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What are the components of a transcription unit?

Last updated date: 20th Jun 2024
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Hint: DNA and RNA are nucleic acids that use nucleotide base pairs as a complementary strand. A DNA sequence is read during transcription by an RNA polymerase which produces a complementary, antiparallel strand of RNA called a sole transcript.

Complete answer:

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Transcription constitutes the first step in gene expression. A fragment of mRNA is made from the gene to be used by the RNA polymerase enzyme, using the DNA as a template. is called a transcription method. When the segment of DNA takes part in transcription is known as the transcription unit. There are three components of the transcriptional unit which include a promoter, the structural gene, and a terminator.

> Promoter: This is present upstream of the structural gene. It is named by the 5′ end (of the coding strand which is the 3′ end of the template strand). It has multiple sections to bind to different transcription factors.

> Structural gene: This is a part of the DNA strand that has a polarity of 3′->5′. This DNA strand is referred to as the template strand or master strand or antisense, or minus (-) strand. During transcription, the other strand with a polarity of 5′ is displaced. The non-template strand which does not participate in transcription is sometimes referred to as sense or coding strand or plus (+) strand because the genetic code in this strand is similar to genetic code (based on mRNA), except that uracil is replaced with thymine.

> Terminator: This region is located at the 3′ end (of the coding strand which is the 5′ end of the template strand) downstream of the structural gene.

Additional information:
The process of transcription is divided into initiation, elongation, and termination. Since the code within the DNA molecule is hidden, the first step is to open the helix to reveal the bases. Only the gene is opened to be transcribed, and the rest of the chromosome remains coiled. Opening the helix reveals both base strands but only one of them is used by the enzyme that makes the mRNA. And the molecule of mRNA is single, not double-stranded. It adds the complementary base to the mRNA as the enzyme travels along the opened DNA chain, reading its code. Therefore, if the DNA base is cytosine, guanine is added (and vice versa) to the mRNA molecule; if it is thymine, adenine is added; if it is adenine, uracil is added. When the enzyme hits a 'stop' signal, the mRNA molecule synthesis is stopped, and the mRNA is released. Other enzymes zip the DNA again, and the mRNA then leaves the nucleus.

Note: The template and coding strand are characterized by the presence of a promoter in a transcription unit. The concept of the coding and template strands is reversed by swapping its position with a terminator. The terminator is positioned to coding strand 3'-end or downstream and typically determines the end of the transcription process.