Name: Lac Operon
Uploaded: 2019-07-19
Duration: 9 min 11 s
Description: A lac operon is a group of genes with a single promoter The genes in the operon encode proteins that allow the bacteria to use lactose as an energy source. L...

Question

Define transcription. Explain the process of transcription in bacteria with suitable diagram.

Accepted Answer

The genetic information flows from DNA to mRNA and then to the protein-synthesizing machinery. The process of synthesis of RNA strands from the DNA is known as transcription. In a prokaryotic cell, this takes place in the cytoplasm as they lack a membrane-bound nucleus. It is a process coupled with the translation in a prokaryotic cell. A transcription unit in prokaryotic cells is the specific region of DNA that undergoes this process catalyzed by DNA-dependent RNA polymerase enzyme. The complete process of transcription requires:

Ribonucleotides (NTPs)- Adenine, Uracil (in place of Thymine), Guanine, Cytosine

RNA polymerase enzyme (it has different subunits) and other transcription factors.

Template DNA strand- Only a specific region (gene) is transcribed in a 3' to 5' direction and not the entire genome. Another strand of the dsDNA will be complementary to the newly synthesized mRNA transcript and would not undergo the transcription process.

Transcription in a prokaryotic cell involves three steps:

Initiation– The dsDNA is unwounded to single-stranded forms near the initiation site to provide a template strand. The DNA-dependent RNA polymerase (RNAP) binds to the specific sequence on the template strand named promoter sequence (Pribnow box / TATA box) and initiates the transcription of the template strand. The sigma factors of RNAP enzyme help them to bind the promoter sequence.

Elongation– After the synthesis of about 10 base pairs of long RNA, the sigma factor of RNAP enzyme slides over from the promoter site over the template DNA strand in a 5’ – 3’ direction continuously all the while polymerizing NTPs to form mRNA strand.

Termination– This is the last stage that terminates the transcription process. There are two mechanisms:

Rho-independent transcription termination– transcription stops due to a specific sequence in terminator DNA. The terminator DNA contains intrachain self-complementary sequences in the newly formed transcript RNA that form the hairpin structure that hinders the RNAP enzyme and pauses its function. This may result in the unloading of the RNAP enzyme from the DNA template.

Rho-dependent transcription termination – transcription is terminated by rho protein that recognizes the DNA sequences act as a signal of termination. It binds to RNA after the translation has completed, but transcription is still going on. This factor causes the dissociation of the RNAP enzyme from the DNA template.

Define transcription. Explain the process of transcription in bacteria with suitable diagram.

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