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4.2Genetics, Variation and Interdependence

DNA and Protein Synthesis — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 12 parts Lesson 11 of 46 Unit quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Lesson summary

The Genome, RNA and Transcription — AQA A LEVEL BIOLOGY 7402 notes: Describe the structure of RNA, how transcription produces mRNA and how pre-mRNA is spliced.A genome is the complete set of genes in a cell.

  • translation
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • RNA polymerase
  • mRNA is a transcript copy of a gene that encodes a specific polypeptide

Part 1 of 5Genome and proteome

A genome is the complete set of genes in a cell. The full genome is present in every cell of an organism, but not every gene is expressed in every cell; which genes are expressed depends on the cell type. The proteome is the full range of proteins a cell can produce.

Part 2 of 5Structure of RNA

RNA (ribonucleic acid) is a polynucleotide like DNA, made of many nucleotides in a chain. Both contain adenine, guanine and cytosine, but RNA nucleotides contain uracil (U) instead of thymine, and ribose instead of deoxyribose. RNA molecules are single-stranded: a sugar-phosphate backbone of alternating ribose and phosphate with bases projecting sideways.

Structure of RNA
mRNA
mRNA is a transcript copy of a gene that encodes a specific polypeptide.
tRNA
tRNA has a folded shape;
  • although it looks double-stranded it is single-stranded, with hydrogen bonds between complementary bases in certain regions holding it together.
  • It has an anticodon complementary to a specific triplet (codon) on mRNA and carries the corresponding amino acid for protein synthesis.

Part 3 of 5Transcription

Protein synthesis occurs in two stages: transcription, in which DNA is transcribed and an mRNA molecule is produced, and translation, in which mRNA is translated into an amino acid sequence.

1
DNA helicase (or RNA polymerase) unwinds and separates the two DNA strands at the gene by breaking hydrogen bonds.
2
The template strand is the one transcribed. RNA polymerase moves along it in the 3' to 5' direction, so mRNA grows 5' to 3'.
3
Free RNA nucleotides pair with complementary exposed bases on the template strand (A-U, T-A, C-G, G-C) and RNA polymerase joins them by phosphodiester bonds.
4
The mRNA detaches and the DNA re-forms the double helix.

Be carefulDNA polymerase is involved in DNA replication, whereas RNA polymerase is involved in transcription.

Transcription
The mRNA codons have the same base sequence as the non-transcribed strand (with U replacing T),
  • while tRNA anticodons have the same sequence as the transcribed strand (U for T).
You do not need to know the directions that DNA strands are read, but they help explain how strands grow.

Part 4 of 5Transcription in eukaryotes and prokaryotes

Eukaryotic genomes contain many non-coding sections.

Pre-mRNA
During transcription the whole gene (introns and exons) is transcribed to produce pre-mRNA.
Spliced
Before it leaves the nucleus, the pre-mRNA is spliced: introns are removed and exons joined to form mature mRNA.
Exons can be spliced in different ways (alternative splicing), producing different mature mRNA molecules and so different proteins from the same gene.
In prokaryotes, genes have no introns, so transcription produces mRNA directly with no splicing.

Part 5 of 5Worked examples

  1. Describe how mRNA is produced from DNA. [4]4 marks

  2. A template strand reads ATG CCT. Give the mRNA sequence. [1]1 mark

  3. Explain how alternative splicing increases the number of proteins. [2]2 marks

Key terms

TermMeaning
GenomeComplete set of genes in a cell
ProteomeFull range of proteins a cell can make
mRNAMessenger RNA; copy of a gene
tRNATransfer RNA carrying amino acids with an anticodon
RNA polymeraseEnzyme that makes mRNA
Pre-mRNAPrimary transcript containing introns and exons
SplicingRemoval of introns and joining of exons

Avoid these mistakes

1
Mixing up RNA polymerase with DNA polymerase.
2
Writing thymine in RNA.
3
Thinking tRNA is double-stranded.
4
Thinking prokaryotes splice their mRNA.

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