Question

The nuclear genome of a single human cell (i.e., the entire diploid complement) contains about 6.6 billion (6.6 × 109) base pairs of DNA. If synthesis at each replication fork occurs at an average rate of 50 nucleotides per second, all the DNA is replicated in 5 minutes. Assume that replication is initiated simultaneously at all origins. Assuming that the origins are approximately equally distributed across the chromosomes, what is the average number of origins per human chromosome?

a) 1
b) 2
c) 3
d) 4

Answer 1

To determine the average number of origins per human chromosome, considering 6.6 billion base pairs are replicated in 5 minutes, we calculate the total number of replication forks and origins. Human cells, having 46 chromosomes, result in an estimation of approximately 478 origins per chromosome, which doesn't match the provided answer choices, suggesting an error in the question or its options.

Step-by-step explanation:

Calculating the Average Number of Origins Per Human Chromosome

The student is interested in understanding how DNA replication occurs in human cells and specifically, how many origins of replication are present per chromosome. Assuming the nuclear genome of a human cell contains about 6.6 billion base pairs of DNA and that the DNA is replicated in 5 minutes at a rate of 50 nucleotides per second at each replication fork, we can estimate the required number of origins to achieve this rate.

Firstly, the total number of nucleotides replicated (6.6 billion) needs to be divided by the length of time for replication in seconds (5 minutes × 60 seconds per minute = 300 seconds), and by the number of nucleotides replicated per second (50) to calculate the total number of replication forks:

Total replication forks = (6.6× 109 base pairs) / (300 seconds × 50 nucleotides/second)
= (6.6 × 109) / (300 × 50)
= 44,000 forks

Since replication is bidirectional, we divide the number of forks by 2 to get the number of origins:
Total origins = 44,000 forks / 2 = 22,000 origins

Given that human cells have 46 chromosomes, to find the average number of origins of replication per chromosome, we divide the number of origins by the number of chromosomes:

Average origins/chromosome = 22,000 origins / 46 chromosomes

The calculation yields approximately 478 origins per chromosome. Considering the answer choices provided, none are correct, which implies a possible error in the question's assumptions or the choices provided. However, the closest answer to our calculation based on the given options would be option (d) 4, potentially rounding down but not accurately representing the calculated value.