Download Cell Divisions

Cell Divisions
Cell division is process by which a parent cell divides into two daughter cells.
Opinion that microbes and worms are generated spontaneously
from dust & would multiply by breaking apart during collisions.
First cell division was observed by Lazarro Spallanzani – a
catholic priest – & another of the Lord’s men for the sciences. S.
spent months trying to divide drops with hairs so that he had only
one microbe under the microscope. It divided by fission.
S.was first to recognize fertilization as unification of sperm and
egg cell; proved by first artificial insemination in female dogs.
(1) For simple unicellular organisms such as bacteria, amoeba etc. cell
division is equivalent to reproduction– a new organism is created. cell
division in prokaryotes is known as binary fission.
(2) Cell division in multicellular eukaryotes is part of mitosis and cell cycle
resulting in daughter cells that belong to the same, growing organism.
Mitotic cell division can create progeny, such as plants grow from cuttings.
(3) A novel type of cell division developed only in eukaryotes: meiosis. A cell
is permanently transformed into a gamete and cannot divide again until
fertilization. This is the ontogenetic reflection of their prokaryotic origin.
Cell Division in animals vs plants
Mitosis was discovered by German botanist Eduard
Strasburger in 1875 in onion cells and later by
German zoologist Walther Flemming in animal gill
cells in 1879. Flemming named it ‘Mitosis’ in 1882.
Cell Divisions
Cytokinesis usually occurs in conjunction with mitosis. However, there are
many cells where mitosis and cytokinesis occur separately, forming single cells
with multiple nuclei & sometimes multiple copies of chromosomes & DNA
(polyploid). This occurs mostly among fungi, but is found also in animals.
Errors in mitosis can kill a cell through apoptosis (1) or cause mutations (2).
Mitosis – chromosomes are divided
G = gap phases 1 & 2
S = synthesis phase doubles
chromosomes back to diploid status
Cell Growth
For a multicellular organism to grow, it takes more than cell division, which
does not increase cell volume Î It needs cell ell expansion.
vacuole
expansion
Cell expansion in animals is slow since based on
the synthesis of more cytoplasm (s –phase).
Cell expansion in plants is quick since it involves
only water uptake into the vacuole (novel phase).
This not only enlarges the organism quickly but is
also the basis for plant movements (tropisms).
Animal movement is based on musles = contractile
actin/myosin protein chains.
Development
Plant development is an umbrella term for a broad spectrum of processes
that include: the formation of a complete embryo from a zygote ; seed
germination; the elaboration of a mature vegetative plant from the embryo; the
formation of flowers, fruits, and seeds; and many of the plant's responses to its
environment.
Development
Cell division and expansion are
particularly simple & fast in plants
Most embryos start with a fused cell
called zygote- a fertilized ovule.
Most higher plants start their life as
an embryo contained in a seed or a
vegetative prop unit.
Most higher plants start their adult
life as germinating seedlings
Plants reach sexual maturity by
flowering & subsequent seed production
E. Haeckel’s biogenetic rule” “ Ontogenesis repeats phylogenesis”
Î plant seedlings look very much alike in the monocots or dicots!
How higher plants reproduce!
How higher plants reproduce, Pollen & Ovary
How Pollen germinates
Pollen grain has 3 nuclei
1. A bigger one that drives
the tube growth
2. 2 smaller sexual nuclei
that fertilize either the
ovule or the polar cell to
give rise to embryo and
endosperm respect.
Pollen germinates only in a highly osmotic medium (in water it bursts 7
is destroyed). It also germinates only on the stigma of its own species, in
non-selfing species only on the stigma of a different plant individual.
Pollen – flying male gametes that replace outdated sperms
Pollen germinates on stigma of only his
own species, otherwise there would be
chaos in the plant world.
Artificial fertilization in plants
Artificial pollination (i.e. by humans
does not strike us as strange as
artificial insemination in cows etc.
since plant are pollinated by other
creatures anyway.
Date palms and vanilla plants
have been-hand pollinated ever
since we know about them. Also
Mendel strictly hand pollinated
his peas in order to get pure-bred
lines and then clean crosses.
