Ch 17 - Transcription and mRNA Processing

Overview of Transcription

  • Explanation of the concept of transcription in relation to genetic information.

    • Transcription means to rewrite or convert information into another form.

    • It specifically refers to converting genetic information from DNA into messenger RNA (mRNA).

  • Focus on the transcription of genes that code for proteins.

    • The process involves a two-step mechanism: from DNA to mRNA and subsequently from mRNA to protein (translation).

Transcription in Prokaryotic vs. Eukaryotic Cells

  • Prokaryotic Cells:

    • DNA is located in the cytosol and is readily available for transcription.

    • Steps involved in transcription include:

    • The DNA strand serves as a template to synthesize complementary mRNA.

    • mRNA is directly involved with ribosomes during translation to produce proteins.

  • Eukaryotic Cells:

    • Transcription occurs within the nucleus.

    • The process is more complex and includes the formation of pre-mRNA, which undergoes processing to become mature mRNA before exiting the nucleus.

    • Sequence of events:

    • DNA to pre-mRNA processing to mRNA, which then migrates out of the nucleus for translation.

Processing of Pre-mRNA in Eukaryotic Cells

  • Steps involved in the modification of pre-mRNA:

    1. Transcription Initiation

    • RNA polymerase attaches to a specific region of DNA called the promoter.

    • Promoter acts as a signal for RNA polymerase, indicating where to start transcription.

    1. Transcription Factors

    • RNA polymerase separates the two strands of DNA during transcription.

    • It synthesizes the mRNA strand by coding in the 5' to 3' direction, adding nucleotides at the 3' end of the growing mRNA strand.

    1. Strand Interaction

    • RNA polymerase interacts with and uses the template strand of DNA to form an RNA strand that is complementary to this template strand.

    • The resulting RNA contains the same sequence as the coding strand of DNA, except that uracil (U) replaces thymine (T).

Completion of RNA Transcription

  • Termination of Transcription:

    • The process continues until a terminator sequence is reached.

    • Various terminator mechanisms include:

    • Formation of a hairpin loop in the newly synthesized mRNA, which causes RNA polymerase to dissociate.

    • Other sequences may signal conformational changes leading the polymerase to release the mRNA.

  • In prokaryotic cells, this mRNA is immediately ready for translation.

  • In eukaryotic cells, the mRNA undergoes further modifications before translation.

Processing Steps for Mature mRNA

  • 5' Cap Addition:

    • A modified guanine nucleotide (the 5' cap) is added to the beginning of the mRNA.

    • This cap is crucial for the initiation of translation.

  • Poly-A Tail Addition:

    • A series of adenine nucleotides (the poly-A tail) is added to the 3' end of the mRNA.

    • The poly-A tail aids in the stability of the mRNA molecule and protects it from degradation.

  • Splicing:

    • Introns (non-coding sequences) are removed during the splicing process.

    • Only exons (coding sequences) remain in the mature mRNA.

    • This process is essential as it ensures that only the coding sequences are translated into proteins.

Summary of Final Product

  • Post-processing, the final product is mature mRNA, which consists solely of exons and is adequately modified for translation.

  • Mature mRNA exits the nucleus and enters the cytoplasm where it can be translated into a protein by ribosomes.

Conclusion

  • Recap of the transcription process and its significance in gene expression and protein synthesis.