<< Chapter < Page Chapter >> Page >

Life cycles of sexually reproducing organisms

Fertilization and meiosis alternate in sexual life cycles . What happens between these two events depends on the organism. The process of meiosis reduces the chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. There are three main categories of life cycles in multicellular organisms: diploid-dominant    , in which the multicellular diploid stage is the most obvious life stage, such as with most animals including humans; haploid-dominant    , in which the multicellular haploid stage is the most obvious life stage, such as with all fungi and some algae; and alternation of generations    , in which the two stages are apparent to different degrees depending on the group, as with plants and some algae.

Diploid-dominant life cycle

Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells    , are produced within the gonads, such as the testes and ovaries. Germ cells are capable of mitosis to perpetuate the cell line and meiosis to produce gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state ( [link] ).

This illustration shows the life cycle of animals. Through meiosis, adult males produce haploid (1n) sperm, and adult females produce haploid eggs. Upon fertilization, a diploid (2n) zygote forms, which, through mitosis and cell division, grows into an adult.
In animals, sexually reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring. The germ cells are generated early in the development of the zygote.

Haploid-dominant life cycle

Most fungi and algae employ a life-cycle type in which the “body” of the organism—the ecologically important part of the life cycle—is haploid. The haploid cells that make up the tissues of the dominant multicellular stage are formed by mitosis. During sexual reproduction, specialized haploid cells from two individuals, designated the (+) and (−) mating types, join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores. Although haploid like the “parents,” these spores contain a new genetic combination from two parents. The spores can remain dormant for various time periods. Eventually, when conditions are conducive, the spores form multicellular haploid structures by many rounds of mitosis ( [link] ).

Art connection

This illustration shows the life cycle of fungi. In fungi, the diploid (2n) zygospore undergoes meiosis to form haploid (1n) spores. Mitosis of the spores occurs to form hyphae. Hyphae can undergo asexual reproduction to form more spores, or they form plus and minus mating types that undergo nuclear fusion to form a zygospore.
Fungi, such as black bread mold ( Rhizopus nigricans ), have haploid-dominant life cycles. The haploid multicellular stage produces specialized haploid cells by mitosis that fuse to form a diploid zygote. The zygote undergoes meiosis to produce haploid spores. Each spore gives rise to a multicellular haploid organism by mitosis. (credit “zygomycota” micrograph: modification of work by “Fanaberka”/Wikimedia Commons)

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?

Questions & Answers

forms which cell exist
Zainab Reply
Wats a molecules in terms of biology?
Ayomide Reply
Important of kreb's cycle
yeboah
what conversation
Sally Reply
is agurment btn two or more people
Okello
what is biology?
KINEI Reply
the scientific study of the life and structure of plants and animals.
Boame
when is the right time to play sex in order to prevent pregnancy?
Guor
After finishing school
Hellen
study of living things n non living things
Benjamin
sex is sweet at anytime
Okello
study of living organsm
Hellen Reply
which element is the largest
Haja Reply
francium
Izzati
francium
Paul
how many groups do we have in the periodic table
Theola Reply
seven groups
Moses
the one that makes eight is the noble gases
Moses
okay thank you
Theola
we have eight groups in the periodic table
Ramsey
there are actually 18 groups or numbered columns
Izzati
so it will be like the s- p-and d block elements of the periodic table
Izzati
how many times can a man release before the lady get pregnant
Smart Reply
nearly a hundred million
Izzati
one
Moses
what do you by one?
Izzati
two times
Theola
just only one time
Okello
what is a function of cell sap
Martin
it repair worn out tissue
Moses
when is the right time for playing sex to prevent pregnancy?
Guor
at anytime
Okello
what is biology
Darkoa Reply
it's study of all living organisms and their characteristics
Ice
by what proces do ameaba reproduce?
Okello Reply
hi dear
Janet
hi my sist gudmorning
Okello
Hi
Muah
hello
Bob
yes
Mathew
hi
promise
what is biology
promise
amoeba reproduce asexually
Chidi
gudevening member
Okello
good chidi
Okello
hello
Felix
hello
Felix
please guys help me with my assignment 😃
Mary Reply
please guys help me with my assignment 😃
Mary
please guys help me with my assignment 😃
Mary
same here
Rachel
lol I am just learning idk how to do assignment
Izzati
lol idk how I am just learning lol
Izzati
lol I think I understand why we repeat this
Izzati
what is active transport
Amona Reply
is the movement's of water & mineral salts from the reason of highly concentrated to the reason of low concentration across semi permeable membrane & it take place in plant
Okello
in what situation would the use of a scanning microscope be ideal and why?
Anthony Reply
what is euglina
Ruth Reply
no idea
Enock
Euglena is a unicellular organism that has both animal- like and plant- like features.
Eugenia
define hybridization?
Guor
Which of the following statements regarding membrane transport is false? 1. Glucose is transported only by facilitated diffusion 2.Each protein carrier will only bind and b transport one type of solute.
#2. Each protein carrier will only bind and b transport one type of soluble
Only
#2 is false for the regarding membrane transport.
Only
Tanks a lot
Thanks a lot

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play




Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?

Ask