DNA Replication & Transcription


In principle: DNA replication is semi-conservative [HOMEWORK #4]
       H - bonds 'unzip', strands unwind,
        complementary nucleotides added to existing strands
             After replication, each double-helix has one "old" & one "new" strand

       DNA is not  the "Genetic Code" for proteins
               Information in DNA must first be transcribed into RNA
               messenger RNA transcript is base-complementary to template strand of DNA
                                                                       & therefore co-linear with sense strand of DNA

       DNA & RNA syntheses occur only in the  5'  3' direction

Central Dogma



DNA synthesis
     Nucleotides are added simultaneously to the 3' ends of both strands, but
     DNA grows in the 5'  3' direction ONLY  [iG1 10.10]

 Distinguish:
   Replication: duplication of a double-stranded DNA (dsDNA) molecule
                         an exact 'copy' of the existing molecule (cf. xerox copy)
   Synthesis: biochemical creation of a new single-stranded DNA (ssdNA) molecule
                        a base-complementary 'copy' of an existing strand (cf. silly putty copy)
                        occurs only in the 
5'3' direction

   Homework #5


Transcription: synthesis of messenger RNA (mRNA)


    What is a "Gene"? [Structure of a Eukaryotic Gene]


     RNA transcribed from DNA by RNA Polymerase (RNAPol I)
            (1) Recognition of transcriptional unit: ~ 'gene'
                      Promoters - short DNA sequences that regulate transcription
                          typically 'upstream' = 'leftward' from 5' end of sense strand
            (2) Initiation & Elongation
                      mRNA synthesized 5'3'  from DNA template strand
                      mRNA sequence therefore homologous to DNA sense strand

                          Co-linear: Prokaryotic mRNA and DNA sense strand "line up"

                          Process similar to DNA replication, except
                              Transcription may occur from either and (or) both strands

                               Most DNA not transcribed into RNA

            (3) Termination

    Regulation of transcription
          In prokaryotes, transcription & translation may occur simultaneously
          In eukaryotes, transcription occurs in nucleus [ex.: Lampbrush chromosomes]
                                     translation occurs in cytoplasm
              RNA must cross nuclear membrane
                        transcription  & translation are physically separated
                        primary RNA transcript is extensively processed
                        heterogeneous nuclear RNA (hnRNA mRNA

    Post-transcriptional processing of eukaryotic RNA is complex [Summary]
          'splicing' of hnRNA : eukaryotic genes are "split"
              intron DNA sequence equivalents removed from hnRNA : "intervening"
              exon   DNA sequence equivalents represented in mRNA"expressed" in protein
                        1 ~ 12's of exons / 'gene'
                         >90 ~ 99% of transcript may be 'spliced out'
                              [An important note on terminology] [or, to put it another way]

             Eukaryotic genes & mRNA are not co-linear!
                DNA / RNA hybridization produces heteroduplexes
                    DNA introns 'loop out'
                    DNA exons pair with mRNA
                 Eukaryotic exons may be widely separated

                Generalized structure of a eukaryotic transcription complex
    
     Alternative splicing of the same transcript produces different products
        Different exon transcripts are combined as different mRNAs
        Alternative transcript combinations differ functionally


Ongoing Homework question:

       What is a 'gene'? How do introns and exons and alternative splicing in eukaryotic genomes modify the concept?


All text material © 2024 by Steven M. Carr