Developmental Biology
Reproduction:
The production of new individuals by pre-existing individuals to continue their generation is called
reproduction. There are two types of reproduction and they are: Apomixis (vegetative and asexual) and
amphimixis (sexual).
Apomixis: It is the formation of new individuals by asexual reproduction without involving formation
and fusion of gametes. It is of two types: Agamospermy and vegetative propagation
Agamospermy: It is a type of apomixes in which seeds and embryo are formed without meiotic
formation of gametes and their fusion.
Vegetative propagation:
The process of development of new individuals directly from vegetative parts of the plants without
formation of gametes or spores. The vegetative parts involve in this type of reproduction is called
propagules and new individuals produced by this method are identical in respect to their parents. It is
divided into two types:
a) Natural vegetative propagation
b) Artificial vegetative propagation
a) Natural vegetative propagation:
The process of production of new individuals from general body parts of plants without influence of
human beings is called natural vegetative propagation. It may take place through root, stem and
leaves.
i) Root: In some plants modified tuberous roots are found in Dahlia, Asparagus, Ipomoea, etc can be
propagated vegetatively when planted in soil. In some plants such as rose, guava, sissoo, etc consist
of adventitious root buds from which new plants develop in suitable condition.
ii) Stem: Stem is of following types:
1. Underground stem: In some plants, stems are modified into underground form. The
underground stem such as rhizome as in ginger, bulb as in onion, corm as in Colocasia and tuber
as in potato grow again during favorable conditions. These underground stems bear nodes
called eyes that grow into individuals.
2. Sub-aerial stem: Sub-aerial stems such as runner in Cynodon, strawberry,etc. sucker as in mint,
Chrysanthemum, etc. Stolon as in Fragaria and offset as in Pistia reproduce vegetatively.
3. Aerial stem: In some plants such as sugarcane aerial stem also take part in vegetative
propagation when they get suitable condition.
iii) Leaves: In some plants like Bryophyllum, the margin of leaves develops bud. When these leaves fall
on to the soil, the buds grow root and becomes independent plants.
iv) Bulbils: In some plants specialized fleshy bulb like structure are found at the base of petiole,
inflorescence axis, over the swollen root and adjacent the fruit. E.g. pineapple, lily, garlic, etc.
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Anil Joshi (Botany)
b) Artificial vegetative propagation:
The production of new individuals from general body part of plant by the influence of human
beings is known as artificial vegetative propagation. Some important methods of artificial
vegetative propagation are as follows:
i) Cutting: This method is very common method of vegetative propagation of a desired variety.
Defoliated stems are obliquely cut off and cut end is placed inside soil and optimum water and
rooting hormones are applied. Due to meristematic activities adventitious roots will produce and
branch become independent plant. Eg: Rose, sugarcane, china rose, etc.
ii) Layering: It is the method in which roots are induced on the stem while it is still attached to parent
plant. This part of stem is later detached from the parent plant and grown into new plant. The
branch which develops the root is called layer. It may be mound layering or air layering.
1. Mound layering: In this, lower branch of stem is bent to bury letting growing tip of branch
above soil surface. After few days, adventitious roots are formed from lower surface of buried
part. After this the branch is cut from its parent and planted as a new independent plant. This
method is used in jasmine, strawberry, etc.
2. Air layering (gootee): It is carried out in those plants where the branches are hard such as
guava, orange, litchi, etc. It is done during early season. A 2-5 cm ring of bark is removed from a
rigid branch. The injured part is covered over by grafting clay (made up of clay soil 2 parts, cow
dung 1 part, finely cut hay 1 part along with root initiating hormone auxin) and then wrapped by
waterproof plastic bag. After 1-3 months root will appear at the injured region. The branch is
now cut below bandage and planted.
iii) Grafting: In this method, two plants of closely related varieties or species are taken. The plant of
which the root system is to be taken is called stock and the other of which shoot system is selected
is known as scion or graft. Stock is raised from seeds whereas stock is removed of its leaves and
buds. Stock and scion are joined with each other in such a manner that their cambia come together.
