细胞生物学资料:02课堂思考(8.30—9.13)

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2002级细胞生物学

Testing & Thinking of Cell Biology

Testing and Thinking(8-30-2004)

1. Assume you were given a mixture consisting of one molecule each of all possible sequences of a smallish protein of molecular weight 4800. How big a container would you need to hold this sample? Assume that the average molecular weight of an amino acid is 120.

2 An adult human is composed of about 101

3 cells, all of which are derived

by cell division from a single fertilized egg.

A. Assuming that all cells continue to divide (like bacteria in rich

media), how many generations of cell divisions would be required to produce 1013 cells?

B. Human cells in culture divide about once per day. Assuming all cells

continue to divide at this rate during development, how long would

it take to generate an adult organism?

C. Why is it, do you think, that adult humans take longer to develop

than these calculations might suggest?

Answer

1. A protein with a molecular weight of 4800 is made of about 40 amino acids; thus there are 1.1 × 1052(= 2040) different ways to make such a protein. Each individual protein molecule weighs 8 × 10-21 g (= 4800/6 × 1023); thus a mixture of one molecule each weighs 9 × 1031 g (= 8 × 10-21 g ×

1.1× 1052), which is 15,000 times the total weight of the planet earth,

1.1weighing 6 × 1024kg. You would need a quite large container, indeed.

Answer:2

A. The number (n) of generations of cell divisions required to produce 1013

cells is 2n = 1013

It is useful to remember that 2110≅ 103 (2n produces the series: 2,

4, 8, 16, 32, 64, 128, 256, 512, 1024; thus, 2110= 1024 ≅ 103). If

103 cells result from ten generations of dividing, 1012 cells will

result from 4 x 10 = 40 generations. Thus, you can estimate quickly

that it will take a little over 40 generations to reach 1013 cells.

You can get a more accurate answer, 43.2, by plugging different

values of n into your calculator. Alternatively, you can solve the

equation for n, which tests your familiarity with logarithms.

Remember that

2 = 10log2 and 2n = 10nlog2

Substituting,

10nlog2 = 1013

Taking the log of both sides,

Nlog2 = 13

n = 13/log2 = 13/0.301

n = 43.2

B. If cells divided once per day and all cells continued to divide, it would

take 43.2 days to generate the number of cells in an adult human.

C. Obviously we don't become adults in 43 days. The simple answer is that

all cells don't continue to divide once per day and some cells are

programmed to die. As cells differentiate, they generally slow their

rate of division, ultimately in the adult dividing just often enough

to replace cells that are lost or die. Of course, the real answer

is much more complex, involving time for cell movements, for local

environments to be established, for extracellular matrices to be

laid down, for cells to differentiate, for global patterns to develop,

and so on.

Testing and Thinking(9-1-2004)

1. Friedrich Miescher (1844-1895), Gregor Mendel (1822-1884), Louis Pasteur (1822-1895), and Rudolf Virchow (1821-1902) were European scientists (from Switzerland, Austria, France, and Germany, respectively) whose most important discoveries were made in the 20-year period from 1855 to 1875. Assume that the four men met at a scientific conference in 1875 to discuss their respective contributions to biology.

(a) What might the four scientists have found they had in common?

(b) What do you think Pasteur might have found most intriguing or relevant about Virchow's work? Explain your answer.

(c) What do you think Virchow might have found most intriguing or relevant about Mendel's work? Explain your answer.

(d) In whose work do you think Miescher might have been the most interested?

Explain your answer.

Answers for (9-5-2003)

1.(a) The four scientists might have found that they had common interests in the origin of cells (from preexisting cells, not by “spontaneous

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