1.3.3

Transcription

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Transcription

Transcription is the first step in protein synthesis. As a result of this process, mRNA that is a copy of the target gene is produced in the nucleus. The stages involved are:

1) Binding of RNA polymerase

1) Binding of RNA polymerase

  • RNA polymerase is the enzyme that allows transcription to take place.
  • RNA polymerase binds to the locus of the gene to be transcribed (the target gene).
    • The region of DNA to which RNA polymerase binds is known as the promoter.
  • In most cases, promoters exist upstream of the genes they regulate.
  • The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all the time, some of the time, or infrequently.
2) Separation of DNA strands

2) Separation of DNA strands

  • When RNA polymerase binds to DNA, the hydrogen bonds that bind the two strands together break. This is different to DNA replication, where DNA helicase separates the strands.
  • The DNA strands separate.
  • The bases of the target gene are exposed.
    • The strand that is copied depends on the gene being transcribed.
3) Binding to template strand

3) Binding to template strand

  • RNA polymerase binds free-floating RNA nucleotides to the template strand.
    • The template strand is complementary to the base sequence of the target gene. It is sometimes referred to as the noncoding, minus, or antisense strand.
    • Nucleotides are added in the 5'-3' direction.
  • The RNA nucleotides form a strand of mRNA that is complementary to the template strand.
    • The template strand is complementary to the gene so this means mRNA is a copy of the gene.
4) Joining the nucleotides

4) Joining the nucleotides

  • The free-floating nucleotides are joined together by RNA polymerase.
  • Phosphodiester bonds form between the nucleotides in a condensation reaction to form the completed strand of mRNA.
  • As elongation proceeds and the mRNA strand grows, the DNA is continuously unwound ahead of the core enzyme and rewound behind.
5) Termination

5) Termination

  • RNA polymerase eventually reaches a stop codon - a triplet of bases that signal 'stop'.
  • The RNA polymerase stops producing mRNA.
  • Another mechanism of stoping transcription is controlled by specific sequences in the DNA template strand.
  • As the polymerase nears the end of the gene being transcribed, it encounters a region rich in C–G nucleotides.
  • The mRNA folds back on itself, and the complementary C–G nucleotides bind together.
    • The result is a hairpin that causes polymerase to stall.
6) Removal of the mRNA

6) Removal of the mRNA

  • The mRNA strand is separated from the template strand by RNA polymerase.
  • The hydrogen bonds between the two strands of DNA form again and the strands join together.
7) mRNA leaves the nucleus

7) mRNA leaves the nucleus

  • In eukaryotes, the completed mRNA strand leaves the nucleus and enters the cytoplasm.
  • mRNA is used in translation, the next step in protein synthesis.
    • Remember, prokaryotes don't have a nucleus. This means transcription occurs in the cytoplasm.
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Practice questions on Transcription

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    Steps Involved in TranscriptionPut in order
  2. 2
    Which of the following statements about transcription are true?True / false
  3. 3
    What enzyme is used to bind RNA nucleotides to the template strand?Multiple choice
  4. 4
    Which of the following is NOT another name for the strand of DNA to which nucleotides are added to in transcription? Multiple choice
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Types of RNA

RNA Processing in Eukaryotes