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: transcription followed by translation. One DNA strand serves as a template 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 to be added to the growing protein (polypeptide) chain.


      Base pairing in DNA. The pairs of nitrogenous bases in a DNA double helix are held together by hydrogen bonds. The geometry of the bases are the basis of base-pairing rules.
    • Adenine (A) pairs with Thymine (T) by forming two bonds,
    • Cytosine (C) pairs with Guanine (G) by forming three bonds.
    • When DNA base-pairs with RNA:
    • A pairs with U.
     
     
     
     
  2. In prokaryotes, transcription and translation take place                 . In eukaryotes, an intermediate step of RNA             is needed due to the presence of the          .

      A prokaryotic cell lacks a nucleus, and mRNA produced by transcription is immediately translated without additional processing.


      In an eukaryotic cell, the nucleus provides a separate compartment for transcription. The original RNA transcript, called pre-mRNA, is processed before leaving the nucleus through nuclear pores as mRNA for translation in the cytoplasm.
     
     
     
     
  3. Transcription has three stages:

      Transcription.
    • Initiation After RNA polymerase binds to the promoter, the DNA strands unwind, and the polymerase initiates RNA synthesis on the template strand.
    • Elongation The polymerase moves downstream, unwinding the DNA and elongating the RNA transcript 5' to 3'.
    • Termination Eventually, the RNA transcript is released, and the polymerase detaches from the DNA.
     
     
     
     
    1. Initiation occurs at the promoter sequence on 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 II.
       
       
       
       
    2. Elongation is performed by RNA             .

        Transcription: elongation. The RNA polymerase unwinds the double helix, adding nucleotides to the 3' end of the growing RNA molecule. The same base-pairing rules as DNA replication is used, except that Uracil (U) substitutes for Thymine (T).


        Base pairing in DNA. The pairs of nitrogenous bases in a DNA double helix are held together by hydrogen bonds. The geometry of the bases are the basis of base-pairing rules.
      • Adenine (A) pairs with Thymine (T) by forming two bonds,
      • Cytosine (C) pairs with Guanine (G) by forming three bonds.
      • When DNA base-pairs with RNA:
      • A pairs with U.
       
       
       
       
    3. Termination


      Transcription: termination. In eukaryotes, transcription terminates when a polyadenylation signal is transcribed, and the RNA transcript falls off from the polymerase.
     
     
     
     
  4. In eukaryotes, the transcript is a           and needs to be processed before it can be used in translation.
     
     
     
     
    • The 5' end receives a modified nucleotide      , and the 3' end gets a         tail.

        The first step in RNA processing is the addition of a 5' cap and 3' poly-A tail to the premRNA. These modified ends and the UnTranslated Regions facilitate ribosome attachment in the cytoplasm.
       
       
       
       
    •          are removed from the pre-mRNA and its        are spliced together by               .


      RNA processing: RNA splicing. The pre-mRNA has regions called introns which do not code for polypeptides and must be removed. The coding regions are 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 on a premRNA with small nuclear ribonucleoproteins (snRNPs) and other proteins.
    • The snRNA bind to specific nucleotides in the introns of a premRNA.
    • The RNA transcript is cut, releasing the introns and at the same time splicing the exons together, producing mature mRNA.
     
     
     
     
  5. 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.
     
     
     
     
  6. The genetic information on mRNA is encoded as a sequence of RNA triplets, or         . The     possible codons constitute the genetic code.

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


      The dictionary of the 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. Codons must be read in a correct reading frame to produce the correct sequence of amino acids. The genetic code is nearly universal among all organisms.


      A tobacco plant expressing a firefly gene. Because diverse forms of life share a common genetic code, it is possible to program one species to produce proteins of another species by transplanting DNA. Here a gene from a firefly is incorporated into the DNA of a tobacco plant. This gene codes for an enzyme that catalyzes a chemical reaction that releases light energy. Rabbit.
     
     
     
     
  7. 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 specific amino acid at its 3' end.


      Hydrogen bonds twist and fold the tRNA into a three-dimensional molecule. This exposes the anticodon (3' - 5') on the tRNA to align with a mRNA codon (5' - 3') on the ribosome.
     
     
     
     
  8. Translation also has three stages:


        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.
       
       
       
       
    • Initiation occurs when the ribosome reads a        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.
       
       
       
       
    • Elongation produces a              by the formation of          bonds between amino acids.

        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.
       
       
       
       
    • Termination occurs when the ribosome reads a       codon.


      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.
     
     
     
     
  9. 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.
     
     
     
     
  10.            are changes in the DNA of a cell which may lead to an abnormal          . Point mutations are changes in one pair of nucleotides.

      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               mutation may result in a         mutation, a           mutation, or a           mutation.

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


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


        A nonsense base-pair substitution changes a codon into a stop codon and results in premature termination of translation.
       
       
       
       
    • Insertions and            of nucleotide pairs may produce             mutations.


      Base-pair insertions or deletions usually 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.