Reproduction in organisms class 12 neet & boards

TYPES OF REPRODUCTION

There is a large
diversity in the biological
world andeach organism
has evolved its own
mechanism tomultiply and
produce offspring.
• The organism's habitat,
its internal physiology
andseveral other factors
are collectively responsible
for how it reproduces.
• Reproduction is of
following two types :
When offspring is
produced by a single
parent with or without the
involvement of gamete
formation, the reproduction
is asexual.
When two parents
(opposite sex) participate in
the reproductive process
and also involve fusion of
male and female gametes,
it is called sexual
reproduction.
A. Mexican Roce
pTrroadnuscptoiornt
In this method, a single
individual (parent)
is capable of producing
offspring.

As a result, the
offspring that are produced
are not only identical to one
another but are also
exact copies of their parent.
The term clone is used
to describe such
morphologically and
genetically similar
individuals.

Occurence of asexual
reproduction

Asexual reproduction is
widespread among
differentgroup of
organisms.
• Asexual reproduction is
common among single
celledorganisms and in
plants and animals with
relatively

simple organisations.

In Protists and

Monerans, the organism or

theparent cell divides into

two to give rise to

newindividuals. Thus in

these organisms cell-

division is itself a mode of

reproduction.

Types of asexual

reproduction

Many single-celled

organisms reproduce by

binary fission (in.: Amoeba,

Paramecium), where a

celldivides into two halves

and each rapidly grows

indian adult.

In yeast, the division is

unequal and small buds

areproduced that remain

attached initially to

theparent cell which

eventually gets seperated

andmature into new yeast

organism (cells). Budding

alsofound in Hydra.

Other common asexual

reproductive structures

are conidia (Penicillium),

and gemmules (sponge).

Members of the

Kingdom Fungi and simple

plants such as algae

reproduce through special

asexualreproductive

structures. The most

common of

these structures are

zoospores that usually are

microscopic motile

structures.

Vegetative

Propagation

While in animals and

other simple organisms

theterm axesual is used

unambiguously, in plants,

theterm vegetative

reproduction is frequently

used.

In plants the units of

vegetative propagation

such asrunner, rhizome,

sucker, tuber, offset, bulb

are all capable of giving rise

to new offspring.

These Structures are called

vegetative

propagules. Obviously,

since the formation of

these structures does not

involve two parents, the

process involvedis asexual.

You must have heard

about the scourage of

the water bodies or about

the 'terror of Bengal'. This

isnothing but the aquatic

plant 'water hyacinth'.

It in one of most

invasive weeds found

growing wherever there is

standing water. It drains

oxygen from the water,

which leads to death of

fishes.

It is interesting to know

that this plant

was introduced in India

because of its beautiful

flowers and shape of

leaves. Since it can

propagate vegetatively at a

phenomenal rate and

spread all overthe water

body in a sh period of

time, it is very difficult to get

rid off them.

In banana, ginger,

potato, sugarcane and

dahlia thenew plantlets

invariably arise from the

nodes present in the

modified stems of these

plants.

When these nodes

comes in contact with

damp soil or water, they

produce roots and new

plants.

Similarly, adventitious

buds arise from the

notches present at margins

of leaves of Bryophyllum

and Kalanchoe. (CBSE 2005)

This ability is fully

exploited by gardeners

and farmers for commercial

propagation of such plants.

Artificial methods or

Horticultural methods

of vegetative Propagation

1. Root cuttings : Lemon,

Tamarind. (Gujarat 2006)

2. Stem cuttings : Rose,

China Rose, Tea, Coffee,

Grape, Sugarcane.

Sugarcane is

propagated by sowing part

of stem having at least one

node.

3. Layering: In layering

adventitious roots are

inducedto develop on a

stem while it is attached to

plant.

Induction of rooting on

stems before separating

themfrom parent plant is

called layering. eg.:

Jasmine,China Rose,

Orange, Lemon,

Blackberry, Litchi, Guava,

Pomegranate.

4. Grafting : In this

technique, a root system

and shoot system of 2

different plants are

connected andlater they

develop a composite plant.

Grafting is only

possible in cambium

containing woody plants.

Grafting is not possible in

monocots as they lack

cambium. (BHU 1995)

For union between

scion and stock in grafting,

first formation of callus

takes place. (CBSE 1990)

Shoot system is called

graft or scion while

rootsystem is called stock.

eg.: Mango, Guava,

Apple, Rose, Peach, Pear,

Plum

Plants with poor root

system are produced

through grafting

Asexual reproduction is

the common method

reproduction in organism

that have a relatively

simple organisation, like

algae and fungi and

thatthey shift to sexual

method of reproduction

justbefore onset of adverse

conditions.

Asexual (vegetative) as

well as sexual modes

of reproduction are

exhibited by the higher

plants. On the other hand,

only sexual mode of

reproduction ispresent in

most of the animals.

Sexual Reproduction

Sexual reproduction

involves formation of the

maleand female gametes,

either by the same

individual or by different

individuals of the opposite

sex. Thesegametes fuse to

form the zygote which

develops toform the new

organism.

It is an elaborate,

complex and slow process

ascompared to asexual

reproduction.

Because of the fusion

of male and female

gametes,sexual

reproduction results in

offspring that are not

identical to the parents or

amongst themselves.

A study of diverse

organisms-plants, animals

fungi-show that though

they differ so greatly in

external morphology,

internal structure

and physiology, when it

comes to sexual mode

of reproduction, they share

a similar pattern.

