Bio102: Introduction to Cell Biology and Genetics
Replication and Chromosomes
Key Terms:
Chromatin
Histones
Chromosome
Homologous pair
N, 2N
Origin
Initiator Protein
Helicase
Topoisomerase
Primase
Leading Strand
Lagging Strand
Okazaki Fragment
Ligase
DNA Polymerase III
DNA Polymerase I
Replication Bubble
Replication Fork
Key Figures: Figures 16.9, 16.12, 16.13, 16.14, 16.15, 16.16, 16.17, 16.21
Key Questions:
 What features of Watson and Crick's DNA structure suggested a model for how it would
replicate?
 What is a homologous pair of chromosomes?
 List the enzymes and other proteins involved in DNA replication and briefly describe their
functions.
 Draw a replication bubble. Label 5' and 3' ends. For each of the two replication forks, show the
leading strand, lagging strand, helicase, RNA primers, Okazaki fragments and two DNA
polymerase III molecules
Lecture Outline:
In a cell, DNA is highly condensed. Associated with many proteins as chromatin
two alleles of each gene, so two homologous pairs of chromosomes (may have different alleles)
Each species has a constant number of unique chromosomes (N). Diploid cells are 2N.
in humans, N=23. So diploid human cells have 46 chromosomes or 23 homologous pairs
DNA Polymerase III is the major enzyme to connect nucleotides
Origin of Replication is bound by Initiator Protein
recruits Helicase to ‘unzip’ helix
creates a replication bubble with two equivalent replication forks
causes tension; released by topoisomerases
DNA Polymerases can’t start polymerizing. Need a 3’ end to add on to
Primase makes short RNA primers that can be extended by DNA Polymerase III
DNA Polymeraes can only add to a 3’ end, not a 5’ end
polymerization of the two strands at one fork move in different overall directions
leading (continuous) strand vs. lagging (noncontinuous) strand
multiple priming events needed on the lagging strand making Okazaki fragments
DNA Polymerase I removes residual RNA and fills in with DNA
Ligase connects adjacent Okazaki fragments with a sugar-phosphate connection