A gene consists of several different regions. The main region of gene is the coding sequence which carries information regarding amino acid sequence of polypeptide chains.

  The region lies on the left side of coding sequence i.e. upstream or minus region and on the right side i.e. downstream or plus region consists of fairly fixed regulatory sequences.

Regulatory sequences of gene made up of promoters which are different in prokaryotes and eukaryotes. There are some genes which are different from normal genes either in terms of their nucleotide sequences or their functions. Split gene, jumping gene, overlapping gene and pseudo gene are some examples of such genes. 

(1) Split Genes:

  Usually a gene has a continuous sequence of nucleotides. In other words, there is no interruption in the nucleotide sequence of a gene. Such nucleotide sequence codes for a particular single polypeptide chain.

  Nevertheless, it was observed that the sequence of nucleotides was not continuous in case of some genes; the sequences of nucleotides were interrupted by intervening sequences. Such genes which carry nucleotides with interrupted sequence are referred to as split genes or interrupted genes. Thus, split genes have two types of sequences, viz., normal sequences and interrupted sequences. 

Normal sequence

 Normal sequence represents the sequence of nucleotides which are included in the mRNA which is translated from DNA of split gene. These sequences code for a particular polypeptide chain and are known as exons. 

Interrupted sequence:

  The intervening or interrupted sequences of split gene are known as introns. These interrupted sequences do not code for any peptide chain, moreover, interrupted sequences are not included into mRNA which is transcribed from DNA of split genes.

An interrupted gene showing introns (light) and exon (dark) portions

( 2 ) Jumping Genes :

  In general, a gene occupies a specific position on the chromosome called locus. However, in some cases within the chromosome and also between the chromosomes of the same genome, a gene keeps on changing its position. Such genes are known as jumping genes or transposons or transposable elements.

( 3 ) Overlapping Genes :

  The genes which code for more than one protein are known as overlapping genes. In case of overlapping genes, the complete nucleotide sequence codes for one protein and a part of such nucleotide sequence can code for another protein.

( 4 ) Pseudogenes :

  Especially in eukaryotes, there are some DNA sequences present which are non - functional or defective copies of normal genes. These sequences do not have any function, such DNA sequences or genes are known as pseudogenes. These pseudogenes have been reported in humans, mouse and drosophila.

Genome :

The basic set of chromosomes is refer as genome. In a genome, each type of chromosome is represented only once. The overall organization of plant nuclear genome revealed that coding capacity is relatively constant among plants as seen in comparison of genome of Arabidopsis and Maize.

  Comparing genomic nature of these two plants also reveals genomic codes for same numbers of genes but differ in their genome size. Similarly, maize and sorghum plant contains 10 chromosomes but the maize genome is three times larger than the size as that of sorghum.

Several striking similarities were observed on the arrangement of genes on chromosome of sorghum and maize. The extra DNA that accounts for differences in maize and sorghum genome size is mainly non coding repetitive sequence between genes. This clearly indicated that in most of the organisms only 1 % of the DNA is utilized for protein production and rest may have a significant role in structure and organization of the genome.

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