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TRANSCRIPTION

  • DNA stays in the nucleus at all times.
  • But protein is manufactured in the cytoplasm!
  • If DNA is the instructions for how to make protein, how do we reconcile this?
  • Simple! Rewrite (literally "TRANSCRIBE") the DNA code into a more portable form: RNA!
  • RNA can leave the nucleus and carry the DNA message to the cytoplasm, where protein is made.
  • That's what is meant by TRANSCRIPTION: REWRITING THE DNA CODE IN A DIFFERENT NUCLEIC ACID LANGUAGE, THAT OF ITS COUSIN, RNA!

    A few important notes about TRANSCRIPTION

    • This manufacture of DNA into RNA takes place only in the nucleus
    • the RNA made from the DNA template is called messenger RNA (mRNA)
    • It's job is to bring the DNA message (how to make protein) out of the nucleus and into the cytoplasm, where the protein-manufacturing "machinery" (ribosomes) is located.
    • Unlike DNA, RNA consists of only ONE STRAND of sugar-phosphate backbone linked to the nitrogenous bases.
    • Unlike DNA, RNA does not form a double helix.
    • At any given gene locus, when the DNA "unzips" at the hydrogen bonds, ONLY ONE OF THE DNA STRANDS IN THE DOUBLE HELIX IS READ AND USED AS THE TEMPLATE TO MANUFACTURE mRNA.

    The PROCESS OF TRANSCRIPTION

    • Before transcription can begin, the portion of the DNA double helix to be transcribed must be "UNZIPPED" by separating the two strands at their hydrogen bonds.
    • This is done by an enzyme named HELICASE.
    • Where helicase has separated the two DNA "backbones" there are now rows of nitrogenous bases just waiting to be "read" and "rewritten" into another language: mRNA.
    • mRNA is transcribed from only one of the two available ("unzipped") DNA strands.
    • the enzyme that attaches to the unzipped DNA and creates a complementary strand of mRNA is called RNA POLYMERASE.

    Let's watch a movie and see this process in action.

  • Interesting Note: organelle DNA is transcribed and translated within the organelle, yet another reminder of the probable prokaryotic origins of these little energy transducers: ancient bacteria!

    POST-TRANSCRIPTIONAL MODIFICATION OF mRNA in EUKARYOTES

    mRNA is not finished when it comes off the DNA template. Three major things are done to the new mRNA strand before it's ready to be used to make protein:

    • 1. a string of adenines (the "poly-A tail") is added to the 3' end of the transcript by the enzyme poly A polymerase.

      (function: stability; may be involved in transfer of the RNA to the cytoplasm)

    • 2. 7-methyl guanosine "cap" added to the 5' end of the transcript. (functions: prevents nucleases from destroying the transcript; may be involved in ribosome recognition and transfer of the transcript to the cytoplasm

    • 3. INTRONS (intervening sequences in mid-gene which are transcribed, but not translated) are removed from the primary transcript, leaving EXONS to be spliced together to become the finished (informative) mRNA.

      (Note: P. Sharp et al. published this work in 1977; in 1993, it earned Sharp the Nobel prize.)

      The final product looks something like this.

      What is the evolutionary significance of introns/exons:

      NOTE: A characteristic of any organism is said to be

    • PRIMITIVE - if it is relatively unchanged from that found in an ancient ancestor or
    • DERIVED - if it is quite different from that found in an ancient ancestor.

      So what about introns? Are they....

    • primitive or derived? The experts are still out on this one.

    • Several hypotheses as to function of introns

      a. separate the exons into the functinal subunits of the product for which they code. (e.g. active site, membrane binding site etc. each coded by one of the exons in a transcript)

      b. They allow exon shuffling, which would account for the tremendous variability in proteins (and their nearly instantaneous production) in eukaryotes.

      c. 1990 - Walter Gilbert published a paper in Science that suggested that a mere 1000-7000 exons could be shuffled and combined in various way to account for the millions (billions?) of proteins known in eukaryotes.

      d. If RNA was the original genetic material (and there is lots of evidence to suggest it is more primitive than DNA), then introns may have pinched out to function as the first enzymes.

      (However, this doesn't explain why most prokaryotes lack introns!)

      Introns as a derived character...

      a. the intron phenomenon may have given early eukaryotes a competitive edge, allowing them to produce a great variety of proteins in a rapidly changing environment.

      b. ...or are introns "parasitic" DNA which move about freely and are transcribed/translated without concern for the host!

      (There is a cellular cost to transcribe/translate the introns, but the host organism is certainly not lethally affected by its own introns. Parasite or harmless companion? We might never know.)