HOW DO ORGANISMS REPRODUCE
Reproduction and the Importance of
Variations in DNA
Do all living organisms reproduce? Why
do offspring’s resemble their parents? Does reproduction always involve the
participation of two parents?
Reproduction is a characteristic feature
of all living organisms. It involves the creation of organelles, cells, or
organisms of the same kind.
Reproduction is necessary for the survival of a particular species. During reproduction, the information for the inheritance of characteristics is passed on from the parents to the offsprings in the form of DNA.
DNA
DNA (Deoxyribonucleic acid) is a genetic
material found in the chromosomes, which are present in the nucleus of a cell.
It is a chemical, which carries genetic information required by the cells to
divide and produce proteins.
Genes are made up of DNA. A set of genes
are responsible for the production of a specific protein. Each protein is
specific for its specific function. Therefore, it is the DNA that decides the
formation of the structural, enzymatic, hormonal, and other components in an
organism. Genes are responsible for the physical appearance and body functions
of an organism.
Therefore, it is the
DNA that gets transferred from the parents to the offsprings and makes them
look similar to their parents.
DNA—genetic material
Transfer of DNA
The reproducing cells
produce a copy of DNA through some chemical reactions. This, in turn, results in
the formation of two copies of DNA. Now, these copies need to be separated from
each other. The newly-formed copy of DNA cannot be simply pushed out, as it
lacks an organized cellular structure. Hence, the copying of DNA always takes
place with the creation of an additional cellular structure. This process is
then followed by the division of a single cell to form two cells.
Do you know that larvae of the Axolotls
Mexican salamander are capable of reproducing at 12 months of age?
Variations
DNA carries genetic material and passes
it from the parents to the offsprings. But are we similar to our parents in
all aspects?
The case I: An organism reproduces an
offspring, which is similar to its parents in all aspects, with no changes
occurring in subsequent generations.
In this situation, the offspring is
adapted like its parent. It has all the characteristic features present in the
parent and is well suited for that environment.
The case II: An organism
reproduces an offspring similar to its parent, but a variation (like the
offspring can withstand temperature changes i.e. it can survive in very high
and low temperatures) has occurred, and this variation is passed to the
subsequent generations.
In this situation, the offspring is
similar to the parent but is more adapted than the parent due to the
variation. Therefore, it has better chances of survival than the parent.
Results of analysis: In nature,
variations occur during sexual reproduction. If this variation is beneficial to
the species, it is selected and remains in the population. This is because the
variant species are more adapted. Therefore, they can survive better and
reproduce to pass the genes to the offsprings.
Therefore, we can say that variations
are important.
Let us learn the importance of variations with the help of a few examples.
1. The peppered moth (Biston betularia) is generally pale in colour, but some variants may be dark coloured as well. In early 1800s, the pale form of the species comprised more than 98% of the population, while a variant of this moth, the darker form comprised less than 2% of the population. The pale form was difficult for birds to see as the trees were covered with lichens. However, with the onset of the industrial revolution, pollution levels increased, which resulted in a drastic decrease in the population of the lichens in that area. Also, smoke from the industries deposited over the trees. This made the pale form easily visible against the dark colour of the trees and thus, made them vulnerable. As a result, the only species of peppered moths that could survive were the dark forms, which arose as a result of variations. In this case, variations proved to be helpful. Such helpful variations are selected by nature as they help in the survival of species.
2. Due to genetic variations, a species
of rabbits were produced that could run faster, escape their predators better
(because of their body structure), and find food faster. As a result,
Hence, variations are important because:
· They allow the survival of a species in all conditions
· They lead to
evolution
If variants are so important, then why
do organisms produce similar offsprings?
Similar offsprings are produced to
maintain the general body design. This needs to be done so that the organism is
able to occupy the same habitat since the organism is best adapted for that
habitat. This is the reason why all fishes live only in water and all humans
live only on land.
Thus, we know that variations that are
beneficial to the population are passed on to the next generation. How does
nature select only beneficial variations? What happens to the variations that
are not beneficial?