Phases of Life-cycle

An organism has 3

phases in their life cycle-

Juvenile phase,

reproductive phase and

senescent phase.

1. Juvenile

phase/Vegetative phase

It is the period between

birth and

reproductive maturity.

All organism have to

reach a certain stage of

growth and maturity in

their life, before they can

reproducesexually. That

period of growth is called

the juvenile phase. It is

known as vegetative phase

implants.

2. Reproductive phase

The end of juvenile /

vegetative phase which

marks the beginning of the

reproductive phase can be

seen easily in the higher

plants when they come to

flower.

Annual and biennial

types of plants, show clear

cut vegetative, reproductive

and senescent phases,

but in the perennial species

it is very difficult to

clearly define these phases.

Flowering plants are of

3 types :

i - Monocarpic plants :

These plants flower

only once in their lifetime

(CPMT 1993). After

flowering they produce

fruit and then die.

All annual plants (rice,

wheat) and biennial

plants(carrot, radish) are

monocarpic.

ii- Polycarpic plants :

Those plants in which

flowering and

fruitingoccurs repeatedly at

intervals every year. eg.:

Mango.

iii-Unusual plants :

A few plants shows

unusual flowering

phenomenon

Bamboo species flower

only once in their life

time,generally after 50-100

years produce large

number of fruits and die.

Strobilanthes

kunthiana (neelakurinji),

flowers once in 12 years.

This plant flowered during

september october 2006.

Its mass flowering

transformed large tracks of

hilly areas in Kerala,

Karnataka and Tamil Nadu

into bluestretches and

attracted a large number of

tourists.

In animals, the juvenile

phase is followed

morphological and

physiological changes prior

toactive reproductive

behaviour.

Birds living in nature

lay eggs only

seasonally. However, birds

in captivity (as in poultry)

can bemade to lay eggs

throughout the year. In this

case,laying eggs is not

related to reproduction but

is commercial exploitation

for human welfare.

The females of

placental mammals exhibit

cyclical changes in the

activites of ovary and

accessory ductsas well as

hormones during the

reproductive phases.

Sexuality In

Organisms

• Sexual reproduction in

organisms generally

involvesthe fusion of

gametes from 2 different

individuals.But this is not

always true.

Plants may have both

male and female

reproductivestructures in

the same plant (bisexual)

or on different plants

(unisexual).

In several fungi and

plants, terms such as

homothallic and

monoecious are used to

denote the

bisexualcondition and

heterothallic and dioecious

are theterms used to

describe unisexual

condition.

In flowering plants

male sex organ is stamen

andfemale sex organ is

carpels.

In flowering plants, the

unisexual male flower

isstaminate, i.e., bearing

stamens, while the

female is pistillate or

bearing pistils.

In most of the

flowering plants both

stamen (malesex organ)

and carpel(female sex

organs) are present in same

flower.Such plants are

called hermaphrodite or

bisexual.

If staminate flower

(male flower) and

pistillate flower (female

flower) are borne on

different plants that these

plants are called dioecious flower.

For convenience these

sequential events may

begrouped into three

distinct stages namely, the

pre fertilisation, fertilisation

and the post

fertilization events..

I. Pre-fertilisation

Events

These include all the

events of sexual

reproduction prior to the

fusion of gametes.

The two main pre

fertilisation events

are gametogenesis and

gamete transfer.

Gametogenesis refers

to the process of formation

ofthe two types of gametes

- male and female.

Gametes are haploid

cells.

In some algae the two

gametes are so similar

appearance that it is not

possible to categorise

theminto male and female

gametes. They are hence

calledhomogametes

(isogametes).

However, in majority of

sexually

reproducing organisms the

gametes produced are of

two morphologically

distinct types

(heterogametes). Insuch

organisms the male

gamete is called

theantherozoid or sperm

and the female gamete is

called the egg or ovum.

1. Cell division during

Gamete formation

Gametes are haploid

though the parent plant

body from which they arise

may be either haploid or

diploid.

A haploid parent

produces gametes by

mitotic division.

Several organisms

belonging to monera, fungi,

algae and bryophytes have

haploid plant body.

A haploid parent

produces gametes by

mitotic division.

Organisms belonging

to pteridophytes,

gymnosperms,angiosperms

and most of the animals

including human beings,

the parental body in

diploid.

Meiosis, the reduction

division, has to occur if

diploid body has to

produce haploid gametes.

In diploid organisms,

specialised cells

called meiocytes (gamete

mother cell) undergo

meiosis.

At the end of meiosis,

only one set of

chromosomes get

incorporated into each

gamete.

2. Gamete Transfer

After their formation,

male and female gametes

mustbe physically brought

together to facilitate

fusion(fertilisation).

In a majority of

organisms, male gamete is

motile and the female

gamete is stationary.

Exceptions area few fungi

and algae in which both

types of gametes are

motile.

There is a need for a

medium through which

themale gametes move. In

several simple plants

likeaglae, bryophytes and

pteridophytes, water is

themedium through which

this gamete transfer

takesplace.

A large number of the

male gametes, however,

fail toreach the female

gametes. To compensate

this loss of male gametes.

In seed plants, pollen

grains are the carriers of

male gametes and ovule

have the egg.

In bisexual, self

fertilising plants, eg.:

peas,transfer of pollen

grains to the stigma is

relatively easy as anthers

and stigma are located

close to eachother.

In cross pollinating

plants (including

dioecious plants).

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