Biological life cycle

In biology, a life cycle is a series of stages that an organism goes through from the beginning of its life until it becomes an adult and can have its own young. It usually starts as a tiny cell called a zygote, often inside an egg, and ends when the organism is fully grown and can reproduce. This process then repeats with the new generation, creating a never-ending circle of life.

For many plants and simple water organisms called algae, the life cycle includes two very different stages. This is called alternation of generations. Some organisms, like certain types of red algae, have even more stages in their life cycles.

When an organism reproduces by combining cells from two parents, its life cycle includes changes in the number of tiny parts inside its cells. These changes happen through a process called meiosis. There are different types of life cycles depending on when these changes occur and which stages of the organism are multicellular. These cycles help scientists understand how different living things grow and develop.

Discovery

Scientists have studied how animals and plants grow and make new babies for a long time. In 1851, a scientist named Wilhelm Hofmeister showed that plants go through different stages in their lives, just like animals do. Other scientists added new words to help describe these life stages, which helped prove that living things don't just appear out of nowhere.

Haplontic life cycle

A zygotic meiosis is a meiosis of a zygote right after karyogamy, which is when two cell nuclei join together. This process ends the diploid stage and creates several haploid cells. These cells can divide mitotically to grow into larger, multicellular beings or to make more haploid cells. Two different types of gametes (such as male and female) from these beings or cells then join to form a new zygote.

In this whole cycle, zygotes are the only diploid cells, and mitosis only happens during the haploid phase. The beings or cells produced by mitosis are called haplonts, which is why this life cycle is known as the haplontic life cycle. Haplonts include many different organisms, such as:

In archaeplastidans: some green algae (for example, Chlamydomonas, Zygnema, Chara)
In stramenopiles: some golden algae
In alveolates: many dinoflagellates, like Ceratium and Gymnodinium, and some apicomplexans (for example, Plasmodium)
In rhizarians: some euglyphids, ascetosporeans
In excavates: some parabasalids
In amoebozoans: Dictyostelium
In opisthokonts: most fungi (some chytrids, zygomycetes, some ascomycetes, basidiomycetes)

Diplontic life cycle

In a diplontic life cycle, a zygote first grows by dividing into many cells to form a multicellular diploid individual. Later, cells from this individual go through meiosis to create haploid cells called gametes. These gametes then join together to form a new diploid zygote, continuing the cycle.

In this cycle, gametes are usually the only haploid cells, and cell division mostly happens in the diploid phase.

Haplodiplontic life cycle

Main article: Alternation of generations

In some living things, a tiny beginning cell splits and grows into a full organism. This organism then makes tiny cells that can grow into another type of organism. These new organisms can then make cells that grow into the first type again, making a repeating pattern.

Haplodiplonts include:

In archaeplastidans: red algae, some green algae (like Ulva), land plants
In stramenopiles: most brown algae
In rhizarians: many foraminiferans, plasmodiophoromycetes
In amoebozoa: myxogastrids
In Opisthokonts: some fungi (like some chytrids, ascomycetes such as brewer's yeast)
Other eukaryotes: haptophytes

Vegetative meiosis

Some red algae and green algae have a special process called vegetative meiosis. In this process, certain cells skip the usual steps of reproduction and change directly from one type to another. This helps the algae grow and make new cells without needing to reproduce.

Another process, called vegetative diploidization, happens in some brown algae. Here, cells can change their number of chromosomes on their own, creating new types of tissue.

Parasitic life cycle

Parasites rely on living in or on other animals, called hosts, to live and grow. Some parasites need to move between different kinds of hosts to finish their life cycles. For example, heartworms, a type of parasite, must first be eaten by a mosquito. Inside the mosquito, they grow and then are passed to another animal when the mosquito bites it.

Other parasites can finish their whole life in just one kind of host. An example is a hookworm that infects dogs. These parasites grow in the environment and then enter a dog’s skin directly, where they mature and live inside the dog. Some parasites can only live in certain hosts and are called obligate parasites of those hosts. Others can live without a specific host and are called facultative parasites. Hosts where parasites reproduce are called definitive hosts, while others may help move the parasite but not let it grow further.

Evolution

The simplest life cycles may involve only one type of reproduction without parents, used by many tiny organisms. Some living things can reproduce in this way, and others sometimes do, even though they usually have parents too.

Most living things get older and eventually stop working, but special cells that help make new babies can keep going for a very long time. Scientists think these cells can stay healthy because they have special ways to fix mistakes in their instructions, helping them stay strong over many generations.