- 1 Describe the Various Steps Involved in the Sexual Reproduction of Animals and What is Puberty?
- 1.1 Male and Female
- 1.2 Gamete Formation & Production
- 1.3 Understanding the Process of Fertilisation
- 1.4 Internal and External Fertilisation
- 1.5 The Advantages of Sexual Reproduction
- 1.6 Why the Amount of DNA does not get doubled during Sexual Reproduction
- 1.7 How Sexual Reproduction in Animals Takes Place
- 1.8 Puberty: Signs and Stages for Boys and Girls
The Biology Topics of biotechnology involve using living organisms to develop new products or solve problems.
Describe the Various Steps Involved in the Sexual Reproduction of Animals and What is Puberty?
In order to understand sexual reproduction, we should know the meanings of some important terms like male sex, female sex, gametes, sperms, ova (or eggs), fertilisation, zygote, and embryo, which are involved in sexual reproduction. These are discussed below:
Male and Female
Our father is a male and our mother is a female. We can also say that our father has male sex and our mother has female sex. Now, our father is a man and our mother is a woman. This means that a man is male whereas a woman is female. Thus, a man is said to have male sex and a woman is said to have female sex. Similarly, a boy has a male sex and a girl has a female sex. Just like us human beings, other animals also have male and female sexes. Even the plants have male and female sexes. As we will learn after a while, being male or female depends on the type of sex cells present in one’s body. An animal having male sex cells called ‘sperms’ in its body is called male. On the other hand, an animal having female sex cells called ‘ova’ (or ‘eggs’) in its body is called female. We will now discuss gametes.
A man is male whereas a woman is female.
Gamete Formation & Production
Sexual reproduction takes place through the combination of special reproductive cells called ‘sex cells’. These sex cells are also known by another name which is ‘gamete’. We can now say that: The cells involved in sexual reproduction are called gametes. In other words, gametes are the sexual reproductive cells (or sex cells). Gametes are of two types: male gametes, and female gametes. The male gamete in animals is called ‘sperm’ and the female gamete in animals is called ‘ovum’ or ‘egg’ (see Figures).
A human sperm (male gamete). A sperm is about 0.05 mm long. A sperm has a head, a middle piece, and a tail. A sperm is a single cell with a nucleus, cytoplasm, and cell membrane. This figure is a highly enlarged sketch of a sperm.
A human ovum or egg (female gamete). An ovum or egg is round and about 0.15 mm in diameter. The ovum or egg is also a single cell having a nucleus, cytoplasm, and cell membrane. This figure is a highly enlarged sketch of an ovum or egg.
Sperms and ova (or eggs) are extremely small cells that can be seen only with the help of a high-power microscope. Please note that a female gamete (or female sex cell) is usually known by two names: ovum and egg. So, whether we use the term ‘ovum’ or ‘egg’, it will mean the same thing. Another point to be noted is that the plural of the ovum is ova. The ovum or egg contains water and stored food. The important part of the ovum or egg is its nucleus (see Figure). The sperm cell is hundreds or even thousands of times smaller than the ovum or egg and it has a long tail (see Figure). The sperms are motile and can move independently with the help of their tails. The nuclei of the sperm and ovum (or egg) contain chromosomes that carry the genes.
It is clear from the above discussion that sperms are the male gametes of animals. And ova (or eggs) are the female gametes of animals. In other words, sperms are the male sex cells of animals, and ova (or eggs) are the female sex cells of animals. As we will study after a short while, a fusion of gametes gives rise to a single cell called a zygote. We can also say that the cell which is formed by the fusion of a male gamete and a female gamete is called a zygote. In most simple words, the zygote is a ‘fertilized ovum’ or ‘fertilised egg’. All multicellular animals start their life from a single cell called a zygote through sexual reproduction. The process of fusion of gametes is called fertilisation. This is discussed below.
Understanding the Process of Fertilisation
For sexual reproduction to occur, a male gamete must combine (or fuse) with a female gamete. The fusion of a male gamete with a female gamete to form a zygote during sexual reproduction is called fertilisation. Since the male gamete of an animal is called sperm and the female gamete of an animal is called ovum (or egg), therefore, we can also say that: The fusion of a sperm with an ovum (or egg) to form a zygote during sexual reproduction, is called fertilisation. It is clear that the process of fertilisation produces a new cell called a zygote. The zygote is actually ‘fertilised ovum’ or ‘fertilised egg’. The zygote (or fertilised egg) grows and develops to form a new baby. The unborn baby in the uterus in the early stages of development (when its body parts are not much developed) is called an embryo. On the other hand, the unborn baby in the uterus in the later stages of development (when all its body parts are well developed and can be identified) is called a foetus (The word ‘foetus’ is pronounced as ‘fetus’).
