Download The Eukaryote life-cycle—diploidy, haploidy

The Eukaryote life-cycle—diploidy, haploidy & sex
Eukaryotes
•Have membrane bound organelles eg chloroplasts, mitochondria
of prokaryote origin (endosymbiosis)
•Larger and more complex genetic (multiple chromosomes) and
membrane systems than prokaryotes
•Usually with some diploid stages, and often sex somewhere in
their life-cycle
The Eukaryote life-cycle—diploidy, haploidy & sex
Although sex is often associated with diploidy, and fertilization
(syngamy) produces a diploid cell, only haploid cells can combine
sexually (become gametes)
meiosis is required to restore the haploid state
meiosis restores the haploid state but also generates infinite
genetic variety among the pool of gametes that it forms
Where is the life-cycle does meiosis take place?
What advantages/disadvantages does diploidy bring?
What are the advantages/disadvantages of sex?
Meiosis reduces chromosome number from
diploid to haploid: a closer look
• Many steps of meiosis resemble steps in mitosis.
• Both are preceded by the replication of chromosomes.
• In meiosis, diploid cells divide twice in a row after replicating
their chromosomes only once.
• Each final daughter cell is haploid—one copy of each
chromosome
• In mitosis cells divide but ploidy doesn’t change
• Haploid cells divide to form haploid cells and diploid cells
divide to form diploid cells
• Meiosis reduces chromosome
number by copying the
chromosomes once, but
dividing twice.
• The first division, meiosis I,
separates homologous
chromosomes.
• The second, meiosis II,
separates sister chromatids.
Fig. 13.6
Sexual life cycles produce genetic variation
among offspring
• Three mechanisms contribute to genetic variation:
– independent assortment of chromosomes
(2n combinations where n=haploid number)
– random fertilization—any sperm can fertilize any ovum
– crossing over—each gamete is unique
• By independent assortment
alone each individual
chromosome in a gamete
would be exclusively
maternal or paternal in
origin.
• However, crossing over
produces recombinant
chromosomes which
combine genes inherited
from each parent.
Fig. 13.10
Diploidy vs Haploidy—eukaryotes usually have both in their life cycle
Diploid cells have two copies of their entire genome
•provides genetic redundancy which increases resistance to
mutagens like ultraviolet light
•having two copies allows species to benefit from sex(genetic
exchange) with other individuals—two copies different. This
requires meiosis
•however cell cycle and cell division more complex
Haploid cells have only one copy of their genome
•more vulnerable to genetic damage
•cell cycle & cell division less complex
•can sometimes function as gametes—combine with other
haploid cells–syangamy or fertilization
Meiosis restores the haploid condition (2n to n)
can take place at different stages of the life cycle
in the zygote (zygotic meiosis)
later, after a diploid organism (sporophyte) has developed
sporic meiosis (gametophyte (n) & sporophyte (2n))
gametic meiosis (no gametophyte)
Zygotic meiosis is the simplest sexual life cycle displayed by eukaryotes.
Why?
Eg Chlamydomonas—a unicellular green alga
Chlamydomonas: the sexual life-cycle
resistant zygospore
Gametes are of two mating types, + and ─
We call this type of life-cycle zygotic meiosis, why?
This is the simplest type of sexual life-cycle found in eukaryotes? Explain?
http://academic.kellogg.cc.mi.us/herbrandsonc/bio111/algae.htm
The life-cycle of Oedogonium:
a different version is found in
filamentous algae.
In what way is this life-cycle similar
to that of Chlamydomonas, and how
is it different?
Because Oedogonium is a
multicellular organism (filamentous)
its life cycle allocates asexual growth
potential to both increase in filament
length (mitotic divisions within
filaments) and to increase in filament
number (zoospore formation).
In contrast, since Chlamydomonas is
a unicellular organism, all mitotic
divisions result in more new
organisms
http://academic.kellogg.cc.mi.us/herbrandsonc/bio111/algae.htm
Oedogonium: unbranched, filamentous cylindrical cells,distinctive rings at the
apical ends of certain cells. Each cellular division creates a new ring on the cap cell.
Cells contain a parietal, netlike chloroplast with several pyrenoids. asexual
reproduction by zoospore; sexual reproduction by fertilization by egg and sperm;
Germination of a zoospore
Vegetative cells
Apical rings
http://protist.i.hosei.ac.jp/pdb/images/Chlorophyta/Oedogonium/sp_10c.html
Oedogonium: a mat-forming filamentous green alga
http://images.google.ca/imgres?imgurl=http://www.csupomona.edu/~jcclark/classes/bot125/resource/graphics/g/chl_oedogonium.jpg&imgrefurl=http://www.csup
omona.edu/~jcclark/classes/bot125/resource/graphics/chl_oedogonium.html&h=298&w=450&sz=20&hl=en&start=11&tbnid=oTj8oIOpG56qM:&tbnh=84&tbnw=127&prev=/images%3Fq%3Doedogonium%2B%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DN
Oogonium
antheridium Class Charophyceae
Chara
A green macrophytic alga with a slighly
different zygotic type of life cycle.
