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Transcription

Transcription is the synthesis of RNA from a DNA template strand by RNA polymerase, using a transcription unit made of promoter, structural gene, and terminator regions.

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Student-friendly explanation

In transcription, only one DNA strand acts as the template strand, while the other is called the coding strand because its sequence resembles RNA except for thymine in place of uracil. RNA polymerase binds at the promoter, moves along the template, and synthesizes RNA in the 5' to 3' direction. In prokaryotes, transcription and translation may be coupled. In eukaryotes, the first product is hnRNA, which undergoes capping, tailing, and splicing to form mature mRNA. Splicing removes introns and joins exons, allowing the message to be translated accurately.

How to write this in exams

  1. 1

    Start with the exact idea

    Transcription is the synthesis of RNA from a DNA template strand by RNA polymerase, using a transcription unit made of promoter, structural gene, and terminator regions.

  2. 2

    Then show how to use it

    Identify promoter, bind RNA polymerase, open DNA locally, synthesize RNA complementary to template, stop at terminator, process hnRNA in eukaryotes by capping, tailing, and splicing.

  3. 3

    Add one concrete example

    In a eukaryotic gene, hnRNA may contain both introns and exons. After splicing, only exon sequences remain in mature mRNA for translation.

  4. 4

    Avoid this incomplete answer

    Writing that introns are joined and exons are removed during splicing.

Definition

Transcription is the synthesis of RNA from a DNA template strand by RNA polymerase, using a transcription unit made of promoter, structural gene, and terminator regions.

Example

In a eukaryotic gene, hnRNA may contain both introns and exons. After splicing, only exon sequences remain in mature mRNA for translation.

Rule to remember

Transcription rule: RNA polymerase reads the DNA template strand and synthesizes RNA 5' to 3' using A-U and G-C pairing.

Memory hook

Template is read; coding resembles the message.

Examples and method

Worked example

If the coding strand has 5'-ATG CCA-3', the mRNA will be 5'-AUG CCA-3'. The template strand would be complementary and antiparallel.

Method to apply

Identify promoter, bind RNA polymerase, open DNA locally, synthesize RNA complementary to template, stop at terminator, process hnRNA in eukaryotes by capping, tailing, and splicing.

Diagram support

Label promoter, structural gene, terminator, template strand, coding strand, RNA polymerase, nascent RNA, 5' end, 3' end, intron, exon, cap, and poly-A tail.

How CBSE asks it

Asked as transcription unit diagram, strand identification, RNA processing steps, or differences between prokaryotic and eukaryotic transcription.

Avoid common mistakes

Common confusion

Students often confuse template strand with coding strand. RNA is made complementary to the template strand, not copied directly from the coding strand.

Common wrong answer

Writing that introns are joined and exons are removed during splicing.

Exam tip

For transcription unit questions, always label promoter upstream, structural gene in the middle, terminator downstream, and identify template versus coding strand.

Quick check

Why is RNA sequence similar to the coding strand but not identical?

RNA is complementary to the template strand, so it matches the coding strand in base order except that RNA has uracil instead of thymine.

Answer writing and exam use

1-mark answer

Define transcription, draw or describe transcription unit, identify coding and template strands, explain RNA synthesis by RNA polymerase, and add eukaryotic RNA processing if asked.

2-mark answer

Define transcription, draw or describe transcription unit, identify coding and template strands, explain RNA synthesis by RNA polymerase, and add eukaryotic RNA processing if asked.

3-mark answer

Define transcription, draw or describe transcription unit, identify coding and template strands, explain RNA synthesis by RNA polymerase, and add eukaryotic RNA processing if asked.
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