beyond Mendel - the molecular basis of inheritance, and DNA biology 1

The structure of DNA (review) DNA replication –Intitiation (priming) –Elongation –Ligation –Proof-reading DNA replication leads to a basis for inheritance

The structure of DNA Deoxyribose nucleic acid consists of a sugar- phosphate backbone, with 4 types of nitrogenous base –Purine Adenine Guanine –Pyrimidine Thymine Cytosine Double helix structure with polarity to each strand, running in antiparallel –Phosphate end 5’ –Sugar end 3’

DNA replication Watson and Crick suggest a model of semiconservative replication –(when replicated, each of the two new daughter molecules will possess one of the old, or conserved strands) DNA replication in essence involves an unzipping of the double strand, with new secondary strands being added to each template strand

Strand separation DNA replication starts at special sites called origins of replication (defined by a specific sequence of nucleotides) Specific proteins required to initiate replication bind to each origin The DNA helix opens at the origin and replication forks spread in both directions away from the central initiation point, forming a replication bubble Eukaryotic chromosomes have hundreds of replication origins - the replication bubbles that form from this process eventually merge and join, forming two continuous DNA molecules

Elongation of the strand Two types of proteins are involved in the separation of strands –helicases (unzip the strand) –Single-strand binding proteins keep strand apart A third category of protein, DNA polymerase, catalyzes the synthesis of a new DNA strand DNA polymerase joins free floating nucleotides to their corresponding pair on the template strand Strands grow in the 5’ to 3’ direction, since nucleotides are only added to the 3’ end

Energy for reaction of joining nucleotide pairs comes from NTP - nucleoside triphosphate - a nucleotide with a triphosphate group. –Exergonic hydrolysis of the phosphate bond drives the endergonic synthesis of DNA Continuous synthesis of DNA on both template strands is not possible because the strands are in antiparallel –Continuous synthesis occurs on the leading strand –Discontinuous synthesis occurs on the lagging (complementary) strand

On the leading strand, DNA is synthesized continuously On the lagging strand, DNA is produced in chunks called Okazaki fragments, each individually poduced in the 5’ to 3’ direction These fragments are linked by DNA ligase, an enzyme that catalyzes the covalent bond between the 3’ end of the new Okazaki fragment and the 5’ end of the growing chain

Primers DNA polymerase can only add nucleotides to a pre-existing sequence Thus process is initiated by a primer, a short segment of RNA polymerized by the enzyme primase RNA will act as the key to which DNA polymerase adds nucleotides –On the leading strand, only one primer is needed, which is later replaced by DNA –On the lagging strand, each Okazaki fragment has a primer, which is replaced as part of the ligation process

Proof-reading Highly accurate, (base pairing errors occur 1/10,000). However, overall DNA errors are 1/10 9 DNA proof-reading/repair accounts for high accuracy –Mismatch repair (during DNA synthesis, performed by polymerase and other enzymes) –Excision repair (after accidental changes in pre- existing DNA. 50+ repair enzymes excise the incorrect sequence, which is then filled by DNA polymerase and DNA ligase)

DNA replication = inheritance? Chromosomes contain the basic ‘stuff’ of inheritance By replicating DNA, the primary constituent of chromosomes, genetic information can be passed to daughter cells as a replication from the parent cell How then, does DNA result in ‘traits’ that are exhibited in the phenotype –Answer = through transcription and translation of the genetic code to manufacture an enzyme that corresponds to a gene