Gene:
A segment of DNA that is involved in producing a polypeptide chain; it can include
regions preceding and following the coding DNA as well as introns between the exons; it is
considered a unit of heredity; "genes were formerly called factors"
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“A complete sequence region necessary for generating a functional product”
A human body possess almost 75 trillion cells with nucleus bounded by nuclear
membrane (means eukaryote)
Each nucleus contain 46 chromosomes made up of chromatin.
Human genome contains 30000 to 35000 nuclear genes.
Average size of chromosome carries 1300 to 1500 genes.
Human gene vary in size ranging from around 1 kb (gene coding for B-globin and
insuline) to to 2.5 mb for dystrophin gene responsible for Duchenne muscular
dystrophy.
Gene posses Exons (coding regions) and introns (intervening sequence which is
not expressed).
Introns are transcribed in to primary RNA / Pre-mRNA in cell nucleus, but introns
are spliced out from mature mRNA in the cytoplasm.
Average no. of exons per human gene is nine and introns is eight, with great
variation i.e. dystrophin gene contain 79 and that of B-globin contain only 03.
Average size of exon in human is 145 bp.
Closely related to upstream (5 prime end) there are regulatory promoter regions
and other regulatory sequences which include enhancers, silencers and sometimes
a locus controlling region.
Promoter (TATA box, CG box and CAAT box) provide binding sites for
transcription factors.
Regulatory sequences serve as enhancer and silencers.
First and last exons contain untranslated regions i.e. 5' UTR and 3' UTR.
5' UTR marks start of transcription, contain initiator codon, which indicate the
site of start of transcription.
3' UTR contain a termination codon which marks the end of translation, plus
nucleotide which encode a sequence of adenosine residues known as Poly (A) tail.
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http://www.google.com.pk/images?hl=en&q=gene+structure&um=1&ie=UTF8&source=univ&ei=_iO7TMe5AcqlcJzVpIMN&sa=X&oi=image_result_group&
ct=title&resnum=2&ved=0CBMQsAQwAQ
http://mbe.oxfordjournals.org/content/23/2/450.abstract