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Cell Division: the Cell Cycle

The second half of this course deals with the problem of heredity. Although spontaneous

generation, the generation of a living organism from inanimate matter without the help of

another living organism, probably occurred at one time, about 4 billion years ago, it

almost certainly has not occurred for several billion years. All organisms are nowspawned by existing organisms. Moreover, the spawn almost always closely resemble

their parents in all but the most superficial respects. Kangaroos give rise to more

kangaroos and Kumquats give rise to more Kumquats. By the end of the course youshould understand how Kumquats inherit Kumquatness from their parents.

Ever since people understood the relationship between sex and reproduction, they havepondered the mystery of heredity. The Greek natural philosopher Aristotle (384BCE-

322BCE) understood that some "essence" was passed from male to female through

semen, which combined with some female "essence" to give form to the resulting

progeny. Over the following centuries there were unresolvable debates over the relative

contribution of the male and the female. Some preformationists went so far as topropose that a tiny preformed human was transferred from the male. However, further

understanding came only with the invention and exploitation of the microscope, primarily

by Anton van Leeuwenhoek (1623-1723), and the discovery of cells. Eventually,scientists like Theodor Schwann (1810-1882) came to the conclusion that all organisms

are made of cells. These cells divide to produce more cells. Moreover, the "essence" that

must be transferred from a male to a female consists of cells, namely sperm, and thesecombine with cells present in the female, namely eggs. So, to a first approximation, cells

are the units of heredity. So we must understand how cells divide to understand heredity.

According to classical cell theory, one mother cell divides into two daughter cells. For

cell division to be a productive use of the cell's time, both daughter cells must inheritfrom the mother cell everything they need to survive including the information to of what

properties the cell should have, i.e. the stuff of heredity.

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We now know that cells are full of stuff necessary to stay alive like proteins,carbohydrates, lipids, golgi, E.R. mitochondria etc. All of these components must end up

in both daughter cells. What is striking about cell division when you watch it is how littleeffort is made to make sure each daughter inherits equally - simply making sure the

division occurs roughly in the middle of the cell seems to do the trick. With one exception:

cells spend a striking amount of their time and space dividing up their chromosomes. To

understand what is so striking about this, we need to look more closely at howchromosomes are organized.

One common way of looking at chromosomes involves a karyotype. The karyotype is

just a way of staining and organizing chromosomes that makes patterns in their structure

more apparent. The chromosomes for a karyotype come from dividing cells because only

during division can you see chromosomes. A number of interesting things becomeapparent from looking at a karyotype:

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1) The number of chromosomes is always the same for a given species but varies greatly

from one species to another. Bacteria typically have one chromosome. Humans have 46

chromosomes.

2) In eukaryotes, chromosomes often come in (apparently) identical pairs called

homologs. Humans have, 22 homologs and two sex chromosomes (which look

different but act as homologs in other ways) for 23 pairs of chromosomes.

3) At cell division each chromosome has been duplicated exactly once. These duplicates

are called chromatids. They are held together by a conglomeration of proteins called the

centromere. One each pair of chromatids goes to each daughter cell.

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By now you should be pretty confused. First the chromosomes were these things you cansee in the karyotype and which come in pairs called homologs. Then each chromosome

consists of two chromatids each of which is a duplicated chromosome. Suffice it to say

that in the special case of the karyotype, chromosome refers to the pair of duplicatecopies known as chromatids. To distinguish this special case, we refer to the

chromosomes in the karyotype as mitotic chromosomes.

By now you know that a chromosome is just a single piece of DNA. These pieces can be

circular like bacterial DNA or linear like human DNA. One cannot normally see an

individual chromosome. After all, a chromosome is a single double-stranded polymermolecule and one cannot normally see individual molecules. However, it turns out that

during cell division, the DNA condenses around a set of special proteins and this huge

conglomeration of DNA and proteins is visible. For example, DNA wraps around

proteins called histones that help keep the long strands of DNA tightly packed instead oftangled (Figures 9.7 and 9.8). The combination of DNA and proteins (e.g. histones and

other proteins as well) is known as chromatin. By the time cell division occurs, all

chromosomes, i.e. double-strand pieces of DNA, have been duplicated. This includesduplicate copies of both homologs (i.e. essentially there are four of each homolog). Theidentical duplicates, which appear as chromatids, remain stuck together by a centromere

at the beginning of cell division. The duplicate or sister chromatids constitute a single

mitotic chromosome at the beginning of cell division (as seen in the karyotype).

