AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Transcription as the production of mRNA from DNA. The role of RNA…
Full specification wording
DNA and protein synthesis: Transcription as the production of mRNA from DNA. The role of RNA polymerase in joining mRNA nucleotides.
Transcription produces mRNA from DNA.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 3 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Transcription produces mRNA from DNA. The DNA double helix unwinds and hydrogen bonds between complementary bases break. Only the template strand is copied. Free RNA nucleotides align opposite exposed bases: RNA uracil pairs with DNA adenine, adenine with thymine, cytosine with guanine, and guanine with cytosine. RNA polymerase catalyses phosphodiester bond formation between adjacent RNA nucleotides, building a single-stranded RNA molecule. In eukaryotes, this occurs in the nucleoplasm producing pre-mRNA, which is spliced. In prokaryotes, it occurs in the cytoplasm directly producing mRNA. The enzyme separates the strands as it moves, and DNA strands rejoin behind it. At a stop signal, RNA polymerase detaches and the RNA is released.
What earns the marks
- Hydrogen bonds between complementary DNA bases break, separating the strands and exposing the template.
- Only one DNA strand, the template strand, is transcribed; the other is the coding strand.
- Free RNA nucleotides align opposite exposed complementary bases: RNA uracil pairs with DNA adenine, RNA adenine with DNA thymine, RNA cytosine with DNA guanine, and RNA guanine with DNA cytosine.
- RNA polymerase catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides, joining them into a single-stranded RNA molecule.
- The RNA molecule is released and the DNA strands rejoin behind the enzyme; in eukaryotes the product is pre-mRNA, which is spliced.
Common mistakes and how to put them right
Naming DNA polymerase as the enzyme that joins RNA nucleotides.
Put it right: It is RNA polymerase that catalyses phosphodiester bond formation between RNA nucleotides.
Stating that both DNA strands are transcribed.
Put it right: Only the template strand is transcribed, and the other is the coding strand.
Putting thymine into the mRNA sequence.
Put it right: RNA contains uracil instead of thymine, so RNA uracil pairs with DNA adenine on the template.
Assuming transcription always occurs in the nucleus.
Put it right: Eukaryotic transcription occurs in the nucleoplasm, but prokaryotic transcription occurs in the cytoplasm.
Exam tips
- Name the specific bonds: hydrogen bonds between complementary bases, and phosphodiester bonds between adjacent nucleotides.
- Remember the difference in products: in eukaryotes the immediate product is pre-mRNA which requires splicing, whereas prokaryotes produce mRNA directly.
Try the lesson
The start of the DNA and protein synthesis: Transcription as the production of mRNA from DNA. The role of RNA polymerase in joining mRNA nucleotides. lesson, made from checked teaching notes for this specification point.
Have a go first
Which enzyme is responsible for joining mRNA nucleotides during transcription?
Listen
Transcription produces mRNA from DNA. The DNA double helix unwinds and hydrogen bonds between complementary bases break. Only the template strand is copied. Free RNA nucleotides align opposite exposed bases: RNA uracil pairs with DNA adenine, adenine with thymine, cytosine with guanine, and guanine with cytosine. RNA polymerase catalyses phosphodiester bond formation between adjacent RNA nucleotides, building a single-stranded RNA molecule. In eukaryotes, this occurs in the nucleoplasm producing pre-mRNA, which is spliced. In prokaryotes, it occurs in the cytoplasm directly producing mRNA. The enzyme separates the strands as it moves, and DNA strands rejoin behind it. At a stop signal, RNA polymerase detaches and the RNA is released. A strong answer earns marks for these points. Hydrogen bonds between complementary DNA bases break, separating the strands and exposing the template. Only one DNA strand, the template strand, is transcribed; the other is the coding strand. Free RNA nucleotides align opposite exposed complementary bases: RNA uracil pairs with DNA adenine, RNA adenine with DNA thymine, RNA cytosine with DNA guanine, and RNA guanine with DNA cytosine. RNA polymerase catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides, joining them into a single-stranded RNA molecule. A common slip: Naming DNA polymerase as the enzyme that joins RNA nucleotides. Instead, it is RNA polymerase that catalyses phosphodiester bond formation between RNA nucleotides. Exam tip: Name the specific bonds: hydrogen bonds between complementary bases, and phosphodiester bonds between adjacent nucleotides.
Key words
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template strand
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RNA polymerase
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complementary base pairing
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phosphodiester bonds
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See every definition
- template strand
- The single DNA strand that is copied during transcription, while the other strand is the coding strand.
- RNA polymerase
- The enzyme that catalyses phosphodiester bond formation between adjacent RNA nucleotides, joining them into a single-stranded RNA molecule.
- complementary base pairing
- Free RNA nucleotides align opposite exposed bases, with RNA uracil pairing with DNA adenine, adenine with thymine, cytosine with guanine, and guanine with cytosine.
- phosphodiester bonds
- Bonds formed between adjacent RNA nucleotides, catalysed by RNA polymerase, joining them into a single-stranded RNA molecule.
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