From DNA to Proteins

   

Beyond Byssus

 

  A. Byssus, a fantastic underwater adhesive, is a protein, synthesized in accordance with the message encoded in the base sequences of DNA.

 

  B. DNA is like a book of instructions written in the alphabet of A, T, G, and C, but merely knowing the letters does not tell us how the genes work.

 

  C. It takes two processes–transcription and translation–plus a critical role played by RNA to synthesize proteins.

 

    1. In transcription, molecules of RNA are produced on the DNA templates in the nucleus.

 

    2. In translation, RNA are molecules shipped from the nucleus to the cytoplasm to be used in polypeptide assembly.

 

I. How Is DNA Transcribed Into RNA?

 

  A. The Three Classes of RNA

 

    1.  Messenger RNA (mRNA) carries the "blueprint" to the ribosome.

 

    2.  Ribosomal RNA (rRNA) combines with proteins to form ribosomes upon which polypeptides are assembled.

 

    3.  Transfer RNA (tRNA) brings the correct amino acid to the ribosome and pairs up with an mRNA code for that amino acid.

 

  B. The Nature of Transcription

 

    1. RNA differs from DNA in some ways:

 

      a. RNA uses ribose sugar, not deoxyribose.

 

      b. RNA bases are A, G, C, and uracil (U).

 

    2. Transcription differs from DNA replication in three ways:

 

      a. Only one region of one DNA strand is used as a template.

 

      b. RNA polymerase is used instead of DNA polymerase.

 

      c. The result of transcription is a single-stranded RNA.

 

    3. Transcription begins when RNA polymerase binds to a promoter region (a base sequence at the start of a gene) and then moves along to the end of a gene; an RNA transcript is the result.

 

  C. Finishing Touches on mRNA Transcripts

 

    1. Newly formed mRNA is modified by the addition of a cap to the 5Õ end (a "start" signal for protein synthesis) and a poly-A tail to the 3Õ end.

 

    2. Additionally, the mRNA transcript must be edited.

 

       a. The introns (noncoding portions) are removed before the transcript leaves the nucleus.

 

      b. Only the exons (portions that will eventually be translated) remain in the finished transcript that leaves the nucleus.

 

II. Deciphering the mRNA Transcripts

 

  A. What Is the Genetic Code?

 

    1. Both DNA and its mRNA transcript are linear sequences of nucleotides carrying the hereditary code.

 

    2. Every three bases (a triplet) specifies an amino acid to be included into a growing polypeptide chain; this is called the genetic code.

 

 

      a. The genetic code consists of sixty-one triplets that specify amino acids and three that serve to stop protein synthesis.

 

      b. Each base triplet in RNA is called a codon.

 

      c. With few exceptions, the genetic code is universal for all forms of life.

 

  B. Structure and Function of tRNA and rRNA

 

    1. Translation occurs on the surface of ribosomes (rRNA + proteins) composed of two subunits that unite during translation.

 

    2. Each kind of tRNA has an anticodon that is complementary to an mRNA codon and also carries one specific amino acid.

 

    3. After the mRNA arrives in the cytoplasm, an anticodon on a tRNA bonds to the codon on the mRNA, and thus a correct amino acid is brought into place.

 

III. How Is mRNA Translated?

 

  A. Stages of Translation

 

    1. In initiation, a complex forms in this sequence: initiator tRNA + small ribosomal subunit + mRNA + large ribosomal subunit.

 

    2. In elongation, a start codon on mRNA defines the reading frame; a series of tRNAs deliver amino acids in sequence by codon-anticodon matching; a peptide bond joins each amino acid to the next in sequence.

 

    3. With termination, a stop codon is reached and the polypeptide chain is released into the cytoplasm or enters the cytomembrane system for further processing.

 

  B. What Happens to the New Polypeptides?

 

    1. In cells that are rapidly making proteins, polysomes consisting of many ribosomes are translating the same mRNA transcript to make polypeptides in a hurry.

 

    2. Newly synthesized polypeptides either join the cytoplasmÕs pool of proteins or enter the cytomembrane system in preparation for export.

 

IV. Do Mutations Affect Protein Synthesis?

 

   A. A gene mutation is a change in one to several bases that may be added, deleted, or replaced in the nucleotide sequence of DNA.

 

   B. Common Gene Mutations and Their Sources

 

    1. An example of a spontaneous mutation is sickle-cell anemia, which is the result of a single "base pair substitution" which places valine as the sixth amino acid in the hemoglobin chain instead of glutamate.

 

    2. In a "frameshift mutation," there may be an insertion or deletion of several base pairs causing a misreading of the mRNA during translation.

 

    3. A rather dramatic mutation is that of transposable elements, which are regions of DNA that "jump" to new locations in DNA.

 

  C. Causes of Gene Mutations

 

    1. Many gene mutations are spontaneous.

 

     2. Others are caused by mutagens such as UV light, ionizing radiation, and alkylating agents.

 

  D. The Proof Is in the Protein

 

    1. Spontaneous mutations are rare and will not endure unless they occur in gametes.

 

    2. A protein that is specified by a heritable mutation may have h