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Transcription

Transcription. Transcription. The process of making an mRNA strand from the DNA gene sequence 3 steps: initiation, elongation, termination. INITIATION. The enzyme RNA polymerase attaches to the DNA double helix at the promoter region.

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  1. Transcription

  2. Transcription The process of making an mRNA strand from the DNA gene sequence 3 steps: initiation, elongation, termination

  3. INITIATION The enzyme RNA polymerase attaches to the DNA double helix at the promoter region. PROMOTER REGION: A sequence of DNA “upstream” from the gene that contains a lot of A-T base pairs. (easier to break apart than G-C)

  4. RNA polymerase unwinds and separates the helix exposing the template strand. Since RNA polymerase only binds at the promoter region, it allows only the gene to be transcribed. If the RNA polymerase randomly transcribed DNA, the cell would not make the correct polypeptide

  5. Rich in A-T Coding strand

  6. ELONGATION Once the helix is open, RNA polymerase starts building a single stranded mRNA in the 5’-3’ direction using RNA nucleotides and complimentary base pairing rules. Unlike DNA replication, the RNA polymerase does not require a primer to begin building the complementary strand

  7. Only one strand of DNA is used as a template for mRNA synthesis. This is the template strand. The other DNA strand that is not used for transcription is the coding strand This coding strand is identical to the mRNA strand except that it has thymine instead of uracil. Note: the promoter region does not get transcribed. *After the DNA is transcribed, it is rewound into its helix

  8. Termination The mRNA strand is synthesized until RNA polymerase reaches the terminator sequence at the end of the gene. The mRNA strand will detach from the DNA template. This mRNA strand is called the primary transcript RNA polymerase is now free to bind to another promoter region and transcribe another gene

  9. Post-transcription After transcription, the mRNA primary transcript must go to the cytoplasm to be translated. In eukaryotes, before it leaves the nucleus it needs a “cap” and a “tail” to protect it and prevent it from being broken down.

  10. 5’ CAP A 5’ cap is added to mRNA. This is a modified guanine nucleoside triphosphate. The cap protects the mRNA from digestion from enzymes when it enters the cytoplasm

  11. Poly-A tail A sequence of ~ 200 adenine ribonucleosides is added to the 3’ end to protect the mRNA from breaking down This is called a poly-A tail The tail is added with the help of the enzyme poly-A polymerase

  12. Exons and Introns In eukaryotic DNA, a gene consists of introns and exons EXONS: coding regions – they code for the specific protein INTRONS: noncoding - “filler DNA” that does not code for proteins.

  13. Before mRNA goes into the cytoplasm, the introns must be removed from the mRNA Proteins called spliceosomes cut the introns out and join the remaining exons together.

  14. mRNA transcript Once the cap and tail have been added, and the introns removed, the mRNA is called an mRNA transcript It can now go to the cytoplasm for translation

  15. Unlike in DNA replication, there is no quality control enzyme for the mRNA transcript Errors in mRNA, however, are not too detrimental since multiple mRNA transcripts are made for each gene. Therefore, sufficient amounts of protein will be produced by the correct copies.

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