D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 26
DNA REPLICATION
1.
Basic Outline: (ppt. 1)
REPLICATION
DNA -----------------------> 2 identical DNAs (in nucleus)
TRANSCRIPTION
DNA -----------------------> mRNA (& other RNAs) (in nucleus)
TRANSLATION
mRNA -----------------------> protein (at ribosome)
• base sequence of DNA codes for amino acid sequence of protein; one gene
codes for one protein
2.
DNA as Hereditary Material:
• genes on chromosomes (Morgan's X-linkage studies); chromosomes made of
nucleoprotein; thus genes must be nucleic acid or protein
• “Transforming principle”: Griffith experiment with S & R strains of
Pneumococcus (fig. 16 – 2 & ppt. 2); infection with S: lethal, mice died; infection with R:
harmless, mice lived; infection with killed S: harmless, mice lived; infection with R + killed
S: lethal; something from killed S rendered R infective (live S cells were found in
blood of dead mouse)
• Avery, MacLeod, & McCarty identified transforming principle as DNA; killed
S cells were treated in separate experiments to distinguish between DNA, RNA or
protein as transforming principle; only when DNA was not inactivated did
transformation occur
• DNA was confirmed as genetic material in virus infection experiment
(Hershey & Chase); 35S-labeled protein & 32P-labeled DNA study (fig. 16 – 4 & ppt. 3)
Bio 102, spr. 2013
3.
lec. 25 - p. 2
Double Helix & Template Theory:
• DNA structure elucidated by Watson & Crick (fig. 16 – 1 & ppt. 4)
(modeled X-ray diffraction data of Wilkins & Franklin) (fig. 16 – 6 & ppt. 5)
• Chargaff showed species-specific differences in DNA base content; A = T,
C = G (Chargaff’s rules)
• two antiparallel polynucleotide strands arranged in double helix, Hbonded by complementary base pairing (A - T; C - G) (figs 16 – 5, 16 – 7, 16 – 8 &
ppts. 6 - 9); this model explained Chargaff’s rules
• each strand = template for its complementary strand
4.
Replication of DNA (template copy mechanism) (fig. 16 - 9 & ppt. 10):
• semiconservative replication (figs. 16 –10 & ppt. 11); 14N15N DNA study
(Meselson & Stahl) (fig. 16 – 11 & ppt. 12)
• mechanisms: replication origins - base sequences recognized by enzymes
which open double strand replication bubbles (replication forks at each end) (fig.
16 – 12 & ppt. 13)
- helix opened up by combined action of helicase & single strand binding
proteins (fig. 16 – 13)
Bio 102, spr. 2013
lec. 25 - p. 3
- DNA polymerases add nucleotides in 5’ to 3’ direction (fig. 16 – 14 & ppt. 14)
- antiparallel strands result in continuous synthesis (leading strand) on one
template & discontinuous synthesis (lagging strand) on the complementary template
(Okazaki fragments so formed are joined by DNA ligase) (figs. 16 – 15, 16 – 16 & ppt. 15)
- primer RNA laid down by primase (figs 16 – 15 to 16 – 17 & ppt. 16)
- summary of replication (fig. 16 - 17, table 16 - 1 & ppts. 17 & 18)
- errors corrected by repair enzymes (fig. 16 – 18 & ppt. 19)