What is so striking about the chromosomes during cell division? The fact that each cellgets one and only one of each chromosome in the form of one of the two chromatids from

each mitotic chromosome!

A brief digression into probability theory.

To get a feel for why the segregation of chromosomes is so extraordinary, it isuseful to understand how unlikely it is to occur by chance. Suppose each

chromatid had an equal chance of ending in either of the two daughter cells and

the chromatids had no effect on each other. That is, if one chromatid went to

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daughter cell A then this does not affect the chance that the other sister chromatidwould end up in cell A or cell B. In probability theory these are referred to as

independent events and the probability oftwo independent events is the product

of the probability of each individual event. If the probability that a chromatid endsup in cell A or B is 50:50, then the probability that both sister chromatids would

end in cell A is 0.5*0.5 = 0.25 or 25%. The probability they would both end up incell B is also 0.25. The probability that at least one daughter cell would not get achromatid for a given chromosome is 0.25+0.25 = 0.5 or 50% and similarly the

probability that each daughter would get one chromatid is also 50% or 0.5. What

about the other chromosomes? Lets assume segregation of the chromatids from

different chromosome is also independent. Then the probability that both daughtercells will get a chromatid from chromosome I is 0.5 and the probability that they

will both get a chromatid from chromosome II is 0.5 and the probability that both

daughter cells will get a chromatid from both chromosomes I and II is

0.5*0.5=0.25. For three chromosomes: 0.5*0.5*0.5=0.125. For fourchromosomes: 0.5*0.5*0.5*0.5 etc. The probability that both cells will inherit one

chromatid from each of the 46 chromosomes in a human cell just by chance is0.546

= 1/70,000,000,000,000. This is approximately the probability of winning

the Lotto 649 twice in a row!

By contrast let us consider mitochondria. Mitochondria have genetic materialand are essential to the survival of the cell. On the other hand, they are like little

bacteria that live in the cytoplasm. Even a few can reproduce and repopulate the

cytoplasm with as many mitochondria as the cell needs. So we might pose the

question , "even if the cell does nothing special to segregate the mitochondria,what are the chances that either of the daughter cells won't have any mitochondria

after cell division?" This depends on the number of mitochondria so lets be

conservative and say there are ten mitochondria. The probability of daughter cellA not inheriting mitochondrion#1 = 0.5, mito#2 = 0.5 etc. The probability of not

inheriting #1 or #2 or #3 etc (i.e. not inheriting any mitochondria) = 0.510

=1/1000. In other words you are bound to inherit at least one and one is all you

really need.

OK, the probability of dividing up the chromosomes one copy per cell is small, so the cellmust have a mechanism for doing it properly. But is it really important? What is the worst

that could happen if you mess up a little? Very bad things in fact. Most organisms need

exactly one of each chromosome. How do we know? Consider humans: no human can

survive (from conception) if a non-sex chromosome (autosome) is missing. The only casewhere you can survive past infancy with an extra chromosome is Down Syndrome,which results from an extra chromosome 21. An extra copy of any other (autosomal)

chromosome is lethal. (Sex chromosomes are exceptions and we'll discuss why that is

later).

1) Organisms need at least one of each chromosome (presumably because they allcarry essential genetic material).

2) Organisms typically need exactly one of each chromosome.

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Clearly, dividing up the chromosomes properly is very important and this is exactly thekind of property you might expect from the genetic material.

Organizing Cell Division: the cell cycle.

Clearly if dividing up the chromosomes is so essential, then the first problem the cell must

solve is to make sure that there are exactly the right number of chromosomes when thecell needs to divide, no extras and none missing. To make sure that every cell division is

preceded by one and only one duplication of the chromosomes, the eukaryotic cell goes

through a very precise series of steps:

1) DNA replication (S phase)

2) Mitosis (M phase) - the process by which somatic cells make identical copies

(clones) of themselves by creating daughter cells that inherit one copy of eachchromosome

or

Meiosis - the process by which germ cells make non-identical copies of

themselves by creating daughter cells that inherit one copy of each homolog. In

other words, the daughter cells end up with half the DNA of the mother cell.