Variations in nature are random. Both positive
and negative variations may occur. However, only positive or
beneficial variations are retained. This is because the variant species are
more adapted, can survive better, and can reproduce to pass the genes to the
offsprings. When a negative variation occurs, the individual is not fit and
cannot survive. Therefore, it cannot pass the genes to the next generation.
Thus, the negative variant is gradually lost from the gene pool.
Some Interesting Facts:
· · Do you know that identical twins are usually of the same sex and share 100% DNA? They come from one fertilized egg, which splits into two.
· · Fraternal twins are just like normal siblings. They come from two separate eggs, fertilized by two sperms, and share 50% of their DNA.
Importance of reproduction
Reproduction is essential for the
continuation of species. Without reproduction, a particular species will not
exist for a long time. Reproduction produces new individuals that resemble
their parents. This is the reason why cats reproduce more cats and dogs
reproduce more dogs.
Sexual Reproduction in Flowers
Reproduction in plants can be summarized in the following stages.
1. The ovule or egg is fertilized by pollen through pollination.
2. The ovules develop into seeds.
3. These seeds germinate to give rise to new plants.
Let us understand the stages and
component structures involved in the process of reproduction in plants.
The male reproductive organs of flowers
are stamens while the female reproductive organs are carpels.
· Male germ cells (male gametes): The stamen (male reproductive organ) produces numerous pollen grains. The pollen contains male reproductive cells or germ cells.
· Female germ cells (female gametes): The carpel (female reproductive organ) consists of a style, stigma, and ovary. Inside the ovary, one or many ovules are produced. Each ovule contains one egg cell, which is the female germ cell.
The female germ cell is non-motile and
is protected by the ovules and ovary.
The transfer of pollen from the anther
of one flower to the stigma of the same flower, or another flower in the same
plant or another flower in a different plant is called pollination.
Therefore, pollination is of two types-
self and cross-pollination.
Self-pollination: It
occurs in bisexual flowers (male and female reproductive structures are present
in the same flower). It is of two types:
Cross-pollination: It can occur in
both unisexual (male and female reproductive structures are present in
different flowers) and bisexual flowers.
In this type of pollination, pollen is
transferred from the anther of one flower of one plant to stigma of another
flower of another plant.
Pollinators
The transfer of pollen occurs through the air, water, birds, insects such as bees, wasps, moths, snails, etc. The petals
of a flower are brightly coloured or scented in order to attract insects. Some
flowers even secrete nectar to attract insects such as bees.
These insects and birds, which help in
the transfer of the pollen to stigma, are called pollinators.
A zygote is formed after fertilization.
This zygote divides many times to form an embryo.
Ovary and ovule after fertilization
The outer layers of the ovule become
impervious, hard and function as a seed coat. An ovule with an embryo inside is
called a seed.
The ovary enlarges and
ripens to become a fruit. Other floral parts such as sepals, petals, stamens,
style, and stigma may fall off. However, in some cases, they remain persistent
in the fruit.
Do you know that some plants are able to
produce fruits without the process of fertilization? This phenomenon is known
as parthenocarpy. Parthenocarpic fruits are either seedless or contain
non-viable seeds such as a seedless variety of grapes and navel oranges.
Seed germination
The embryo inside a seed is inactive. It
becomes active under favourable conditions and grows out of the seed coat as a
seedling. This process is known as germination.
Favourable conditions for germination
are moisture, air, and temperature.
When a seed germinates,
the portion above the cotyledons, which grows into a shoot, is called a plumule.
The portion below the cotyledons, which grows into a root, is called a radicle.
Structure of the Human Reproductive
System
Structure and Functions of the Male
Reproductive System
Male Reproductive System
It consists of various
organs, which are involved in the production and transfer of germ cells into
the female body.
The organs present in the male
reproductive system consist of a pair of testes, vas deferens, prostate gland,
and seminal vesicles.
Testes
They are located outside the abdominal
cavity within a pouch called the scrotum.
The testes produce sperms. But for this, they require a temperature, which is 2 – 2.5°C lower than the temperature of
the rest of the body. Therefore, they are protected inside the scrotum.
The function of the testes is to produce
sperms and a hormone called testosterone, which brings about secondary
sex characteristics in boys.
