2.1 Essential ideas

2.1.7 DNA replication, transcription and translation

Genetic information is coded by DNA, which is transcribed and translated to make proteins needed by the cell.

DNA replication

  • DNA replication is semi-conservative: each new molecule of DNA formed after replication contains one strand from the original molecule and one newly formed strand.

semiconservativeFigure 2.1.7a – DNA replication is semi-conservative

  • The process depends on complementary base pairing: free nucleotides found in the nucleus of the cell form hydrogen bonds with their complements on the template strand.

Figure 2.1.7b – The role of enzymes in DNA replicationFigure 2.1.7b – The role of enzymes in DNA replication

  • Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds between complementary base pairs.
  • DNA polymerase links nucleotides together to form a new strand.

Protein synthesis: Transcription and translation

Two processes are necessary to produce a polypeptide from a gene:

  1. Transcription – the synthesis of mRNA from a DNA template, using RNA polymerase. It occurs in the nucleus.
  2. Translation – the synthesis of polypeptides from the genetic code on mRNA. It occurs on ribosomes and involves complementary base pairing with tRNA.

Figure 2.1.7c – Overview of protein synthesisFigure 2.1.7c – Overview of protein synthesis

Transcription

Figure 2.1.7d – Transcription is the synthesis of mRNA from DNA.Figure 2.1.7d – Transcription is the synthesis of mRNA from DNA.

  • RNA polymerase binds to DNA at the start of a gene. DNA is unwound and the two strands separate.
  • Free RNA nucleotides form hydrogen bonds with one strand of DNA – the template, or anti-sense, strand. The coding, or sense, strand is not transcribed.
  • RNA polymerase binds the nucleotides to form single-stranded mRNA (messenger RNA). The enzyme travels along the DNA, adding nucleotides to mRNA in a 5' to 3' direction.
  • Transcription stops at the end of the gene. RNA polymerase detaches and the DNA double helix reforms.

Translation

  • mRNA is translated three bases at a time. A triplet of bases is called a codon and each codon corresponds to one amino acid (see Figure 2.1.7f).
  • During translation, ribosomes attach to the mRNA molecule and draw in tRNA (transfer RNA) molecules.
  • Each tRNA molecule is linked to an amino acid at one end and has a triplet of bases at the other end. The triplet is called the anticodon – it is the complementary base sequence of the codon on mRNA.

Figure 2.1.7e – Translation depends on complementary base pairing between codons and anticodons.Figure 2.1.7e – Translation depends on complementary base pairing between codons and anticodons.

  • When the ‘stop’ codon is reached, the polypeptide is released from the ribosome.

Skill: Reading the genetic code

  • The amino acid sequence of polypeptides is determined by mRNA according to the genetic code.
  • You should be able to read a table to determine the sequence of amino acids coded for by a short sequence of mRNA.

Figure 2.1.7f – The genetic code: a table of codons and corresponding amino acidsFigure 2.1.7f – The genetic code: a table of codons and corresponding amino acids

  1. Deduce the codons for: tyrosine (Tyr), alanine (Ala) and the stop codon.
  2. Determine the sequence of amino acids that corresponds to the following mRNA sequence: UCUCUUAAAUGA.

Skill: Deducing base sequences

  1. Deduce the base sequence of the template strand of DNA for an mRNA molecule with the following base sequence: AAUGCGCAUUCC
  2. Deduce the base sequence of the mRNA strand for the following bit of DNA: ACGCGCCCTAAT.

Howard HelicaseFigure 2.1.7g – Howard Helicase

Key questions

  • Explain the semi-conservative nature of DNA replication.
  • Describe the role of enzymes in DNA replication.
  • Distinguish between sense and anti-sense DNA.
  • Outline transcription, including the role of RNA polymerase.
  • Distinguish between codon and anticodon.
  • Outline translation, including the roles of mRNA, tRNA and ribosomes.

Concept help

  • DNA replication is not a part of protein synthesis.
  • A gene is a region of DNA that codes for a specific polypeptide. This is the ‘one gene–one polypeptidehypothesis.
  • The coding strand is not transcribed – but complementary base pairing of the template strand results in mRNA with the correct genetic code.

2.1.7hFigure 2.1.7h
Don’t confuse the sense (coding) strand with the anti-sense (template) strand. Remember: U replaces T in RNA.

2.1.7iFigure 2.1.7i – Table of codons
Can you read this table of codons? Figure 2.1.7i shows the same information as Figure 2.1.7f.

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