Where Mitosis, Meiosis, and Fertilization Fall in Plants, Algae, Fungi, and Animals
by Professor Stephen T. Abedon Steve Abedon, Ph.D., has taught well over 100 individual classes and ~10 individual courses (including introductory biology, microbiology, physiology, biochemistry, etc.) over his 30+ years as a college professor. He has over 150 publications, including multiple monographs, and is ranked in the top 1 or 2 percent among researchers of all disciplines worldwide. His research focuses on bacteriophage biology and phage therapy. (abedon.1@osu.edu)
phage.org | phage-therapy.org | biologyaspoetry.org | abedon.phage.org | google scholar | publications
Jump to: 📋 Overview | ⭕ One Circle | ➡️ In a Line | 🔄 Detailed Cycles | 🧬 n or 2n? | 🌿 Four Plants | 🍎 Fruits | 🎥 Videos | 📖 Terms | 🧮 More Calculators
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A life cycle can start at any point in an organism’s existence and end at the same point in the existence of its offspring: from zygote to zygote, say, or from spore to spore. In thinking about life cycles, the focus is usually on what it takes for an organism to reach reproductive maturity and then on how it reproduces, whether sexually or asexually.
This laboratory is mostly about sexual life cycles, and especially about the cycles of plants, the algae they came from, and fungi, which differ from our own in ways that can seem strange at first. Animal cycles serve as the familiar point of comparison. The aim is to see both what all of these cycles have in common and exactly where they differ.
Every sexual life cycle has two events in common. Fertilization fuses two haploid cells into a diploid zygote, doubling the number of chromosome sets. Meiosis halves the number again, from diploid back to haploid. Draw a cycle as a circle with fertilization at the top and meiosis at the bottom, and the right half of the circle is diploid and the left half haploid. Every sexual organism fits on that circle.
What differs is where mitosis builds a body. Mitosis copies cells without changing their ploidy, so a multicellular body grows on whichever side of the circle mitosis is at work. In animals it works only on the diploid side. In Chlamydomonas, a single-celled alga, it never builds a body at all. In plants it builds bodies on both sides, and how big each body is changes from moss to fern to pine to flowering plant. In the rings below, a thick band is a stage built by mitosis into a multicellular body; a thin line is a stage of single cells.
Fungi need one more idea. In many fungi the two steps of fertilization come apart: two cells fuse (plasmogamy) without their nuclei fusing (karyogamy). The cells in between hold two separate haploid nuclei, one from each parent, and are described as n + n. In a mushroom-forming fungus this n + n stage is most of the fungus. Every organism on this page is drawn this way on the One Circle tab; click any ring to see that cycle in detail.
Since meiosis halves the chromosome number and fertilization doubles it, a sexual cycle can be classified by what meiosis makes.
| Meiosis makes… | So the haploid stage is… | And the diploid stage is… | Examples |
|---|---|---|---|
| Gametes (gametic meiosis) | gametes only | the organism | animals; the brown seaweed Fucus; diatoms |
| The first haploid cells, in the zygote (zygotic meiosis) | the organism | the zygote only | Chlamydomonas, Chara, most fungi, the malaria parasite |
| Spores (sporic meiosis) | a multicellular gametophyte, which makes gametes by mitosis | a multicellular sporophyte, which makes spores by meiosis | all plants; sea lettuce; kelps |
The third pattern is alternation of generations, and its vocabulary follows from its definition. A sporophyte is diploid and makes spores by meiosis. A spore is the first cell of the next generation and grows, by mitosis, into a gametophyte, which is haploid and makes gametes, also by mitosis, since it is already haploid. Gametes fuse into a zygote, the first cell of the next sporophyte. A spore and a gamete are both single haploid cells; the difference is that a spore grows into something on its own, while a gamete must first fuse with another gamete.
The closest living relatives of the land plants are green algae such as Chara, whose zygote is its only diploid cell and divides by meiosis straight away. Each fertilization therefore yields just four haploid cells. In land plants, by contrast, the zygote first divides by mitosis into a multicellular sporophyte, many of whose cells then divide by meiosis, so one fertilization yields many spores, in a fern hundreds of thousands or more.
One way to see the advantage: plants live on land, but their ancestors’ sperm swim, and on land the water that swimming sperm need is often scarce. When fertilizations are hard to come by, each one is worth more if it is multiplied many times over before meiosis. Seen this way, it makes sense that the diploid generation has grown and the haploid generation shrunk over plant evolution:
In seed plants three generations end up packaged together: a seed holds the new sporophyte (the embryo) along with tissue of its parent gametophyte or, in flowering plants, endosperm, all inside a coat made by its grandparent sporophyte. The Four Plants tab lays the four plants side by side.
In animals meiosis makes eggs and sperm directly, and the products of meiosis mature into gametes without dividing again. An animal therefore has no multicellular haploid stage, and no alternation of generations. Three animal cycles are worth a closer look because they seem at first to break that rule:
Many organisms also reproduce asexually, by mitosis alone: molds release spores made by mitosis, a yeast buds, a polyp colony buds new polyps, and the malaria parasite multiplies through red blood cell after red blood cell. Asexual reproduction does not change ploidy, so it does not go around the circle. It loops back on itself on one side of it, and on the rings these loops are drawn as small dashed circles beside the haploid or diploid side.
The reverse also happens: sex without reproduction. When two Paramecium cells conjugate, they swap haploid nuclei made by meiosis, each fuses one received nucleus with one of its own, and the two cells part, genetically changed but no more numerous than before.
Your answers are kept as you go. In most browsers they are still here if you leave and come back on the same device, though a private window, a different device, or cleared browser data will start you fresh. When you are done, download a summary to hand in or keep. It lists your answers and, for each, whether it was right the first time.