Embryo development: double fertilization
Fertilization in higher plants is different from lower plants and animals.
The pollen has 3 nuclei rather than one:
one to drive pollen tube expansion, one to fertilize the egg cell and form
the embryo and one to fertilize the polar nucleus to form the endosperm
Embryo development in dicot seeds
For the ovule in the ovary Î to become a seed in a fully developed fruit
there are seven simple steps it has to undergo
Embryo development in dicot seeds
1. Fertilization of egg cell Î
zygote
2. Unequal cell division
3. Terminal cell Î embryo Basal
cell Î suspensor = umbilical
cord
4. Embryo divides Î globular
stage
5. Embryo Î heart stage
(cotyledons develop)
6. Torpedo stage Î stretching of
embryo axis Î stem & root
meristem appear
7. Seed development takes a
break Î dormancy induced
Germination
Germination is the process in which a seed or spore emerges
from a period of dormancy. This includes the sprouting of
seedling from a seed (angiosperm, gymnosperm) & growth of
sporelings (felt-like protonema, heart-shaped prothalli ) from a
spore in bryophytes & ferns..
Seed germination requires:
(1) Water. Dormant seeds have water content of approx. 10 %.
Hydration increases internal pressure of a seed to burst the coat &
activate preformed enzymes.
(2) Oxygen is required for aerobic respiration, the sole source of energy.
If a seed is buried in waterlogged soil or has impermeable seed coats the
seed it will be oxygen starved.
(3) Temperature. Seeds have a temperature range & will not germinate
above or below this range. Most seeds germinate at temperatures slightly
above room-temperature 60-75 F (16-24 C).. Some seeds require
exposure to cold temperatures (vernalization) to break dormancy.(4 C ).
(4) Light or darkness: dill, lettuce seeds need light – shallow planting
Embryo development in dicot seeds
Note that early dicot seed still has endosperm, which gets digested to
feed the swelling cotyledons as the new storage organ for food.
Embryo development in monocot seeds
1. Fertilization & unequal division
1. Fertilization & unequal division
1. Fertilization & unequal division
2. Embryo undergoes cell division
1. Fertilization & unequal division
Embryo development in monocot & dicot seeds
Polarity – a basic feature of plant cells in stem & roots
Polarity of plants is well established during embryogenesis where it is set
with the first division of the zygote and apparent in the torpedo stage.
Polarity of plants is reflected in the
polarity of its stem and root cells.
At the level of the individual cell
polarity is organized by asymmetric
distribution of organelles& and Golgi
network of transport proteins,
especially for the polar transport of
auxin. Polar cells exist also in
animals, but only at tissue level.
Hermann von Vochting 1878: established term “Polarity “ for plants. Went 1974
environmental factors regulate polar auxin flows to induce physiol. responses like
reorientation, leaf abscission, new apical leaders 1974 Rubery & Sheldrake
chemisosmotic model: PM with auxin uptake carriers and an efflux carrier confined to
the lower side of cells 1999 PIN proteins are PM proteins acting as auxin efflux
carriers. Their localization in the cell determines direction of auxin flow in the tissue!!
REF: E. Ferraru & Jiri Friml: PIN polar targeting, Plant Physiol 147, 1553-1559 (2008)
Embryo Size
Embryos of plants look different from those of animals. But like the later
any similarity to the adult parents is only a matter of flattery.
Animal embryos at maturity
Bean Phaseolus vulgaris
coconut embryo at maturity
Embryo Size - Dicots
Gingko biloba
Pea Pisum sativum
Phaseolus vulgaris
Avocado Persea amer.
Red Pepper Capsicum
Onion Alium
Dormancy – a state between life & death
Dormancy is a period in an organism's life cycle when growth,
development, and physical activity is temporarily suspended to
conserve energy and survive adverse environmental conditions
Bacterial dormancy: bacteria produce metabolically inactive forms
that survive intensely adverse conditions unharmed; cysts or
endospores. To inactivate these resistant forms you need autoclave
During hibernation animals have decreased heart rate (95%) & body
temperature: bats, hamsters, other rodents, European hedgehog, bears.
