Bio1151 Chapter 17 From Gene to Protein
  1. A major function of DNA is to direct the synthesis of           in two steps:                and              ; this flow of information is known as the central        .

    Central dogma of genetics.

    Sequences of DNA called genes serve as templates for transcription of messenger RNA (mRNA), following the base-pairing rules - remember A pairs with U in RNA.

    During translation, sequences of base triplets (codons) in the mRNA specify amino acids that make up a protein (polypeptide) chain.

    The process of transcription and translation results in expression of the gene.

  2. In prokaryotes, transcription and translation take place                 .

    A prokaryotic cell lacks a nucleus, and mRNA produced by transcription is immediately translated to a polypeptide without additional processing.
  3. In eukaryotes, an intermediate step of RNA             is needed due to the presence of the          .

    In eukaryotes, the nucleus provides a separate compartment for transcription.

    The initial RNA transcript (pre-mRNA) is processed before leaving the nucleus as mRNA.

    The mRNA is used for translation on ribosomes, producing a polypeptide.

    Overview and exercise:

  4. Transcription has three stages:             ,             , and              .


  5. Initiation occurs at the promoter sequence on the DNA.
  6. Elongation is performed by RNA polymerase.
  7. Termination occurs when the RNA transcript is released, and the RNA polymerase detaches from the DNA.

      Transcription: initiation. RNA polymerase (RNA polymerase II in eukaryotes) binds to promoter sequences on the DNA.

      The promoter includes a TATA box, a nucleotide sequence containing many T-A base pairs. Several transcription factors form an initiation complex with RNA polymerase.

      Transcription: elongation.

      The RNA polymerase unwinds the double helix, adding nucleotides to the 3' end of the growing RNA transcript.

      The same base-pairing rules as DNA replication is used, except that Uracil (U) substitutes for Thymine (T).

      Transcription: termination.

      Transcription terminates when a polyadenylation signal is transcribed.

      The RNA transcript is released, and the polymerase falls off from the DNA.

    Transcription review and exercise:
  8. In eukaryotes, the transcript is a           and needs to be processed to form the mature       that will used in translation.

    RNA processing begins with the addition of a 5' cap and 3' poly-A tail to the pre-mRNA. The protein-coding segment contains coding sequences called exons. The exons are spliced together to form the mature mRNA.

      The pre-mRNA has sequences called introns which do not code for polypeptides and must be removed. The coding sequences (called exons) are spliced together produce the mature mRNA, which exits the nucleus.

      Pre-mRNA splicing.

    • Small nuclear RNAs (snRNAs) form a complex called a spliceosome with small nuclear ribonucleoproteins (snRNPs) and other proteins.
    • The snRNAs bind to specific nucleotides in the introns of a pre-mRNA.
    • The RNA transcript is cut, releasing the introns and splicing the exons together, producing mature mRNA.
    RNA Processing review:
  9. The genetic information on mRNA is encoded as a sequence of RNA triplets, or         .

    Triplet genetic code.

    For each gene, one DNA strand functions as a template for transcription to produce mRNA.

    The base-pairing rules for DNA synthesis also guide transcription, but uracil (U) takes the place of thymine (T) in RNA.

    In translation, the mRNA is read in the 5' - 3' direction as a sequence of base triplets, or codons.

    Each codon specifies an amino acid to be added to the growing polypeptide chain.

  10. The     possible codons constitute the genetic code.

    Genetic code.

    The 3 bases of a codon are read in the 5' - 3' direction along the mRNA.

    The 64 codons specify 20 different amino acids.

    The codon AUG not only stands for the amino acid methionine (Met but also functions as a "start" signal to begin translating the mRNA.

    Three codons are "stop" signals, marking the end of a polypeptide.

  11. The mature mRNA exits the nucleus, and translation takes place on            , together with           RNA (tRNA).

    A ribosome (site of polypeptide synthesis) consists of a large and a small subunit.

