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D3.3 · Explain protein synthesis and gene-expression control

Learn to explain protein synthesis and gene-expression control through clear examples and targeted practice.

Ontario Grade 12 Biology

Molecular Genetics

How cells use DNA instructions to make proteins—and regulate when they do so

In SBI3U, you learned that genes are sections of DNA and that inherited information contributes to an organism’s characteristics. A gene does not usually affect a characteristic by acting directly. Its information can be used to make a protein, and proteins help cells carry out many tasks. Protein synthesis is the process of making a protein using genetic instructions. Gene expression means using information in a gene to produce a functional product, often a protein. Cells can control whether, when, and how much a gene is expressed. This lesson follows the information from DNA to protein, then shows how a cell can regulate that process.

What you will learn

  • Explain how transcription and translation produce a protein from a gene.
  • Distinguish transcription from DNA replication and translation.
  • Describe how cells can control gene expression, including by regulating transcription.
  • Use a short DNA sequence to predict the corresponding mRNA and amino acid sequence.

1. From a gene to a protein

DNA stores genetic information in the order of its bases. A gene is a DNA segment whose information can be used to make a particular product. Before this lesson, recall that DNA can be copied during DNA replication. Replication makes a new DNA copy. Protein synthesis is different: it uses information from a gene to make a product.
Protein synthesis has two main stages. During transcription, a cell makes messenger RNA, or mRNA, using one DNA strand as a guide. RNA is a molecule similar to DNA, but it uses the base uracil, written U, instead of thymine, written T. During translation, a ribosome reads the mRNA and joins amino acids in an order specified by the message. Amino acids are the small units that make up proteins.
A simple example helps separate the stages. A DNA gene contains an instruction, but the ribosome reads an RNA message, not the DNA itself. The mRNA carries a working copy of the instruction to the ribosome. This copy lets the cell use the information without treating transcription as DNA replication.
DNA→transcriptionmRNA→translationprotein\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{protein}
  • Transcription makes RNA from a DNA template.
  • Translation uses an mRNA message to assemble amino acids.
  • DNA replication copies DNA; it is not either stage of protein synthesis.

2. How translation reads the message

An mRNA message is read in groups of three bases. Each group is called a codon. A codon can specify an amino acid or a stop signal. The genetic code is the set of rules that connects mRNA codons with amino acids. A ribosome is the cell structure that carries out translation.
Transfer RNA, or tRNA, brings amino acids to the ribosome. Each tRNA has an anticodon, a three-base sequence that pairs with a matching mRNA codon. As the ribosome moves along the mRNA, tRNAs bring amino acids in the order indicated by the codons. The amino acids are joined into a chain, which can fold into a protein.
For example, the mRNA codon AUG specifies the amino acid methionine and commonly serves as a start signal. The codon UAA is a stop codon. The ribosome stops adding amino acids when it reaches a stop codon. A short sequence example can show how a message is read, but a short chain alone does not show all the steps needed for a functioning protein.
AUG→methionine\mathrm{AUG} \rightarrow \text{methionine}
  • A codon is a group of three mRNA bases.
  • tRNA pairs its anticodon with an mRNA codon and delivers an amino acid.
  • The codon order determines the amino acid order in the chain.

