Speed of chromosome replication

Fifty-two years after Watson and Crick’s Nature note, a group of about 50 molecular geneticists published the complete DNA sequence of the human X chromosome (Fig. 11). This discovery revealed that the X chromosome is one double stranded DNA molecule that is about 155 million nucleotide pairs long. Every time a human cell divides, one or two of these 155,000,000 nucleotides must be replicated. This must be done with speed and accuracy.

Figure 11. Article title "The DNA sequence of the human X chromosome". This article announced that 99.3% of the X chromosome was sequenced by the time of publication, 17 March 2005.

The main DNA replication enzyme (DNA pol III, see below) works fast. Estimates of its replicating speed are around 750 nucleotides per second.  


Let us do the math to see how long it would take to copy one human X chromosome based on the article that says the human X chromosome has 155,000,000 nucleotides and DNA polymerase III reads 750 nucleotides per second. 

155,000,000 nucleotides / X chromosome × 1 second / 750 nucleotides = 206,666 seconds per X chromosome   

Let us convert these seconds to hours.

1 hour / 360 seconds × 206,666 seconds / chromosome=547 hours per chromosome

That means one replication enzyme would take 547 hours or about 24 days to replicate an X chromosome. What are the biological ramifications of this 24 days? If you cut yourself, it would take 24 days to replicate the chromosomes before the skin cells could divide. The formation of new cells needed for healing cuts in our skin would keep us in a wounded state. 

Speeding up replication

To speed up the chromosome replication process, two tactics are used by living cells. Both tactics are shown in Figure 12. First, replication will not start on just one end of the chromosome. Instead, there are hundreds of origins of replication (ori) along the chromosome. In addition, the replication process moves bidirectionally from the ori. This bidirectional replication creates forks of replication that move toward each other as the chromosome is replicated.

Figure 12. Origin of replication and replication forks. The first line illustrates how many origins of replication appear along a segment of DNA. The second line of DNA illustrates that each origin of replication replicates in both directions of the DNA strand to create a new copy of the gene. The last DNA segment shows the replication forks continuing in each direction and new DNA strands getting longer. By W. Suza, 2021.

As a result of these two tactics, a large chromosome can be replicated in hours rather than weeks and cell division can happen quickly when it is needed for growth or cell replacement.