The cambial strips make an organic connection between two plants. The most common methods of
grafting are:
1. Tongue, slice or whip grafting: The stock and scion are given oblique cuts followed by an
appropriate notch in the tongue or whip grafting.
2. Wedge grafting: The stock is given a ‘V’ shaped notch and a scion is cut like a wedge.
3. Crown grafting: Several wedge shaped scions are selected and slits are made towards the
outside of stock as required.
4. Side grafting: Only one scion is inserted in a lateral slit.
5. Bud grafting: In this scion consist of a single vegetative bud with a stem peeling having cambium
and T-shaped cut is incised in the bark of scion and bud scion is inserted into the slit and grafting
wax is applied at the area of incision.
6. Approach grafting: In this method, both scion and stock are injured at equal level and the
injured parts are joined and grafting wax is applied and plants are tied together. When organic
connection has established, the scion is cut from below and stock is cut from above.
iv) Micro-propagation (tissue culture): It is advanced invitro method of multiplication of plants. It is
based on cellular totipotency. In micropropagation, explants is taken and provided with culture
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media under aseptic conditions, it produce undifferentiated mass of cells called callus. After getting
suitable phytohormones, callus differentiated into new plantlets having root and shoot initials.
Cutting
Gootee
Soil layering
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Advantages of vegetative propagation:
1. It is quicker method of multiplication.
2. It is used for developing pure lines or homozygous characters.
3. It is applicable for quick fruiting in plants.
4. By tissue culture large number of disease free and disease resistant individuals can be produced.
5. It helps in protection of endangered and rare plants.
6. It preserves parental characters.
7. It is only one method of multiplication of plant from seedless varieties.
Disadvantages of vegetative propagation:
1.
2.
3.
4.
5.
6.
New species cannot be formed.
It does not play any role in evolution.
It decreases adaptability and resistivity of organism.
Diseases of parents can transfer to all daughters.
Bad characters cannot be eliminated in plants.
Productivity is always less than average value.
Amphimixis (Sexual reproduction):
It is advanced method of reproduction, which involves two essential stages: meiosis and fertilization
(syngamy). Sexual reproduction has following five steps:
Sporogenesis, gametogenesis, pollination, fertilization or syngamy and embryogenesis.
SPOROGENESIS and GAMETOGENESIS:
Sporogenesis is the process of formation of spores within sporangium. It is of two types:
microsporogenesis and megasporogenesis. Gametogenesis is a process of formation of gametophyte. It
is of two types: microgametogenesis and megagametogenesis.
Microsporogenesis:
The process of formation of micro spore or pollen grains is called microsporogenesis. A very young
anther consists of homogenous mass of meristematic cells surrounded by epidermis which becomes 4
lobed later and the hypodermal cell in each lobe becomes prominent which is called as archesporial
cells. The archesporial cell divides periclinally forming outer-primary parietal layer which divides
anticlinally to form endothecium, middle layer and tapetum and inner primary sporogenous tissue which
divides to form microspore mother cell (MMC) or pollen mother cell. MMC divides meiotically to form
tetrad of microspores or pollen grains. Each pollen grain is covered by outer thick exine and inner thin
intine.
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Anil Joshi (Botany)
(Draw from A-E)
Microgametogenesis:
The process of formation of male gamete from pollen grain is called microgametogenesis. Haploid pollen
grain is the first stage of microgametogenesis. The nucleus of microspore divides mitotically to form a
large vegetative cell and small generative cell. The nucleus of vegetative cell remains at centre while the
nucleus of generative cell moves towards periphery. These nuclei are separated by a thin wall. Hence
the pollen grain shed at this condition is 2-celled stage but in some cases they become three nucleated
before dehiscence and this stage is called as 3-celled stage. After pollination further development of
pollen grain takes place where they absorb stigmatic fluid and swells up. Due to swelling outer exine
ruptures and intine protrude out in the form of pollen tube. The nucleus of vegetative cell lies at the tip
of tube and regulates growth and the nucleus of generative cell divides to form two male gametes. Thus
two male gametes and a vegetative nucleus constitute the male gametophyte.
