Question

Suppose that \(E .\) coli synthesizes DNA at a rate of 100,000 nucleotides per minute and takes 40 minutes to replicate its chromo- some. (a) How many base pairs are present in the entire \(E .\) coli chromosome? (b) What is the physical length of the chromosome in its helical configuration- that is, what is the circumference of the chromosome if it were opened into a circle?

Short answer

Answer: There are 2,000,000 base pairs in the entire E.coli chromosome, and the physical length or circumference of the chromosome in its helical configuration is 680,000 nm.

Step-by-step solution

Calculate total number of nucleotides synthesized in 40 minutes

First, we need to find the total number of nucleotides synthesized by E.coli during the 40-minute replication time. We can use the synthesis rate (100,000 nucleotides per minute) and multiply it by the replication time (40 minutes) to get the total number of nucleotides. Total number of nucleotides = synthesis rate × replication time = 100,000 × 40

Find the number of base pairs in the E.coli chromosome

Since each base pair consists of two nucleotides, we need to divide the total number of nucleotides from Step 1 by 2 to find the number of base pairs. Total number of base pairs = Total number of nucleotides / 2

Determine the length of one base pair in DNA helix

In a DNA helix, each base pair is parallel to its neighboring base pairs, and they are all separated by a distance of 0.34 nm along the helix axis. So, the length of one base pair in DNA helix = 0.34 nm.

Calculate the physical length of the chromosome

To find the physical length of the chromosome in the helical configuration, we need to multiply the total number of base pairs in the E.coli chromosome (from Step 2) with the length of one base pair in a DNA helix (0.34 nm). Physical length of chromosome = total number of base pairs × length of one base pair

Find the circumference of the chromosome in a circle

Since we're asked to find the chromosome's circumference when it's opened into a circle, we'll treat the physical length (from Step 4) as the circle's circumference. Circumference of the chromosome = Physical length of the chromosome Now, we can calculate the values for each step and find the answers. Total number of nucleotides = 100,000 × 40 = 4,000,000 Total number of base pairs = 4,000,000 / 2 = 2,000,000 Physical length of chromosome = 2,000,000 × 0.34 nm = 680,000 nm Circumference of the chromosome = 680,000 nm So, (a) There are 2,000,000 base pairs in the entire E.coli chromosome, and (b) The physical length or circumference of the chromosome in its helical configuration is 680,000 nm.