3) Cytokinesis - dividing the cytoplasm in two (optional).

In between the M and S is a gap, a period of time when the cell is sort of resting. This gapperiod is called G1. In between S and M is a second gap period when the cell is preparing

to divide called G2. The amount of time spent in each of these processes can berepresented in a timeline but, since it is a repeating process, it makes sense to represent it

as a time circle. It is called the cell cycle (Figure 9.4). You will notice that cytokinesis is

not represented on the cell cycle (I suppose because it is optional) but it normally occursright after mitosis. Note also that the length of time in each of the various stages

(especially G1) can vary considerably, even among cells in the same organism.

Interphase refers to all phases of the cell cycle except M phase.

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It should be clear why the cell needs a cell cycle. It would be disastrous for the cell todivide before its chromosomes had been replicated. The cell must have an orderly

procedure like the cell cycle to make sure everything happens sequentially. But how doesthe cell prevent, for instance, M-phase (mitosis) from occurring before S phase (DNA

replication) is finished?

Part of the answer is that cells have a quality control system whereby they check for

completion of each step before they continue to the next. There are special "checkpoints"

at the end of each phase of the cell cycle. Certain conditions must be fulfilled before thenext phase can begin. For example, the cell has a checkpoint at the end of Sphase; the cell

will not proceed to G2unless it senses that all the DNA has been replicated no matter howlong that takes.

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We can show this by adding a drug (hydroxy urea) to the cell that prevents DNA

replication. The cell gets big but never even starts to divide; the checkpoint keeps the cell

in S phase and prevents it from proceeding to G2.

We can inactivate this S-to-G2 checkpoint by adding another drug, caffeine. Normallythis is not a problem for the cell, DNA is replicated well before the checkpoint is reached

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so if the checkpoint is inactivated, the cell is none the wiser Figure.

But if you add both caffeine and hydroxyurea the cell proceeds through the caffeine-

inactivated S-to-G2 checkpoint (in spite of the fact that the hydroxyurea has preventedchromosome duplication) and enters mitosis. Since there aren't enough chromosomes for

two daughter cells they both die. Figure

It is not well understood yet how the cell senses that, for instance, DNA is replicated andhow it activates the processes necessary to carry it to the next step of the cell cycle. It is

clear however that proteins called "cyclins" and "cdks" play a central role. The

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concentrations of different versions of these proteins rise and fall with the cell cycle andinterfering experimentally with their concentrations affects the ability of the cell to

proceed through the cell cycle. The book uses the example of cyclin D and cdk4 to

illustrate (Figure 9.5). However there are cyclins and cdks for every step of the cell cycle.

The cell cycle and cancer.

Most cells in your body have not divided for a long time and have no intention of dividing

anytime soon. Cells that have stopped cycling almost always arrest in G1. The G1 to S

checkpoint is critical; a cell that passes this checkpoint almost always goes on to divide.

So what does the G1 to S checkpoint look for? Unlike the S to G2 checkpoint which looks

for internal signals (e.g. replicated DNA) the G1 to S checkpoint mostly looks for externalchemical signals from other cells. What are the external signals good for? For instance, in

mammals cyclin E becomes active in response to hormonal signals during pregnancy,which results in the proliferation of breast cells that are necessary for lactation. In general,

cell division is very tightly choreographed by complex signals among cells to ensure thatyou have the right numbers of the right cells in the right places. Its why you have ten

fingers and not eleven.

The flip side of a properly regulated cell cycle is the unregulated proliferation of cells

that is the basis of cancer. If something goes wrong with the checkpoint controls that

regulate cell division, cells divide continuously and at the wrong time. For example, adefect in cells that causes continuous over-expression of cyclin E can contribute to breast

cancerbreast cells divide as if during pregnancy even when there is no pregnancy.

There is a great deal of interest in how the cell cycle is regulated because of the beliefthat drugs that interfere in cell cycle progression would be useful for treating cancer.