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Anil Joshi (Botany)
Megasporogenesis:
The process of formation of megaspore in megasporangium or ovule is called measporogenesis.
Structure of ovule: An ovule consists of mass of parenchymatous cells which is called as nucellus. The
nucellus is protected by the covering of one or two multicellular coats called as integument. The
integument covers nucellus except a part known as micropyle. The base of nucellus from where
integument arises is called chalaza. The point of attachment of funicle with the ovule is called hilum.
Some parenchymatous cells of micropyle region which lies below epidermis enlarge and acts as
archesporial cells. The archesporial cell either develops directly as megaspore mother cell and the
nucellus at this phase is called tenuinucellus or divides periclinally to form outer parietal cell and inner
sporogenous cell. The sporogenous cell divides to form sporogenous tissue which act as megaspore
mother cell and such type of nucellus is called crassinucellus. The megaspore mother cell divides
meiotically to form megaspore tetrad out of which three megaspore degenerates and only one near
chalaza becomes functional.
Megagametogenesis:
The process of formation of female gametophyte (egg cells) in megaspore is called as
megagametogenesis. Haploid functional megaspore enlarges in size and the haploid nucleus undergoes
3 mitotic divisions forming 8 nuclei, 4 lies towards micropylar end and 4 lies towards chalazal end. One
nucleus from both end moves towards centre and fuse to form polar nucleus. The remaining 3 nuclei at
micropylar end organize to form egg apparatus (1 central egg and 2 lateral synergids) and another 3
nuclei at chalazal end form antipodal cells.
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POLLINATION
Transfer of pollen grains from anther to stigma of same flower or other flower of same species is known
as pollination. It is pre-fertilization stage. It is of two types: self and cross pollination.
SELF POLLINATION: The process of transfer of pollen grains from anther to stigma of same flower (called
autogamy) or genetically similar flower of same plant (called geitonogamy) is called self pollination. IT
may occur when flower are open (called chasmogamy) or closed (called cleistogamy).The self pollinated
flowers are generally small, inconspicuous, colourless, odourless and nectarless.
Advantage of self pollination:
1.
2.
3.
4.
5.
6.
It preserves parental characters.
It does not depend on external agents.
Pollen grains are not wasted.
It does not require more energy.
It ensures pure line maintenance.
It is advantageous for those flowers which do not contain colour, nectar and fragarance.
Disadvantage of self pollination:
1.
2.
3.
4.
5.
6.
Useful character cannot be obtained.
High chances of transferring parental diseases.
Productivity is always less than normal value.
Continuous self pollination leads to death.
Defected characters cannot be omitted.
It has no significant role in evolution, etc.
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Cross pollination:
The process of transfer of pollen grains from anther to stigma of genetically different flowers is called
cross pollination. It is also known as allogamy or xenogamy. It depends on external agencies like wind,
water and animals. It is divided into following types:
a) Anemophily: It is the type of cross pollination that occurs by the help of wind. The anemophilous
plant shows following characters: i) Pollen grains are small, light and dry, ii) both stigma and anthers
are exposed, iii) plants grow in large group, iv) stigma are either sticky, hairy or feathery, v)pollen
grains are produced in large amount, vi)flower are produced above foliage leaves, etc. eg: wheat,
rice, maize, etc.
b) Hydrophily: It is the type of cross pollination that occurs by the help of water. It is of two types:
Hypohydrophily: If cross-pollination occurs inside water then it is called hypohydrophily. Eg:
Hydrilla.
