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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 sugarphosphate 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
1/109
• 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 preexisting 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