Where sex came from

Sex
Sex (Photo credit: danielito311)

When I decide on a topic, I usually hold onto it in my mind, and maybe flesh out a few ideas mentally. I’ve mentioned this before. This topic suddenly came to mind for no good reason, and I haven’t thought of any significant lines of discussion. I wish I could snapshot the “Related Articles” that have popped up, but it looks like they will lead me to places I do not intend to go. An example is “Casual sex isn’t just for college kids”. Mmm. As if college kids a) invented it b) have a monopoly in it.

The Cool Kids
The Cool Kids (Photo credit: TheMarque)

But I digress. Sex. Most living organisms have it. Amoeba, the popularly held archetype of the simple single celled organism, was believed to reproduce simply by fission. I’m unable to understand much of the scientific literature about amoeba reproduction, and there doesn’t seem to be much material about it anyway, but fission, I believe, results in each child organism having half the genetic material of the parent cell.

AMOEBA PROTEUS
AMOEBA PROTEUS (Photo credit: PROYECTO AGUA** /** WATER PROJECT)

Maybe nuclear genetic material is doubled before the split. Maybe each ‘individual’ is half an individual and needs to find another ‘individual’ in the same state to merge with? Merging has been observed in amoeba.

What is certain is the enormous size of the genome of an amoeba species. Some of them have genomes which are more than 200 times the size of the human genome. Amoeba are presented to us in school as possibly the simplest organism that there is. Based simply on the size of genome, this isn’t true.

human genome
human genome (Photo credit: vaXzine)

I can conjecture, based on little to no knowledge at all of the genetics of amoeba, that fission and fusion would enable amoeba species to mix and match their genetic material with much greater freedom than simple sexual reproduction.

So, amoeba splitting and merging could create an enormous genome, even in a simple organism. The size of a genome could be just a result of a less restrictive reproductive process than applies to more “advanced” multi-celled organisms (not to mention more “advanced” single-celled organisms.

Martin Krzywinski, Circles of Life - a compari...
Martin Krzywinski, Circles of Life – a comparison of human and dog genomes (Photo credit: chrisjohnbeckett)

If I’m correct or anywhere near close to correct about the amoeba genome and its reproduction, amoeba may represent an early stage of sexual reproduction. Amoeba were inventing reproduction, in a way. One can imagine that early organisms would absorb other weaker organisms, and in doing so, acquire their genetic material or proto-genetic material.

a haploid cell
a haploid cell (Photo credit: Wikipedia)

Of course in most cases they would simply digest it,  but in those days, the early days of life, when the chemical processes and genetic processes of life were shaking down into the rules that we know today, things would have been more fluid and the genetic material could have been incorporated into the organism’s own genetic material. Indeed, in the beginning the genetic material would probably not be distinguishable from other material in the organism. There wouldn’t have been a nucleus, as such.

English: In telophase, the nucleus of one cell...
English: In telophase, the nucleus of one cell is divided equally into two nuclei.It is the last stage of mitosis and directly proceeds interphase. (Photo credit: Wikipedia)

One can imagine that in the beginning, organisms just didn’t reproduce, by fission or any other method. They would have fairly quickly died out. Then organisms could have happened which just grew and grew until they split. Parts would have died off, parts would have lived.

The parts that survived would have been changed, modified by the environment, until the bits that would have earlier died, survived as new individuals. Maybe they couldn’t themselves reproduce, but eventually, the split off bits would have survived and been able to reproduce.

Diagram of bacterial binary fission.
Diagram of bacterial binary fission. (Photo credit: Wikipedia)

In retrospect, it appears that the best way to be able to reproduce is to separate the reproductive materials and functions mainly into a single location, the nucleus.

Organisms can they reproduce simply by duplication of the genetic information in the nucleus, producing a clone of themselves, which they can hive off as a new individual. Some organisms bud off a clone of themselves as a reproductive process.

Production of new individuals along a leaf mar...
Production of new individuals along a leaf margin of the air plant, Kalanchoe pinnata. The small plant in front is about 1 cm tall (Photo credit: Wikipedia)

This doesn’t allow for change in environment though. A self-cloning organism can’t react to environment changes. However if organisms can exchange genetic material while creating a child, it may be that the child’s genetic make up may allow it to survive where its parents would struggle.

The process used by amoebas, that is to say the process of division and merging of individual organisations could be the first step in that direction. Of course, uncontrolled merging could result in possibly viable individuals with large genomes, which is what we see in some amoeba.

Immature and mature fruits of Cocculus orbicul...
Immature and mature fruits of Cocculus orbiculatus….Trái của dây Sâm, dây xanh, Mộc Phòng kỷ … (Photo credit: Vietnam Plants & The USA. plants)

There are two routes here. Either an organism would clone its nucleus including its genetic material, then split, producing two identical organisms. or it could halve its genetic material and merge with a similarly haploid organism, resulting in a diploid individual.

The advantages of the haploid/diploid cycle are obvious – genetic material is mixed so at least some individuals may survive an environmental change, because the expression of the genome in the individual (the phenotype) allows them to differ from their parents and survive the change.