Vas deferens
It carries the sperms from the testes to
the urethra. It also receives the ducts, which bring products from the seminal
vesicles and prostate glands.
Seminal vesicles and prostate glands
The fluids from these organs provide
nutrients in the form of fructose, calcium, and some enzymes. The sperms
produced in the testes are carried by the vas deferens. The secretions from the
seminal vesicles and prostate glands lubricate the sperms. The sperms are now
dispersed in a fluid, which makes their transport easier. This milky white
fluid that contain mixture of sperms along with secretions from seminal
vesicles and prostate glands is called semen. The urethra extends
through the penis to an external opening and carries either urine or semen.
Structure of a sperm
It consists of a head,
middle piece, and a tail.
· The head contains a nucleus, containing the chromosomal material.
· The middle piece has many mitochondria to provide energy for the movement of the tail.
· · The tail facilitates the movement of the sperm into the female body.
Do you know that every day around 500
million sperms mature in a normal human male? The average life span of a sperm
is around 36 hours.
Structure and Functions of the Female
Reproductive System
Female
Reproductive System
It consists of a pair
of ovaries, a pair of oviducts, uterus, and vagina.
The ovaries are located on each side of
the lower abdomen. It produces thousands of eggs in the female body. It also
produces a hormone called oestrogen, which brings about secondary sex
characteristics in the female body.
The eggs produced in the ovary start
maturing on reaching puberty. One egg from each ovary grows and matures, and is
carried from the ovary to the uterus by a thin oviduct or the fallopian
tube.
Sperms from the male reproductive system
enter the body of the female through the vagina.
Do you know that a female child at birth
has around 1 -2 million potential eggs? The average life span of an egg is 12
-24 hours, after it is released from the ovary.
Structure of the ovum
It contains a single
nucleus, surrounded by the cytoplasm.
We have studied the different parts of
the human reproductive system. But how does reproduction take place in humans? Let us explore.
Passage of sperms inside the female body
Sperms, which are ejaculated from the
male reproductive organ (penis), enter the female body through the vagina. They
travel towards the fallopian tubes, where they meet the egg. The process of
fusion of the sperm and ovum is called fertilization. Fertilization
takes place in the fallopian tubes.
During fertilization, the haploid
nucleus of the sperm and ovum fuse to form a zygote. This zygote divides to
form an embryo.
Menstrual cycle
The ovary
releases one egg every month. The uterus also prepares itself every month to
receive a fertilized egg. The inner uterus lining (endometrium) becomes thick
and is supplied with blood to nourish the embryo.
If the egg is not fertilized, then the
uterus lining is not required. Hence, it breaks down and is released in the
form of blood and mucous through the vagina.
This process lasts for 2-8 days. This
cycle occurs every month and is known as menstruation. Hence, the
menstrual cycle is the reproductive cycle in the female body. It begins with
the onset of puberty.
Development of secondary sex
characteristics
Growth of beard may vary among different
individuals (males). While some have thick growth, others may have a very
sparse growth.
This is because each individual matures
at a different pace. This pace is determined by the individual’s biological
clock.
But, why do such changes occur at a
certain age and not gradually since childhood?
Development into an adult or reaching
maturity is essential for reproduction. The period of life where changes set in
for the development of a young individual into an adult is called puberty.
During puberty, many changes occur that allow an individual to attain sexual maturity
as the sex organs become well developed, and the body becomes capable of
reproduction.
Therefore, puberty is a period
during which body changes such as the growth of beard, hair growth in armpits, the appearance of pimples, development of breasts etc. occur in a child’s body.
With the onset of puberty, the body becomes capable of reproduction.
Time of occurrence of puberty
Puberty occurs when the body is ready to change.
· For girls: Puberty can start anywhere between the
ages 8 and 13. However, the age of puberty may vary in different individuals.
· For boys: Puberty in boys starts somewhat later than girls. It occurs between 9.5 and 14 years.
What controls the occurrence of puberty?
When a child
reaches the age of puberty, the pituitary gland in the brain secretes a hormone
called follicle-stimulating hormone. This hormone acts as a signal to
the reproductive organs (the testes in males and ovaries in females). In
response to this signal, the reproductive organs produce hormones that cause
changes in the male and female body. These changes are referred to as secondary
sex characteristics.
