1. How long would it take to replicate the largest human chromosome (#1, approximately 240 Mb in size) assuming a replication rate of 50 nucleotide pairs per second at each replication fork, that replication is bi-directional and that replication starts at an origin in the exact middle of the chromosome? (Show your work/ reasoning to insure maximum credit). (5 pts. possible)

 

Rate of replication (both forks together is 100 bp/ second. Length of chromosome is 240 Mb = 240 X 10⁶ bp. Time to replicate the chromosome = length/ rate = 240 X 10⁶ bp/ 100 sec/ bp = 2.4 X 10⁶ seconds There are 3600 seconds in an hour. 24 X 10⁵ seconds / 3.6 X 10³ seconds/ hr. = 6.67 X 10² hours = 667 hours = 27.8 day. Full credit possible for answer in seconds, minutes, hours or days.

 

 

2. You are studying a linear DNA fragment of a 5 kb region of a human chromosome. Digestion with the enzyme EcoRI gives bands of 3.5 and 1.5 kb after gel electrophoresis. Digestion with the enzyme BamHI gives bands of 3 and 2 kb. Simultaneous digestion with both enzymes gives bands of 3 kb, 1.5 kb and 0.5 kb. Draw a restriction map of the region demonstrating where the enzymes cut the DNA.

The Eco must not cut inside the larger Bam fragment or it would become smaller. However, it does cut within the smaller Bam fragment. The digestion with Eco alone shows that the cut site is 1.5 kb from one end. Therefore, the restriction map must be something like this: