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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance4 Marks
Question
Briefly describe the following: Transcription
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Answer

Transcription: Genetic informations are transferred from DNA to RNA. The process of synthesis of RNA from DNA is called transcription. DNA directs the synthesis of proteins. This DNA is found in nucleotide of prokaryotic cells and nucleus of eukaryotic cells. DNA molecules never leave their place and therefore DNA molecules code the messages of protein synthesis in mRNA which carries the message from nucleus to cytoplasm over ribosome therefore the synthesis of RNA on DNA template is called transcription. When mRNA is synthesized on DNA template, it contains Uracil in place of Thymine. This synthesis occurs in presence of enzyme RNA polymerase.
A segment of DNA that transcripts the RNA, is called Transcriptional unit. This unit may be like a gene or may have many continuous genes. There are three main components of transcriptional unit in DNA:
(1) Promoter,
(2) Structural gene,
(3) Terminator
DNA dependent RNA polymerase catalyses the polymerization in only one direction i.e. 5'→3'. The strand with 3'→5' polarity serves as template and is therefore, called template strand.
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The promoter and terminator flank the structural gene in a transcription unit. The promoter is said to be located towards S'-end (upstream) of the structural gene (the reference is made with respect to the polarity of coding strand). It is a DNA sequence that provides binding site for RNA polymerase, and it is the presence of a promoter in a transcription unit that also defines the template and coding strands. The terminator is located towards 3'-end (downstream) of the coding strand and it usually defines the end of the process of transcription. There are additional regulatory sequences that may be present further upstream or downstream to the promoter.

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