Complete notes for Chapter 18 Reproduction. Covers asexual and sexual reproduction in plants and animals, human reproductive systems, menstrual cycle, birth process, and STDs.
Reproduction is the mechanism that produces new generation and maintains a species. Every species of organism can reproduce and produce new individuals of that species. There are different methods of reproduction in organisms. In some organisms these are quite complex.
Reproduction differs from all other functions of animals in one respect: it is not necessary for survival of the individual but it is very important for the survival of a species or population.
Reproduction is of two types:
Asexual reproduction does not involve the production of gametes and requires only single parent. The offspring is produced through mitosis and he is genetically identical to his parent. Different methods of asexual reproduction are fission, parthenogenesis, budding etc.
Sexual reproduction involves the production of gametes through two parents. The gametes are produced through meiosis. Reducing division (meiosis) reduces the number of chromosomes to half and here crossing over takes place which produce new recombination. Thus in this way in sexual reproduction, the number of chromosomes are maintained and genetic variations take place.
Plants show sexual and asexual both types of reproduction.
The different methods of asexual reproduction are layering, grafting, budding etc.
In sexual reproduction plants have diplohaplontic life cycle in which diploid sporophyte alternate which haploid gametophyte generations. There are two types of alternation of generations:
The flowering plants produce male and female gametes which fertilize to form zygote. Evolution of pollen tube is an important step as pollen tube act as vehicle for male gametes for their safe transport to ovule. These male and female gametes fuse to form zygote. Zygote develops in an embryo, there is storage of food for embryo and it is protected in seed coats.
Seed plants are predominantly present all around us due to their better sexual reproduction mechanism:
Pollen tube acts as vehicle for safe transport of male gametes to female gamete in ovule in land environment. It is an important step in land adaptation by spermatophytes. Evolution of pollen tube is parallel to evolution of seed. It is a tool for the success of seed plants.
The inactivity of seed during unfavourable condition is called seed dormancy. Early in the development of angiosperm embryo an event takes place that embryo stops developing. This is called seed dormancy.
This is a special condition of rest which enable an embryo to survive long period of unfavorable environmental conditions such as water scarcity or low temperature. During the dormant stage the embryo metabolic activities stop.
It prevents dormant seed from germinating in response to conditions such as warm spell in winter. This warm spell is favourable only temporarily. Thus seed dormancy has a great survival importance for plant. The dormant seed needs very precise combinations of environmental cues for germination or resumption of normal growth. In this way, it avoids any accidental stimuli which may be fatal later on.
Germination is a normal growth by dormant embryo which requires certain stimulus such as availability of water and oxygen.
Following plant hormones take part in the formation and ripening of seed and fruit:
Germinating pollen grain is an important source of auxin. It also stimulates the tissues of style and ovary to produce more auxin. This auxin is necessary for fruit set i.e. retention of the ovary. The ovary becomes fruit after fertilization. Without auxin the abscission of flowers normally occurs. This abscission leads to low fruit yields. After fertilization, the ovary and the ripe seeds continue to produce auxin. This auxin stimulates the fruit development.
Developing seeds are rich source of auxins, gibberellins and cytokinins. These correspond with development of the embryo and accumulation of food reserves in the seed or in the pericarp (fruit wall).
The respiratory activity starts rapidly during ripening of fruit. It is associated with ethene production. Ethene helps in ripening of the fruit.
In some plants fruit is developed without fertilization and no seed is produced. This type of development is called parthenocarpy. Parthenocarpy occurs due to hormonal imbalance. Usually high auxin is also induced artificially. The plants which show parthenocarpy are banana, pineapple etc.
The response of an organism to changes in relative lengths of light and dark is known as photoperiodism. Photoperiodism and temperature effects flowering, fruit and seed production, bud and seed dormancy, leaf fall and germination.
Photoperiod affect flowering. When shoot meristem starts producing floral buds instead of leaves and lateral buds. This effect of photoperiodism was first studied in 1920 by Garner and Allard. They examined that tobacco plant flowers only after exposure to a series of short days. Tobacco plant flowers naturally under same conditions in autumn but flowering could be induced by conditions artificially i.e. short days exposing.
The flowering responses of plants fall into two basic categories in relation to day length:
In both kinds of plants it is actually the length of darkness that is significant and not the length of day. Thus short day plants are really long night plants. If they are grown in short days but the long night is interrupted by a short light period, flowering is prevented. Long day plants will flower in short days if long night period is interrupted.
The response to light intensity and quality lead to the discovery of blue light sensitive protein pigment phytochromes. Phytochromes exist in two forms i.e. P660 and P730. P660 is a quiescent form acts absorbs red light at wave length of 660 nm and is converted to active P730. P660 to P730 conversion takes place in daylight. P730 to P660 conversion occurs in the dark. Thus Phytochromes in the form of P660 helps plant detect darkness.
This observation led to hypothesize that P730 - P660 inter-conversion might be the plant time regulator for flowering. P730 promotes flowering in long day plants and inhibits flowering in short day plants. P730 accumulates in the day and degrades at night.