English: Illustration of the chromosomal organ...
English: Illustration of the chromosomal organisation of haploid and diploid organisms. (Photo credit: Wikipedia)

What is less obvious is why organisms split into male and female sexualities. It’s possible that the difference is caused by the necessity of one set of haploid individuals to supply an environment in which the child organism can develop. The other set of haploid individuals merely supplies the other half of the necessary genetic material.

So the female supplies the support environment plus the genetic material, or egg (ovum) and the male supplies only the genetic material, the sperm. One can imagine that originally organisms would directly exchange genetic material by fusion and fission, like amoeba, but at some time it became more efficient to disseminate genetic material outside the organism.

English: Electron microscope image of sperm.
English: Electron microscope image of sperm. (Photo credit: Wikipedia)

Cells within multicellular organisms or possibly unicellular organisms developed the ability to create new haploid cells with a copy of half the genetic information leaving behind unicellular haploid organisms or haploid cells within a diploid organism.  In female organisms the haploid cell would be an egg and would have the support environment to create a new diploid individual, and in male organism the haploid cell would just have half the genetic material and be a sperm.

Description unavailable
Description unavailable (Photo credit: EYECCD)

There are some hermaphroditic animals, for example some snails and slugs, which produce both eggs and sperms and many plants have both male and female characteristics, but many, many animals have separate male and female individuals. (I’m not keen on saying the majority of animals display sexual differentiation, because I don’t know if it is true.)

English: hermaphrodite symbol
English: hermaphrodite symbol (Photo credit: Wikipedia)

So, when life began it would have been simple unprotected self-replicating molecules. Growth would have been by accretion. At some stage the molecules would have evolved to the point where they developed some structure around themselves, maybe by rejecting some unwelcome molecules. Organelles, small biological factories would have developed as the organisms became more complex, all enclosed in a membrane that allowed the necessary chemicals in and unwanted ones out. This membrane would eventually enclose the nucleus of the cell. More complexity, more biological factories, and the cell would have formed an outer membrane, that enclosed all the necessary mechanisms that modern cells contain and require. (OK, I’m no expert so some of these conjectures may be wrong).

High magnification transmission electron micro...
High magnification transmission electron microscope image of a human leukocyte, showing golgi, which is a structure involved in protein transport in the cytoplasm of the cell. JEOL 100CX TEM (Photo credit: Wikipedia)

Cells would initially have not had any reproductive mechanism. They would grow and then split when they got too big. When cells developed specialised mechanisms for reproduction they needed some way of passing on the genetic material. Some cells would have developed a method of creating haploid individuals and these would have then merged with other haploid individuals to create normal diploid individuals.

English: Male and Female Superb Fairy-Wren.Tak...
English: Male and Female Superb Fairy-Wren.Taken in Ensay, Victoria. (Photo credit: Wikipedia)

Or maybe so-called haploid individuals arose first and diploid individuals arose from the merger of two haploid individuals.  When multi-cellular organism arose, they evolved special organs related to reproduction. Such organs created haploid versions of the organism and a method of delivery to the outside world of these eggs and sperm.

Once individuals have evolved to specifically create eggs or sperm, they are sexual individuals. If an individual evolved to create a support system for their haploid genetic material, for example eggs, it would find it difficult to find similar individuals to merge with since eggs are not particularly mobile. Sperm on the other hand are specialised to be mobile, so are ideal for merging with the eggs.

English: Male and female Sockeye salmon (Oncor...
English: Male and female Sockeye salmon (Oncorhynchus nerka) specimens. (Photo credit: Wikipedia)

Once the individuals have sexual differentiating characteristics this would flow through to the phenotype (physical expression of the genetic material – the multi-cellular organism). And that is my guess, as a complete amateur in the field of genetics is where sex came from. So the above may make sense at some level, or not. Even it does make a sort of sense, I may well be wrong about the detail! But it has been fun speculating.

here comes life
here comes life (Photo credit: AlicePopkorn)

 

 

 

 

 

 

Enhanced by Zemanta

Why does cancer spread?

The Human Body -- Cancer
The Human Body — Cancer (Photo credit: n0cturbulous)

Everyone, I’m sure, knows of someone who has died of cancer. It’s a disease that is wide-spread and seems to be more common now that other diseases are coming under control. It may be that cancer is certain to appear in the human body if it lives long enough. There’s a cheery thought.

Wikipedia describes cancer as follows: “known medically as a malignant neoplasm, (it) is a broad group of diseases involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumours, and invade nearby parts of the body”.

Cross section of a human liver, taken at autop...
Cross section of a human liver, taken at autopsy examination, showing multiple large pale tumor deposits. The tumor is an adenocarcinoma derived from a primary lesion in the body of the pancreas. (Photo credit: Wikipedia)

There’s two parts of this definition. Firstly there is the unregulated cell growth and secondly there is the spread of the growth to other parts of the body. The unregulated cell growth is usually attributed to a number of causes, but the mechanism is usually given as damage to the genetic material. The causative agent whatever it might be, damages the genetic material and this results in a huge proliferation of cells.