This picture shows a number of sperms (greatly magnified)
This picture shows an ovum or egg (greatly magnified).
This is a zygote (or fertilised egg) formed by the fusion of a sperm with an ovum or egg
This is the foetus formed from a zygote (or fertilised egg). It will grow further and become a baby.
Internal and External Fertilisation
We have just studied that the fusion of a sperm with an ovum (or egg) is called fertilisation. Now, the ovum (or egg cell) is made in the body of the female animal. So, the fertilisation of an egg by a sperm can take place either inside the body of the female animal or outside its body. This leads to two modes of fertilisation in animals: internal fertilisation and external fertilisation.
The fertilisation which occurs inside the female body is called internal fertilisation. In internal fertilisation, the female animal’s eggs are fertilised by sperm inside her body. In mammals (including human beings), birds, and reptiles, fertilisation occurs inside the female body. In other words, internal fertilisation takes place in mammals (including human beings), birds, and reptiles. In internal fertilisation, the male animal puts its sperm into the female animal’s body. This transfer of sperm from the testes of the male animal into the female animal’s body occurs at the time of copulation (or mating). Copulation is the act by which the male animal transfers his sperm into the female animal’s body. During copulation, a very large number of sperm are discharged into the female body. These sperms fertilise the eggs inside her body. For example, during copulation, a man puts his sperm inside a woman’s body through an organ called the penis. These sperm then fertilise the egg inside the woman’s body. So, this is a case of internal fertilisation.
The fertilisation which occurs outside the female body is called external fertilisation. In external fertilisation, the female animal’s eggs are fertilised by sperm outside its body. In amphibians (like frogs and toads) and fishes, the fertilisation of eggs occurs outside the female animal’s body. In other words, in amphibians (like frogs and toads) and fishes, external fertilisation takes place. In external fertilisation, the male and female animals release their sperms and eggs in water where fertilisation takes place by collisions between sperms and eggs. For example, the males and females of frogs and fishes release their sperm and eggs in the water in which they live. The sperms then collide with the eggs and fertilise them outside the body of a female frog or fish (see Figure).
As the female frog lays eggs in water, the male frog releases its sperm. The collisions between sperm and eggs lead to fertilisation. This is an example of external fertilisation.
From the above discussion, we conclude that there are two different modes of fertilisation in nature: internal fertilisation and external fertilisation. The fertilisation in humans, cats, dogs, and birds is an example of internal fertilisation. The fertilisation in frogs and fishes is an example of external fertilisation.
We know that the new cell which is formed by fertilisation is called a ‘zygote’. And this zygote then grows and develops into a full organism (or baby animal). The method by which a zygote grows and develops into a full organism also varies in different animals. For example, in human beings, the zygote grows and develops into a baby inside the female body (mother’s body). And then the mother gives birth to the baby. Just like humans, the animals like cats and dogs also give birth to their young ones. But the process is entirely different in the animals (like birds) which lay eggs. For example, a hen sits on its fertilized eggs for a considerable time to give them warmth. During this period, the zygote grows and develops to form a complete chick. This chick then comes out of the egg by breaking its shell. It is clear from this discussion that all organisms do not give birth to individuals like humans do.
The Advantages of Sexual Reproduction
Sexual reproduction has many advantages over asexual reproduction. In asexual reproduction, the offsprings are almost identical to their parent because they have the same genes as their parent. So, much genetic variation is not possible in asexual reproduction. This is a disadvantage of asexual reproduction because it inhibits the further evolution of the organism.
In sexual reproduction the offsprings, although similar to their parents, are not identical to them or to one another. This is because the offsprings receive some genes from the mother and some from the father. Because of the mixing of genes of mother and father in various different combinations, all the offsprings have genetic variations. In this way, sexual reproduction leads to a greater variety in population. This means that a species (animal or plant) can adapt more quickly to changes in its surroundings (or environment). This is because there are always likely to be some individuals who are more suited to the changes than others, and these individuals will survive and reproduce themselves.
From the above discussion, we conclude that sexual reproduction promotes the diversity of characters in the offspring by providing genetic variation. Sexual reproduction plays an important role in the origin of new species having different characteristics. This genetic variation leads to the continuous evolution of various species to form better and still better organisms. All this is not possible in the case of asexual reproduction.