Biennials and perennials plants are stimulated to flower by exposure to low temperature. This is called vernalisation.
In some plants vernalisation is absolute requirement and in some cases it assist in inducing flowering. The duration of low temperature treatment is required from four days to three months. Temperature of about 4°C is much effective. It stimulates the production of "vernalin" hormone which induces vernalisation.
Photoperiodism and vernalisation serve to synchronise the reproductive behaviour of plants with their environment ensuring reproduction at favourable times of year. They also ensure the members of same species flower at same time encouraging cross pollination for genetic variability.
There are different methods of asexual reproduction in animals such as binary fission, multiple fission, budding, parthenogenesis, tissue culturing, cloning and identical twins.
A type of asexual reproduction in which egg is developed without fertilization is called parthenogenesis. It accelerates the normal reproductive rate.
The procedure of producing a line of genetically identical cells or organisms from single parent is called cloning.
Technique: In this technique the nucleus from somatic cell is removed and introduced in an egg cell. The nucleus of egg cell is already destroyed by ultraviolet radiation. The egg cell now contains only nucleus of somatic cell which is diploid. This egg cell develops into a complete organism which is genetically identical to his parent who has contributed the nucleus.
Advantages:
Disadvantages: The application of this technique to humans would be open to serious moral questions.
The method of reproduction in which union of gametes take place is called sexual reproduction.
These gametes may be similar called isogametes or different called heterogametes. In heterogametic stage male gametes (sperms) are motile and females gametes (Ova) are non-motile.
Differentiation of sex is found in those organisms which undergo sexual reproduction. Some organisms have one sex, those are called unisexual (dioecious). Some have both sexes called hermaphrodite or bisexual e.g. earthworm, tapeworm etc. Animals usually are unisexual.
Union of gametes leads to fertilization. Fertilization is of two types:
According to the development of zygote the animals are classified into three groups:
Male reproductive system consists of following parts:
The sperms are produced in the testis. Each testis consists of highly complex duct system called seminiferous tubules which consists of cells of germinal epithelium. These cells undergo repeated division to produce spermatogonia. Spermatogonia undergo mitotic division to form primary spermatocyte. Primary spermatocytes are changed into secondary spermatocytes and spermatids meiotically. Each spermatid differentiates into mature sperms having half the number of chromosomes.
Sperms are the male gametes which are present in testes where Sertoli cells provide liquid medium, protection and nourishment to sperms while they are in tubules.
Vas deferens is the main duct of male reproductive tract where the sperms are transferred. The highly convoluted portion of vas deferens is epididymis. The sperms then pass through urinogenital duct and are discharged out.
The male hormone is testosterone which is secreted in interstitial cells of seminiferous tubules. This hormone controls:
The female reproductive system consists of:
In the ovary there are germ cells which give rise to oogonia. Oogonia undergo mitosis to form primary oocyte. The first meiotic division takes place in primary oocyte which forms haploid secondary oocyte and first polar body. Second meiotic division in the oocyte proceeds as far as metaphase but is not completed until the oocyte is fertilized by sperm. This phenomenon is completely different from the oogenesis of frog as in it second meiotic division takes place before fertilization.
In human only one ovum is discharged from ovary at one time. This phenomenon is called ovulation.
As the ovum is produced it is transferred to the oviduct. The sperm enter in the proximal part of the oviduct where ovum is fertilized and ovum completes its second meiotic division forming egg and second polar body.
The zygote formed after fertilization enters the uterus where it undergoes further development. The zygote undergoes series of divisions (cleavage) to form foetus. A placenta is formed between the wall of uterus and foetal tissues. It acts as excretory organ for fetus. Uterus opens into the vagina through cervix. Urethra and vagina have independent openings.
Female reproductive cycle involves production of egg which is a cyclic activity. It is also called menstrual cycle. The cycle which involves changes in structure and function of whole reproductive system is called menstrual cycle. It is completed nearly in 28 days. The events of menstrual cycle involve the ovaries and uterus and these are regulated by pituitary gonadotropins.
The end or complete cessation of menstrual cycle is called menopause after which female stops producing ova.
The total gestation period (pregnancy) is usually about 280 days.
A placenta is developed between the wall of uterus and foetal tissues. Placenta secretes progesterone which maintains pregnancy. If progesterone is not secreted properly then it may cause premature birth or miscarriage.
Human embryo is protected in amniotic filled sac which contains amniotic fluid. Amniotic fluid is not only protective but also shock absorptive.
At beginning of 3rd month the embryo technically becomes fetus. Most of major organs are formed by 12th week of pregnancy.
Birth process can be divided into three stages:
During this period pituitary gland produce luteotropic hormone (LTH) or prolactin. Placenta also secretes human placental lactogen. These stimulate mammary development.
Those parents which are unable to have normal process of fertilization and birth due to some abnormalities can have test tube babies.
Procedure: Parent sperm and ovum is fertilized in vitro (outside the female body) and then zygote is implanted back in mother's uterus, placenta establishes and remaining development takes place in body of mother leading to normal birth.
The sexually transmitted diseases can be transmitted from carrier to healthy persons.