I write computer programs as tools for doing my job. Each program ends up as a string of data which a human has a hard time decoding, though of course the computer hardware has no problems. The effect of changing a single bit (actually, a byte) of a program would almost certainly cause it to crash. In so far as the analogy that the genetic code resembles a computer program holds, this indicates that a random change to the genetic code would most likely result in the death of the cell. It would require a specific hit, to say a piece of code that control the termination of a loop that would let the program “grow out of control”.

Out of memory ATM
Out of memory ATM (Photo credit: RuiPereira)

I’d guess, from a position of almost total ignorance, that changes to the cells in the body occur all the time. Changes happen to the genetic code in a cell, and it almost certainly dies. (A side question is : what exactly happens to a cell when it dies? Presumably some process or other ‘detects’ the death and breaks it up? Or does the first failure cause other processes to fail until the integrity of the cell is lost? Lots of questions). However, unless the failure is dramatic, explosion-like rather than simple deflation, it should not affect its neighbouring cells, should it? So in general the death of a single cell is probably not noticeable.

Going back to the computer program analogy, in a computer there are dozens, if not hundreds of programs running all the time, but the user is not aware of any other than the one he is interacting with. Equating cells with computer programs, it is most likely that a random change would cause a program or a cell to die, with little effect on the computer or body as a whole.

Facit computer memory
Facit computer memory (Photo credit: liftarn)

All the programs running in a computer need ‘memory’ to run in, and there is a limited supply of memory, so (conceptually at least) one program is given the task of managing the memory allocations. Damaging the memory manager program could theoretically lead to it repeatedly allocating memory until there was none free for allocation. The computer would, once again, crash. If a program is damaged in a particular way it could ask repeatedly for memory and again cause itself or other programs. or even the whole computer to crash.

In a living organism there does not seem to be a single equivalent of the ‘memory manager’ or ‘resource manager’. In a living organism everything seems to be done by consensus between cells. (That is both anthropomorphic and probably naive, but it will do, I think).

So, although I’d estimate that the vast majority of changes to the genetic code would result in cell death and nothing else, a very, very small number of changes could result in the cell soaking up as much of the cell-level resources as it can and damaging the cell itself, its neighbouring cells or the whole organism.

Genetic code
Genetic code (Photo credit: Martina Gobec)

That, however, is not cancer. For damage to result in a cancer it has to damage the cell in a particular way. In a computer, cancer would be analogous to a program continually creating copies of itself and using up all the system resources, which would result in the system crashing. Almost all cells have the ability to duplicate themselves, but whether or not they do replicate is, so far as I know, determined by the conditions in the cell itself and conditions in its environment, ie the surrounding cells, possibly signalled by chemicals or chemical gradients.

For a cell to become cancerous, it first of all must be inclined to duplicate itself and it would also have to be able to ignore any signals from its environment. The damage to the genetic structure to achieve this seems to me to be remarkably specific. Its like damaging a computer program in such a way as to destroy a control loop. Possible, but not very likely.

Main sites of metastases for some common cance...
Main sites of metastases for some common cancer types. Primary cancers are denoted by “…cancer” and their main metastasis sites are denoted by “…metastases”. List of included entries and references is found on main image page in Commons: (Photo credit: Wikipedia)

Then there is the issue of metastasis. This is how cancer spreads. It first of all attacks the boundary of the organ it is embedded in, then some cancer cells migrate into other organs. Interestingly, when they start to form a cancerous tumour in the new organ, they are still identifiable as cells from the original organ.

Think for a moment about what that means. A cancer which metastasizes has to have its genetic material damaged in such a way that it can do all of the following :

  • divide in an uncontrolled manner
  • attack the walls of the organ it is contained in
  • migrate to other organs (and it can be very specific about the organs it migrates to)
  • settle there and start to divide again

That’s a remarkably specific set of actions to occur as the result of damage. Damage usually results in less efficient operation, both of computer programs and bodies. Of course the damage doesn’t have to create these action ‘programs’ in the genetic material. They may be already there. All that the damage needs to do is somehow kick off those actions in sequence to cause the cancer to form and metastasize. To a programmer it would look like a small chunk of code to call those routines which I’ve called ‘actions’. The only other time in one’s life that one’s body experiences explosive growth and cell migration is in the womb and as a young child. One can imagine that damage could somehow kick off the genes or part of the genetic code that was used when one was a developing foetus.

foetus
foetus (Photo credit: Leo Reynolds)

This is easier to contemplate than damage which somehow creates the whole process from scratch. It does imply that the damaging agent somehow attacks a specific part of the genetic material and replaces it with some very specific other material that has the specific effect of kicking off explosive growth and metastasis.

I’m not geneticist or cancer specialist of course, so my musings above may be way, way off beam. They could be and probably are complete rubbish. I can’t and won’t defend them if anyone were to attack them. My main thesis is that it seems incredibly unlikely that damage to the genetic material could cause the specific effects of cancer, which are uncontrolled growth and metastasis.

Yet cancer happens and happens frequently and in varied ways. There are many sorts of cancer and they appear to be often triggered by specific stimuli. All my arguments above founder on that logical rock.

Shipwreck
Shipwreck (Photo credit: Wikipedia)