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Alternation of generations

The third life-cycle type, employed by some algae and all plants, is a blend of the haploid-dominant and diploid-dominant extremes. Species with alternation of generations have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes , because they produce gametes from specialized cells. Meiosis is not directly involved in the production of gametes in this case, because the organism that produces the gametes is already a haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte    . Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will subsequently develop into the gametophytes ( [link] ).

This illustration shows the life cycle of fern plants. The diploid (2n) zygote undergoes mitosis to produce the sphorophyte, which is the familiar, leafy plant. Sporangia form on the underside of the leaves of the sphorophyte. Sporangia undergo meiosis to form haploid (1n) spores. The spores germinate and undergo mitosis to form a multicellular, leafy gametophyte. The gametophyte produces eggs and sperm. Upon fertilization, the egg and sperm form a diploid zygote.
Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism. In some plants, such as ferns, both the haploid and diploid plant stages are free-living. The diploid plant is called a sporophyte because it produces haploid spores by meiosis. The spores develop into multicellular, haploid plants called gametophytes because they produce gametes. The gametes of two individuals will fuse to form a diploid zygote that becomes the sporophyte. (credit “fern”: modification of work by Cory Zanker; credit “sporangia”: modification of work by "Obsidian Soul"/Wikimedia Commons; credit “gametophyte and sporophyte”: modification of work by “Vlmastra”/Wikimedia Commons)

Although all plants utilize some version of the alternation of generations, the relative size of the sporophyte and the gametophyte and the relationship between them vary greatly. In plants such as moss, the gametophyte organism is the free-living plant, and the sporophyte is physically dependent on the gametophyte. In other plants, such as ferns, both the gametophyte and sporophyte plants are free-living; however, the sporophyte is much larger. In seed plants, such as magnolia trees and daisies, the gametophyte is composed of only a few cells and, in the case of the female gametophyte, is completely retained within the sporophyte.

Sexual reproduction takes many forms in multicellular organisms. However, at some point in each type of life cycle, meiosis produces haploid cells that will fuse with the haploid cell of another organism. The mechanisms of variation—crossover, random assortment of homologous chromosomes, and random fertilization—are present in all versions of sexual reproduction. The fact that nearly every multicellular organism on Earth employs sexual reproduction is strong evidence for the benefits of producing offspring with unique gene combinations, though there are other possible benefits as well.

Section summary

Nearly all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis appears to be one of the advantages of sexual reproduction that has made it so successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. Fertilization, the fusion of haploid gametes from two individuals, restores the diploid condition. Thus, sexually reproducing organisms alternate between haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly. There are three main categories of life cycles: diploid-dominant, demonstrated by most animals; haploid-dominant, demonstrated by all fungi and some algae; and the alternation of generations, demonstrated by plants and some algae.

Art connections

[link] If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?

[link] Yes, it will be able to reproduce asexually.

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Questions & Answers

effect of planning beans of using fertilizer
Elizerbeth Reply
what do we mean by transgenic organisms?
FADILAT Reply
what is or are the functions of the Islets of Langarhaans
FADILAT
They are the regions of the pancreas that contains the endocrine cell
Iyadi
is the studly of life
Aisha Reply
what is biology
Asunta Reply
is the study of living organism and their interection with one another and their environment.
Belbon
what is soil
Mukisa Reply
the top layer of the earth in which plant's, tree's
Ahmad
type of soil
Asunta
function of cell wall
Nthati Reply
function of cell wall
Asunta
To protect the cell from bursting
Maurice
to protect the cell from bursting
Deborah
to protect all other internal components of the cell
Olaoye
What is escherichia coli
Tumise Reply
in what type of cells is meiosis taking place?
Rhyeann Reply
sex cells
Eric
hlo
Amit
reproductive system of earthworm plzz describes
Amit
procryotic cell and some eucaryotic cell
Olaoye
applications of biology
Namawejje Reply
what is dormancy?
Aliyu Reply
hello guys what's the difference between prokaryotes and eukaryotes
Nwachukwu Reply
hlo what are the applications of biology?
Namawejje
eukaryotic cells have DNA in their nucleus while prokaryotic cells have their DNA present freely in their cytoplasm.
FADILAT
deviation from mendelian
Ogali Reply
what is lethal allele
Ogali
Explain how chemical , bioligical and physical interaction between themselves and the non living components ?
Beyan Reply
what is Tissues
Faith Reply
Group of similar cells performing some related functions We have some type of tissues Connective Muscle Epithelial Nervous tissues
peter
group of cells that is called tissue.
Nikita
tissue is the arrangement or combination of similar cells that perform the same function .
FADILAT
what is abiotic?
Williams Reply
state 2 abiotic factors that affect the rate of transpiration in plants?
Benenge
non living things
Chris
1) temperature:the temperature of the atmosphere affects the opening and closing of the stomata in the leaves of plants .high temperature encourages water loss . 2)wind : air flow encourages transpiration in plants . example,plants that are found on high areas like mountains transpire easily
FADILAT
I agree with Fadilat Muhammad In addition, water plants tend to reserve water during dry seasons so if they are not given plenty of water, they keep the water to them selves
Iyadi

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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