Secondary sex characteristics in boys
·
Under
the influence of hormones, the larynx develops prominently. The vocal cord
becomes longer and thicker in boys, causing the voice to become hoarse.
·
Growth
of hair in other areas of skin like underarms, face, hands, and legs.
·
Enlargement
of the testicles followed by penis.
· Testes grow and start producing sperms.
Secondary sex characteristics in girls
·
Development
of breasts, darkening of the skin around nipples (present at the tip of breasts).
·
Growth
of hair in other areas of skin like underarms, face, hands, and legs.
·
Increase
in the size of uterus and ovary.
· Beginning of the menstrual cycle.
However, somebody changes are common to
both boys and girls.
These common changes are:
·
Growth
of pubic hair
·
Growth
of the reproductive organs
· Secretions of sebum (oil) from the skin, which results in pimples
Puberty or sexual maturation should not
be confused with adolescence. Adolescence is the period between the beginning
of puberty and adulthood. An Adolescent experiences not only physical growth,
but changes in emotional, psychological, and social status as well.
Fertilization and Embryo Development
Do you know what
fertilization is?
Fertilization is the process
of the union of an egg and sperm. Let us study how fertilization takes place.
Whom does the new individual resemble –
the father or the mother? The new individual shows characteristics of both,
the father and the mother. Fertilization takes place inside the body in dogs,
cats, and human beings. This type of fertilization is known as internal
fertilization.
But do you know that in some animals,
fertilization takes place outside the body of the organism?
In animals such as frog and fish, the
process of fertilization takes place outside the body. This type of
fertilization is known as external fertilization. In external
fertilization, both the male and the female release their gametes in water. The
gametes then fuse in water and result in the formation of a zygote, which
develops into a new individual.
Do you know how eggs of animals, in
which external fertilization occurs, are protected?
Eggs of animals such as frogs are
covered with jelly, which holds all the eggs together and also protects the
eggs from the external environment.
You know that the process of
fertilization leads to the formation of a zygote. But what happens to this
zygote afterwards? How does it give rise to a new individual? Let us
explore.
The zygote formed after
fertilization divides repeatedly to form a mass of cells known as the embryo.
The cells of the embryo start dividing into groups, which leads to the
formation of future tissues and organs of the baby.
The uterus is a sac-like structure which opens into the vagina. The wall of the uterus has three layers of tissues:
1. The outer-thin layer is known as the perimetrium.
2. The middle layer is thick and is made up of smooth muscles. It is known as the myometrium. These muscles contract strongly during the delivery of a baby.
3. The inner layer lines the uterine cavity, which has many glands and is known as the endometrium. It thickens every month and is supplied with blood to nourish the embryo. It undergoes cyclic changes during the menstrual cycle.
Therefore, the zygote in the uterus or
womb divides many times to form an embryo. This leads to pregnancy.
Within a span of some
months, the embryo starts developing limbs, hands, and legs and begins to
resemble a miniature human being. When all the body parts of an embryo can be
recognised it is known as a foetus. The full term of development of an
embryo into a fully developed foetus is known as gestation. In humans,
the gestation period is of about 280 days. When the foetus is fully developed,
the mother gives birth to the baby. During childbirth, the baby is pushed out
of the female's body through the vagina by powerful contractions of uterus muscles.
Development of an embryo inside the
female body
The zygote formed after fertilization in
the fallopian tubes is implanted in the uterus. The uterus is also known as the
womb. Here, the zygote divides to form an embryo, which develops into a foetus.
How is the embryo attached in the
uterus?
The process discussed above explains the
development of an embryo in humans. But what about the process of embryo
development in animals, which lay eggs like birds?
In hens, after fertilization, when the
egg travels through the oviduct, it gets covered with various protective
layers, including the shell.
The hen lays eggs, and it takes around
three weeks for the embryo to develop into a chick.
You must have
seen hens sitting on their eggs. A hen does this to keep the egg warm and to
maintain a proper temperature for the growth of the embryo inside the egg.
Do you know that the heart of an embryo
starts beating when the embryo is 23 days old?
