Use terminology for DNA replication, transcription, and translation

Ontario Grade 12 Biology

Molecular Genetics

A Grade 12 guide to the terminology of three related processes

In SBI3U, you learned that genes are sections of DNA and that inherited information can affect an organism’s traits. A useful starting idea is that DNA stores information in the order of its bases: adenine (A), thymine (T), cytosine (C), and guanine (G). In this lesson, you will use the terms for three processes that handle that information. DNA replication copies DNA. Transcription makes an RNA copy of information in a gene. Translation uses a messenger RNA sequence to assemble a protein. These processes are related, but they are not interchangeable.

What you will learn

Describe how DNA replication differs from transcription and translation.
Use key terms for the molecules and steps involved in each process.
Follow a short DNA sequence through transcription and translation.

1. A shared starting point: DNA and base pairing

DNA is a molecule that stores genetic information. A DNA molecule has two strands. The strands are held together by matching bases: A pairs with T, and C pairs with G. This matching is called complementary base pairing. A nucleotide is a building block of DNA or RNA; it contains a sugar, a phosphate group, and a base.

RNA is another nucleic acid, meaning a molecule made from a chain of nucleotides. RNA is usually shown as one strand in these course-level models. RNA uses uracil (U) instead of thymine (T). When RNA bases pair with DNA during transcription, A pairs with U, and C pairs with G.

The terms template strand and coding strand help describe a gene’s DNA strands. The template strand is the DNA strand used to make an RNA strand. The coding strand has a sequence that matches the RNA sequence, except that DNA has T where RNA has U. These labels describe the strands’ roles in transcription.

\mathrm{A-T,\ C-G;\ A-U,\ C-G}

DNA uses A, T, C, and G; RNA uses A, U, C, and G.
A pairs with T in DNA, while A pairs with U when RNA is made from DNA.
The template strand guides the RNA sequence.

2. DNA replication copies DNA

Before a cell divides, it needs a copy of its DNA. DNA replication is the process of making a DNA copy from DNA. The two original strands separate. Each original strand acts as a template for a new complementary strand. DNA polymerase is an enzyme that joins DNA nucleotides to build the new strands. An enzyme is a molecule that helps a cell carry out a process.

The result is two DNA molecules. Each contains one original strand and one newly made strand. This pattern is called semiconservative replication: each copied DNA molecule conserves one of the original strands. The term describes how the strands are shared between the original and copied DNA.

For example, if one DNA strand has the sequence

\mathrm{A-T-C-G}

, its complementary strand is

\mathrm{T-A-G-C}

. During replication, each strand can guide the making of its partner. Replication copies the DNA molecule; it does not make messenger RNA or assemble a protein.

\mathrm{DNA + DNA\ template \rightarrow DNA\ copies}

Replication uses DNA as its template and produces DNA.
Complementary base pairing guides the new strands.
Semiconservative means each resulting DNA molecule has one original strand and one new strand.

3. Transcription makes an RNA message

A gene is a section of DNA that contains information used to make a functional product, such as a protein. Transcription is the process of making an RNA copy of a gene’s information. In protein production, that RNA copy is messenger RNA, or mRNA. RNA polymerase is an enzyme that builds the RNA strand using the DNA template strand.

During transcription, RNA nucleotides pair with exposed bases on the template strand. The RNA sequence is complementary to the template. It matches the coding strand except that U replaces T. The mRNA carries the copied information to a ribosome, the cell structure where translation takes place.

For a short example, let the DNA coding strand be

\mathrm{ATG\ GCT\ TAA}

. Its template strand is

\mathrm{TAC\ CGA\ ATT}

. Transcription produces mRNA

\mathrm{AUG\ GCU\ UAA}

. The mRNA is not a second DNA copy: it is RNA, and it uses U instead of T.

\mathrm{DNA\ template \rightarrow mRNA}

Transcription uses a DNA template and produces RNA.
RNA polymerase builds the RNA strand.
The mRNA sequence matches the coding strand, with U in place of T.

4. Translation reads mRNA to assemble a protein

Translation is the process of using the information in mRNA to assemble a chain of amino acids. Amino acids are the building blocks of proteins. A ribosome reads the mRNA in groups of three bases. Each group of three mRNA bases is a codon. A codon can specify an amino acid or a stop signal.

Transfer RNA, or tRNA, helps bring amino acids to the ribosome. Each tRNA has an anticodon, a three-base sequence that pairs with a complementary mRNA codon. The ribosome links the brought amino acids in the order specified by the mRNA. The resulting amino-acid chain is a protein or can become part of one.

In the sequence

\mathrm{AUG\ GCU\ UAA}

, the codons are

\mathrm{AUG}

,

\mathrm{GCU}

, and

\mathrm{UAA}

. In the standard genetic code, AUG specifies methionine, GCU specifies alanine, and UAA is a stop codon. The stop codon signals the end of translation; it does not add an amino acid.

\mathrm{mRNA\ codons \rightarrow amino\ acid\ chain}

Translation uses mRNA and takes place at a ribosome.
A codon is a group of three mRNA bases.
tRNA anticodons pair with mRNA codons and help bring amino acids.
A stop codon signals the end of translation.

Compare the three processes

Process	Starting information	Product	Key location or structure
DNA replication	DNA	DNA copies	DNA strands act as templates
Transcription	DNA template strand	RNA, often mRNA	RNA polymerase builds RNA
Translation	mRNA codons	Amino-acid chain	Ribosome reads mRNA; tRNA brings amino acids

Worked example

Identify the process

A cell uses one DNA strand as a template and builds a strand containing uracil. Name the process and the product.

