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Chapter 13

BIOL 1411 Chapter Notes - Chapter 13: Proofreading, Dna Clamp, Semiconservative Replication

Course Code
BIOL 1411
Logsdon John

of 2
Chapter 13
DNA is an important component of the nucleus and chromosomes
oChromosomes carry genes
DNA varies among species, have their own specific amount of nuclear DNA
Transfection – the transformation of eukaryotic cells by DNA
Genetic marker – a gene whose presence in the recipient cells confers an observable phenotype
DNA is a double-stranded helix
oSugar-phosphate backbone, base pair rungs
It is a right-handed helix
DNA is antiparallel, two strands run in opposite directions
Major and minor grooves: the outer edges of the nitrogenous bases are exposed
Hydrogen bonding occurs between the base pairs
Van der Waals forces between adjacent bases on the same strand (stacking because of weak
5’ end – free phosphate group in sugar molecule
3’ end – free hydroxyl group
DNA double helix, 5’ end is paired with the 3’ end, and vice versa
The double-helical structure of DNA is essential to its function
oThe genetic material stores an organism’s genetic information
oIt is susceptible to mutations
oThe genetic material is precisely replicated in the cell division cycle
oIt is expressed as the phenotype
Templates – direct the sequence of nucleotides in the new molecules
DNA polymerase – enzyme to catalyze the polymerization reaction
Semiconservative replication – in which each parent strand serves as a template for a new
strand, and the two new DNA molecules each have one old and one new strand
There are 2 steps in DNA replication
1. Double helix is unwound to 2 separate template strands, made available for new base
2. As new nucleotides form base pairs with template DNA, form covalent phosphodiester
bonds, forming a polymer
Prokaryotes typically have one ori of DNA replication
o2 replication forks
Eukaryotes have one ori
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DNA helicase – unwinds and separates the DNA strands
Single-strand binding proteins – bind to the unwound strands and keep them separated
Leading strand – grows continuously at its 3’ end as the fork opens up
Lagging strand – grows away from the fork, consists of gaps between stretches of DNA
oStretches are called Okazaki fragments
DNA ligase – catalyzes formation of bonds between Okazaki fragments making the lagging
strand whole
DNA clamp – “donut” that encircles the polymerase and double helix to keep them bound
tightly to polymerize nucleotides
Telomeres – repetitive sequences at the ends of chromosomes
Proofreading – corrects errors in replication as DNA polymerase makes them
Mismatch repair – scans DNA immediately after it has been replicated and corrects any base-
pairing mismatches
Excision repair – removes abnormal bases that have formed, replaced with functional bases
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