Transcription

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Transcription is the process of copying the information present on DNA in the form of RNA. It involves the joining of ribonucleotides by phosphodiester bonds.

Important points

  • Transcription occurs throughout interphase as well as in a non-dividing cell.
  • Site: prokaryotes → cytoplasm; eukaryotes → nucleus + mitochondria + plastids.
  • It is a template-dependent process, i.e. DNA is required as a template for the synthesis of RNA.
  • Only one strand of DNA is used as a template.
  • It is based on complementary base pairing.
  • Only a small fragment of DNA is copied into RNA.
  • Only NTPs are used as substrates.

Requirements

1. Transcription unit

  • The part of dsDNA involved in transcription is called the transcription unit (TU).
  • A transcription unit consists of 3 main parts: (a) promoter, (b) structural gene and (c) terminator.

Promoter

  • Rich in A=T base pairs.
  • A high % of A=T base pairs helps in easy unwinding of DNA during initiation.
  • Consists of conserved sequences for recognition by RNA polymerase.

Structural gene

  • Consists of information for the synthesis of RNA [mRNA, rRNA, tRNA].
  • In prokaryotes:
    • The structural gene consists of only coding sequences, i.e. they do not have a split gene arrangement.
    • Each structural gene codes for more than one polypeptide chain and is therefore called polycistronic.
  • In eukaryotes:
    • The structural gene consists of both coding sequences (exons) and non-coding sequences (introns).
    • The structural gene usually codes for only one polypeptide chain and is thus called monocistronic.

Terminator

  • Site at which transcription stops.

2. Enzymes

  • The enzyme involved in transcription is called DNA-dependent RNA polymerase.
  • RNA polymerase synthesises the RNA strand by joining ribonucleotides, based on the sequence of deoxyribonucleotides present on DNA, in the 5′ → 3′ direction, i.e. it has 5′ → 3′ polymerase activity.
  • DNA-dependent RNA polymerase also causes unwinding of the DNA helix during transcription.
  • In prokaryotes, only one type of DNA-dependent RNA polymerase is present, which synthesises all types of RNA, i.e. mRNA, tRNA and rRNA (23S rRNA, 16S rRNA, 5S rRNA).
  • In eukaryotes, 3 types of RNA polymerases are present, i.e. RNA pol I, RNA pol II and RNA pol III.

RNA pol I → rRNAs (28S, 18S, 5.8S)
RNA pol II → hnRNA (precursor of mRNA)
RNA pol III → tRNA, 5S rRNA and snRNAs

3. Proteins

σ-factor (Sigma factor)

  • Required for the initiation step of transcription.
  • σ-factor helps the RNA polymerase to bind to the promoter region of the transcription unit.
  • In the absence of σ-factor, RNA polymerase binds randomly to any site in DNA and starts transcription.

ρ-factor (Rho factor)

  • Helps in the termination of transcription.

4. Ions

  • Mg2+

Mechanism of transcription (in prokaryotes)

Initiation

  • σ-factor binds to the RNA polymerase.
  • The RNA polymerase–σ-factor complex binds to the promoter site by recognising the conserved sequences present in the promoter.
  • RNA polymerase starts unwinding a short stretch of the DNA helix, resulting in the formation of a transcription bubble.
  • RNA is formed by the activity of RNA polymerase, using the template strand of DNA as the template and NTPs as substrates.
  • σ-factor is released from the RNA polymerase.

The template strand runs 3′ → 5′. The coding strand runs 5′ → 3′ and has the same sequence as the RNA, except that it has T in place of U. That is why gene sequences are always written as the coding strand.

Elongation

  • RNA polymerase continues to add ribonucleotides to the RNA chain by using NTPs as substrates.

Termination

  • ρ-factor binds to the RNA polymerase as it reaches the termination site.
  • The newly formed RNA (primary transcript) is released.

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