All sixteen cycles on this page, drawn the same way: fertilization at the top, meiosis at the bottom, diploid down the right side, haploid up the left. Compare where the thick bands fall, which is where mitosis builds bodies. Click any ring to see that cycle in detail.
Slide from a moss to a flowering plant and watch the haploid band thin as the diploid band thickens. Nothing else about the circle changes.
The band widths are qualitative: they show which generation is large and which small, not measurements.
The same slide through the algae. First the haploid side builds a body (Chara), then the diploid side builds one too (Ulva), then the diploid body takes over (kelp), until in Fucus only the gametes are haploid, as in animals.
These five are not one line of descent. Chlamydomonas, Chara, and Ulva are green algae; kelp and Fucus are brown algae, only distantly related to plants. A large diploid generation evolved in the land plants and, separately, among the brown algae.
Read the description, then choose how big each side of the circle should be and what meiosis makes. The ring redraws as you choose. Check to compare it with the answer.
Cut the circle open at the cells that fuse and lay it out flat, and every sexual cycle reads the same way from left to right. Fertilization doubles the number of chromosome sets, meiosis halves it, and mitosis, however many times it happens in between, leaves it alone. So the ploidy of any stage can be read from its place in the line: after fertilization and before meiosis, 2n; after meiosis and before the next fertilization, n.
In many fungi fertilization comes in two steps. Plasmogamy, the fusing of two cells, brings two haploid nuclei into one cell (n + n), and karyogamy, the fusing of those nuclei, makes the 2n zygote, often much later.
In general terms, with the specifics left out. The patterns differ only in where mitosis builds a body between fertilization and meiosis, or between meiosis and the next fertilization.
Each cycle with its own names for the stages. Stages side by side, such as egg and sperm, happen at the same point in the cycle. These lines are halfway between the rings of the One Circle tab and the diagrams of the Detailed Cycles tab, which add the stages left out here.
Each cycle laid out stage by stage. The left column holds the diploid stages, read from top to bottom; the right column the haploid stages, read from bottom to top; and in fungi a middle column holds the n + n stages. Meiosis always crosses from diploid to haploid at the bottom, and fertilization from haploid to diploid at the top. Filled boxes are multicellular; open boxes are single cells. Click any box for more about that stage.
The diagrams are schematic: they show order, ploidy, and process, not size or appearance. “Becomes” marks a change of role without a division worth naming, such as a cell of an embryo sac becoming the egg.
For each structure, decide how many sets of chromosomes its nuclei carry. Each structure is numbered on its cycle’s line, as on the In a Line tab, but with the ploidy hidden. Work it out from the steps along the way: fertilization doubles the number of chromosome sets, meiosis halves it, and mitosis leaves it alone. Watch for structures that sit on one generation but belong to another, such as a seed coat.
Moss, fern, pine, and flowering plant all alternate generations, and they use the same words for the same things: sporophyte, sporangium, spore, gametophyte, gamete. What changes is how large and how independent each generation is. Fill in the table, then check.
In a seed plant the gametophyte never leaves its parent, and after fertilization the next sporophyte begins its life inside the gametophyte, which is still inside its own parent. A seed packages the result. For each part, choose the generation, or tissue, it belongs to. In flowering plants the food store is not gametophyte but endosperm, made by the second of the two fertilizations.
A fruit is the product of a flower after fertilization, and a plant organ for carrying seeds away from the parent: to new places, to new opportunities, and away from competing with the parent itself. Strictly speaking, only flowering plants make fruits.
From the outside in, a complete (perfect) flower has sepals, the bud covering; petals; stamens, the male parts, whose anthers make pollen; and carpels (pistils), the female parts. A carpel is a stigma, a style, and an ovary, and the ovary holds one or more ovules. The ovules become the seeds, and the ovary usually becomes most of the fruit. In accessory fruits, much of what we eat comes from another part of the flower instead.
Fruits are also fleshy, edible or at least chewable when ripe and raw (berries, drupes, pomes, hesperidia, pepos), or dry (achenes, samaras, capsules, grains, legumes, nuts). The categories overlap, so one fruit can carry more than one label: a raspberry is aggregate and fleshy. Botanical and kitchen usage also differ. A tomato, a squash, and a green bean are fruits to a botanist and vegetables at the dinner table.
Legumes (beans, peas, peanuts, clover, alfalfa) deserve a note. Their plants host nitrogen-fixing bacteria in nodules on their roots, which is why their seeds tend to be rich in protein and why they enrich the soil they grow in with nitrogen.
Five views, each taking a closer look at part of the one before it. Color shows ploidy throughout, and the young sporophyte inside a seed is drawn darker than the parent plant around it: both are diploid, but they are different generations.
| In the flower… | …becomes | Ploidy | Generation |
|---|---|---|---|
| Ovary wall | Fruit wall (pericarp) | 2n | Parent sporophyte |
| Ovule | Seed | mixed | Parent, offspring, and endosperm together |
| Integuments of the ovule | Seed coat | 2n | Parent sporophyte |
| Egg + sperm | Zygote, then embryo | 2n | New sporophyte |
| Central cell + sperm | Endosperm | 3n | Neither: a food tissue for the embryo |
| Microspore | Pollen grain, then pollen tube | n | Male gametophyte |
| Megaspore | Embryo sac | n | Female gametophyte |
Many things called seeds are fruits, and some things called nuts are seeds. Decide what each one really is.
Name the kind of fruit. A definition of each appears once you have it right.
Those marked core are the place to start; additional ones fill in background. The notes say what to watch for, and where a video gets something wrong.
Specimens of fungi, mosses, ferns, and pines, most of them brought into focus under the microscope, along with a look at fruits and three very short takes on spores, gametophytes, and sporophytes.