A life cycle is a period involving one generation of an organism through means of reproduction, whether through asexual reproduction or sexual reproduction.
–Wikipedia
There you have the gist of it. But there are details. Oh, how there are details.
Evolution has given us myriad ways of being born, growing up, reproducing, and dying. To expand on all of them here would involve transcribing the sum total of humankind's knowledge of biology. I'll leave that to Wikipedia.
Out of curiosity, we allowed a bucket of used motor oil to sit outside for a year.
Bacteria from the outside found their way into the oil and were warmed by the sun. Apparently, though motor oil is refined and processed endlessly in metal containers and chemical baths, it is still a bonanza to unicellular organisms. After some months, I endeavored a tap on the bucket with my foot; crumbly brown sludge which had accumulated on the bottom rolled to the surface, carrying the heavy smell of pure shit.
Humans are made of cells; bacteria are unicellular. Thankfully, large colonies of bacteria typically do not specialize, and become sentient beings. Just imagine — if the contents of that oil bucket breached the surface, reaching.
We have found bacteria capable of flourishing in petri dishes containing nothing but bleach. More and more, antibacterial soap is a psychological device rather than a biological one. But bacteria don't need to couple to reproduce. Put them in a nice, warm, protein-rich sludge, and they cheerfully cleave and split apart. They've got the advantage.
When we observed fat white insect larvae floating lifelessly in our oil, we decided the experiment had gone too far. At that point, we were a hair's breadth away from taping NASCAR and firing shotguns into the sky.
Put life somewhere nice, and it will roll. Superficially, the cycle of life is a snapshot from birth until death. In this way, organisms become similar: like bats, we come from the womb, reach puberty, have sex, give birth, and gradually waste away to death.
But flowers have sex — albeit impersonally. Flowers also reach sexual maturity. But when you get into the mechanisms of cell division, we become rather unlike flowers, and eerily similar to brewer's yeast.
"Life cycle" has different meanings, depending on who you're talking to. Talk to your average person about life cycles, they're likely to bring up things like pupae, and chrysalises, and incubation periods. A macroscopic view of life cycles. But, to a cellular biologist, a life cycle — more specifically, a biological life cycle — is much more subtle. It involves, well, cells. Cell divisions, mitosis, unicellular organisms versus multicellular organisms. Biological life cycles are concerned with the life cycles of cells, which are projected onto the life cycles of larger organisms, like you.
For these purposes, you'll need to know the meanings of the prefixes hap and di; hap is one while di is many.
Gametes, some of the basic building blocks of reproduction are haploid — singular. They fuse, becoming zygotes, which are diploid — plural.
So, the life cycle of humans and animals and some yeasts is diplontic.
Immediately after meiosis of sperm and egg, the new zygote pulls an about-face and starts to divide mitotically. Haploid cells — gametes — are produced, which quickly fuse to form zygotes. The zygotes split, its cells forming a cohesive machine: hence the prefix di, for many. Reproduction is tasked by diploids. The result: cows, bats, ducks, people.
Sometimes, the gametes are lazy and replicate to form more haploid cells; this is what happens with yeast, whose bulk is simply a pile of haploid cells.
The science of cell division is subtle.
And convoluted beyond belief.
Lifeforms with a haplontic life cycle — things like most fungi, and protists — grow in a way almost identical to that of animals. Except, and get ready for it, cell division immediately following the fusion of cell nuclei is meiotic rather than mitotic.
This is what separates the mechanisms of our growth from that of scum. The bacterial sludge in my bucket of oil commands a new, perverse respect.
In protists, meiosis of the zygote produces haploid cells. The haploid cells then reproduce mitotically, as in animals. Gametes fuse, as in animals. The difference?
In protists, mitosis occurs only in haploid cells, whereas in animals, mitosis occurs in diploid individuals. More simply, gametes split apart rather than zygotes. You still have zygotes forming from gametes, you still have both haploids and diploids forming larger organisms, like mushrooms.
Now, plants. We've forgotten plants.
The life cycle of plants is diplobiontic. Plants undergo a phenomenon known as sporic meiosis. This is a fancy way of saying that both haploids and diploids undergo mitosis simultaneously. You could say that plants are a hybrid of animals and protists — at least, from a cellular biology standpoint.
This hybrid nature makes things confusing sometimes for botanists. Sometimes, you can't tell if a clump of seaweed is plant or fungus, since, of course, you have both haploids and diploids reproducing all over the place. The solution: bring out the microscope.
Even though it involves profoundly boring things like mitosis and diploids and the like, a life cycle is easy to adapt. Anything which grows and dies can be said to have a life cycle.
In marketing, you have life cycles for ideas and products. In this sense, the term resembles the broad life cycles of animals more closely than those of tiny cells. New Coke is born, and is promptly killed by poor sales. One is reminded of mayflies; except New Coke died from without. Mayflies die so much because they're mayflies.
Communities have life cycles. Relationships have life cycles. A first kiss, that's karyogamy — fusion of two cell nuclei. A couple is a diploid organism.
Sources
Wikipedia
http://en.wikipedia.org/wiki/Biological_life_cycle
Biocrawler
http://www.biocrawler.com/encyclopedia/Biological_life_cycle
tutor2u
http://http://www.tutor2u.net/business/marketing/products_lifecycle.asp
For the quest.