3. Gene-expression control in cells

Cells do not need to make every protein all the time. Gene-expression control is the regulation of whether, when, or how strongly a gene is used. A major point of control is transcription: if a gene is transcribed more often, more mRNA may be available for translation. If transcription is reduced, less mRNA may be made.
Regulatory proteins can bind to particular DNA regions and affect whether transcription begins. A promoter is a DNA region near a gene where the machinery for transcription can assemble. Some regulatory proteins help transcription begin; others make it less likely. Signals from inside or outside a cell can affect regulatory proteins and therefore influence gene expression.
This control helps explain why cells with the same DNA can behave differently. For instance, cells in different tissues can express different sets of genes. The proteins they make help them carry out different roles. Gene expression is not the only influence on a cell’s characteristics, but it is an important link between stored genetic information and cell activity.
A useful model is to imagine a gene as an instruction that is available to be copied only when the cell’s controls allow it. This is a model, not a claim that all genes are controlled in exactly the same way. The details of regulation can vary among genes and organisms.
gene activity→mRNA amount→potential protein amount\text{gene activity} \rightarrow \text{mRNA amount} \rightarrow \text{potential protein amount}
  • Gene-expression control determines whether, when, or how strongly a gene is used.
  • Regulatory proteins can affect transcription by interacting with DNA.
  • Different gene-expression patterns help cells with the same DNA carry out different roles.

4. Reading sequence information carefully

To interpret a sequence question, first identify whether the DNA sequence shown is the template strand or the coding strand. The template strand is the DNA strand used to build mRNA. The mRNA bases pair with that template: DNA A pairs with RNA U, DNA T with RNA A, DNA C with RNA G, and DNA G with RNA C. If the coding strand is shown, its sequence matches the mRNA except that DNA T appears where RNA has U.
Next, divide the mRNA into codons starting at the stated reading position. Use an appropriate genetic-code table to identify amino acids. If the question does not give enough information about the strand or reading position, a unique sequence answer may not be possible.
Sequence reasoning describes the message and the amino acid order it specifies. It does not, by itself, establish how much protein a real cell makes. Gene regulation and other cellular steps affect the outcome. A sequence model is useful evidence about what a gene can encode, but it is not a measurement of protein production in a particular cell.
DNA template→mRNA\mathrm{DNA\ template}\rightarrow\mathrm{mRNA}
  • Check which DNA strand is provided before transcribing.
  • Read mRNA in groups of three bases from the stated starting point.
  • A predicted amino acid sequence is not the same as measured protein production.

Two stages of protein synthesis

StageInformation usedMain result
TranscriptionDNA templatemRNA
TranslationmRNA codonsAmino acid chain

Worked example

Transcribe and translate a short message

A DNA template strand has the sequence TAC CTT ACT. Write the complementary mRNA sequence and identify the amino acids specified by its codons. Use the standard genetic code.
  1. Build the mRNA
    Pair each DNA template base with its RNA partner. The template triplet TAC produces AUG, and the full message is AUG GAA UGA.
    TAC CTT ACT→AUG GAA UGA\mathrm{TAC\ CTT\ ACT}\rightarrow\mathrm{AUG\ GAA\ UGA}
  2. Read the codons
    AUG specifies methionine, GAA specifies glutamic acid, and UGA is a stop codon. The stop codon ends translation and does not add an amino acid.
    AUG GAA UGA→methionine–glutamic acid–stop\mathrm{AUG\ GAA\ UGA}\rightarrow\text{methionine–glutamic acid–stop}
Answer: The mRNA is AUG GAA UGA. The codons specify methionine and glutamic acid, followed by a stop signal.
Check: The mRNA is complementary to the template, uses U rather than T, and is read in the given triplets.

Worked example

Identify where a process occurs

A student says, “The ribosome transcribes DNA into mRNA and then copies the DNA.” Correct the statement by identifying the two protein-synthesis stages.
  1. Name transcription
    Transcription uses DNA information to make mRNA. It is not DNA copying. In eukaryotic cells, transcription takes place in the nucleus, where the DNA is located.
    DNA→mRNA\text{DNA}\rightarrow\text{mRNA}
  2. Name translation
    Translation takes place at a ribosome. The ribosome reads mRNA codons and links amino acids into a chain. It does not transcribe DNA.
    mRNA→amino acid chain\text{mRNA}\rightarrow\text{amino acid chain}
Answer: Transcription makes mRNA from DNA. Translation occurs at a ribosome and uses mRNA to assemble an amino acid chain. DNA replication is a separate process that copies DNA.
Check: The corrected statement distinguishes all three processes rather than treating them as one.

