Introduction:
- Transcription is a vital biological process that involves the synthesis of RNA chains from DNA templates. This article discusses the process of transcription, the transcription unit, and the different types of RNA polymerases involved in transcription.
The Process of Transcription
- Complementary Base Pairing and RNA polymerase: Transcription involves the synthesis of an RNA chain through the process of complementary base pairing, which is catalyzed and scrutinized by the enzyme RNA polymerase.
- Direction of RNA Synthesis: The RNA chain or transcript is synthesized from the 5 ‘ end toward the 3 ‘ end.
Transcription Unit
- Selectivity of Transcription: Transcription is a selective process where each transcribed segment of DNA is called a transcription unit.
- Simple and Complex Transcription Units: Transcription units may be simple or complex. In eukaryotes, a transcription unit typically carries the information of just one gene, while in bacteria; a set of adjacent genes is often transcribed as a unit.
- Primary Transcript and Multiple mRNA Formation: The immediate product of transcription is called the primary transcript. In the case of complex transcription units, the primary RNA transcript can be processed in more than one way, leading to the formation of more than one type of mRNA.
Transcription Start and Strand Synthesis
- Start Point and Terminator Sequence: Transcription starts from the first base pair called the start point and moves along the template, synthesizing RNA until it reaches a terminator sequence.
- Upstream and Downstream Sequences: Sequences prior to the start point are upstream of it, while those after the start point within the transcribed sequence are downstream of it.
- Coding and Template Strand: During transcription, only one strand of the transcription unit is transcribed, resulting in the transcript being identical in sequence with one strand of the DNA. This strand is called the coding strand and is complementary to the template strand.
Types of RNA Polymerases
- Prokaryotic RNA Polymerase: RNA synthesis is catalyzed by the enzyme RNA polymerase, which is responsible for almost all synthesis of mRNA, rRNA, and tRNA in a eubacterium. Eubacterial RNA pol is a multisubunit enzyme made up of four different polypeptides.
- RNA Dependent RNA Polymerase: Some viruses contain RNA genomes and are replicated in the host cell by the action of enzymes called RNA-dependent RNA polymerases or RNA replicases.
- Reverse Transcriptase: RNA-dependent DNA polymerase or reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the RNA in RNA viruses like retroviruses.