Process of copying of information present on the DNA in the form of RNA i.e. Transcription involves 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
• 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.
• Based on complementary base pairing.
• Only a small fragment of DNA is copied into RNA.
• Only NTPs are used as substrate.

REQUIREMENTS

1. TRANSCRIPTION UNIT

• Part of dsDNA involved in Transcription is called the Transcription Unit (TU).
• Transcription unit consist of 3 main parts:
  a. Promoter
  b. Structural gene
  c. Terminator

PROMOTER

• • Rich in A=T base pairs.
  • High % of A=T base pairs help in easy unwinding of DNA during initiation.
  • Consist of Conserved sequences for recognition by RNA polymerase.

STRUCTURAL GENE

• • Consist of information for synthesis of RNA [mRNA, rRNA, tRNA].
  • In Prokaryotes,
    • The structural gene consist of only coding sequences i.e. they do not have Split gene arrangement.
    • Each structural gene codes for more than one polypeptide chains therefore called Polycistronic.
  • In Eukaryotes,
    • The structural gene consist of both coding (exons) and non-coding sequences (introns).
    • Structural gene usually codes for only one polypeptide chain and thus called Monocistronic.

TERMINATOR

• • Site at which transcription stops.

2. ENZYMES

• The enzyme involved in Transcription is called DNA dependent RNA polymerase.
• RNA polymerase synthesizes RNA strand by joining Ribonucleotides based on the sequence of deoxynucleotides present on DNA in 5’ → 3’ direction i.e. 5’ → 3’ polymerase activity.
• DNA dependent RNA polymerase also causes unwinding of DNA helix during transcription.
• In prokaryotes, only one type of DNA dependent RNA polymerase is present which synthesizes 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

3. PROTEINS

σ-factor (Sigma factor)

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

ρ-factor (Rho factor)

• • Helps in termination of transcription.

4. IONS

• Mg²⁺

MECHANISM OF TRANSCRIPTION (IN PROKARYOTES)

 INITIATION

• σ-factor binds to the RNA polymerase.
• RNA polymerase- σ-factor complex bind to the promoter site by recognizing the conserved sequences present in the promoter.
• RNA polymerase starts unwinding a short stretch of the DNA helix resulting in formation of a Transcription Bubble.
• Formation of RNA by using coding strand of DNA as template and NTPs as substrate by the activity of RNA polymerase.
• σ-factor is released from the RNA polymerase.

ELONGATION

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

TERMINATION

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