and that the size is controlled by the cell length, such that the change in
volume with time is simply the change in length with time times a constant
prefactor; then consider the change in protein concentration as a function of
the change in the total volume into which the original
N
proteins are diluted.
(b) We can repeat a calculation like that given above using a discrete
language. Imagine that before cell division, the number of copies of a given
transcription factor in the cell is
N
. In particular, for every cell doubling,
the number of proteins is reduced by a factor of 2. Using such a picture,
write a formula for the average number of proteins per cell as a function of
the number of cell divisions and relate this result to that obtained in part
(a). Furthermore, by using the fact that 2 = exp (ln 2), reconcile the discrete
and continuous pictures precisely.
(c) Interestingly, the model used in part (b) opens the door to one of the
most important themes in physics, namely, that of ﬂuctuations. In particular,
as the cells divide from one generation to the next, each daughter does not
really get
N/
2 copies of the protein since the dilution eﬀect is a stochastic
process. Rather the partitioning of the
N
proteins into daughter cells during
division follows the binomial distribution. Analyzing these ﬂuctuations can
actually lead to a quantiﬁcation of the number of copies of a protein in a cell.
In this part of the problem, work out the expected ﬂuctuations after each
division by noting that the ﬂuctuations can be written as
q
<
(
N
1

N
2
)
2
>
,
where
N
1
and
N
2
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 Winter '09
 DNA, Cell nucleus, Rosenfeld, Physical Biology of the Cell

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