A life cycle is a period involving one generation of an organism through means of reproduction, whether through asexual reproduction or sexual reproduction. In regard to its ploidy, there are three types of cycles: Generation (From the Greek γιγνμαι), also known as procreation, is the act of producing offspring. ... For other uses, see Reproduction (disambiguation) Reproduction is the biological process by which new individual organisms are produced. ... It has been suggested that Parthenogenesis be merged into this article or section. ... Sexual reproduction is a union that results in increasing genetic diversity of the offspring. ... Ploidy is the number of homologous sets of chromosomes in a biological cell. ...

• haplontic life cycle
• diplontic life cycle
• diplobiontic life cycle (also referred to as diplohaplontic, haplodiplontic, or dibiontic life cycle)

These three types of cycles feature alternating haploid and diploid phases (n and 2n). The haploid organism becomes diploid through fertilization, which joins of gametes. This results in a zygote which then germinates. To return to a haploid stage, meiosis must occur (see Cell division). The cycles differ in the product of meiosis, and whether mitosis (growth) occurs. Zygotic and gametic meioses have one mitotic stage and form: during the n phase in zygotic meiosis and during the 2n phase in gametic meiosis. Therefore, zygotic and gametic meiosis are collectively term haplobiontic (single mitosis per phase). Sporic meiosis, on the other hand, has two mitosis events (diplobiontic): one in each phase. Haploid (meaning simple in Greek) cells have only one copy of each chromosome. ... Diploid (meaning double in Greek) cells have two copies (homologs) of each chromosome (both sex- and non-sex determining chromosomes), usually one from the mother and one from the father. ... Categories: Biology stubs ... A gamete is a specialized germ cell that fuses with another gamete during fertilization (conception) in organisms that reproduce sexually. ... It has been suggested that Biparental zygote be merged into this article or section. ... Sunflower seedlings, just three days after germination Germination rate testing on the germination table Germination is the process where growth emerges from a period of dormancy. ... Not to be confused with miosis. ... This does not adequately cite its references or sources. ... Mitosis is the process in which a cell duplicates its chromosomes to generate two identical cells. ...

Haplontic life cycle

Zygotic meiosis

A zygotic meiosis is a meiosis of a zygote immediately after karyogamy, which is the fusion of two cell nuclei. This way, the organism ends its diploid phase and produces several haploid cells. These cells divide mitotically to form either larger, multicellular individuals, or more haploid cells. Two opposite types of gametes (e.g., male and female) from these individuals or cells fuse to become a zygote. Image File history File links Drawn by self for Biological life cycle Scan black/white/grey outline PSP Reduce size (by 20%) Brightness/contrast to get rid of artifacts Copy-&-paste the multicellular balls Fill-in colours Labelling Re-fix details by going back to Layer 1. ... Image File history File links Drawn by self for Biological life cycle Scan black/white/grey outline PSP Reduce size (by 20%) Brightness/contrast to get rid of artifacts Copy-&-paste the multicellular balls Fill-in colours Labelling Re-fix details by going back to Layer 1. ... Not to be confused with miosis. ... It has been suggested that Biparental zygote be merged into this article or section. ... Karyogamy is the fusion of nuclei of two cells, as part of syngamy. ... HeLa cells stained for DNA with the Blue Hoechst dye. ... Overview of the major events in mitosis In biology, mitosis is the process of chromosome segregation and nuclear division that follows replication of the genetic material in eukaryotic cells. ...

In the whole cycle, zygotes are the only diploid cell; mitosis occurs only in the haploid phase.

The individuals or cells as a result of mitosis are haplonts, hence this life cycle is also called haplontic life cycle. Haplonts are:

• Most fungi
• Some green algae
• Many protozoa

Divisions Chytridiomycota Zygomycota Ascomycota Basidiomycota The Fungi (singular: fungus) are a large group of organisms ranked as a kingdom within the Domain Eukaryota. ... Divisions Chlorophyta Charophyta Green algae are microscopic protists; found in all aquatic environments, including marine, freshwater and brackish water. ... Wikisource has an original article from the 1911 Encyclopædia Britannica about: Protozoa Protozoa (in Greek proto = first and zoa = animals) are single-celled eukaryotes (organisms whose cells have nuclei) that commonly show characteristics usually associated with animals, most notably mobility and heterotrophy. ...

Diplontic life cycle

Gametic meiosis

In gametic meiosis, instead of immediately dividing meiotically to produce haploid cells, the zygote divides mitotically to produce a multicellular diploid individual or a group of more unicellular diploid cells. Cells from the diploid individuals then undergo meiosis to produce haploid cells or gametes. Haploid cells may divide to form more haploid cells, as in many yeasts, but the haploid phase is not the predominant life cycle phase. In most diplonts, mitosis occurs only in the diploid phase, i.e. gametes usually form quickly and fuse to produce diploid zygotes. Image File history File links Drawn by self for Biological life cycle Scan black/white/grey outline PSP Reduce size (by 20%) Brightness/contrast to get rid of artifacts Copy-&-paste the multicellular balls Fill-in colours Labelling Re-fix details by going back to Layer 1. ... Image File history File links Drawn by self for Biological life cycle Scan black/white/grey outline PSP Reduce size (by 20%) Brightness/contrast to get rid of artifacts Copy-&-paste the multicellular balls Fill-in colours Labelling Re-fix details by going back to Layer 1. ... A gamete is a specialized germ cell that fuses with another gamete during fertilization (conception) in organisms that reproduce sexually. ...

In the whole cycle, gametes are usually the only haploid cells, and mitosis usually occurs only in the diploid phase.

The diploid multicellular individual is a diplont, hence a gametic meiosis is also called a diplontic life cycle. Diplonts are:

• Animals
• Some brown algae
• Some fungi, e.g. brewer's yeast

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Life history theory

In animal and human biology life history theory is a method of understanding evolved behaviors and strategies to optimize reproductive success. Life history theory is a method of analysis in animal and human biology, psychology, and especially evolutionary sociobiology which postulates that many of the physiological traits and behaviors of individuals may be best understood in relation to the key maturational and reproductive characteristics that define the life course. ...