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Reproduction

Chapter: Reproduction

Introduction

Reproduction is the process by which living organisms give rise to their off springs. Reproduction, unlike the essential life processes like nutrition, respiration or excretion is not necessary to maintain the life of an individual but still it is a very important process in the life of a living organism. Reproduction is necessary to maintain the existence of living organisms. Reproducing organisms produce new individuals which look very much like themselves. Reproduction is of two types 1. Asexual reproduction 2. Sexual reproduction.

Asexual reproduction

Definition

It is the process resulting in the production of genetically identical offspring from one parent.

Types of asexual reproduction:

1. Binary fission:

  1. Most of the unicellular organisms use cell or fission as the mode of their reproduction. There are many different patterns of fission.
  2. Bacteria and protozoa split into two equal halves during cell division. In organisms like amoeba splitting take place in any plane.
  3. In organisms like Leishmania, which have a whip like structure at one end of the cell, binary fission occurs in a definite orientation in relation to these structures.
  4. In other single celled organisms like the malarial parasite, plasmodium, divides into many daughter cells simultaneously by binary fission.
  5. Yeast, can put out small buds that split and grow further.

2. Spore formation:

In many multi-cellular organisms, specific reproductive parts can be identified. The thread like structures called hyphae have tiny bob like structures called reproductive structures. They contain cells or spores, which can develop into new Rhizopus individuals. The spores are enclosed by thick walls which protect them till they come into contact with another moist surface and can begin to grow

3. Vegetative propagation:

It is the mode of asexual reproduction in plants. Many parts of the plant like the root, stem, and leaves can develop into new plants under appropriate condition this is called vegetative propagation.Plants raised by vegetative propagation can bear flowers and fruits earlier than those produced from seeds. Vegetative propagation is used in methods such as layering or grafting to grow plants like sugarcane, roses or grapes for the purpose of agriculture. One of the advantages of vegetative propagation is that all the plants produced by this method are genetically similar enough to the parent plant. Buds produced in the notches along the leaf margin of Bryophyllum fall on the soil and develop into new plant. Let us understand vegetative propagation better by studying tuber formation in potatoes.

Tuber formation in potato:

➤ The scars which are found on the potato develop into new shoots. These shoots are called as eyes.

➤ These seed potatoes are allowed to produce short shoots. Once the shoots come up they are planted to produce a new potato crop.

➤ Once the shoots have developed enough to perform photosynthesis, they send food along underground shoot, the tips of which swell to form new potatoes.

Sexual reproduction

Definition

It is the process involving the fusion of haploid nuclei to form diploid zygote and the production of genetically dissimilar offspring.

Sexual reproduction in plants

 

 

1. Sepals:

They are green in colour. Their main function is to protect the flower bud. They usually disappear after pollination.

2. Petals:

They are brightly coloured and scented. Petals may sometime produce nectar, a sugary solution from small glands at their bases. Colour, scent and nectar attract the pollinating insects.

3. Anthers:

It contains four pollen sacs filled with pollen grains. Each pollen grain contains a male gamete.

4. Stigma:

It serves as a platform on which pollen grains land.

5. Ovary:

It is the female reproductive organ. Ovule which is the female gamete is produced from the walls of ovary.

Pollination

Pollination is defined as the transfer of pollen grains from male part of the plant (anther of stamen) to the female part of the plant (stigma). Pollination is of two types. 1. Cross pollination 2. Self pollination.

Cross pollination:  it is the transport of pollen from anther of one flower to stigma of another flower and it is achieved by agents like wind, insects or water.

Self pollination:  If the transport of pollen occurs from anther to stigma within the same flower it is called as self pollination.

Agents of pollination

Some agents or vectors are required to carry pollen grains from the anthers to stigma.  These agents are mostly insects or wind. Flowers show adaptations to successful insect pollination or wind pollination. Some of the insects which act as pollinators have also become adapted to make relationship with flowers.

Structural adaptations of the insect pollinated flowers and wind pollinated flowers.

 

part of flower

Insect pollinated flower

Wind pollinated flower

reason

Petals

Brightly coloured, usually large, scented

Small, green or dull coloured with no nectaries.

Insects are attracted to colour and scent

Anthers

Stiff,  firmly attached  and positioned where insects must brush against them

Hang loosely on long thin filaments

Wind is more likely to dislodge pollen from exposed, dangling anthers than from enclosed ones

Pollen

Small amount of large sticky grains

Smooth pollen grains

Sticky grains attach to hairs on insects body.

Stigma

Mostly flat or lobe shaped and positioned where insects must brush against them

Long and feathery but hanging outside flower

Feathery stigmas form a large network to catch pollen being blown past the flower.

