What you should
know about the processes
“Broader
applications” of cell division concepts
A topic of
considerable interest in our considerations of population ecology, animal
behaviour, and evolutionary biology is reproduction. Since the key measure of
success for organisms is whether they replace themselves, and thus achieve
persistence on the planet, the various means they use to do so become central
to our thinking, but in order to think through the implications of complex
life-cycles, and the forces acting upon organisms favouring variable balances
between sexual and asexual reproduction, it is important to be clear from the
start about the basic underlying processes: mitosis (involved in the growth
of a body as well as in asexual production of new bodies), meiosis
(sometimes producing gametes and sometimes other cell-types, but always
involved with sexual diversification of populations), and cytokinesis
(often, but not always, associated in its occurrence with the nuclear-division
processes of mitosis and meiosis).
Theoretically, students entering Biology 121 really ought to know
almost every technical point about nuclear and cell division: the names and
sequence of the stages, the large- and small-scale events characteristic of
each stage, the role of cell architecture like microtubules and microfilaments
in bringing about the steps, and so forth. In practice, though, I will not
ask you to enumerate the stages or the details in an exam. I will, however,
proceed in lectures on the assumption that you are aware of the information, or
at least that you will go and refresh your knowledge when necessary. For
example, when we consider the phenomenon of independent assortment, it would be
difficult for you to predict progeny ratios unless you understand the meiotic
steps which cause chromosomes to align as they do in the two divisions.
So…
u Mitosis – involves “simple” nuclear division, so daughter nuclei
are clone copies of the original (except for any mutations). Whatever the ploidy of the first nucleus,
the daughter nuclei will be of the same ploidy. Always preceded by DNA
replication.
u Meiosis – more complex nuclear division. Daughter nuclei each
inherit a quarter of the amount of DNA included in the original nucleus,
usually one haploid set of single-stranded chromosomes derived randomly from
the original diploid set of double-stranded ones. First division preceded by
DNA replication, the second division not. Delay between divisions may be
lengthy or almost nonexistent. Daughter nuclei may all survive (spores, sperm
cells), or only some do (most eggs).
u Cytokinesis – largely independent of nuclear division.
Involves division of the cytoplasm and included organelles, usually to send
equal amounts to each daughter cell but sometimes highly unequal (especially in
egg production). May or may not occur between the meiotic divisions, and in
most organisms (except coenocytic
forms) does occur following mitosis and following the second meiotic
division.
Freeman’s text is really excellent in the area of
providing detailed background for all aspects of division. [See the What incoming
students are expected to know document for references.] In addition to
having a broad overview of what occurs, you should also try to develop
an understanding of why things occur as and when they do. For instance,
we might consider some or all of these puzzles:
§
Why
is a dividing cell unlikely to be expressing any genes as it divides?
§
Why
do DNA replication and messenger RNA production often occur more or less
simultaneously (even if it seems potentially confusing, even chaotic)?
§
What
role is played by quaternary structure (considered broadly) at the various
stages of the cell cycle?
§
Why
does meiosis require two divisional steps instead of one?
§
Why
would unequal cell-division (often lethal for the smaller products) ever
evolve?
… and so forth – you begin to get the idea!
Once you have grasped the essentials of cell-division
processes, you can begin to explore the ways in which divisional processes have
evolved to suit ecological and evolutionary requirements.
Growth – in some organisms, like plants
and animals, growth of the body may involve making each cell larger, but more
often involves making more cells by mitosis. Large body-size confers many advantages
on organisms that can achieve it, but also imposes costs: a large body
makes an organism succeed in competition for resources, but also makes it hard
to hide from specialist predators or bad weather; large bodies allow for the
storage of energy and nutrients to cope with an uncertain environment, but also
require costly internal plumbing to move stored materials around to where they
are needed, and a large intake of food in total. You could even propose that
the repeated cell-division needed to grow a large body may make the body more
prone to diseases of unmoderated division – cancers.
