How do organisms reproduce - notes
HOW
DO ORGANISM REPRODUCE
Reproduction is the process by which living organisms produce new
individuals similar to themselves.
Reproduction ensured continuity of life on earth.
Reproduction – A bridge to hereditary transmission.
It involves continuation of characters from the parents to
daughter cells
by Copying of DNA (Deoxyribose
Nucleic Acid) molecules present in
the chromosomes of
the cell.
Copying of DNAs is also not a foolproof exercise, even minute
changes
bring about Variation
in the blue print of the offsprings.
The useful variations are retained while the harmful one does not
go
beyond.
Actually variations help the species to withstand drastic
environmental
changes, thus save
the species from becoming extinct and promotes its
survival for a longer
time.
This inbuilt tendency of variation is the ‘‘fuel’’ for Evolution.
REPRODUCTION
Asexual
Reproduction
|
Sexual
Reproduction
|
1.
A single parent is involved
|
1.
Both Parents involved
|
2.
Gametes not formed
|
2.
Gametes are formed
|
3.
Progeny is Identical to parent
eg.
Fission in Amoeba
|
3.
Progeny is only genetically similar to the parent.
|
|
|
Asexual Reproduction is
extremely useful as a mean of rapid multiplication. It is common in lower
plants and animals
MODES
OF ASEXUAL REPRODUCTION
1.
FISSION : the
parent cell divides/splits into two daughter cell-Binary Fission;
splits into many cells-multiple
Fission.
FISSION
Binary
Fission
|
Multiple
Fission
|
The
parent cell divides into two equal halves (daughter cells)
E.g.
Amoeba
|
The
parent cell divides into many daughter cell simultaneously.
Eg.
Plasmodium, Leishmania
|
Stages:
Nucleus lengthensà
nucleus divides-àcytoplasm divides-àdaughter amoebae
2.
BUDDING : A bud develops as an outgrowth on parent
body due to repeated cell division at a specific site. These buds develop into
thing individuals, which detach from
parent body when they mature.
Eg. Hydra, yeast.
3.
Spore Formation : Spores are small, bulb like
structure that develop at the top of the erect hyphae of the fungus plant. These are
released into the air and germinate, into new individuals after landing into food or soil.
4.
FRAGMENTATION
: It
is the accidental process when the broken pieces of an organism (fragments)
grows into a complete organism.
5.
REGENERATION : When the simple animals like
Hydra, Planaria develop a new individual from their broken older part it is
known as regeneration. It is carried out
by
specialised cells which divide and differentiate to form the complete
individual.
6.
Vegetative Propagation: A mode of
reproduction in many plants which part like the stem, root,leaves develop into new
plant under favourable conditions.
Methods of Vegetative Propagation
1. By Roots : Eg.
adventitious roots of Dahlias
2. By Stems : Eg. Potato
(tuber), ginger (rhizome)
3. By Leaves : Eg. leaves
of bryophyllum bear adventitious buds (in the
notches of leaf margin) which
develop into new plants.
4. Grafting : Eg. Mango.
5. Cutting : Eg. Rose
6. Layering : Eg. Jasmine
7. Tissue adture : Eg.
Orchids, Ornamental Plants.
Benefits of
Vegetative Propagation
1. Plants can bear flowers,
fruits earlier than those produced from seeds.
2. Growing plants like Banana,
orange, rose, jasmine that have lost the
capacity to produce seeds.
3. Genetical similarity is
maintained in the plants.
Sexual
Reproduction
When
reproduction takes place as a result of fusion of two gametes, one
from
each parent, it is called sexual reproduction
The process of fusion of male and female gametes is called fertilization.
The
formation of gametes involves exchange of chromosomal (genetic)
fragments between homologous chromosomes causing genetic
recombination which
leads to variation.
Gametes: Special Reproductive cells
are different from normal cells as they have half the
number of
chromosomes.
e.g Human cells have 46 chromosomes but sperm and egg have 46/2=23
chromosomes.
So when sperm and egg fuse, zygote will have 23+23= 46 chromosome.
Male gamete : small in size and motile (has tail)
Female gamete: big in size ( because of stored food reserve) and
non motile.
Sexual Reproduction in Plants
It occurs mostly in flowering plants. In fact flowers are the
reproductive organ of plants.
FLOWERS
Bisexual Flowers
|
Unisexual Flowers
|
Both male and female reproductive part i.e., stamen &
carpel are present.
|
Either male or female reproductive part is present.
|
Eg. Hibiscus, mustard
|
Eg.
Papaya, Watermelon
|
A typical flower consists of four main whorls namely 1. Calyx
(Sepals),
2. Corolla (Petals),
3.
Androceium (Stamens) and
4. Gynoecium( Carpels).
Reproductive
Part of Flower
1. STAMENà Anther (2n) + Filament
Pollen grains
(Male gamete)
(n)
2.
CARPEL-à Stigma +
Style + Ovary(2n)
Egg cell
(ovule)
(n)
Pollination:
Transfer of pollen grains from anther to stigma of carpel.
(Self-Pollination)
Pollen
grains transferred to the stigma of the carpel of the same flower (Cross-Pollination)
Pollen grains transferred to the stigma
of carpel of the another flower This
transfer of pollens is achieved by agents like wind, water or animals.
After Pollination, a pollen
tube grows out of pollen grains, through which male
germ cell reaches the ovary and fusses
with the female germ cell.
