Gene

 

In 1877, Johannsen coined the term gene. A gene is a short segment of DNA having a specific nucleotide sequence that codes for a polypeptide chain. Number of genes is variable in different organisms. The following table shows the amount of DNA, haploid number of chromosomes, and number of genes of some organisms.

Structure and organization of genes:

All organisms have genetic material in the form of DNA or RNA. Some of the sequences code for a polypeptide chain and are called coding sequences (genes) the other do not code for any polypeptide and are called non-coding sequences (Junk DNA). These no-coding sequences are found interspersed in the genetic material. In prokaryotes, almost all the non-coding DNA is found between genes as intergenic DNA. In eukaryotes, non-coding DNA is scattered throughout the eukaryotic chromosomes. It is present between the genes and also in the actual genes. These non-coding sequences in the genes are known as introns,

 

The structure of a typical eukaryotic gene can be represented by the human β-globin gene, This gene consists of the following elements:

 

a.      Promoter

 

A promoter region is responsible for the binding of RNA polymerase and the initiation of transcription. The promoter region of the human β-globin gene has three distinct units and extends from 95 to 26 base pairs before ("upstream from") the transcription initiation site (i.e., from -95 to -26). The promoters in prokaryotes are TATAAT (Pribnow box )and TTGACA located at -10 and -35 respectively in eukaryotes they are TATA and CAAT located at -25 and -70.

 

b.      Transcription initiation site

 

The transcription initiation site is the region from where the RNA polymerase enzyme starts the synthesis of RNA molecules. This site for human β-globin is ACATTTG. This site is often called the cap sequence because it represents the 5´ end of the RNA, which will receive a "cap" of modified nucleotides soon after it is transcribed.

 

c.      Translation initiation site

 

The translation initiation site, ATG. This codon (which becomes AUG in the mRNA) is
located 50 base pairs after the transcription initiation site in the human β-globin gene (although
This distance differs greatly among different genes).  The intervening sequence of 50 base pairs between the initiation points of transcription and translation is the 5´ untranslated region, often called the 5´ UTR or leader sequence.

 

d.     Exons and introns    

 

The regions of the DNA that contain coding information are known as exons. Exons have intervening sequences that do not code for any amino acid known as introns. Intron Segment of a gene that does not code for protein but is transcribed and forms part of the primary transcript. Exon is the Segment of a gene that codes for protein and that is still present in the messenger RNA after splicing.

 

·       β-globin gene the first exon, contains 90 base pairs coding for amino acids 1 30. ·       An intron containing 130 base pairs with no coding sequences for the globin protein. ·       An exon containing 222 base pairs coding for amino acids 31 104. ·       A large intron 850 base pair has nothing to do with the globin protein structure. ·       An exon containing 126 base pairs coding for amino acids 105 146

 

e.      Translation termination codon

 

A translation termination codon, TAA. This codon becomes UAA in the mRNA. The ribosome dissociates at this codon, and the protein is released.

 

f.       3´ untranslated region

 

A 3´ untranslated region that, (3´ UTR) although transcribed, is not translated into protein.
This region includes the sequence AATAAA, which is needed for Polyadenylation. The poly(A) tail is inserted into the RNA about 20 bases downstream of the AAUAAA sequence.

Gene and chromosome differences

Chromosomes are thread-like structures composed of DNA and proteins. These are present inside the nucleus of every cell of an organism. The DNA consists of many Specific segments of DNA that can synthesize proteins called genes.  Genes, therefore, are composed of specific nucleotide sequences. All genes instruct the cell to make certain proteins. The location of a gene on a chromosome is called locus.