Epihydrophily: If cross-pollination occurs above water then it is called epihydrophily. Eg: Pistia.
c) Zoophily: It is a type of cross pollination that occurs by the help of animals. It is of following types:
Entomophily: If pollination occurs by the help of insects then it is called entomophily. Eg: mustard,
sunflower, etc.
Chiropterophily: If pollination occurs by the help of bat then it is called chiroptero phily. Eg: Banana,
Agave, etc.
Ornithophily: If pollination occurs by the help of small birds then it is called as ornithophily. Eg:
Bombax, Begnonia, etc.
Psychophily: If pollination occurs by the help of butterfly then it is called as psychophily.
Malacophily: If pollination occurs by the help of snails then it is called malacophily.
Anthropophily: If pollination occurs by the help of human then it is called anthropophily.
Myrmecophily: If pollination occurs by the help of ant then it is called as myrmecophily, etc.
Devices to ensure cross pollination:
1. Unisexuality or dicliny: The plant which bears male and female sex organs in different flowers
of same or different plant is called unisexual plant. Eg: maize, mulberry, etc.
2. Dichogamy: If anther and stigma mature at different time then it is called dichogamy. If anther
matures earlier than stigma as in sunflower, maize, etc then it is called protandry and if stigma
matures earlier than anther as in Magnolia, Ranunculus, etc then it is called protogany.
3. Self sterility: In some plants like potato, tea, etc. the pollen grains are incapable of growing over
stigma of same flower then it is called self sterility.
4. Heterostyly: If a plant has different length of style and stamens then it is called as heterostyly.
Eg: Oxalis, Jasmine, etc.
5. Herkogamy: In this, a physical barrier occurs in between male and female reproductive organ.
Eg: Calotropis, Orchid, etc.
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Advantages of cross pollination:
1.
2.
3.
4.
5.
6.
Desired characters can be inserted.
Variations can be observed.
Defected characters can be omitted.
It helps in adaptation of plants.
Productivity is always more than normal value.
It is responsible for hybrid vigour.
Disadvantages of cross pollination:
1.
2.
3.
4.
5.
6.
This method needs external agency.
The useful character can be spoiled.
Parental characters cannot be preserved.
Chances of fertilization are less.
It is less economical method as a plant consumes large amount of pollination.
This method is not responsible for development of pure lines.
FERTILIZATION
The process of fusion of two dissimilar gametes is called fertilization. It is also called as syngamy. When
pollen grains reach at stigma then intine forms pollen tube through germ pore of exine. Pollen tube
consist of two male gametes. Pollen tube elongates and pass into style and ovary before reaching ovule.
Pollen grain may be monosiphonous (develop one pollen tube) or polysiphonous ( develop more than
one pollen tube). The pollen tube enters into ovule by two methods porogamy (through micropylar
pore) or apogamy (through chalazal side called chalazogamy,; through integument called mesogamy).
Pollen tube after entering into the ovule reaches to embryo sac through nucellus. It always occurs
through egg apparatus. Pollen tube either enters in between egg and one synergid or in between
embryo sac wall and a synergid or directly into synergid. The pollen tube is attracted by synergids by
secreting some chemical. As a rule on synergid is destroyed and the tip of pollen tube disintegrated by
which two male gametes finally comes out of pollen tube. Out of two male gametes, one fuses with egg
to accomplish true fertilization to produce diploid zygote. The other male gamete unite with secondary
nucleus or polar nucleus to form triploid primary endosperm nucleus (PEN) through triple fusion. This
peculiar system of twice fertilization by two male gametes within same embryo sac to form diploid
zygote and triploid primary endosperm nucleus is called double fertilization and triple fusion. It is
characteristic features of angiosperm.
Significance of double fertilization:
1) It helps in production of viable seeds.
2) No wastage of energy.
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Anil Joshi (Botany)
EMBRYOGENESIS (DEVELOPMENT OF EMBRYO):
Zygote is the initial stage of embryogeny. The process of development of embryo after fertilization is
called embryogeny. It includes development of monocot and dicot embryo.