Worked example

Reason about gene-expression control

In a simplified model, a regulatory protein binds near Gene X and makes transcription less likely. Predict the direct effect on mRNA and explain what can—and cannot—be concluded about protein amount.
  1. Predict the transcription effect
    If transcription is less likely, fewer mRNA molecules may be made over time, compared with the same gene without that regulatory effect.
    reduced transcription→less mRNA\text{reduced transcription}\rightarrow\text{less mRNA}
  2. Limit the conclusion
    With less mRNA available, less protein may be produced. This is a prediction from the simplified model, not a direct measurement. The exact protein amount cannot be calculated from the information given.
    less mRNA→potentially less protein\text{less mRNA}\rightarrow\text{potentially less protein}
Answer: The model predicts reduced mRNA production and potentially reduced protein production. It does not provide an exact protein amount or establish the measured response of a real cell.
Check: The conclusion follows the model but distinguishes a prediction from experimental evidence.

Common mistakes and how to avoid them

Calling transcription DNA replication.
Correction: Transcription makes RNA from DNA information. DNA replication makes a DNA copy.
Saying that translation makes mRNA.
Correction: Transcription makes mRNA. Translation reads mRNA and joins amino acids.
Pairing RNA bases as though RNA contains thymine.
Correction: RNA uses uracil, U, instead of thymine, T. Check that the mRNA sequence uses U.
Treating a prediction of mRNA as an exact protein measurement.
Correction: A model can predict a likely direction of change. An exact amount requires appropriate evidence and information.

Lesson summary

  • Protein synthesis uses genetic information to make a protein.
  • Transcription makes mRNA from a DNA template; translation reads mRNA codons at a ribosome.
  • tRNA brings amino acids that match the mRNA codons, helping build an amino acid chain.
  • Cells control gene expression, including by using regulatory proteins to affect transcription.
  • Sequence models support predictions, but they do not by themselves measure protein production in a real cell.

Check your understanding

Question 1

Which statement correctly distinguishes transcription from translation?
  1. Transcription makes mRNA from DNA; translation uses mRNA to assemble an amino acid chain.
  2. Transcription copies DNA; translation makes mRNA from the copy.
  3. Transcription joins amino acids; translation copies DNA.
  4. Both terms mean that DNA is copied.
Show answer and explanation
Transcription makes mRNA from DNA; translation uses mRNA to assemble an amino acid chain.
Transcription produces an RNA message from DNA information. Translation reads that message to build an amino acid chain.

Question 2

A regulatory protein makes transcription of a gene less likely. What is the best immediate prediction?
  1. Less mRNA may be produced from that gene.
  2. The DNA must be copied into a second DNA molecule.
  3. The ribosome will change the gene’s DNA sequence.
  4. The cell will immediately stop making every protein.
Show answer and explanation
Less mRNA may be produced from that gene.
Transcription produces mRNA, so reducing its likelihood may reduce mRNA production. The model does not support conclusions about every protein or an exact amount.

Question 3

A DNA template triplet is CAA. What mRNA triplet is complementary to it?
  1. GUU
  2. CAA
  3. GTT
  4. CUU
Show answer and explanation
GUU
Using DNA-to-RNA pairing, C pairs with G and A pairs with U. The resulting mRNA triplet is GUU.

Key terms

Gene
A section of DNA whose information can be used to make a particular product.
Gene expression
The use of information in a gene to produce a functional product, often a protein.
Transcription
The process of making RNA using a DNA strand as a guide.
Translation
The process in which a ribosome reads mRNA and links amino acids in the specified order.
Codon
A group of three mRNA bases that specifies an amino acid or a stop signal.
Anticodon
A three-base sequence on a tRNA that pairs with an mRNA codon.
Regulatory protein
A protein that can affect whether or how strongly a gene is expressed.
Promoter
A DNA region near a gene where the machinery for transcription can assemble.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation D3.3. 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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