The Meaning Behind Meiosis
Thousands of millions of years in the past of our planet, life was relatively simple. The Hadean had long since
ended, and asteroid impacts were now infrequent. The Archaean Eon had witnessed the evolution of simple prokaryotic
cells: some of which could use sunlight to convert carbon dioxide into sugars, while others adapted to eat those sugar
producers. Even after the evolution of more complex Eukaryotic cells, organisms were still unicellular, and still
reproduced through mitosis. Along the journey, though, some key traits had evolved in the vast sea of unicellular
microorganisms that inhabited Earth’s oceans. During the Proterozoic Eon, some of these will lend a new source of
diversity and variation for evolution to work on, and by the end of that eon, the diversity of life will explode.
A primary problem of mitosis as a means of replication is that it in general produces clones…with the exception
of occasional mutations, one cell that divides mitotically will simply produce more identical cells, possibly resulting in a
huge number of what are essentially cloned organisms. The main problem with this is that since all the organisms in the
population have identical genomes, they are all equal. (A genome is a full set of all the genes an organism has.) In
evolutionary terms, there can be no meaningful selection if there is no variation. It is likely that this lack of variation is
why most of the history of life on Earth was confined to a rather unexceptional crowd of microorganisms: evolution was
crawling along, since rare non-deleterious mutations were the only source of variation.
An early solution to this problem of variation was to find a way to gain new genes that didn’t rely directly on
mutation or duplication errors…after all, the more genes a cell has, then the more proteins it can make. The more
proteins it can make, it more things it can build and do. Early in life’s history, the ability to accomplish transformation
and conjugation both provided cells with a means to accumulate new genes from their environment, thereby increasing
the amount of variation within populations.
While we are uncertain which of these evolved first, a decent candidate might be transformation. This is the
process where a cell grabs a piece of genetic material that is floating around in the environment and uses the process of
endocytosis to bring it into the cell. These floating chunks of DNA are called plasmids. (In answer to the question of
where plasmids may have first come from, the answer may simply be they were the leftover bits of genetic code from
dead, broken open cells.) Once inside the cell, they can either be inserted into the cell’s chromosomes, or can float free
in the cytoplasm. In either case, the cell usually can transcribe the plasmid’s information into mRNA so that a ribosome
can translate it into a functioning protein. (There is some decent evidence that the ability to absorb new DNA from
plasmids into an organism’s genome evolved from a pre-existing ability to repair damaged DNA during replication.) We
know this is a relatively ancient trait, because lots of modern prokaryotes have this ability, implying that they are all
descended from an ancient ancestor that originally evolved the ability to use transformation to enhance its genome.
(The need for this enhancement may be linked to stressful conditions, where DNA can be damaged by the environment.
Thus, the evolution of this ability to gain new DNA may have evolved also as an adaptation to the problem of how to
replace damaged DNA.)
A similar process is called conjunction. In this case, instead of cells finding plasmids in their environment, they
found them in each other. When two cells would interact, they could briefly join their membranes together to allow
things like DNA plasmids to be exchanged. Just as in transformation, the genes gained in this way could be integrated
into a cell’s genome. As in transformation, this resulted not only in genes spreading through a population, but also
increased variation: not only could cells have different genes, but they could have different combinations of genes. All
this variation gave more fodder for the grist mill that is selection, accelerating the likely progress of evolution.
A critical part of this that you must remember is: the ability to join cells together and the ability to swap around
bits of DNA is an ancient piece of life’s technology…but it is the foundation for the later invention of the springboard
that would likely launch the evolution of complex multicellular life on this rock falling through space that we call Earth.
It is the foundation for the invention of sexual reproduction.
Around 1.2 BYA, at least one Eukaryotic cell developed that had the ability not just to replicate itself, but it could
also join together with another cell. This was not the temporary joining for genetic exchange that is seen in prokaryotic
transformation. When this new form of joining happened, the result would be a new, larger cell with two sets of
chromosomes: one from each cell that joined together. This was a new organism, created by the union of originally
separate cells. This was sexual reproduction.
Before we continue, it might help to have a simple way to tell the difference between a cell that has just one set
of genes and a cell that has combined with another to form a sort of supercell with two sets of genes. The technical
term for a cell that has a single set of genes is that it is haploid. A cell that contains two sets of genes is known as
diploid. (Humans, by the way, are diploid.) There are organisms that have four sets of genes: these are tetraploid.
Anything more than that is typically just known as polyploidy. (Obviously, ploidy just refers to how many sets of genes
an organism typically has. As we’ve seen before, the name for a full single set of genes is the organism’s genome. When
talking about ploidy or genomes, a full genomic set is usually represented as n. A haploid organism is therefore 1n,
while a diploid organism is 2n, and a tetraploid organism is 4n, and so on.)
So, somewhere back in the mists of time, certain eukaryotic cells started spending part of their time joined
together as a diploid organism. From now on, they could replicate clonally through mitosis, or they could replicate
sexually through a different process: meiosis. Mitosis creates a genetically identical copy of the original cell. A diploid
2n cell will mitotically divide to form two new 2n cells. Meiosis is similar, but it is a two-step process that results in 4
haploid 1n cells that are not identical to the original cell. It is these haploid cells that then go off and join together with
some other haploid cells to form a new 2n diploid organism.
Originally, this new stage in the life of an organism was probably similar to modern zoospores. These are
haploid cells produced by diploid parent cells through meiosis, and they are mobile. Typically, they float or swim
through the water or air, living independently until they meet up with another zoospore from the same species…they
then unify to create a diploid organism, settle down and start replicating through mitosis to form a colony. Later, as
resources near the colony grow scarce, a new generation of mobile, haploid zoospores are released to go found new
colonies somewhere else. This process of alternating between a stationary and colonial diploid state and a haploid but
mobile zoospore state is sometimes called alternating generations, and it is still used by more primitive organisms today.
(Please note that when we say something is primitive, what we really mean is that it is supposed to resemble something
like an original state, how things used to be.)
As humans, we are descended from a very distant unicellular eukaryotic ancestor who developed this ability to
generate haploid cells, and we maintain this trait today with our ability to generate haploid human cells. Like those
ancient creatures, we spend part of our time as individual diploid organisms. (Unlike them, we spend the vast majority
of our time in this state.) Also like them, we can produce haploid cells that can exist as a separate organism for a time
(although in our case, the time these organisms can survive once separated from us can be measured in seconds and
hours rather than days or months.) In our case, these haploid cells are known as gametes, and like all other animals, the
gametes we produce are known as sperm and eggs. They are the haploid part of our life cycle, and when unified they
create a new, independent organism that is similar to but unlike each of the diploid parent organisms that contributed
their gametes to allow It to form. To understand the mysteries of how traits can combine and disappear, hide and
reappear from generation to generation will be the next topic, when we discuss inheritance and the mechanisms of
meiosis.
Use this text (and any other) to create a personal definition that explains or demonstrates why this term is
important to this text’s topic.
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“n”
Alternating Generations
Clone
Conjunction
Diploid
Egg
Gamete
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Genome
Haploid
Meiosis
Plasmid
Primitive
Sexual reproduction
Sperm
15. Transformation
16. Zoospore