Bio102: Introduction to Cell Biology and Genetics

Replication and Chromosomes

Key Terms:

Chromatin / Origin / Leading Strand / DNA Polymerase III
Histones / Initiator Protein / Lagging Strand / DNA Polymerase I
Chromosome / Helicase / Okazaki Fragment / Replication Bubble
Homologous pair / Topoisomerase / Ligase / Replication Fork
N, 2N / Primase

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