A.  Structure of RNA
     1.  Subunits - referred to as nucleotides (same as in DNA)
          a.  Ribose - five-carbon sugar
          b.  Phosphate Group - 1 or more phosphate groups
          c.  Nitrogenous Base - one of four molecules containing nitrogen
                1)  Adenine - purine base - two rings
                2)  Uracil - pyrmidine base - one ring
                3)  Guanine - purine base - two rings
                4)  Cytosine - pyrimidine base - one ring

                           
     2.  Differences with DNA
                RNA                                        DNA
          a)   Single-stranded                        Double-stranded
          b)  Ribose                                     Deoxyribose
          c)  A, C, G and U                          A, C, G and T

B.  Types of RNA
      1.  mRNA - messenger RNA - carries information from DNA of the structural gene to the ribosome where the protein is made

      2.  tRNA - transfer RNA - carries amino acids to mRNA at the ribosome to assembly the protein being made

      3.  rRNA - ribosomal RNA - major structural component of the ribosome where protein synthesis occurs

C.  Transcription
      1.  Purpose - to synthesize RNA from a DNA template  
            a.  Must be completed before the protein can be produced
            b.  DNA   --->  RNA   --->  Protein     
       
      2.  Components
           a.  DNA Template - strand of DNA providing directions for transcription of mRNA

       

           b.  RNA Polymerase - enzyme that helps to pull apart DNA strands and link new mRNA nucleotides

               
           c.  Promoter - sequence of DNA that signals where (and on which strand) transcription should begin
           d.  Termination Signal - sequence of DNA that causes RNA Polymerase to detach with the newly transcribed mRNA strand

D.  Genetic Code
      1.  Genetic "Alphabets" - there are three alphabets involved in the entire process of protein synthesis
           a.  DNA   --->  RNA   --->  Protein
           b.  Alphabets:
                1)  DNA - A, C, G and T
                2)  RNA - A, C, G and U
                3)  Protein - Twenty different amino acids

      2.  Triplet Code - three nucleotides code for one amino acid
            a.  Codon - three nucleotides of mRNA determining which amino acid is added to a protein
            b.  Sample Genetic Code



            c.  Example
                 1)  mRNA Codon = AUG
                 2)  Amino Acid = Methionine

    3.  Code Characteristics
         a.  It is described in terms of the mRNA codons.
         b.  STOP codons - UAA, UAG, UGA - all three of these codons signal the end of a polypeptide chain
         c.  Universal - The genetic code is the same in all living organisms, from bacteria to humans.
         d.  Degenerate - More than one codon is assigned to each amino acid.  This allows for possible mutations to be less damaging.
              AAA - Lysine
              AAG - Lysine
              UAA - Lysine
         e.  Third Base is usually less specific than the first two.  This is also known as the "Wobble Hypothesis" because often the third base can change, but the amino acid remains the same.
         f.  Reading Frames - There are three possible reading frames in every strand of mRNA that are created by the codons.  These reading frames can be altered by Frameshift Mutations.
              Reading Frame #1 -  C U A G G C A A C U U A
              Amino Acids #1 - Leucine - Glycine - Asparagine - Leucine

              Reading Frame #2 -  C U A G G C A A C U U A
              Amino Acids #2 - STOP - Alanine - Threonine

              Reading Frame #3 -  C U A G G C A A C U U A
              Amino Acids #3 - Arginine - Glutamine - Leucine

D.  Details of RNA Types
      1.  mRNA - messenger RNA
           a.  Prokaryotes 
                1)  DNA is found in cytoplasm because there is no nucleus
                2)  Ribosomes attach to mRNA and begin protein synthesis BEFORE transcription is complete.

            b. Eukaryotes 
                1)  Transcribed mRNA must first be processed before it can leave the nucleus for the cytoplasm.
                2)  Steps of mRNA Processing: 
                    a)  Guanine Cap - Several Guanine nucleotides are added to the front end of the mRNA strand in order to bind to the ribosome more effectively.
                    b)  Poly-Adenine Tail - Several Adenine nucleotides are added to the tail end of the mRNA strand to prevent destruction by RNases (enzymes which break down RNA)
                    c)  Splicing - Introns are removed and Exons are joined together
                        1.  Intron - segment of mRNA which does NOT code for protein; therefore, it is removed
                        2.  Exon - segment of mRNA which does code for protein; therefore, it remains for expression in protein
    

      2.  tRNA - transfer RNA
           a.  Function
                1)  Carries amino acids to mRNA at the ribosome
                2)  tRNA molecules are specific to the amino acid they carry; therefore, there are 20 tRNA molecules 
            b.  Structure
                1)  T - shaped molecule held together by complementary base pairing (Hydrogen bonds)
                2)  Aminoacyl Attachment Site - location where the amino acid is attached en route to the ribosome
                3)  Anticodon - segment of three tRNA nucleotides that are complementary to the mRNA codon; ensures that proper amino acid is added to the growing polypeptide chain
               

      3.  rRNA - ribosomal RNA
           a.  Important structural component of ribosome
           b.  Ribosome - composed of one large and one small subunit; location for protein synthesis in cells

To test your knowledge about RNA, click on the RNA Questions Link at the top of this page.  After you answer the questions, be sure to check your responses by clicking on the RNA Answers Link.

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