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Biology · DNA and Protein Synthesis

Protein Synthesis (Higher tier)

A protein is made in two stages: transcription in the nucleus and translation at a ribosome. This lesson is for separate Biology only, and for the Higher tier only: Combined Science and Foundation students do not need it. It explains the role of RNA polymerase, mRNA, codons, tRNA and ribosomes.

  • Separate onlyHigher
  • 10 key terms
  • All boards

Learning Objectives

  1. 1(Higher tier) Describe transcription, including the role of RNA polymerase and the production of mRNA.
  2. 2(Higher tier) Describe translation, including the roles of the ribosome, codons and tRNA.
  3. 3(Higher tier) Use the base pairing rules to work out a messenger RNA sequence and the amino acids it codes for.
  4. 4(Higher tier) Compare the structures of DNA and RNA.

Retrieval practice

  1. 1

    What is a triplet?

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    A group of three bases that codes for one amino acid.

  2. 2

    Which base pairs with A in DNA?

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    T.

  3. 3

    What is a ribosome?

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    A structure in the cytoplasm where proteins are made.

  4. 4

    Where is DNA found in a eukaryotic cell?

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    In the nucleus.

Why DNA needs a messenger

The DNA of a cell never leaves the nucleus, but proteins are made on ribosomes in the cytoplasm. A gene cannot go to the ribosome, so a copy of the gene is made instead. This copy is a different molecule called messenger RNA (mRNA). The mRNA carries the code to the ribosome, where the protein is assembled. Making the mRNA copy is called transcription, and using it to build the protein is called translation.

DNA and RNA

DNA

  • Two strands in a double helix
  • Contains the sugar deoxyribose
  • Bases A, T, C and G
  • Very long, stays in the nucleus

RNA

  • A single strand
  • Contains the sugar ribose
  • Bases A, U, C and G, with uracil (U) instead of thymine
  • Short, and leaves the nucleus

Transcription in the nucleus

Transcription makes an mRNA copy of one gene.

  1. 1 RNA polymerase binds

    The enzyme RNA polymerase attaches to the non-coding DNA that lies in front of the gene.
  2. 2 The DNA unwinds

    The hydrogen bonds between the bases break and the two strands separate in that region.
  3. 3 mRNA is built

    RNA polymerase moves along the coding DNA of the gene and joins RNA nucleotides, which pair with the exposed bases. A pairs with U, T with A, C with G and G with C.
  4. 4 mRNA leaves

    The complementary mRNA strand is released, the DNA closes up, and the mRNA travels out of the nucleus through a pore to the cytoplasm.

Translation at the ribosome

Translation turns the code in the mRNA into a polypeptide.

  1. 1 mRNA attaches

    The mRNA binds to a ribosome in the cytoplasm.
  2. 2 Codons are read

    The ribosome reads the mRNA in groups of three bases called codons, and each codon codes for one amino acid.
  3. 3 tRNA brings amino acids

    Transfer RNA (tRNA) molecules carry specific amino acids to the ribosome. The tRNA has three bases, an anticodon, that pair with the codon.
  4. 4 Amino acids are joined

    The ribosome links the amino acids in the order of the codons, to form a polypeptide.
  5. 5 The chain is released

    At a stop codon the polypeptide is released and folds into a protein.

The key molecules

Four different molecules do the work, and each has a clear job.

  • RNA polymerase

    An enzyme that makes mRNA by joining RNA nucleotides according to the order of bases in the DNA.

  • mRNA

    A single-stranded copy of a gene, which carries the code from the nucleus to the ribosome.

  • Ribosome

    The structure that reads the mRNA and joins amino acids together to make a polypeptide.

  • tRNA

    A molecule that carries one particular amino acid to the ribosome, and has an anticodon that matches a codon.

From DNA to amino acids

The coding strand of part of a gene has the base sequence T A C G G A C C T. Write the mRNA sequence, and use the codon table to give the amino acids. Codon table: AUG = methionine, CCU = proline, GGA = glycine, UUU = phenylalanine.

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  1. 1 Pair each base with its RNA partner T pairs with A, A with U, C with G and G with C. The sequence is T A C, G G A, C C T.
  2. 2 Write the mRNA T A C becomes A U G, G G A becomes C C U, and C C T becomes G G A, so the mRNA is A U G C C U G G A.
  3. 3 Split it into codons AUG, CCU, GGA.
  4. 4 Use the table AUG is methionine, CCU is proline and GGA is glycine.

AnswerThe mRNA is A U G C C U G G A, and the amino acids are methionine, proline and glycine.

Making a protein in a cell

Protein synthesis is happening in all of your cells, all of the time.

  • Fast

    Many ribosomes can read one mRNA at the same time, so lots of copies of a protein are made quickly.

  • Controlled

    A gene is only transcribed when its protein is needed, so cells make the proteins they need and no others.

  • Energy needed

    It takes a lot of energy, supplied by respiration in mitochondria.

  • Different cells, different proteins

    All body cells have the same genes, but different genes are transcribed in different cells, which is why a muscle cell differs from a nerve cell.

Case study

Insulin made by bacteria

The human gene for insulin was put into bacteria in the 1970s. The bacteria transcribe the gene into mRNA and translate it at their ribosomes, using the same code as in humans, and so they make human insulin. Before this, insulin had to be extracted from the pancreas of pigs and cattle. The first genetically engineered insulin became available for patients in 1982.

1982 The first genetically engineered human insulin was approved.
Same process Bacterial cells use transcription and translation as humans do.

Summary and exam focus

  • Transcription: RNA polymerase binds in front of the gene and makes a complementary mRNA strand from the coding DNA.
  • The mRNA leaves the nucleus and attaches to a ribosome.
  • Translation: the ribosome reads the mRNA in codons, and tRNA brings the matching amino acids.
  • Amino acids are linked to form a polypeptide, which folds into a protein.
  • RNA has uracil instead of thymine, and is a single strand.

Exam focus

Describe the stages of protein synthesis, including transcription and translation. (6 marks) (6 marks)

Keep transcription and translation separate and use their key words. In transcription, include RNA polymerase, the non-coding DNA before the gene, and complementary mRNA. In translation, include ribosome, codons, tRNA and amino acids being linked.

Key terms

The words this lesson expects you to use. Each one is linked from the first place it appears above.

Transcription
Making a complementary mRNA copy of a gene from DNA.
Translation
Reading the code in mRNA at a ribosome to build a polypeptide.
RNA polymerase
The enzyme that makes mRNA by joining RNA nucleotides complementary to the DNA.
mRNA
Messenger RNA: a single-stranded copy of a gene that carries the code to a ribosome.
tRNA
Transfer RNA: a molecule that carries a particular amino acid to the ribosome.
Codon
A group of three bases in mRNA that codes for one amino acid.
Anticodon
The three bases on a tRNA molecule that pair with a codon.
Ribosome
The structure in the cytoplasm that reads mRNA and builds the polypeptide.
Uracil
The base in RNA that takes the place of thymine, and pairs with adenine.
Non-coding DNA
DNA that does not code for a protein, including the region in front of a gene where RNA polymerase binds.

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Flashcards 16 cards to flip through Exam questions 6 questions, 20 marks with answers
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