 

Fertilization

  1. After the pollen lands on a suitable stigma, it has to reach the female germ-cells which are in the ovary. A tube grows out of the pollen grain in this and travels through the style to reach the ovary where the fusion of gametes takes place. This is called fertilization. Fertilization results in zygote.

Structure of a non-endospermic seed

The best example of a non-endospermic seed is pea. Non-endospermic seed is a type of seed in which the endosperm is not developed. Endosperm is a structure that stores food and is found in endospermic seed. In non-endospermic seeds food is absorbed from endosperm and is stored in the cotyledons. Testa or the seed coat surrounds the seed. This seed coat protects the seed and helps it to remain dormant for a long period. The hilum is a scar on the seed where the seed was attached to the fruit wall. There is a small opening on one end of hilum called the micropyle. Micropyle is the opening through water enters the seed before it germinates. The young embryo pant is between the cotyledons. The first shoot is called plumule and the first root is called radicle.

The formation of fruit and seed

  1. The zygote divides several times to form an embryo within the ovule after fertilisation.
  2. The ovule develops a hard coat and is slowly converted into a seed.
  3. The ovary grows fast and ripens to form a fruit. Meanwhile, the petals, sepals, stamens, style and stigma may wither and fall off.
  4. The seed containing embryo develops into a seedling under appropriate conditions and this process is known as germination.

Dispersal of seeds and fruits

Seeds are found attached to the parent plant after the process of fertilization. Before these seeds develop into new young plants they need to be dispersed from its parent plant in order to avoid competition with the parent plant for light energy, carbon dioxide and water. The parent plant uses different means for the dispersal of fruit and seeds. Fruits may be adapted in different way to aid seed dispersal. Plants produce large number of fruits which makes it more likely that at least few will land in a suitable place for germination. For example a single mature oak tree produces a millions of oak fruits in one year.

Let us look at some examples of seeds and fruits dispersed through the means of wind and animals.

  1. Avens produce false fruit because the style and not just the ovary is involved in dispersal. Hooked style is lignified and attaches to the fur of animals for the dispersal.
  2. The sycamore has two extended wings formed two fused carpels which will allow its dispersal through wind.

Sexual reproduction in animals

Male reproductive system

 

 

1. Testes

Testes or testicle is made up of many coiled tubes which produces sperms and the cells between these tubes produces testosterone.

2. Sperm duct or vas deferens

Carries sperms from testes to urethra

3. Urethra

Carries sperm from the sperm duct to the tip of penis.

4. Prostate gland

Helps to produce seminal fluid which makes up 99.5% of the semen

5. Penis:

It delivers sperm into vagina during sexual intercourse

6. Scrotum:

It is a sac like structure which hangs outside the body in between legs. The position of scrotum protects testes from physical damage and keeps them at temperature of 2 -3oC lower than the body temperature which is ideal for the sperm development.

Female reproductive system

 

 

1. Fallopian tube or oviduct:

It transfers the ovum to the uterus. Fusion of sperm with ovum takes place in fallopian tune.

2. Uterus or womb:

It is the site where the foetus develops. The uterus size increases in volume from 10 cm3 to 5 dm3.

3. Funnel or the oviduct:

It collects ovum released from the ovary after ovulation.

4. Ovary:

It contains follicles which develop the ova and produces estrogen and progesterone

5. Cervix:

It is the neck of uterus

6. Vagina or birth canal:

Receives sperm from the penis during sexual intercourse and it is the way out for the baby at birth.

Menstrual cycle

The cycle of producing and releasing mature ova is called menstrual cycle. The wall of the uterus undergoes through four phases during menstrual cycle. Estrogen and progesterone influence the entire menstrual cycle. The entire cycle takes place in 28 days. Let us look into the phases of menstrual cycle

1. Menstruation:

This phase lasts for about five days. During this period the uterus lining is shed as a result blood and fragments of tissue leave the body through the vagina. Decrease in the concentration of progesterone triggers menstruation.

2. Repair phase:

The lining of the uterus becomes thick and stable due the growth of blood vessels in the lining of blood vessels. These changes are influenced by the increase in the concentration of estrogen.

3. Receptive phase:

As the uterus lining and blood vessels are now well developed it can result in the implantation of embryo if fertilization has occurred. The optimum conditions for embryo implantation remain for 6-7 days after ovulation and it is maintained by an increasing concentration of progesterone.

4. Premenstrual phase :

If the embryo implantation has not occurred the uterus lining starts degenerating as the concentration of progesterone starts decreasing. In case if embryo implantation has occurred the progesterone keeps the lining intact to begin pregnancy.

