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CraftExam
medium difficulty6 min

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.

In one line: Template is read; coding resembles the message.

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Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1If the coding strand has 5'-ATG CCA-3', the mRNA will be 5'-AUG CCA-3'.
  2. 2The template strand would be complementary and antiparallel.

Method you can reuse

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.

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

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

Common wrong answer

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

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

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

Answer plan

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.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

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

  3. 3

    Add for 3 marks

    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.

Practice

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