IB Guides

Transcription

7.3.1 State that transcription is carried out in a 5? → 3? direction.

Transcription is carried out in a 5'→3' direction.

7.3.2 Distinguish between the sense and antisense strands of DNA.

The antisense strand is the template DNA strand which is transcribed. The sense strand on the other hand is the DNA strand which has the same base sequence as the mRNA with thymine instead or uracil.

7.3.3 Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleoside triphosphates and the terminator.

mRNA is produced during transcription. In prokaryotes, RNA polymerase recognises a specific sequence of DNA called the promoter. The promoter basically "tells" the RNA polymerase where to start the transcription process. Transcription is initiated with the binding of RNA polymerase to the promoter site. The RNA polymerase then uncoils the DNA and separates the two strands. One of the strands is used as the template strand for transcription. The RNA polymerase will then use free nucleoside triphosphates to build the mRNA in a 5'→3' direction. These nucleoside triphosphates bond to their complementary base pairs on the template strand. As they bind they become nucleotides by losing two phosphate groups to release energy. Since RNA does not contain thymine, uracil pairs up with adenine instead. RNA polymerase forms covalent bonds between these nucleotides. It moves along the DNA to keep elongating the sequence of mRNA until it reaches a sequence of DNA called the terminator. This sequence of DNA "tells" the RNA polymerase to stop transcription. The RNA polymerase is then released from the DNA and the newly created mRNA separates from the template DNA strand. Finally, the DNA rewinds back to its original double helical structure.

Summary:

  1. RNA polymerase binds to the promoter region 

  2. This initiates transcription

  3. RNA polymerase uncoils the DNA

  4. Only one strand is used, the template strand

  5. Free nucleoside triphosphates bond to their complementary bases on the template strand 

  6. Adenine binds to uracil instead of thymine

  7. As the nucleoside triphosphates bind they become nucleotides and release energy by losing two phosphate groups

  8. The mRNA is built in a 5'→3' direction

  9. RNA polymerase forms covalent bonds between the nucleotides and keeps moving along the DNA until it reaches the terminator

  10. The terminator signals the RNA polymerase to stop transcription

  11. RNA polymerase is released and mRNA separates from the DNA

  12. The DNA rewinds

7.3.4 State that eukaryotic RNA needs the removal of introns to form mature mRNA.

Eukaryotic RNA needs the removal of introns to form mature mRNA.

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