I learned this at a lecture, but somehow I have trouble in understanding this. It is said that the bacteria such as E.coli need 20 minutes to divide, but its chromosome require 40 minutes to multiply. It is explained that the E. coli will have copied half of it, which will spend 20 minutes and after divided, it will continue another 20 minutes to complete the chromosome copy. However, it is said that each daughter cell will have one complete and half copy after they divide. I am confused that if the bacteria only spend 20 minutes to copy half of it, how can each daughter cell could have one and a half?
Referring to the six stages in your diagram as Stage 1 through Stage 6, let's look at Stage 3: At that point, there is one bidirectional replication in progress, having made it about halfway around the chromosome. So it has 2 copies of the first half of the chromosome, and 1 copy of the rest, which is yet to be copied. To get to this state of replication took 20 minutes.
Now the cell starts two new replications, one at "origin of replication" on each of the two copies of the first halves of partially replicated chromosome. This is the state shown in Stage 4. There are now three replications going on: the original, which is working on the last half of the chromosome, and two replications, which are working on (their own copy of) the first half of the chromosome. All three replications will continue working for about 20 minutes.
We then get to Stage 5: the original replication has finished and the septum forms to separate the daughter cells. The two replications started in stage 4 are continuing, and are about halfway down the chromosome.
The cells separate, and we are at Stage 6. But stage 6 is the same state for each daughter cell as we had in Stage 3: a chromosomal replication that is halfway through (thus having one and a half copies). It took 20 minutes to get from Stage 3 to Stage 6, where we are ready to double again. During that 20 minutes, only one half of a chromosome had time to be copied, so it takes twice that (40 minutes) for replication of the whole chromosome. This works out because the cell is doing three replications at once.
Your question has already answered reasoning behind it. Just remember this happens only in rich media. Now I tried to build simple model of this to illustrate how DNA levels are changing over the period of time in given scenario. I will assume very simple (but reasonable) numbers for model. Rules are very simple,
- Cell division in 20 min ( Cell will lose half DNA )
- DNA replication in 40 min ( Cell will double DNA copy )
- Firing of origin of replication in 10 min ( Cell will have one more origin of replication )
- At each time point, cell will have DNA replication depending on number of origin of replications
I wrote simple code in python and simulated,
import numpy as np
import matplotlib.pyplot as plt
cell_division_time = 20 #min
full_DNA_replication_time = 40 #min
firing_of_new_Ori = 10 #min
origin_of_replications = 1
total_DNA = 1 #initial condition
time_for_simulation = 400 #min
time_coordinates = range(time_for_simulation)
DNA_number_coordinates =  #This will be updated at each time step
for t in time_coordinates:
if (t%firing_of_new_Ori) == 0:
origin_of_replications = origin_of_replications + 1 #Add more origin of replications
total_DNA = total_DNA + (1.0/full_DNA_replication_time)*origin_of_replications #Add total DNA accordingly
if (t%cell_division_time) == 0:
total_DNA = total_DNA/2.0 #At cell division, total DNA will be half
origin_of_replications = 1 #We will reset origin of replication to 1
if (t%full_DNA_replication_time) == 0:
total_DNA = total_DNA + 1 #DNA will be double when you complete one full replication
origin_of_replications = origin_of_replications -1 #Remove that origin of replication
DNA_number_coordinates.append(total_DNA) #record amount of total DNA in cell
plt.plot(time_coordinates, DNA_number_coordinates,linewidth=2, label='DNA copies')
plt.axvline(x=40,linestyle='--', color='red',label='Cell Division')
plt.ylabel('Total DNA copies in single Cell')
This simple game can give you intuition, check how DNA levels are changing in following image