Sexual intercourse and fertilization

Before the intercourse takes place, penis becomes erect due to the blow flow into it which is caused by sexual stimulation. The penis is hard enough to enter the vagina with the help of lubricating fluids released by vagina. This entire process is called copulation or sexual intercourse. The rubbing action of penis against the walls of vagina sets off a reflex action that releases sperms stored in the testes, and squeezes them by peristalsis along the sperm duct and the urethra. Seminal fluid is added to the sperm as they pass along these tubes. Addition of sperm to the sperm results in complete semen which is ejaculated from the tip of penis. About 3 or 4 cm3 semen is ejaculated and it contains about 300 000 00 sperm. Fertilization takes place after the process of sexual intercourse.

Fertilization

Fertilization is the process of fusion of female gamete (an ovum) with the male gamete (sperm). Fertilization takes place in the oviduct.  Although several hundred sperms swim towards the ovum only one of them penetrates the membrane that surrounds it. Let us look into the process of fertilization in detail.

  1. After the release of sperms into the vagina they begin to swim towards the ovum.
  2. The membrane of the ovum is altered to form a barrier that allows the entry of only one sperm.
  3. The sperm moves towards the nucleus of the ovum and then the two of them fuse.
  4. The fusion results in the zygote formation. The fertilized ovum or zygote starts dividing, first into two cells, then into four and so on. The zygote moves towards the uterus.

Development of zygote

The zygote which is formed after the fertilization keeps dividing a form a ball of cells. About six after fertilization these ball of cell are called as embryo. The embryo gets implanted into the thickened lining of uterus as a result the conception i.e. development of new individual begins. Once the embryo is attached to the lining of the uterus, some of its outer cells combine with mother’s cells resulting in the development of placenta.

Developmental stages of foetus

  1. The actual length of the foetus by the first month is 7mm. The basic pattern of the body is laid down by the first month. The heart begins to beat, the limb buds appear and the three regions of the brain are formed. The organism that is developing is the embryo.
  2. The foetus length increases to 13mm by the second month. All the adult organs are present by second month but they are tiny and immature. Muscles start differentiating and the embryo is capable of movement. Sex organs are distinguishable as ovary or testes. Bones begin to harden. From the second month embryo is called as foetus.
  3. The actual length of the foetus by the third month is 70 mm. in third month sex of the foetus can be determined by external inspection. Foetus may kick, curl, squirt and frown.
  4. The foetus becomes 95 mm in length by fourth month. Face begins to look like a human, lobes of the forebrain develop, ears, eyes and nose begin to look normal. Movements become vigorous in fourth month and these movements can be detected by mother.
  5. The foetus has outgrown the placentas supply systems but has to adjust to much more variable environment. In the last three months the preterm babies have a good chance of survival with appropriate respiratory and thermoregulatory assistance.

Placenta and umbilical cord

Placenta

It is a disc shaped structure which is formed by the cell of embryo and cells of the mother. It begins to develop at implantation and after 12 weeks it becomes like a disc like structure with finger like projections on it called the villi. Placenta has very important roles to play during the pregnancy.  Let us take a look at the functions of placenta

  1. Exchange of soluble materials such as foods, waste and oxygen between the mother and the foetus takes place through placenta. Foetus is attached to the wall of uterus through placenta.
  2. Placenta protects the fetus from mother’s immune system and against dangerous fluctuations in mother’s blood pressure.
  3. Placenta secretes hormones which maintain the lining of uterus as the corpus luteum breaks down by the third month.

Umbilical cord

It connects foetus with the placenta. Umbilical cord contains blood vessels which carry materials for the exchange between mother and foetus.

Antenatal care of pregnant women

Antenatal care includes advice on diets, guidance on motherhood, check on foetus and mother. Missing a period is the first sign of pregnancy. The second sign of pregnancy is another missed period along with nausea, weakness and frequent urination. Pregnancy is confirmed by measuring the amount of HCG in the urine using monoclonal antibodies. Once the pregnancy is confirmed the mother should be on a healthy diet while maintaining her fitness. Along with the diet there should always be a check on the mother and the foetus.

Check on the foetus

  1. Size and position of the foetus can be felt by applying a gentle pressure on the uterus. The best position for the baby towards the end of pregnancy is head downwards facing mothers back.
  2. Heart beat can be measured during second half of pregnancy using a stethoscope. The fetal heart rate is 120-160 bpm, about twice as high as mother.
  3. Ultrasound scanning produces picture of the foetus in the uterus. It is done to provide information on position of placenta, baby’ age, size, sex and position.