Natural Vegetative Propagation
Vegetative propagation
It is the plant’s ability to reproduce
by producing new plants from vegetative plant parts such as roots, stem, and
leaves.
Vegetative
propagation is classified into two types:
Natural vegetative propagation
It is a process
involving structural modification of stems, roots, or leaves of plants.
Propagation by leaves: In
Bryophyllum, the leaf allows the development of many shoot buds. These
buds form roots at their base. When these plantlets break and fall from the
parent leaf on the ground, a new plant is formed.
Propagation by shoots: In a potato
plant, the stem is modified to store food. This modified stem is called the tuber.
Since it is a modified stem, it has many auxiliary buds over its surface called
eyes. Each of these buds, when planted in soil, can develop into a new plant.
Propagation by
roots: Roots
are modified to store foods in sweet potato, asparagus, carrot, turnip, etc.
When these get detached from the parent plant, they form a new plant.
Advantages of Natural Methods of Vegetative Propagation
·
The
plants (like banana, seedless grapes, rose, etc) that can not produce viable
seeds can reproduce with the help of vegetative propagation.
·
To
get genetically identical copies of a plant in order to preserve the selected
varieties, vegetative propagation is useful.
·
It
is a rapid, easier, and less expensive method of reproduction.
· Vegetative propagation is suitable for the plants with small number of seeds or higher periods of seed dormancy.
Disadvantages of Vegetative Propagation
· No
genetic variability is introduced in the vegetatively propagated plants.
Generation after generation same varieties are produced.
·
Since
all plants are genetically alike, hence all of them suffer from same diseases.
· The plants loose their vigour and undesirable characters are transmitted from generation to generation.
Artificial Vegetative Propagation
Artificial Vegetative Propagation
This process is used
commercially for improving the yield, quality, and disease resistance of plants
and their products.
Types of artificial vegetative
propagation
Cutting
It refers to the formation of a complete
plant from the regenerated pieces of roots, stem etc.
Examples: Rose,
sugarcane, money plant etc.
Stem cutting − in this short
length of the stem of plants are removed and placed in suitable conditions to
develop roots. Sometimes root cutting has to be placed in rooting hormone to
stimulate rooting
Root cutting − in this method
root cuttings are put in damp soil to produce anew plant.
Grafting
In this method, a stem cutting from the desired plant (scion) is inserted on a rooted plant (stock) which is resistant to diseases. These are bound firmly with the help of tape or cloth so that they have vascular continuity.
Bud grafting − In this scion
of a dormant bud are grafted on to the disease-resistant stock. The scions of
good coloured roses are grafted on to the stocks of wild rose to produce a
better rose variety.
Layering
In this method, the branch of a plant is
bent and covered with moist soil called mound. After a few days, roots arise
from the underground portion. These separate from the parent plant and grow
independently.
Examples: Jasmine,
strawberry, bougainvillaea etc.
Layering is of two types − air layering
and mound layering
Mound layering − the stem is
bent so that node lies beneath the soil. The growing tip remains above the
soil. Adventitious roots grow at the node which is cut from the parent plant
and grown as in jasmine and strawberry.
Air layering − in this method
plants having thick branches that can not be bent are propagated. The stem of
such plants are girdled, covered with moist cotton and is kept in a polythene
bag. When adventitious roots start appearing, the branch is cut and planted.
Tissue Culture
In this method, the
cells of plant tissue are taken under sterile conditions. These cells are
kept in test tubes containing culture medium, which allows these cells to grow
fast and form an unorganized mass of cells called a callus. This callus is
transferred into another medium containing proper growth hormones for
differentiation and organ formation. As a result, small plantlets are formed in
the test tubes. These can be later placed in soil where they can grow into
mature plants. This technique is called micro-propagation.
Asexual Reproduction in Animals
Methods of Asexual Reproduction
Fission
It occurs in
single-celled organisms, belonging to the kingdom Protista and Monera. It is
further divided into two types:
Binary fission
In binary fission, the
single cell divides into halves. A few organisms that divide by binary fission
are bacteria and Amoeba. In this process, the nucleus of Amoeba first
divides to form two daughter nuclei. Later the body of Amoeba splits into two
halves, each half receiving its own nucleus. This leads to the formation of the
two daughter amoebae.
