DoAssignment.ca

DoAssignment study guide

D3.2 · Compare DNA and RNA in protein synthesis

Learn to compare dna and rna in protein synthesis through clear examples and targeted practice.

Ontario Grade 12 Biology

Molecular Genetics

Comparing the molecules that store and use genetic information

In SBI3U, you learned that DNA carries inherited information. Protein synthesis uses some of that information to assemble a protein. DNA and RNA are both chains of bases, but they have different structures and roles. This lesson follows a short sequence from a DNA template to an RNA message and then to an amino acid sequence. The sequence is a simplified model of the main steps, not a complete picture of everything that happens in a cell.

What you will learn

  • Compare the structure and roles of DNA and RNA in protein synthesis.
  • Distinguish transcription from translation.
  • Use base-pairing rules to determine an mRNA sequence from a DNA template.
  • Explain how a DNA base change can affect an mRNA codon and an amino acid sequence.

1. Bridge from inheritance: DNA stores information

A gene is a section of DNA with information used to make a functional product, often a protein. A protein is a molecule made of amino acids. The order of amino acids is important because it helps determine the protein that is made.
DNA stores information in the order of its bases. Its four bases are adenine, thymine, cytosine, and guanine, shortened to A, T, C, and G. DNA usually consists of two strands paired together. A pairs with T, and C pairs with G.
RNA is a related molecule that also contains a sequence of bases. RNA uses A, U, C, and G. The base uracil, U, takes the place of thymine, T. These similarities and differences help explain why DNA and RNA can take part in different parts of protein synthesis.
  • DNA stores the information used in protein synthesis.
  • DNA uses T; RNA uses U.
  • Base order carries information.

2. The biological system: transcription and translation

Protein synthesis has two main stages. Transcription makes an RNA copy of information from a gene in DNA. Translation uses the information in messenger RNA to assemble amino acids into a protein. Transcription makes RNA; translation makes a protein. They are different processes.
During transcription, one DNA strand serves as a template. A template is a strand that guides the order of bases in a new molecule. The RNA bases pair with the DNA template: DNA A pairs with RNA U, DNA T with RNA A, and C with G. The resulting messenger RNA, or mRNA, carries the message to a ribosome.
A ribosome is a cell structure where translation takes place. It reads mRNA in groups of three bases. Each group is a codon. A codon specifies an amino acid or a stop signal. Transfer RNA, or tRNA, brings amino acids to the ribosome. Ribosomal RNA, or rRNA, is part of the ribosome.
During translation, the ribosome reads codons in order and links the corresponding amino acids. Translation starts at a start codon and ends at a stop codon. For example, AUG is a start codon that specifies methionine. UAA is a stop codon; it signals the end of translation rather than adding an amino acid.
DNA template→mRNA→protein\text{DNA template} \rightarrow \text{mRNA} \rightarrow \text{protein}
  • Transcription uses DNA information to make RNA.
  • Translation reads mRNA codons to assemble amino acids.
  • mRNA carries the message, tRNA brings amino acids, and rRNA is part of the ribosome.

3. Compare DNA and RNA by following a sequence

DNA and RNA are both chains of bases, but they differ in their usual structure and their roles. DNA is usually double-stranded and contains thymine. RNA is usually single-stranded and contains uracil. In protein synthesis, DNA provides the information for transcription. mRNA carries a copy of that information for translation.
Consider a DNA template strand written as 3′-TAC AAA CCG ATT-5′. The direction marks show the order in which the strand is written. Applying the transcription pairing rules gives the mRNA sequence 5′-AUG UUU GGC UAA-3′.
The mRNA codons AUG, UUU, and GGC specify methionine, phenylalanine, and glycine. UAA signals the end of translation. The amino acid sequence in this example is therefore methionine–phenylalanine–glycine. A codon chart can be used to match mRNA codons to amino acids.
3′-TAC AAA CCG ATT-5′→5′-AUG UUU GGC UAA-3′\text{3′-TAC AAA CCG ATT-5′} \rightarrow \text{5′-AUG UUU GGC UAA-3′}
  • The mRNA sequence is complementary to the DNA template.
  • Translation reads mRNA in three-base codons.
  • A stop codon ends translation and does not add an amino acid.

4. What the model shows—and what it does not

The sequence model shows how information can pass from DNA to mRNA and then relate to amino acid order. Complementary base pairing lets a DNA template guide an RNA sequence. Codons connect the mRNA sequence to the amino acids assembled during translation.
The model is simplified. It uses a short sequence and does not show every detail of a cell or the final protein’s shape and role. It also does not mean that every RNA molecule carries a message for translation. tRNA and rRNA have different roles in the process.
A change in a DNA base can change a base in the mRNA made from it. If the mRNA codon changes, the amino acid specified may change. However, a different codon can specify the same amino acid, so a DNA change does not always change the amino acid sequence. Compare the codons using a codon chart before deciding what changed.
  • The model links DNA sequence, mRNA codons, and amino acid order.
  • DNA replication is not shown; the focus is transcription and translation.
  • A changed DNA base does not always change the amino acid sequence.