Fertilization : The
fusion between the pollen grain and female egg cell.
- It occurs inside the ovary. Zygote is
produced in this process.
-Zygote divides several times to form an
embryo within the ovule.
-The ovule develops a tough coat and is
converted into a seed.
- Ovary grows rapidly and ripens to
forms a fruit, while the seed contains the future
plant or embryo which develops into a
seedling under suitable condition. This
process is known as Germination.
Reproduction
in Human Beings
Humans use Sexual Mode
of reproduction.
It needs sexual
maturation which includes creation of the germ cells
i.e., egg (ova) in the female and sperm in the
male .
This
period of attainment of sexual maturation is called Puberty.
Pubertal
changes/Secondary sexual characteristics:
(Common) Hair growth in armpits, genital
region,
Acne/ pimples(excessive oil production
by sebaceous glands in skin)
(Girls) :
Breast development, Menstruation
( Boys) : Facial hair i.e. beards,
moustaches, Adam’s apple (change in voice)
Penis enlargement..
Human beings have a
well developed male and female
reproductive system.
Male Reproductive
System- Male
reproductive organ (Testis)
+ Other glands (Prostate Gland and seminal
vesicles)
+
Ducts (Vas deferens, Ejaculatory duct…)
.
The
Testes
(male reproductive organ)
Function:
a) Formation of male germ cell (sperms)
takes place.
Location:
A pair of testes are located inside scrotum situated outside the
abdominal cavity. It is meant to keep
relatively a low temperature
needed for the production of sperms by
testes.
Function:
b) Testes release a male sex hormone
called testosterone whose
function is to:
1.
Regulate the production of sperm
2. Brings about changes in appearance
seen in boys at the time of puberty.
The
sperms along with the secretion of prostate gland and seminal vesicle,
together constitute semen.
( Sperms + Secretions of
Prostate gland and seminal vesicles = Semen)
Secretions of Prostate gland and seminal vesicles-
1.Provide nourishment to sperms.
2. Help
sperms in their movement by lubricating the ducts.
The semen is released and made to enter into
the female genital tract during Copulation.
Female
Reproductive System- Female
reproductive organ (Ovary)
+ Ducts (Fallopian tube, Uterus, Vagina)
Ovary-Location: One ovary each on
both sides of abdomen.
Function: 1.The female germ
cells or eggs are produced in the ovaries.
(When a girl is
born, the ovaries already contain thousands of immature eggs.
At the puberty, some of these eggs
start maturing. One egg is produced
every
month by one of the ovaries.
2. Female sex hormones (oestrogen and progesterone) are produced
in ovary.
Fallopian tube:
Carries egg from ovary to the uterus (also known as womb)
Uterus:
is an elastic bag like structure in which development of embryo takes place.
The Uterus opens into
the vagina through the cervix (birth canal).
Fertilisation
:
(Fusion of sperm and egg)
Fertilization occurs in
the fallopian tube of female genital tract. (Internal fertilization)
The fertilized egg also
called zygote (2n) starts dividing, reaches uterus and gets embedded
in the thick lining of
the uterus, (which is richly supplied with blood to nourish the growing
embryo) . This process is known as Implantation.
Placenta.
The Embryo gets nutrition from the mother's blood with the help of a special
tissue
called Placenta.
(Function) - It provides a large surface area for glucose and oxygen to
pass from
the
mother to the embryo.
- Similarly the wastes from developing embryo
are removed to
mother's
blood through placenta.
Placenta
also produces hormones to support pregnancy.
The child is born as a
result of rhythmic contractions of the muscles in the uterus. After nine months
(36 weeks) of development inside mother's womb. This is called Gestation
Period.
If no fertisation
occurs (i.e. sperms are not present in the female genital
tract) –
1. Zygote
is not formed,
2. The
inner wall of uterus breaks which causes bleeding through vagina. This process
is called MENSTRUATION. It occurs at a
regular interval of 28 days.
The sexual
cycle in a woman continues upto the age of 45 to 50 years.
After that the ovary do not
release egg. This stage is called Menopause.
It also marks the end of menstruation in the
woman.
Reproductive Health
Reproductive
Health means a total well-being in all aspects of reproduction, i.e.,
physical emotional, social and behavioural.
Contraception
: It is the avoidance of pregnancy.
It can be achieved by preventing the fertilization of ova.
Methods
of contraception
1. Physical
Barrier: To prevent union of
sperm & egg.
Use
of condoms, Diaphragm & cervical
caps.
2. Surgical
Method: Also called sterilization.
In vasectomy, the vas deferens of male is blocked to
prevent sperm transfer.
In tubectomy, the fallopian
tube of female is blocked to prevent egg to reach uterus.
3. Chemical Method : Oral contraceptive
(OCs) - changes the hormonal balance to
check the
egg release in females. OCs cause side effect.
4. IUDs: Intrauterine contraceptive device-
Copper-T or loop is placed in uterus to
prevent
pregnancy.
Healthy
society needs a balanced sex ratio that can be achieved by educating the
people to avoid
malpractices like female foeticide & prenatal sex determination.
Sexually
Transmitted diseases (STDs)
VIRAL STDs: Eg.
H.I.V. – AIDS, Warts
Bacterial STDs: Eg. Syphilis and Gonorrhoea
STDs
are communicated during unsafe sexual contact.
These
diseases (STDs) can be prevented by using physical barrier i.e.
condom.
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