Genes stay in DNA, but the instructions for building a protein need a portable copy. Transcription produces mRNA from a DNA template strand so coded information can move on to the next stage of polypeptide synthesis.
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A student says, "If DNA already contains the code, the cell should just send the DNA out to the ribosome whenever it needs a protein. There is no real reason to make mRNA first."
Before reading on, explain why cells use mRNA rather than moving the DNA itself. What advantage does a temporary RNA copy give the cell?
Wrong: Bacteria and viruses are the same thing.
Right: Bacteria are living cells; viruses are non-living particles that require host cells to reproduce.
The cell does not use the whole genome at once. It reads particular gene sequences when it needs a particular product.
A gene is a section of DNA containing the base sequence information needed to produce a functional product. In the HSC Biology context for this module, the important link is that genes contain the code that will later direct polypeptide synthesis.
The base sequence matters because the order of bases carries information. That information must be copied into a usable form for the next step of protein production.
During transcription, the relevant section of DNA unwinds and one strand acts as the template strand. RNA nucleotides pair with the exposed DNA bases using complementary base pairing rules, except that RNA uses uracil instead of thymine.
If the DNA template strand has adenine, the mRNA formed will contain uracil. If the template has thymine, the mRNA formed will contain adenine. Cytosine still pairs with guanine, and guanine still pairs with cytosine.
Once the RNA sequence is formed, the mRNA separates and carries the coded information away from the DNA.
In eukaryotic cells, DNA remains in the nucleus. mRNA is important because it acts as a temporary copy of the gene that can be used outside the nucleus in the next stage of polypeptide synthesis.
This protects the original DNA from having to move around the cell each time a protein is needed. It also allows the cell to make multiple RNA copies from the same gene if many copies of a protein are required.
mRNA therefore does not replace DNA. It carries the relevant information from DNA in a form that can be used by the cell.
The sequence on mRNA is read in groups of three bases called codons. At this lesson stage, the key idea is that codons hold transferable information copied from DNA. In the next lesson, you will see how those codons are used in translation.
The CFTR gene is one real example where the DNA sequence matters biologically. Before any CFTR protein can be produced, the gene must first be transcribed into mRNA.
The relevant DNA region unwinds and exposes the template strand.
RNA nucleotides pair with exposed DNA bases using A-U and C-G rules.
The mRNA sequence separates as a temporary copy of the gene.
The mRNA carries codons into the next stage of polypeptide synthesis.
Transcription copies the information in a gene from DNA into mRNA.
One DNA template strand guides complementary pairing of RNA nucleotides to form mRNA, which carries codons.
Do not say the cell moves DNA to the ribosome or that both DNA strands are copied into one mRNA.
mRNA is important in transcription because it acts as...
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
For the DNA template strand T A C C G A A T T, write the complementary mRNA sequence in codons.
Then label which sequence is DNA and which is mRNA.
Explain why a change in the DNA sequence of the CFTR gene could change the mRNA produced during transcription, even before translation happens.
1. What is transcription?
2. Why is mRNA important?
3. If a DNA template strand contains the base adenine, which base will be added to the mRNA during transcription?
4. Which statement about codons in this lesson is correct?
5. Why is the statement "the ribosome reads DNA directly during protein synthesis" inaccurate in eukaryotic cells?
6. Define transcription and outline the role of the DNA template strand. 3 marks
7. Explain why mRNA is required as a temporary copy of a gene in eukaryotic cells. 4 marks
8. Evaluate the statement: "A change in the CFTR DNA sequence can affect the cell even before translation, because transcription depends on the DNA base order." 5 marks
You should now be able to reject the idea that DNA is simply sent to ribosomes. In eukaryotic cells, transcription creates mRNA as the transferable copy of the code, preserving the original DNA in the nucleus.
DNA template strand: T A C C G A A T T
mRNA formed: A U G G C U U A A
If the DNA sequence of the CFTR gene changes, the complementary mRNA sequence produced during transcription can also change. That means the codons carried by the mRNA may differ before translation even begins.
1. A - Transcription is production of mRNA from a DNA template strand.
2. D - mRNA is a temporary copy that carries the code away from the DNA.
3. B - In RNA, adenine pairs with uracil.
4. C - Codons are three-base units on mRNA carrying coded information.
5. B - In eukaryotic cells, DNA remains in the nucleus and mRNA carries the code onward.
Q6 (3 marks): Transcription is the process of producing an mRNA copy from a DNA template strand [1]. The DNA unwinds and one strand acts as the template [1]. Complementary RNA nucleotides pair with that template to form the mRNA sequence [1].
Q7 (4 marks): mRNA is required because DNA remains in the nucleus in eukaryotic cells [1]. The cell therefore needs a temporary copy of the gene that can carry the coded information away from the DNA [1]. mRNA performs this role by holding the copied sequence in transferable form [1]. This protects the original DNA and allows the code to be used in the next stage of polypeptide synthesis [1].
Q8 (5 marks): The statement is valid because transcription depends directly on the DNA base order [1]. During transcription, RNA nucleotides pair complementarily with the DNA template strand [1]. If the CFTR DNA sequence changes, the mRNA sequence produced can also change [1]. That means the codons carried by the mRNA may differ before translation begins [1]. Therefore a DNA sequence change can affect cell function at the transcription stage by altering the copied message [1].
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