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Biology & life sciences · 07 / 12

Gene expression: DNA, RNA, protein

High school · intermediate · Worked example and 4 exercises with solutions, at your pace.

Need a foundation?Cells and DNA: levels of organisationMitosis and meiosis: maintain or reduce
UnderstandSee an exampleGuided exercise3 progressive challenges

The principle, simply

Transcription makes RNA from a DNA template strand. In a simple model without processing, RNA matches the coding strand with U replacing T. Translation reads RNA codons to build a polypeptide. Eukaryotic RNA processing can remove introns before translation.

THE PICTURE TO REMEMBERGene expression: DNA, RNA, protein
  1. 01Template DNA
  2. 02Transcription
  3. 03RNA
  4. 04Translation to polypeptide

WORKED EXAMPLE

From principle to reasoning

Coding DNA 5′-ATGC-3′ pairs with 3′-TACG-5′; simplified RNA is 5′-AUGC-3′. DNA strands are antiparallel.

Why 5′ → 3′ matters

RNA is synthesised 5′→3′ from a template read 3′→5′. Starting with the coding strand written 5′→3′ gives the same base sequence with U replacing T. The lab uses this convention, not full protein translation.

Solve a problem in this subject

  1. Frame a question

    State the relationship to explain and its scale: molecule, cell, organism, population or geological time.

  2. Describe evidence

    Give a measurable observation, unit, comparison and conditions before explaining it. Observation and inference do different jobs.

  3. Connect a mechanism

    Use a precise concept and label arrows as production, transfer, action or transformation. Sequence alone does not prove causation.

  4. Answer with a limit

    Connect evidence and knowledge to the question. Distinguish the simplified model from living systems and identify a missing condition.

YOUR TURN

01 · Try before checking

Write simplified RNA from coding DNA 5′-TTAC-3′.

One hint

Keep the coding-strand direction.

Reveal and understand the solution

5′-UUAC-3′. This T-to-U shortcut requires a coding strand, not a template strand.

Compare method, conditions and answer; identify the exact part to revisit.

IMPROVE THROUGH PRACTICE

Three additional challenges with solutions

Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.

Reflect after practising
02 · Recognise and understandLink to this exercise

Transcribe coding strand 5′-ATG GCT-3′ to mRNA.

A hint to get started

For the coding strand, replace T with U and preserve direction.

Step-by-step solution
  1. mRNA is 5′-AUG GCU-3′.
  2. Coding DNA matches RNA except for T replacing U.
  3. A template strand would require complementary pairing and antiparallel direction.

Which step did you miss? Retry without help, then explain why the method works.

03 · Apply the methodLink to this exercise

Translate AUG GCU UAA, given AUG = methionine, GCU = alanine and UAA = stop.

A hint to get started

A stop codon is not an amino acid.

Step-by-step solution
  1. Translation starts here at AUG.
  2. The chain contains methionine followed by alanine.
  3. UAA ends translation without adding a third amino acid.

Which step did you miss? Retry without help, then explain why the method works.

04 · Go furtherLink to this exercise

Does every base substitution change the protein?

A hint to get started

Different codons can encode the same amino acid.

Step-by-step solution
  1. The genetic code is redundant.
  2. Some substitutions are synonymous and preserve the amino acid.
  3. Others change an amino acid or create a stop; the effect depends on the mutation.

Which step did you miss? Retry without help, then explain why the method works.

The trap to avoid

Do not confuse transcription and translation or omit strand directions.

Without notes, can you…

  • Explain the principle in your own words?
  • Rebuild the diagram and its connections?
  • Solve the exercise and justify the method?

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