Meiosis occurs within the zygospore,
which germinate into haploid plants that
produce oogonia and antheridia
What is the main difference between the
Chara and Oedogonium life-cycles?
Charophyceans are considered to be closely related to land plants
Chara sperm
An oogonium with an open antheridium
http://images.google.ca/imgres?imgurl=http://www.visualsunlimited.com/images/watermarked/316/316013.jpg&imgrefurl=http://ww
w.visualsunlimited.com/browse/vu316/vu316013.html&h=238&w=350&sz=12&hl=en&start=13&tbnid=Suqmn7RgKRdyIM:&tbnh=78&t
bnw=116&prev=/images%3Fq%3Dchara%2Boogonium%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DG
Spirogyra
A filamentous green alga with
a long spiral chloroplast
A peculiar form of sexual reproduction—
conjugation to form zygospores, with no
motile gametes or zoospores
Meiosis again takes place in the zygote stage: haploid cells grow into new
filaments (asexualy)—no motile zoospores
Filaments are either male or female mating types
During sex, when male and female filaments are lined up next to each other, the
male type cytoplasm migrates through the conjugation tube and joins with the
female cytoplasm to form a zygote—no motile gametes.
Spirogyra filaments produce a thin layer of slime that tends to keep them clean of
epiphytes, why would this be of particular importance to them?
http://academic.kellogg.cc.mi.us/herbrandsonc/bio111/algae.htm
Ulva—sporic (biphasic)—isomorphic alternation of generations
What is the most fundamental difference between zygotic and sporic meiosis cycles?
What advantage might sporic life cycles have overy zygotic cycles?
Steps in the Cladophora Life Cycle
•Quadriflagellate zoospores (1N) form in ordinary vegetative cells of the sporophyte
individual (2N), break through the sides of the cells and enter the water
•Spores settle on substrate and germinate into diecious gametophytes (1N)
•Gametophytes form biflagellate isogamous gametes (1N), which burst from lateral
apertures in branches of gametangia
•Gametes fuse externally forming quadriflagellate zygote (2N)
•Zygote settles and germinates directly into Sporophyte (2N)
Cladophora: filamentous benthic green alga with branched filaments
500 microns
Advantages and disadvantages of a biphasic life cycle
If the biphasic cycle is heteromorphic such as in kelp or red algae, then the
gametophyte, and sporophyte might be adapted to different ecological niches.
This can be advantageous if both types of niches occur commonly enough to
allow the life cycle to be completed. Eg. the two phases often differ in terms of
size, vulnerability to waves action, and to predators.
If they do not, and say only the sporophyte or the gametophyte is successful,
then the sexual life-cycle could not be completed. There are many asexual algae
which may have lost their sexuality for this reason.
There are some examples of brown algae where the gametophyte is lost, and
meiosis in the sporophyte gives rise directly to gametes rather than spores
(gametic meiosis). These species are usually intertidal—that is exposed to light
and drying during the tidal cycle, to which the gametophyte may be too sensitive.
If the biphasic cycle is isomorphic, the cycle would make little sense unless there
were subtle differences between the sporophyte and the gametophyte. Several
studies have found differences in growth rate, nutrient requirements, vulnerability
to stresses and ability to proliferate asexually.
Zygotic meiosis—the simplest sexual life cycle, common
among fungi and protists.
– The zygote is the only diploid phase.
– After fertilization, the zygote undergoes meiosis to
produce haploid cells.
– These haploid cells undergo
mitosis to either:
• increase in number
• or grow into a
haploid multicellular adult
organism.
– Some haploid cells develop
into gametes by mitosis.
Sporic meiosis—alternation between haploid and a diploid
organisms—diploid zygote first replicates by a series of
mitotic divisions to form a diploid organism, which
undergoes meiosis.
(some green and brown algae, red algae and plants)
– both haploid (gametophyte) and diploid
(sporophyte) multicellular stages.
– Meiosis by the sporophyte produces haploid spores
that develop by mitosis
into the gametophyte.
– Gametophyte produces
gametes by mitosis.
Fertilization forms
a zygote which produces
a sporophyte by mitosis
The timing of meiosis and fertilization varies among
species.
Gametic meiosis—gametes the only haploid stage found in
all animals incl. humans & some protists.
– haploid cells don’t divide,
– diploid zygote that divides by
mitosis to produce a
multicellular organism.
Fig. 13.5a
Question
What is the difference between a spore and a gamete?
Both are reproductive cells, but
•a gamete must combine with another gamete of the
opposite sex
•a spore can produce a new multicellular organism by
mitosis all by itself (no sex required)
http://academic.kellogg.cc.mi.us/herbrandsonc/bio111/algae.htm