M03.03.04 Transcription: Concepts and Terminology

Transcription is the process of synthesizing RNA from a DNA template. This process is facilitated by a DNA-dependent RNA polymerase, which uses the DNA strand to generate complementary RNA. Below is an overview of essential concepts and terminology related to transcription.

Overview of Transcription

1. Role of RNA Polymerase

• Function: RNA polymerase is the enzyme responsible for transcribing RNA from the DNA template.
• Promoter Region: The RNA polymerase locates genes by searching for promoter regions. The promoter serves as the binding site, determining:
  • Where does transcription begin?
  • Which DNA strand serves as the template?
  • The direction of transcription?
• Primer: Unlike DNA replication, RNA polymerase does not require a primer to initiate transcription.

2. Directionality of Transcription

• Template Strand: RNA polymerase moves along the DNA template strand in the 3′ to 5′ direction.
• RNA Synthesis: The RNA product is synthesized in the 5′ to 3′ direction, using nucleoside triphosphates (NTPs), including ATP, GTP, CTP, and UTP.
• Lack of Proofreading: RNA polymerase does not have proofreading ability, so errors made during transcription are not corrected.
• RNA-DNA Relationship:
  • The RNA product is complementary and antiparallel to the template strand.
  • The coding (Antitemplate) strand, which is not used during transcription, has the same sequence as the RNA (except that thymine in DNA is replaced by uracil in RNA).

3. Gene Base Numbering System

• A numbering system is employed to identify the location of important bases in and around a gene:
  • The first base transcribed into RNA is referred to as the +1 base.
  • Bases to the left (upstream, 5′ direction) of this point are numbered as –1, –2, –3, etc.
  • Bases to the right (downstream, 3′ direction) are numbered as +2, +3, etc.

4. Termination of Transcription

• Termination Signal: Transcription concludes when RNA polymerase reaches a termination signal on the DNA template.

Table 1: Comparison of DNA Strands in Transcription

Flow of Genetic Information

For a gene that encodes a protein, the genetic information flows from DNA to RNA and ultimately to protein. The process involves:

1. Transcription: DNA is transcribed into messenger RNA (mRNA) in the 5′ to 3′ direction.
2. Translation: The ribosome translates the mRNA into a protein, reading in the 5′ to 3′ direction, synthesizing the protein from the amino (N-terminal) to carboxyl (C-terminal).

Table 2: Flow of Genetic Information from DNA to Protein

Points to Remember:

• RNA polymerase does not need a primer to start transcription.
• The coding strand of DNA has the same sequence as RNA, except thymine (T) is replaced by uracil (U).
• Transcription is unidirectional and proceeds from 5′ to 3′ on the RNA strand.
• Errors during transcription are not corrected since RNA polymerase lacks proofreading mechanisms.
• Upstream and downstream refer to the direction relative to the start site of transcription.

Bibliography:

1. Lodish, H., et al. Molecular Cell Biology. W.H. Freeman and Company, 8th edition.
2. Alberts, B., et al. Molecular Biology of the Cell. Garland Science, 6th edition.
3. Berg, J.M., et al. Biochemistry. W.H. Freeman and Company, 7th edition.