Check on the mother

  1. Constant weight check of the mother is necessary. From the third month of pregnancy a women gains about 450 gm of weight per week. During the whole pregnancy she gains about 12kg.  This includes the weight of the baby, the placenta and the amniotic fluid. In case if she gains more weight than normal she will be advised with diet.
  2. She needs to undergo blood test like haemoglobin test in order to check if she is anemic or not and blood group has to be known in case of any emergency blood transfusions if needed.
  3. Urine test is done to check for the presence of sugar. This test will confirm if she has diabetes or not. Protein albumin is also checked in the urine to know if there is any kidney damage.
  4. In the initial stages of pregnancy the vagina of the mother is examined to check if there is any infection. A cervical smear is obtained to identify cancer cells. In the later stages of pregnancy the vagina is checked to make sure that it will be big enough for the baby’s head to pass through it.
  5. Blood pressure is checked at every visit as high B.P may indicate toxaemia of pregnancy which can be serious for both, mother and the baby.
  6. The level of estrogen in the blood is checked to know how well the placenta is working in supplying food and oxygen to the foetus.

Birth of the baby

In the final stages of pregnancy the baby lies with its head against the cervix. In the beginning contractions come every 20 minutes but later they become more frequent and powerful. The amniotic membrane breaks as a result of contractions and so the amniotic fluid is released. This breaking of amniotic fluid and release of amniotic fluid is called –breaking the waters. Cervix dilates with the release of amniotic fluid.  When the cervix is wide enough for the baby to pass through the first stage of labour is said to be complete. The labour continues as the baby’s head is pushed through the birth canal which is the cervix. From now the process is very rapid and needs only gentle contractions by the mother helped by the obstetrician. The birth process is quite traumatic for the baby because of the shortage of oxygen due to compression of umbilical cord by the birth canal. The baby’s heart beat is monitored during the birth and once the baby starts breathing immediately the blood gets reoxygenated. Once the baby is breathing properly the umbilical cord is clamped and cut. The placenta comes away from the wall of the uterus and leaves the vagina as the after birth.

Testosterone and estrogen

Testosterone and estrogen are the sex hormones in male and female respectively.  One pituitary hormone stimulates the testes to produce testosterone and ovary to produce estrogen. These hormones circulate in the blood and stimulate the development of male and female secondary sex characteristics.

Role of estrogen

  1. Estrogen begins the cycle of egg production and prepares the uterus lining for implantation.
  2. Estrogen enhances breast development and thereby increases the size of nipples.
  3. Pubic hairs are developed with the influence of estrogen.
  4. Vagina becomes large with the influence of estrogen

Role of testosterone

  1. Testosterone promotes development of facial hairs, development of hair in pubic region and on chest.
  2. . Penis becomes larger and scrotum expands to contain larger testes
  3. Testes are stimulated for the production of sperm.

Birth control measures:

The process of sexual maturation is steady, and occurs when the general body growth is still going on. Therefore, some degree of sexual maturation does not necessarily mean that the body or the mind is ready for sexual acts or for having and bringing up children. There can be pressure from our friends for participating in many activities, whether we really want or no. There can be pressure from families to get married and start having children. Since the sexual act is an intimate connection of bodies, it is not surprising that many diseases can be sexually transmitted. These comprise bacterial infections such as gonorrhea and syphilis, and viral infections like warts and HIV. We can prevent such infections by using condom which covers the penis. The sexual act can always have chances to lead to pregnancy. We can avoid pregnancy by using contraceptives. Some of the contraceptive methods include creation of a mechanical barrier so that the sperm does not reach the egg. Another way of contraceptives is by creating a hormonal imbalance so that the eggs are not released and fertilization does not occur. Other contraceptive devices such as the loop or the copper-T are placed in the uterus to prevent pregnancy. If the vas deferens in the male is blocked, sperm transfer will be prevented. We can stop the sperm transfer by blocking the vas deferens. Surgical methods are used to block fallopian tube so that sperm does not reach the egg. We have to maintain female sex ratio for a healthy society. Although prenatal sex determination has been banned people still go for it and abort if they get to know it is a girl child.  This is decreasing the child sex ratio at an alarming rate. The size of the human population is a cause of concern for many people. This is due to a growing population which is making it tough and to pick up everybody’s standard of living.

Sexually transmitted diseases

Gonorrehea

Signs and symptoms:

  1. Pain or burning sensation while passing urine
  2. A creamy discharge from the vagina
  3. Inflammation of testicles.

Transmission:

  1. Transmitted by penetrative sex-i.e. when penis enters the vagina, mouth or anus.

Treatment:

Gonorrhea can be treated by using a course of antibiotics.

Transmission of HIV

  1. HIV is mainly transmitted by having an unprotected sex with an infected person
  2. Blood transfusion from an infected person
  3. From an infected mother to child during pregnancy
  4. Sharing syringes while injecting drugs

Prevention of HIV

  1. We can prevent HIV by avoiding unprotected sex.
  2. Using sterile syringes and needles in the hospital
  3. Pregnant women should undergo HIV test so that they can be medicated from the initial stages to prevent the baby from getting HIV.