Development of dicot embryo:
Development of dicot embryo is followed from crucifer Capsella bursa pastoris. In this zygote divides
transversely to form larger basal or suspensor cell towards micropyle side and small terminal or
embryonal cell towards chalazal side. The basal cell divides transversely to form 6-10 celled filamentous
structure called suspensor. The last cell of suspensor is called haustorial cell which absorbs nutrients
from endosperm. The cell of suspensor near to embryo is called hypophysis which later gives rise to
apex of radicle. The terminal embryonal cell undergoes two vertical and one transverse division to
produce a two tier of 8 celled embryo. The upper tier form two cotyledons and plumule while lower tier
forms radicle. The 8 celled embryo divide periclinally and anticlinally to form globular multicellular
embryo, later it becomes heart shaped. During seed germination, plumule part is responsible for giving
shoot system and radicle forms root system.
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Development of monocot embryo:
The development of monocot embryo is followed from Sagittaria sagittifolia. Firstly zygote is divided
transversely into larger basal cell towards micropylar end and smaller terminal or embryonal cell
towards chalazal end. The larger basal or suspensor cell does not divide further and enlarge in size and
functions as haustorial organ. The embryonal cell divides by a vertical wall to form T shaped proembryo.
The embryonal cell divide by another transverse and vertical division to produce 8 cells arranged in two
tiers. The 4 cells of upper tier divides to form single terminal cotyledon whereas middle cell of
proembryo forms hypocotyls and radicle. The plumule arises as a depression, midway between the base
of cotyledon and the axis of embryo. In monocot embryo there is a single cotyledon called scutellum.
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:
Some terminologies of genetics:
1. Exon: Essential or coding part of split gene is called exon.
2. Intron: Non-essential or non-coding parts of split gene is called intron.
3. Splicing: The phenomenon of removal of introns and joining exons from split gene is called
splicing.
4. Inducible gene: The inactive gene which becomes active in presence of a chemical substance is
called inducible gene.
5. Repressible gene: The active gene which becomes inactive in presence of chemical substance
called repressor by formation of repressor protein is called repressible gene.
6. Structural gene: The gene which determines the sequence of amino acid in protein is called
structural gene.
7. Regulatory gene: The gene which controls the activity of structural gene by formation of
repressor protein is called regulatory gene.
8. Promoter gene: The gene that binds RNA polymerase are called promoter gene.
9. Operator gene: The gene that control the activity of structural genes by switch on or switch off
is called operator gene.
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Operon concept:
Jacob and Monad (1961) developed an operon concept in E. coli. An operon is a genetic unit consisting
of operator, promoter and structural gene.
Operon=structural gene + regulator gene (R) + promoter gene (P) + operator gene (O)
Lac operon concept:
Jacon and Monad studied lactose metabolism in E. coli. The bacterium E. coli when grown in a medium
containing lactose produces three enzymes β galactosidase, galactose permease and thiogalactoside
transacetylase by three structural gene viz. Z,Y and A respectively. These enzymes break down lactose to
glucose and galactose. (fig: a)
Regulation:
In the absence of lactose:
The regulator gene (R) in the absence of lactose (inducer) produces a repressor protein, which binds
with operator site and prevents its transcription due to blockage of RNA polymerase hence there is no
synthesis of protein or protein synthesis is switch off. (fig: b)
In the presence of lactose:
When inducer lactose is introduced, it binds with repressor that makes operator gene active by which
switch is turned on. Operator gene now induces RNA polymerase to bind the initiation site on promoter
gene and RNA polymerase starts transcription. Three enzymes i.e. enzymes β galactosidase, galactose
permease and thiogalactoside transacetylase are formed by three structural gene viz. Z,Y and A
respectively to express the characters. Synthesis of enzymes is continued unless and until all lactose
molecules are consumed. After consumption of lactose molecule, repressor protein binds to operator
and causes switch off. (Fig: c)
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