Transcription

Essential Question

How is the information in a gene copied into RNA?

A gene is a segment of DNA that holds the instructions for making a protein. To use those instructions, a cell first copies the gene into messenger RNA. This copying step is called transcription.

Overview of Transcription

• A gene is a region of DNA that contains the instructions for making a protein.
• Transcription makes an RNA copy of one gene. The cell does not copy the whole chromosome.
• The enzyme RNA polymerase unwinds a short part of the DNA and builds the RNA.
• In eukaryotes, transcription happens in the nucleus. In prokaryotes, it happens in the cytoplasm.
• The DNA is not changed. It winds back up behind the polymerase.

Transcription copies only the gene that the cell needs, and the DNA stays intact.

The Three Steps

• Step 1, binding: RNA polymerase attaches to the promoter, a DNA sequence that marks where a gene starts.
• Step 2, elongation: the polymerase moves along the gene. It matches RNA nucleotides to the template strand (A with U, T with A, G with C, C with G) and joins them in a growing chain.
• Step 3, termination: the polymerase reaches a terminator sequence that marks the end of the gene. It releases the new mRNA.
• Many mRNA copies can be made from one gene.

Binding, elongation, and termination copy one gene from start to end.

Template Strand and Coding Strand

• Only one DNA strand, the template strand, is read for a gene. The RNA is complementary to it.
• The other DNA strand is the coding strand. The mRNA has the same sequence as the coding strand, except that it has U where the coding strand has T.
• Different genes can use different strands as their template.
• To transcribe, pair each template base with its RNA partner: A with U, T with A, G with C, and C with G.

The mRNA is a complementary copy of the template strand, so it matches the coding strand with U in place of T.

After Transcription in Eukaryotes

• In eukaryotes, the first RNA made (pre-mRNA) is processed before it leaves the nucleus.
• Introns are segments that are cut out. Exons, the segments that stay, are joined together to form the final mRNA. This cutting and joining is called splicing.
• The mRNA then leaves the nucleus and goes to a ribosome.
• In prokaryotes, mRNA can be used right away.

Eukaryotic cells process their RNA before it is used to make a protein.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Transcribe a Gene

The template strand of a gene reads TAC CGA CTC AAA TTT GCA ACC ATT. Write the mRNA that RNA polymerase makes from it. Then write the coding strand of the DNA.

Show the solution

1. Pair each template base with its RNA partner: A with U, T with A, G with C, C with G.
2. TAC CGA CTC AAA TTT GCA ACC ATT becomes AUG GCU GAG UUU AAA CGU UGG UAA.
3. The coding strand is complementary to the template strand and has T instead of U: ATG GCT GAG TTT AAA CGT TGG TAA.
4. Check: the mRNA matches the coding strand, with U where the coding strand has T.
5. Answer: mRNA: AUG GCU GAG UUU AAA CGU UGG UAA. Coding strand: ATG GCT GAG TTT AAA CGT TGG TAA.

Example 2: How Long Is the Message?

The coding region of a gene (with no introns) is 1,032 nucleotides long and ends with a stop codon. Each codon is 3 nucleotides. How many codons does the mRNA have, and how many amino acids will the protein have? (The stop codon does not code for an amino acid.)

Show the solution

1. Number of codons = 1,032 / 3 = 344.
2. One of these is the stop codon, which does not code for an amino acid.
3. Amino acids = 344 – 1 = 343.
4. Answer: 344 codons and a protein of 343 amino acids.

Example 3: Same DNA, Different mRNA

Liver, muscle, and nerve cells from one person all contain Genes 1, 2, and 3 in their DNA. The table shows the relative amount of mRNA from each gene in each cell type (sample values for practice). Write a Claim, Evidence, Reasoning response about how these cells can be different.

Gene	Liver cell: mRNA (units)	Muscle cell: mRNA (units)	Nerve cell: mRNA (units)
Gene 1 (needed by every cell)	50	52	48
Gene 2 (liver protein)	90	2	1
Gene 3 (muscle protein)	1	85	1

Show the solution

1. Claim: The cell types differ because they transcribe different genes, not because they have different DNA.
2. Evidence: Gene 1 mRNA is similar in all three cells (50, 52, and 48 units). Gene 2 mRNA is high in liver (90) and almost absent in muscle (2) and nerve (1). Gene 3 mRNA is high in muscle (85) and almost absent in liver (1) and nerve (1).
3. Reasoning: All three cells have all three genes in their DNA. The protein made from a gene depends on whether the gene is transcribed into mRNA. A gene used in every cell (Gene 1) is transcribed in all of them, but cell-specific genes are transcribed only in the cell type that needs them.
4. Answer: Cells differ because different genes are transcribed in different cell types.

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Practice

Use the practice worksheet to transcribe genes, compare the template and coding strands, calculate message length from nucleotide counts, and analyze mRNA data from different cells.

• Transcription Practice Worksheet (PDF)
• Transcription Practice Answer Key (PDF)

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Lesson Slides

• 9.5 Transcription Slides (PDF)

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Big Ideas

• Transcription copies one gene of DNA into mRNA using the enzyme RNA polymerase.
• RNA polymerase reads the template strand and pairs A with U, T with A, G with C, and C with G.
• The mRNA matches the coding strand, with U in place of T.
• In eukaryotes, introns are removed and exons are joined before the mRNA leaves the nucleus.
• Different cell types transcribe different genes from the same DNA.

Back to the essential question: RNA polymerase uses one DNA strand as a template and builds a complementary mRNA with base pairing. The mRNA carries the gene’s instructions out to the ribosome, while the DNA stays in the nucleus.

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