Notice the template
The process starts with a DNA strand used as a template. That rules out translation, which reads mRNA, and distinguishes the description from replication, which makes DNA.
Use the product clue
The new strand contains uracil, a base used in RNA rather than DNA. Making an RNA strand from a DNA template is transcription.
$\mathrm{DNA\ template \rightarrow RNA}$

Answer: The process is transcription, and the product is an RNA strand. If the RNA carries a gene’s information for protein production, it is mRNA.

Check: The product contains uracil, so it is RNA rather than a DNA copy.

Worked example

Transcribe a DNA sequence

A DNA coding strand is

\mathrm{5'-GAT\ CCA-3'}

. Write the corresponding mRNA sequence.

Find the template
The coding strand is not the strand used as the template. Use complementary base pairing to find its partner: G pairs with C, A with T, and C with G. The template sequence is $\mathrm{3'-CTA\ GGT-5'}$ .
$\mathrm{5'-GAT\ CCA-3' \; / \; 3'-CTA\ GGT-5'}$
Build the RNA
Pair RNA bases with the template. C pairs with G, T with A, A with U, and G with C. The mRNA sequence therefore matches the coding strand except that U replaces T.
$\mathrm{5'-GAU\ CCA-3'}$

Answer: The corresponding mRNA sequence is

\mathrm{5'-GAU\ CCA-3'}

.

Check: The answer has the same sequence as the coding strand with U replacing T.

Worked example

Separate replication from transcription

A DNA molecule has the strands

\mathrm{A-C-G}

and

\mathrm{T-G-C}

. A student says that using the first strand to make

\mathrm{T-G-C}

is transcription. Correct the student’s statement.

Check the product
The proposed product contains T, so it is DNA, not RNA. Transcription produces RNA, which uses U instead of T.
Name the DNA-copying process
Making a complementary DNA strand from a DNA template is part of DNA replication. The complementary sequence is correct because A pairs with T and C pairs with G.
$\mathrm{A-C-G \rightarrow T-G-C}$

Answer: The student has described DNA replication, not transcription. The product is DNA. Transcription would produce RNA and use U rather than T.

Check: The process is identified by both its template and its product.

Common mistakes and how to avoid them

Calling transcription the process that makes a DNA copy.

Correction: Replication makes DNA copies. Transcription makes RNA from a DNA template.

Using T in an RNA sequence.

Correction: RNA uses U instead of T. Check whether the requested product is DNA or RNA.

Treating translation as another name for transcription.

Correction: Transcription makes RNA from DNA. Translation reads mRNA to assemble an amino-acid chain.

Calling a stop codon an amino acid.

Correction: A stop codon signals the end of translation; it does not specify an amino acid.

Lesson summary

DNA replication copies DNA using each original strand as a template.
Transcription uses a DNA template to make RNA; mRNA carries gene information for translation.
Translation takes place at a ribosome, where mRNA codons guide the assembly of an amino-acid chain.
Keep the products distinct: replication produces DNA, transcription produces RNA, and translation produces an amino-acid chain.

Check your understanding

Question 1

Which process makes an RNA strand from a DNA template?

DNA replication
Transcription
Translation
Complementary base pairing

Show answer and explanation

Transcription

Transcription uses a DNA template to make RNA. Replication makes DNA copies, while translation reads mRNA to assemble an amino-acid chain.

Question 2

What is a codon?

A three-base sequence on mRNA
A three-amino-acid chain
A DNA molecule made during replication
An enzyme that builds RNA

Show answer and explanation

A three-base sequence on mRNA

A codon is a group of three bases on mRNA. It specifies an amino acid or a stop signal.

Question 3

A student sees U in a newly made strand. Which conclusion is best supported?

The strand is RNA, so it may have been made during transcription.
The strand must be a DNA copy made during replication.
Translation has produced a new DNA strand.
The strand is a protein because it contains U.

Show answer and explanation

The strand is RNA, so it may have been made during transcription.

U is used in RNA rather than DNA. If the RNA was made from a DNA template, the process was transcription.

Key terms

Anticodon: A three-base sequence on tRNA that pairs with a complementary mRNA codon.
Codon: A group of three mRNA bases that specifies an amino acid or a stop signal.
DNA replication: The process of making DNA copies using the original DNA strands as templates.
Messenger RNA (mRNA): RNA that carries copied gene information to a ribosome for translation.
Semiconservative replication: A pattern of DNA replication in which each resulting DNA molecule has one original strand and one newly made strand.
Template strand: A DNA strand used to guide the making of a complementary DNA or RNA strand.
Transcription: The process of making RNA from a DNA template.
Translation: The process in which a ribosome uses mRNA information to assemble an amino-acid chain.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation D2.1. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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D2.1 · Use terminology for DNA replication, transcription, and translation

Ontario Grade 12 Biology

A Grade 12 guide to the terminology of three related processes

What you will learn

1. A shared starting point: DNA and base pairing

2. DNA replication copies DNA

3. Transcription makes an RNA message

4. Translation reads mRNA to assemble a protein

Compare the three processes

Identify the process

Transcribe a DNA sequence

Separate replication from transcription

Common mistakes and how to avoid them

Lesson summary

Check your understanding

Question 1

Question 2

Question 3

Key terms

Continue through SBI4U

About this lesson and its review