Question

What is transcription? What are the different parts of a DNA transcription unit? What is their role in the process of transcription?

Answer 1

Transcription is the first step of gene expression. Using the DNA as a template, a piece of mRNA is made from the gene to be used by the enzyme RNA polymerase. This process is called transcription. Both DNA and RNA are nucleic acids, which use base pairs of nucleotides as a complementary language. During transcription, a DNA sequence is read by an RNA polymerase, which produces a complementary, antiparallel RNA strand called a primary transcript.

Transcription Unit: The segment of DNA that takes part in transcription is called transcription unit. It has three components (i) a promoter, (ii) the structural gene and (iii) a terminator. Eukaryotes also require an enhancer.

(i) Promoter- It is located upstream of the structural gene. By convention, it is called 5′ end (of coding strand which is 3′ end of template strand). It has different parts for attachment to various transcription factors.

(ii) Structural gene- It is a component of that strand of DNA which has 3′→ 5′ polarity. This strand of DNA is called template strand or master strand or antisense, or (-) strand. The other strand which has a polarity of 5′→ 3′ is displaced during transcription. This non-template strand which does not take part in transcription is also called sense or coding strand or plus (+) strand because genetic code present in this strand is similar to genetic code (based on mRNA) except that uracil is replaced by thymine.

(iii) Terminator- This region is present downstream of the structural gene at the 3′ end (of coding strand which is actually 5′ end of the template strand).

Mechanism of Transcription: Transcription is divided into initiation, promoter escape, elongation, and termination. Because the code is buried within the DNA molecule, the first step is to open up the helix to expose the bases. Only the gene to be transcribed is opened, the remainder of the chromosome remains coiled. Opening up the helix exposes both base strands, but the enzyme that makes the mRNA uses only one of them. Hence, the mRNA molecule is single, not double-stranded. As the enzyme moves along the opened DNA strand, reading its code, it adds the complementary base to the mRNA. Therefore, if the DNA base is cytosine, guanine is added to the mRNA molecule (and vice versa); if it is thymine, adenine is added; if it is adenine, uracil is added. When the enzyme gets to a 'stop' signal, it terminates synthesis of the mRNA molecule, and the mRNA is released. The DNA is zipped up again by other enzymes, and the mRNA then leaves the nucleus.