Comparing DNA and RNA in protein synthesis

FeatureDNARNA
Usual structureUsually double-strandedUsually single-stranded
BasesA, T, C, GA, U, C, G
RoleProvides information used during transcriptionmRNA carries the message; tRNA brings amino acids; rRNA is part of the ribosome
StageA DNA strand is used as a transcription templatemRNA codons are read during translation

Worked example

Example 1: Transcribe a DNA template

A DNA template strand is 3′-TAC CCT ACT-5′. Determine the mRNA sequence made from it.
  1. Pair the bases
    Use the DNA strand as a template. DNA T pairs with RNA A, DNA A with RNA U, and DNA C with RNA G. Apply the rules to each base in order.
    TAC CCT ACT→AUG GGA UGA\text{TAC CCT ACT} \rightarrow \text{AUG GGA UGA}
  2. Write the mRNA direction
    The RNA strand is written in the opposite direction to the template in this representation. The mRNA sequence is written from its 5′ end to its 3′ end.
    5′-AUG GGA UGA-3′\text{5′-AUG GGA UGA-3′}
Answer: The mRNA sequence is 5′-AUG GGA UGA-3′.
Check: The template and mRNA each contain nine bases. The mRNA uses U rather than T.

Worked example

Example 2: Translate mRNA codons

Use a codon chart to determine the amino acid sequence from mRNA 5′-AUG UUU GGC UAA-3′.
  1. Read the codons
    Translation reads mRNA in groups of three bases. The spaces in this sequence mark the codons.
    AUG∣UUU∣GGC∣UAA\text{AUG} \mid \text{UUU} \mid \text{GGC} \mid \text{UAA}
  2. Use the codon chart
    The chart shows that AUG specifies methionine, UUU specifies phenylalanine, and GGC specifies glycine. UAA is a stop signal, so it does not add an amino acid.
    AUG UUU GGC UAA→Met-Phe-Gly\text{AUG UUU GGC UAA} \rightarrow \text{Met-Phe-Gly}
Answer: The amino acid sequence is methionine–phenylalanine–glycine, followed by a stop signal.
Check: There are three amino acids because the fourth codon signals translation to stop.

Worked example

Example 3: Compare a changed DNA base

A DNA template changes from 3′-TAC AAA CCG ATT-5′ to 3′-TAC AAG CCG ATT-5′. Compare the mRNA codons and decide whether the amino acid sequence changes.
  1. Transcribe both templates
    Apply the same pairing rules to each DNA template. The changed template triplet AAA becomes UUU; AAG becomes UUC.
    A: AUG UUU GGC UAAB: AUG UUC GGC UAA\text{A: AUG UUU GGC UAA} \qquad \text{B: AUG UUC GGC UAA}
  2. Compare the codons
    The second mRNA codon changes from UUU to UUC. A codon chart shows that both specify phenylalanine, so the amino acid sequence in this example remains the same.
    UUU→PheUUC→Phe\text{UUU} \rightarrow \text{Phe} \qquad \text{UUC} \rightarrow \text{Phe}
Answer: The mRNA sequence changes, but the amino acid sequence remains methionine–phenylalanine–glycine.
Check: Both versions of the second codon specify phenylalanine. This shows why a DNA base change does not always change the amino acid sequence.

Common mistakes and how to avoid them

Treating transcription and translation as the same process.
Correction: Transcription uses a DNA template to make RNA. Translation uses mRNA to assemble amino acids.
Putting thymine in an RNA sequence.
Correction: RNA uses uracil, U, instead of thymine, T.
Assuming every codon adds an amino acid.
Correction: A stop codon, such as UAA, signals the end of translation rather than adding an amino acid.
Assuming every DNA base change changes the amino acid sequence.
Correction: Compare the resulting mRNA codon with a codon chart. Different codons can specify the same amino acid.

Lesson summary

  • DNA stores information used in protein synthesis; RNA molecules have distinct roles in using that information.
  • Transcription makes mRNA from a DNA template. Translation reads mRNA codons to assemble amino acids.
  • DNA contains T, while RNA contains U. mRNA carries a message, tRNA brings amino acids, and rRNA is part of the ribosome.
  • A DNA base change can affect an mRNA codon, but it does not always change the amino acid sequence.

Check your understanding

Question 1

Which statement correctly compares transcription and translation?
  1. Transcription makes mRNA from DNA; translation uses mRNA to assemble amino acids.
  2. Transcription assembles amino acids; translation copies DNA into mRNA.
  3. Both terms describe copying DNA before a cell divides.
  4. Both terms describe making RNA from a DNA template.
Show answer and explanation
Transcription makes mRNA from DNA; translation uses mRNA to assemble amino acids.
Transcription produces RNA using DNA information. Translation reads mRNA codons to assemble amino acids.

Question 2

What mRNA sequence is transcribed from the DNA template 3′-TAC GGT-5′?
  1. 5′-AUG CCA-3′
  2. 5′-UAC GGU-3′
  3. 5′-ATG CCA-3′
  4. 5′-AUG GGA-3′
Show answer and explanation
5′-AUG CCA-3′
Pair each template base with its RNA partner: T with A, A with U, C with G, and G with C. The result is 5′-AUG CCA-3′.

Question 3

What does a stop codon do during translation?
  1. Signals the end of translation.
  2. Adds an amino acid called stop.
  3. Starts transcription on the DNA template.
  4. Changes the mRNA base U into DNA base T.
Show answer and explanation
Signals the end of translation.
A stop codon signals that translation ends. It does not represent an amino acid.

Key terms

Gene
A section of DNA with information used to make a functional product, often a protein.
Template
A strand that guides the order of bases in a newly made molecule.
Transcription
The process of making RNA using information from a DNA template.
Translation
The process of using mRNA information to assemble amino acids into a protein.
Codon
A group of three mRNA bases that specifies an amino acid or a stop signal.
Ribosome
A cell structure where translation takes place.
mRNA
Messenger RNA; carries a copied message from DNA to a ribosome.
tRNA
Transfer RNA; brings amino acids to a ribosome during translation.

Continue through SBI4U

View the complete SBI4U Ontario Grade 12 Biology curriculum and lessons

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation D3.2. 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.

Official curriculum reference

Report a correction or ask a question