the molecular basis of heredity chapter 16. learning target 1 i can explain why researchers...
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Learning Target 1
I can explain why researchers originally thought protein was the genetic material.
Protein as the genetic material?
T.H. Morgan – fruit flies Discovered genes as part of
chromosomes Chromosomes made of protein and
DNA Protein?
More known Diverse structures Specificity of function
DNA? Little known Seemed too uniform to be the
genetic code of all life
Learning Target 2
I can summarize the experiments performed by the following scientists and identify the evidence that
DNA is the genetic material.
Learning Target 3
I can describe the structure of DNA, explain the base-pairing rule, and describe its significance.
Anti-parallel Structure
Sugar carbons are numbered 1’-5’
One side of DNA runs in the 3’ direction
The other side runs in the 5’ direction
This is important to replication
Learning Target 4
I can describe the semiconservative model of replication and the significance of the experiments
by Meselson and Stahl.
DNA Replication
Making DNA from existing DNA
Semi-conservative At the end of DNA replication,
each daughter molecule has one old strand (from the parent DNA) and one new strand (synthesized during replication)
Model proposed by Meselson and Stahl
Meselson & Stahl
Learning Target 5
I can describe the process of DNA replication, including the role of the origins of replication and
replication forks.
DNA Replication Step 1:
Helicases unwind DNA at origin of replication by breaking hydrogen bonds between nitrogen bases
Replication bubble forms as two parental strands separate
Replication fork forms at end of each replication bubble
DNA Replication Step 2:
Single-strand binding proteins hold the unpaired DNA strands apart while new DNA strands are being synthesized
Topoisomerase protein binds to parental DNA to relieve strain untwisting puts on replication fork
DNA Replication
Step 3: Primase creates a short RNA primer that binds to the
parent DNA to signal DNA polymerase III where to begin adding nucleotides
RNA primer will later be replaced with DNA nucleotides
DNA Replication
Step 4: DNA Polymerase III adds nucleotides to exposed bases in 5’-3’ direction at the RNA
primer Leading strand
Produced continuously in 5’-3’ direction Elongation moves towards replication fork
Lagging strand Produced in pieces
Okazaki fragments Elongation moves in opposite direction of replication fork (5’-3’)
Proofreading & Repair
Initial error rate in replication is 1 in 100,000 nucleotides
DNA polymerases proofread and correct errors Error rate in
completed replication is 1 in 10 billion bases
Mismatch Repair
For that 1 in 10 billion errors that escapes DNA polymerase or are due to environmental mutations
Many enzymes involved Cut out damaged section
(nuclease) Replace with new
nucleotides (DNA polymerase)
Seal in place (DNA ligase)
Telomeres As cells divide, chromosomes erode after
multiple DNA replications Telomeres
End caps of non-coding DNA that protect the coding genes on the chromosomes Think of the plastic pieces on the ends of your
shoelaces These sequences shorten after each round of
replication so the actual genes do not Part of aging process?
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