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 is semi-conservative: each new molecule of DNA formed after replication contains one strand from the original molecule and one newly formed strand.
- 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.
- 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.
- Transcription – the synthesis of mRNA from a DNA template, using RNA polymerase. It occurs in the nucleus.
- Translation – the synthesis of polypeptides from the genetic code on mRNA. It occurs on ribosomes and involves complementary base pairing with tRNA.
- 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.
- 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.
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.
- Deduce the codons for: tyrosine (Tyr), alanine (Ala) and the stop codon.
- Determine the sequence of amino acids that corresponds to the following mRNA sequence: UCUCUUAAAUGA.
- 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.
- 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 polypeptide’ hypothesis.
- The coding strand is not transcribed – but complementary base pairing of the template strand results in mRNA with the correct genetic code.
Don’t confuse the sense (coding) strand with the anti-sense (template) strand. Remember: U replaces T in RNA.