Why the Amount of DNA does not get doubled during Sexual Reproduction
In sexual reproduction, though the genetic material DNA (in the form of chromosomes) from two gametes, male and female gametes, combines together to form a new cell ‘zygote’ but the amount of DNA in the zygote does not get doubled. This can be explained as follows: The gametes are a special type of cells called reproductive cells which contain only half the amount of DNA (or half the number of chromosomes) as compared to the normal body cells of an organism. So, when a male gamete combines with a female gamete during sexual reproduction, then the new cell ‘zygote’ will have the normal amount of DNA (or a normal number of chromosomes in it). For example, the human sperm has 23 chromosomes and the human egg (or ovum) has also 23 chromosomes. So, when a sperm and an egg fuse together during fertilization, then the zygote formed will have 23 + 23 = 46 chromosomes, which is the normal number of chromosomes.
How Sexual Reproduction in Animals Takes Place
Sexual reproduction is the most common method of reproduction in animals (including human beings).
Fertilisation of an ovum (or egg) by a sperm to form a zygote.
Sexual reproduction in animals takes place in the following steps:
- The male parent produces male gametes (male sex cells) called sperms. The sperm is a small cell with a long tail (flagellum) for movement [see Figure (a)].
- The female parent produces female gametes (female sex cells) called ova (or eggs). The ovum (or egg) is a much bigger cell than the sperm, having a lot of cytoplasms [see Figure (a)].
- The sperm enters into the ovum (or egg) and fuses with it to form a new cell called ‘zygote’ [see Figures (b) and (c)]. This process is called fertilisation. So, the zygote is a fertilised ovum (or fertilised egg).
- The zygote then divides again and again to form a large number of cells (all of which remain together). And ultimately zygote grows and develops to become a new baby.’
From the above discussion we conclude that the whole process of sexual reproduction in animals involves the formation of sperms and eggs; the joining together of sperm and egg to form a zygote; and then the growth and development of zygote to form a baby animal. In complex multicellular animals (like human beings) there are special reproductive organs to make sperms and eggs; to bring together sperms and eggs for fertilisation; and for the growth and development of a zygote into a baby. We will study all this in the human reproductive system. Before we describe the human reproductive system, we should know the meaning of the term ‘puberty’. This is discussed below.
Puberty: Signs and Stages for Boys and Girls
When a child is small, sometimes it becomes difficult to tell from appearance whether it is a boy or a girl (see Figure). This is because small boys and girls have the same body shape. A time of rapid growth and body changes starts in the early teens which makes the boy and the girl appear different and also behave differently (see Figure). These changes start earlier in girls than in boys. We call the time between childhood and adulthood ‘adolescence.’ The production of male and female ‘sex hormones’ in the bodies of boys and girls increases dramatically at this stage and causes wide-ranging changes in their bodies. The testes (in boys) and ovaries (in girls) make different hormones, so the boys and girls develop in different ways. Ultimately the boys and girls become sexually mature and their reproductive systems start functioning.
The age at which the sex hormones (or gametes) begin to be produced and the boy and girl become sexually mature (able to reproduce) is called puberty. Puberty tends to start earlier in females (girls) than in males (boys). Generally, boys attain puberty at the age of 13 to 14 years while girls reach puberty at a comparatively lower age of 10 to 12 years. On attaining puberty, the male gonads called testes start producing male gametes called sperms and the female gonads called ovaries start producing female gametes called ova (or eggs). In addition to producing sex cells (or gametes) male and female gonads (testes and ovaries) also produce and secrete sex hormones with the onset of puberty.
Girls attain puberty earlier than boys do.
The testes produce the male sex hormone called testosterone, and the ovaries produce two female sex hormones, oestrogen, and progesterone. Sex hormones play an important role in the process of reproduction because they make the reproductive organs mature and start functioning. Puberty is the age at which the reproductive organs reach maturity and secondary sexual characteristics develop.
The various changes which occur in boys at puberty are Hair growth under the armpits and in pubic regions (genital area) between the thighs. Hair also grows on other parts of the body like chest and face (moustache, beard, etc.) (see Figure). The body becomes more muscular due to the development of muscles. The voice deepens (or cracks). Chest and shoulders broaden. The penis and testes become larger. The testes start to make sperms. Feelings and sexual drives associated with adulthood begin to develop. All these changes in boys are brought about by the male sex hormone ‘testosterone’ made in the testes.
Small boys and girls have the same body shape. So, they look alike.
Growing up boys and girls have different body shapes. So, they look different.
The various changes which occur in girls at puberty are Hair growth under the armpits and pubic region (This change is the same as in boys). Mammary glands (or breasts) develop and enlarge. The hips broaden (see Figure). Extra fat is deposited in various parts of the body like hips and thighs. Fallopian tubes, uterus, and vagina enlarge. Ovaries start to release eggs. Menstruation (monthly periods) starts. Feelings and sexual drives associated with adulthood begin to develop. All these changes in girls are brought about by the female sex hormones ‘oestrogen’ and ‘progesterone’ made in ovaries. Please note that the hormone ‘oestrogen’ is also written and spoken as ‘estrogen’.