Page 204 - Concise Pathology for Exam Preparation ( PDFDrive )

8

Genetic and Paediatric

Disorders

PART I: GENETIC DISORDERS
Q. Write briefly on the structure of a gene.
Ans. A gene is a specific sequence of nucleotides. It codes for a protein through a genetic
code or sequence called codon (Fig. 8.1).
• The boundaries of a gene are known as start and stop codons. The start codon decides
when to initiate the protein synthesis and the stop (termination) codon decides when to
end it.
• Human genes contain exons which are regions that contain the coding information that
are both transcribed and translated into proteins and introns which are stretches
between exons that do not code for a protein (noncoding region).
• On either side of a gene, there are noncoding regions called flanking regions that are
responsible for the regulation of gene expression. They are called regulatory regions.
These include promoters (regions which bind to transcription factors strongly or
weakly), enhancers (regions that enhance the effects of a weak promoter) and silencers
(regions that inhibit transcription).
• In the first stage of transcription, an enzyme called RNA polymerase binds to a TATA
base sequence in the 5’-flanking region (at the ‘front end’ of the gene) adjacent to where
transcription is initiated. There are other sequences in the region that serve as sites to
which proteins that assist in transcription bind. This entire flanking region prior to the
coding region of the gene is called the promotor.
• On the far end of the gene, past the coding region of introns and exons, is the 3’-flanking
region which largely remains untranslated.
• Once an mRNA is transcribed from the DNA coding region of a gene, it goes
through several processing steps before it leaves the nucleus to be translated in the
cytoplasm.

Transcription Transcription
initiation termination
Exon 1 Intron 1 Exon 2 Intron 2 Exon 3
5’ 3’
Promoter
region Translation start Translation
codon (ATG) STOP codon
FIGURE 8.1. Structure of a gene.

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