Now in some organisms, like fungi, relatively large bodies with many nuclei are pretty much coenocytic rather than subdivided into cells or cell types. We often view the presence of distinct cell types, organized into tissues and organs, as a sign of efficiency, but fungi seem to be doing quite well! Why should some organisms have mitosis with, and others more or less without, cytokinesis? How necessary is local specialization of body parts for a relatively large organism? We may not be able to answer these questions satisfactorily, but they could lead to other interesting hypotheses, and stretch our understanding of life usefully.
The cases of multinucleate organisms, whether cellular (one nucleus per cell) or coenocytic, suggest that a single nucleus can control only a certain volume of cytoplasmic machinery. In a fungus, this isn’t too surprising – how could a cytoplasmic organelle “know” which of several nearby nuclei it should “obey” when carrying out functions? Distance must impart information. Cell sizes, however, vary quite a bit in eukaryotes. Why should there be a specific range of cell volumes, in either multicellular or indeed single-celled eukaryotes? You could even ask why a single-celled eukaryote grows to a more or less fixed size before dividing: when only one nucleus is present, why can’t a cell get dramatically larger? Is there a limit on how much cytoplasm a single nucleus can control? If so, why? (A size limit can’t exist because there is confusion about which nucleus is in control.) Of course there are surface-area/volume issues which must limit the maximum size of a whole cell… but why aren’t cells all that one, same size? Thus growth at all levels poses challenges of explanation.
Life
cycles –
Freeman’s biodiversity chapters outline the wide range of observed life-cycle
patterns. In many organisms, mitosis is the dominant (or in asexual species the
only) process. Even in sexual species capable of meiosis, non-sexual vegetative
reproduction can go on for hundreds of cycles before any sign of sex appears –
consider liverworts, or mycelial
fungi, or even Hydra. Clearly for those organisms the flexibility to
engage in sexual (meiotic) or asexual (mitotic) processes provides a
real advantage. This of course leads us to ask why the same flexibility doesn’t
exist in many species… is it an indication that they are inferior? Or does it
suggest that sexual-only cycling confers a huge advantage? But then, why aren’t
all organisms cycling sexually?! Presumably one path out of this dilemma
is to ask “under what circumstances will different modes of
reproduction be advantageous?”, and we will spend some time investigating
this question.
Apart from strictly reproductive issues, how do life-cycles and their division-processes relate to other aspects of the lives of the organisms involved? For example, consider dispersal to new sites: what kind of life-stage is best suited to this? In some species, a sexually-produced propagule is used, while in others an asexual propagule occurs. Why? And in some species, the life-cycle is critically dependent on some outside agency (wind, running water, the actions of other species) – why should organisms have ever evolved that sort of (risky) dependency? And in a different direction, we see some organisms which are almost always diploid, some almost always haploid, others with a more even time-split, and still others with haploid and diploid units breeding together (!!) – how can we make sense of this diversity? And why are there organisms with not just one or two, but sometimes dozens, of chromosome-sets? Once again, several of these questions probably cannot be answered, but they do stimulate thinking.
Genetic
variability
– tradeoffs between sexual and asexual stages are often explained through the
possible role of genetic variability in populations. Thus we can appreciate that
life-cycles and cell-division processes relate not only to environmental forces
but also to genetic and molecular ones. If an organism is asexual, each of its
offspring amounts to a copy of itself – excellent fitness outcome, right? So
why aren’t all organisms asexual? Sexual organisms are usually only
about 50%-related to each offspring (sometimes it can be nearer to 100%,
sometimes nearer to zero) – so apparently on average a much poorer fitness
showing, yet many sexual species thrive and dominate landscapes in spite of
this “shortcoming”. How can we reconcile this state of affairs under the
assumptions of natural selection? One thing we cannot conclude is that
organisms plan for their own future needs – so how can sexual organisms
“anticipate” the advantages their diverse (genetically mixed) offspring may
later enjoy? And how much more diverse are they, actually, than an
asexual brood (does it really make a difference)? As for the issue of
life-cycles, the resolution of the variability question involves situational
thinking, and an understanding of how the balance of forces in an environment
dictates advantages to mitotic and meiotic processes.