Diapause is a strategy that is common in insects suspending
developm. til next spring . Mammals like red deer delay attachm.
of embryos to uterus, ensures offspring born in spring
Aestivation, also estivation, is dormancy in response to very hot or
dry or winter conditions & occurs in invertebrates such as garden snail
Torpor = short-term reduction of body temperature to ambient level
during periods of inactivity & occurs e.g. in hummingbirds & bats
Plant dormancy – Seeds & Buds
Plant embryos are always put into dormancy at the end of the
embryogenesis to prevents germination on the mother plant Î vivipary
Def.:
Viable seeds that do not germinate are said to be dormant.
When a mature, viable (embryo cells alive) seed fails to germinate under favorable
conditions (water, temps), it is dormant. 2 types of dormancy Î
1. seed coat dormancy or external dormancy is caused by the presence
of a hard seed covering or seed coat that prevents water and oxygen from
reaching and activating the embryo.
2. embryo dormancy or internal dormancy, is caused by inhibition of
the embryo which prevents it to germinate under increased hydration
In the dehydrated (< 10 % water) state of dormancy,
the embryo can survive extremely hot or cold
temperatures (300 C for a short time), radioactive
radiation is not causing genetic mutations.
Î Therefore annuals survive adverse
environments like winter or drought as seeds
How to overcome seed dormancy!
Wild plants have exceeding long periods of dormancy, domesticated
plants do not. Dormant seeds were abolished during the selection
process of harvesting and sowing the next year. There are several ways
to overcome the dormancy of seeds.
Scarification is artificial damaging of seed coat to allow access of water.
Use of file (mechanic sc.) or 10 min soak in con. Sulfuric acid (chem. S)
Stratification: cold treatment simulating the winter frost. Seeds are
mixed together with moist sand or peat moss and stored for a few weeks
in the refrigerator (4 C or 40 F). This treatment starts a process that
breaks down abscisic acid and inhibitors of embryonic development.
Seed Banking refers to the fact that some plant
species (weeds, Scots broom) have many generations
of seeds in the soil. A variable length of the
dormancy create seed banks, which allow them to
germinate even if the previous years did not set seeds
or even if the plant was destroyed years before.
Plant dormancy –Trees & Buds
Plant embryos are always put into dormancy at the end of the
embryogenesis to prevents germination on the mother plant Î vivipary
Tree dormancy: Trees that have dormancy needs too. Î leaf
abscission and dormant buds. Deciduous plants will lose their leaves;
evergreens will curtail all new growth.
if a Japanese Maple (Acer palmatum) is given an "eternal summer"
through exposure to additional daylight, it will grow continuously for as
long as two years. Eventually, however, a temperate climate plant will
automatically go dormant, no matter what the environmental conditions..
Going through an "eternal summer" and growth is stressful to the plant
and usually fatal. The fatality rate increases to 100% if the plant does not
receive the necessary period of cold temperatures required to break the
dormancy. Most plants will require a certain number of hours of
"chilling" at temperatures between about 0 °C and 10 °C to be able to
break bud dormancy
How plants survive the winter!
Annuals survive the winter as dormant seeds, biennials as buried
storage roots, bulbs or shoots (rhizomes), perennial plants survive with
their shoots in the open; have new buds ready in the fall.
Seeds & buds form next generation of full plants or shoots only. Bud is
embryonic shoot with short internodes & protective bud scales as
leaves. Bud dormancy develops before the old leaves senesce and fall.
Shortening of day length induces dormancy in buds.
How to overcome bud dormancy?
.
1. Dormancy is broken in s leafless trees by rising day length:
birch, , larch yellow poplar, red oak scales are light-sensitive,
keep them indoors in constant light or try method 3 below :Molisch
2. Bud dormancy can be overcome by low temps
apples need 1000 hours at 7C, lilac branch inside warm greenhouse
will stay dormant, branches outside house will overcome dormancy
3. Immersing branches in warm water baths at 40-55 C for a minute