    Each subunit is an aggregate of ribosomal RNA (rRNA) molecules and proteins.

    Ribosomes contain 3 sites (E, P, A) for binding a 3rd type of RNA, transfer RNA (tRNA).

  12. The            on the transfer RNA ( tRNA ) binds with the mRNA        to add a specific amino acid to the polypeptide chain.

    Each tRNA type has a unique anticodon triplet, and carries a corresponding amino acid at its 3' end.

      Hydrogen bonds twist and fold the tRNA into a three-dimensional molecule.

      This exposes the anticodon on the tRNA to align with a complementary codon on the mRNA.

  13. Translation also has three stages:             ,             , and              .


    Codons on the mRNA move through a ribosome, and are translated into amino acids.

    The interpreters are tRNA molecules, each type with an anticodon at one end and a corresponding amino acid at the other end.

  14. Initiation occurs when the ribosome reads a start codon.
  15. Elongation produces a polypeptide by the formation of peptide bonds between amino acids.
  16. Termination occurs when the ribosome reads a stop codon.

      Translation: initiation. A small ribosomal subunit binds to mRNA. An initiator tRNA, with the anticodon UAC and carrying the amino acid methionine (Met), base-pairs with the start codon AUG. A large ribosomal subunit completes the initiation complex. The initiator tRNA is in the P site; the A site is available to the next tRNA.

      Translation: elongation.

    • The anticodon of an incoming tRNA base-pairs with the complementary codon in the A site.
    • A peptide bond is formed between the new amino acid in the A site and the growing polypeptide in the P site.
    • The ribosome translocates the tRNA in the A site to the P site.
    • The empty tRNA in the P site is moved to the E site and released. The mRNA moves the next codon into the A site.

      Translation: termination. When a ribosome reaches a stop codon, the A site accepts a release factor. The release factor releases the last amino acid of the polypeptide from the tRNA in the P. The two ribosomal subunits and the other components of the assembly dissociate.
    Translation review:
  17. Central dogma summary.

    Summary of eukaryotic transcription and translation.

    A gene in the DNA is transcribed into RNA molecules, including pre-mRNA.

    RNA processing occurs in the nucleus.

    Translation occurs in the cytoplasm on ribosomes in conjunction with tRNA.

    A gene is a region of DNA whose final product is either a polypeptide or an RNA molecule.


  18.            are changes in the DNA of a cell which may lead to an abnormal          .
    •        mutations are changes in one pair of nucleotides.

      Point mutation in hemoglobin gene.

      In this point mutation, the DNA template strand has an A where the wild-type template has a T (base-pair substitution). The mutant mRNA has a U instead of an A in one codon. The mutant (sickle-cell) hemoglobin has a valine (Val) instead of a glutamic acid (Glu).

    • A base-pair substitution mutation may result in a         ,           , or           mutation.

      Base-pair substitution may lead to silent, missense, or nonsense mutations.

    • silent mutation
    • missense mutation
    • nonsense mutation

        Silent mutation.

        A silent base-pair substitution alters a codon but does not result in a change in the amino acid.

        In this case both GGC and GGU code for Glycine.

        Missense mutation.

        A missense base-pair substitution leads to a change in the translated amino acid.

        The mutated mRNA changed from GGC to AGC, resulting in a change in the amino acid from Glycine to Serine.

        Nonsense mutation.

        A nonsense base-pair substitution changes a codon into a stop codon and results in premature termination of translation.

        The mutated mRNA changed from AAG (translated to Lysine) to UAG, a stop codon.

    • Insertions and            of nucleotide pairs may produce             mutations.

    Frameshift mutations.

    Base-pair insertions or deletions can cause frameshift in reading the codon, leading to nonsense or missense mutations. A base-pair insertion can cause immediate nonsense if the resulting a results in a premature stop codon. A base-pair deletion can cause extensive missense by changing the reading frame of the codons.