DNA Transcription

Overview of DNA Transcription and Translation

  • Objective: Understand how DNA codes for proteins to produce an organism.

  • Key Processes: Transcription and Translation.

Structure of DNA

  • Chromosomes: Long molecules made up of millions of base pairs.

    • Genes: Specific portions of chromosomes that code for proteins.

    • Length of Genes in Humans: On average, between 10,000 to 50,000 base pairs.

    • Longest Gene: Approximately 2.5 million base pairs long.

Transcription Process

  • Definition of Transcription: The process by which enzymes use a strand of DNA as a template to synthesize messenger RNA (mRNA).

  • Key Enzyme: RNA polymerase.

  • Role of Transcription Factors: Proteins that assist RNA polymerase in initiating transcription.

  • Promoter: A specific sequence where RNA polymerase binds to start transcription.

  • Strands of DNA:

    • Template Strand (Antisense Strand): The strand that serves as a template for mRNA synthesis.

    • Nontemplate Strand (Sense Strand): The strand not used during the synthesis process.

  • Initiation of mRNA Synthesis: RNA polymerase does not require a primer; it initiates synthesis at the start codon.

  • Elongation Process:

    • RNA polymerase moves along the gene, synthesizing mRNA by reading the antisense strand from 3' to 5'.

    • Generates mRNA from the 5' end, adding RNA nucleotides to the 3' end during synthesis.

    • Chemical Differences: RNA contains ribose instead of deoxyribose, and uracil (U) replaces thymine (T) in RNA.

  • Zipping Back of DNA: RNA polymerase zips DNA back up, exposing only 10 to 20 bases at a time.

  • Termination: RNA polymerase detaches upon reaching the end of the gene, and DNA returns to its original state.

    • Outcome: Production of mRNA carrying encoded genetic information.

RNA Processing

  • Modification Steps: After transcription, mRNA undergoes a few quick modifications before exiting the nucleus, including:

    • Capping

    • Polyadenylation

    • Splicing

  • Exit to Cytoplasm: The modified mRNA leaves the nucleus to enter the cytoplasm where translation occurs.

Translation Process

  • Definition of Translation: The process by which mRNA is decoded to synthesize proteins.

  • Codons and Anticodons:

    • Codons are sets of three bases on mRNA that specify amino acids.

    • Anticodons are complementary sequences on tRNA that match the codons on mRNA.

  • tRNA: Transfer RNA that carries specific amino acids covalently linked to it.

  • Reading Frame: The arrangement of nucleotides in codons.

    • Calculation of Codons: Since there are four bases and each codon has three letters, the total possible codons is calculated as 43=644^3 = 64.

  • Codon Redundancy: Multiple codons can code for the same amino acid; however, there is no ambiguity (each codon corresponds to one amino acid).

  • Special Codons: There are specific codons that serve special functions, such as the start codon (AUG), which signals the beginning of translation, and stop codons (UAA, UAG, UGA), which terminate protein synthesis.

    • Start Codon: AUG, codes for methionine, initiates translation.

    • Stop Codons: Three specific codons that terminate translation.

Ribosome Structure and Function

  • Ribosome: The cellular machinery where translation occurs.

    • Initiation Complex Formation:

    • The small ribosomal subunit binds to mRNA and initiator tRNA that corresponds to the start codon.

    • The large ribosomal subunit joins to complete the initiation complex.

  • Amino Acid Binding: As tRNA corresponding to subsequent codons enter the ribosome:

    • Amino acids become covalently bound to the growing polypeptide chain.

    • The first tRNA detaches, leaving the bonded polypeptide chain inside the ribosome as it shifts for the next tRNA.

  • Polypeptide Chain Formation: The polypeptide chain grows as tRNAs enter and exit ribosome.

  • Termination: Occurs upon encountering a stop codon, causing the completed polypeptide to exit the ribosome.

    • Folds and undergoes modifications in cell organelles.

Conclusion

  • Summary of Processes: In the two-step process, DNA is transcribed into mRNA, and mRNA is translated into a protein.

  • Role of Proteins: Proteins generated are crucial constituents of living organisms, forming muscles, organs, receptors, and enzymes.