Sexual Life Cycles
Sexually reproducing organisms have life cycles that alternate
between haploid and diploid stages
- haploid
haploid- a cell with one complete copy of genetic information
- diploid - a cell with two complete copies of genetic information
- N is the number of chromosomes of a haploid cell,
- 2N is the number
n mber of chromosomes of a diploid cell.
cell
Diploid organisms produce the haploid stage through a reduction
division called meiosis.
meiosis In animals this produces gametes - sperms
and eggs. In plants this produces a single cell that multiplies to
produce a multicellular haploid gametophyte. Plant gametophytes
can then
h produce
d
gametes - sperms andd eggs – via
i mitosis.
i i
A haploid egg and a haploid sperm combine, in a process called
s ngam to
syngamy,
t produce
d
a new diploid
di l id cell
ll called
ll d a zygote.
t
Sex: the combination of genetic information from two different
sources
sources.
In most eukaryotes, sex involves the fusion of two cells, egg and
sperm. Their fusion is called syngamy.
Egg cells and sperm cells are gametes. Their production is
called gametogenesis. Oogenesis produces egg cells.
Spermatogenesis produces sperm cells.
cells
In both spermatogenesis
and oogenesis the amount
of genetic information in
y g y
cells is halved. Syngamy
restores the amount of
genetic information to the
normal amount.
amount
2
Most eukaryotes are diploid (2N) and their gametes are haploid
(N) G
(N).
Gametogenesis
i makes
k hhaploid
l id cells
ll from
f
diploid
di l id cells:
ll
2N → N
Syngamy
y g y combines two haploid
p
cells to make a diploid
p
cell:
N + N → 2N
The new diploid
organism
i commonly
l goes
through many rounds of
g
mitosis to become a large
multicellular diploid
organism. A few cells of
that organism are devoted
to producing egg cells or
sperm cells for sexual
reproduction.
d i
3
In animals, meiosis is a part of gametogenesis
S
Spermatogenesis
t
i occurs in
i the
th seminiferous
i if
tubules
t b l off the
th testes:
t t
Germ cell line cells (2N) multiply by mitosis to produce
spermatogonia
p
g
((2N))
Spermatogonia (2N) go through
mitosis to produce primary
spermatocytes (2N)
Primary spermatocytes go through
meiosis I to p
produce secondary
y
spermatocytes (1N)
Secondary spermatocytes go
thro gh meiosis II to spermatids
through
Spermatids mature to become
spermatozoa (sperm cells)
4
Oogenesis occurs on the surface of the ovary
Germ cell
G
ll line
li
cells produce
oogonia
g
byy
mitosis.
Oogonia
produce primary
oocytes by
mitosis.
Primary oocytes
produce go
through meiosis
I to produce one
secondary
oocyte and a
polar body.
The primary oocyte keeps the nutrients stored
in the egg. The polar body does not become a
gamete.
5
The secondary
oocyte goes
through meiosis II
to produce an
ovum and a
second polar body.
The ovum may go
through additional
growth and
maturation to
become a mature
egg.
6
Plant life cycles consist of alternate stages of multicellular diploid
sporophytes and multicellular haploid gametophytes.
gametophytes This is
often called alternation of generations.
Sporophytes produce spores through meiosis.
Gametophytes produce gametes through mitosis.
Mosses and liverworts are nonvascular plants
Ferns and horsetails are seedless vascular plants
Gymnosperms are pines, ginkgos, and cycads. All are seed plants
Angiosperms are flowering plants
Animal Diversity
Animal life cycles all have a multicellular diploid stage and a
single celled haploid stages – eggs and sperms
single-celled
sperms.
Jellyfish and
many other
animals have
multiple body
forms during
life.
life
The jellyfish
life cycle is
another kind of
alternation of
ge e
generations.
o s.
Both
generations are
diploid.
diploid
Vertebrate Diversity
Fertilization (syngamy) in vertebrates can be external or
internal Only aquatic or semi-aquatic
internal.
semi aquatic organisms – fish,
fish and
amphibians - have external fertilization but many of those have
internal fertilization – e.g sharks, guppies, …
Reptiles, Birds and Mammals are all called “amniotes”
because their embryos develop within an amniotic sac.
Amniotic egg - Water-tight eggs contain food source (yolk) and
four membranes: Chorion, amnion, yolk sac and allantois
E h an important
Each
i
t t embryonic
b
i life-support
lif
t system
t
Chorion allows oxygen to enter, but retains water
Amnion encases developing embryo within fluid-filled cavity
Yolk sac holds food for embryo - extracted by blood vessels
Allantois encloses waste products
Some mammals lay eggs – platypus and spiny anteater
Most mammals are live bearing (viviparous) and have internal
development within an amniotic sac.
Within mammal diversity there are three
developmental modes.
Monotremes – platypus and spiny anteaters
lay amniotic eggs – like reptiles and birds
Marsupials – kangaroos, opossums, etc. have
a short period of internal development
(gestation). Young are born in a fetal state and
continue development in their mother’s pouch.
Placental mammals have a long gestation
period and young are born in a more advanced
condition.
All mammals produce milk and nurse their
y
young
g
There are many modes of reproduction used by eukaryotes, both
sexual and asexual reproduction.
p
Asexual reproduction is common in plants and some animal
groups.
Budding - growth
of a new
individual on the
body of the parent
Fragmentation - in starfish, an arm and
part of the central disk can regenerate a
complete multi-armed body.
body Fragmentation
is common in plants and some worms. 20
Parthenogenesis
g
- the development
p
of eggs
gg that have not been
fertilized -- exhibited by many insects, many lizard species and
some fish species. Parthenogenesis can be induced in some birds
by heat-shocking
heat shocking eggs
eggs.
21
There is extreme varietyy in modes of sexual reproduction
p
The male and female sex organs can be found in the same body.
This is known as monoecy or hermaphroditism.
Hermaphroditism is very common in plants and found in many
animal groups.
22
There is variety in the types of hermaphroditism
A simultaneous hermaphrodite has both male and female sexual
functions at the same time.
A sequential hermaphrodite is a male during one part of its life
and a female during another part.
P t d
Protandrous
h
hermaphrodites
h dit are males
l first
fi t andd then
th later
l t
become females
Protogynous hermaphrodites are females first and then later
become males.
23