In Amoeba, cell division or
splitting of cells can take place in any plane. Binary fission can also occur
in one particular axis. For example, Leishmania (a parasitic flagellated
protozoan), which causes kala-azar in humans, divides only
longitudinally.
Leishmania has whip-like
flagella at one end of the cell. Cell division occurs in relation to these
flagella.
Multiple fission: In
multiple fission, a single cell divides into many daughter cells
simultaneously. Examples: Plasmodium and Amoeba
Budding
It involves the formation of a new
individual from a protrusion called the bud. It is very common in
plants, yeasts, and lower-level animals such as Hydra.
In Hydra, the
cells divide rapidly at a specific site and develop as an outgrowth called a bud.
These buds, while attached to the parent plant, develop into small individuals.
When this individual becomes large enough, it detaches itself from the parent
body to exist as an independent individual.
Regeneration This
mode of asexual reproduction takes place in some invertebrates belonging to the
animal kingdom. Individual body parts of these animals have the ability to grow
into new organisms. Let us discuss how this process takes place in Planaria.
Spore Formation Some organisms
like Rhizopus (fungi) reproduce with the help of spores. They are produced
in sporangia (blob like structures) attached to thread like structures called
hyphae. These spores are capable of giving rise to new individuals. They are
covered by thick walls which provide them protection until they come in contact
with a moist surface and begin to grow.
Reproductive Health
Do you know that some diseases are
sexually transmitted, and can spread from one person to another while having
sex? What are these sexually transmitted diseases? Let us explore.
A Sexually transmitted disease (STD) is a disease
that gets transferred from one person to another through sexual contact. Some
common sexually transmitted diseases are herpes, HIV-AIDS, syphilis, gonorrhoea
etc. Gonorrhoea and syphilis are bacterial infections, while HIV-AIDS and warts
are viral infections.
Sexual act not only leads to the
transfer of sexually transmitted diseases but also leads to unwanted
pregnancies. What measures can be taken to prevent the spread of sexually
transmitted diseases and unwanted pregnancies? The answer to this question
lies in the method of contraception.
Contraception is
the process where different methods are used to prevent pregnancy. They
interfere with processes such as fertilization, implantation of an embryo,
ovulation etc. Thus, these methods help in birth control.
Contraception not only prevents
pregnancies but also prevents sexually transmitted diseases.
Contraceptive methods
can be broadly divided into the following types:
(i) Natural methods: In this method, the sexual act is avoided from day 10th to 17th of the menstrual cycle since during
this period ovulation is expected. Therefore, the chances of fertilization are
very high.
(ii) Barrier methods: In this method,
the fertilization of ovum and sperm is prevented with the help of barriers.
Barriers are available for both males and females. Condoms act as barriers.
They are made of a thin rubber, which is used to cover the penis and vagina in
males and females respectively.
(iii) Oral contraceptives: In this method,
tablets or drugs are taken orally. These contain small doses of hormones, which
prevent the release of eggs and prevent fertilization.
(iv) Implants and
surgical methods: Contraceptive devices such as the loop
or Copper-T rods are placed in the uterus to prevent pregnancies. However, they
can cause side effects in the uterus.
Some surgical methods can also be used
to block the gamete transfer. It includes the blocking of vas deferens to
prevent the transfer of sperms by a process known as vasectomy. Similarly,
the fallopian tubes in the female body can be blocked by surgery to prevent the
egg from reaching the uterus. This process is known as tubectomy.
Surgical methods are safe if the
surgery is performed properly.
Surgery can also
be performed during unwanted pregnancies. However, these methods are misused by
people for illegal sex-selective abortion of female foetuses, which has to lead to
a rapid decline in the female-male sex ratio.
A healthy society needs a balance
between the female-male sex ratio. Therefore, the process of prenatal
sex-determination has been prohibited by the law in our country.
Some interesting facts:
·
Do you know that our population was around 350
million at the time of independence? The recent data (July 2007) suggests that
our population is more than one billion (1,129,866) now.
· India is the second-largest country (after China) to cross the one billion mark. 👇
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