Suppose that E. coli synthesizes DNA at a rate of 100,000 nucleotides per minute and takes 40 minutes to replicate its chromosome. (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?

Key Concepts

DNA Replication Rate

DNA replication rate refers to the speed at which DNA polymerase synthesizes new DNA strands. In this case, E. coli synthesizes DNA at a rate of 100,000 nucleotides per minute. Understanding this rate is crucial for calculating the total number of nucleotides replicated over a given time, which directly relates to the size of the E. coli chromosome.

Base Pairs in DNA

Base pairs are the building blocks of the DNA double helix, consisting of pairs of nucleotides (adenine-thymine and guanine-cytosine) that bond together. The total number of base pairs in a chromosome can be determined by multiplying the number of nucleotides by 2, as each nucleotide on one strand pairs with a complementary nucleotide on the opposite strand. This concept is essential for understanding the overall structure and size of the E. coli chromosome.

Physical Length of DNA

The physical length of DNA refers to the actual distance that the DNA molecule would occupy if it were laid out in a linear fashion. For circular DNA, such as that in E. coli, the circumference can be calculated using the number of base pairs and the average length of a base pair in its helical form. This concept is important for visualizing the spatial arrangement of the chromosome and understanding its compact structure within the bacterial cell.