Download Reproduction Chapter Notes and more Study notes Biology in PDF only on Docsity!
NOTES FOR REPRODUCTION CLASS 10
How do Organisms Reproduce?
Reproduction is not necessary to maintain the life of an individual organism, it creates new individuals that look very much like themselves. So, its main purpose is continuity of the species and linked to the stability of populations of species. Chromosomes in the nucleus of a cell contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. The DNA in the cell nucleus is the information source for making proteins. If the information is changed, different proteins will be made. Different proteins will eventually lead to altered body designs. Therefore, a basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA. This creates two copies of the DNA in a reproducing cell, and they will need to be separated from each other. However, keeping one copy of DNA in the original cell and simply pushing the other one out would not work, because the copy pushed out would not have any organised cellular structure for maintaining life processes. Therefore, DNA copying is accompanied by the creation of an additional cellular apparatus, and then the DNA copies separate, each with its own cellular apparatus. Effectively, a cell divides to give rise to two cells. While copying, no bio-chemical reaction is absolutely reliable. Therefore, it is only to be expected that the process of copying the DNA will have some variations each time. As a result, the DNA copies generated will be similar, but may not be identical to the original. Some of these variations might be so drastic that the new DNA copy cannot work with the cellular apparatus it inherits. Such a new born cell will simply die. On the other hand, there could still be many other variations in the DNA copies that would not lead to such a drastic outcome. Thus, the surviving cells are similar to, but subtly different from each other. This inbuilt tendency for variation during reproduction is the basis for evolution. Importance of variation Variation is important and useful for the survival of species over time. This is clearly evident from the example of the bacterial population which developed variations with the reproduction over a long period of time and that gave them the ability to survive when the temperature of the water increased by global warming over the normal population of bacteria which did not develop the variations Modes of reproduction used by single organisms (ASEXUAL) It involves only one parent. There is no formation and fusion of gametes.
The young ones formed are almost identical to each other as well as to the parent cell. Asexual reproduction generally occurs during favourable environmental conditions and when there is an abundance of food. It is a faster method of reproduction.
- Binary FISSION Seen in bacteria, protozoa like Amoeba, Paramecium. First is (karyokinesis) the nucleus of the parent cell divides and then the cytoplasm divides (cytokinesis) resulting in the formation of two daughter cells). It occurs during highly favourable conditions. a. The cell division can occur in any plane as in case of Amoeba. Cytokinesis: Division of cytoplasm. Karyokinesis: Division of Nucleus. b. However, organisms like Leishmania. (cause Kala- azar), have more organised bodies, which have a whip like flagella at one end, binary fission occurs in a definite orientation in relation to the flagellum.
- Multiple fission Seen in Plasmodium, (a malarial parasite). In this during unfavourable conditions, the parent cell develops a thick resistant wall around itself forming a cyst. Within the wall, the cytoplasm divides many times to form many daughter cells. When conditions become favourable, the cyst wall breaks and the daughter cells are released. Many daughter cells are released. Only parent
division at one specific site. These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals. Differences between them Budding in hydra In yeast hydra is a multi-cellular organism Yeast is a unicellular organism. In case of budding in Hydra , a small bud is developed from the body of parent hydra through repeated mitotic division of its cells. The small bud then derives nourishment from the parent hydra and begins to grow. The mature bud starts developing small tentacles and mouth. When the bud completely matures into a new hydra, it detaches itself from the parent hydra and becomes an independent organism. In case of budding in yeast, a small bud appears as an outgrowth from the parent body. After this, the nucleus of the parent yeast elongates and gets divided into two parts. One of these nuclei migrates into the bud. The newly created bud divides and grows into a new yeast cell. In hydra the bud always detaches from the parent and lives independently In yeast it may detach or may remain attached to the parent forming colonies. Regeneration is not the same as reproduction: Many fully differentiated organisms can give rise to new individual organisms from their body parts. That is, if the individual is somehow cut or broken up into many pieces, many of these pieces grow into separate individuals. For example, simple animals like Hydra and Planaria can be cut into any number of pieces and each piece grows into a complete organism. This is known as regeneration. Regeneration is carried out by specialised cells called regenerative cells
. These cells proliferate and make large numbers of cells. From this mass of cells, different cells undergo changes to become various cell types and tissues. These changes take place in an organised sequence referred to as development.
Regeneration in Planaria Normal fragmentation in planaria
. However, regeneration is not the same as reproduction since most organisms would not normally depend on being cut up to be able to reproduce. Reproduction is normal process on attaining maturity
- Spore formation Even in many simple multi-cellular organisms, specific reproductive parts can be identified. The thread-like structures that develop on the bread are the hyphae of the bread mould (Rhizopus). They are not reproductive parts. On the other hand, the tiny blob-on-a-stick structures are involved in reproduction. The blobs are sporangia, which contain cells, or spores, that can eventually develop into new Rhizopus individuals. They are produced during unfavourable times and help to tide over the unfavourable environmental conditions. The spores are covered by thick walls that protect them until they meet another moist surface and can begin to grow.
Plants which have lost the ability to produce viable seeds can also reproduce by vegetative propagation e.g banana, orange, rose and jasmine. All plants are genetically almost similar to the parent plant. Seedless varieties can be obtained. The property of vegetative propagation is used by horticulturists in developing methods like layering, grafting to grow many plants like sugarcane, roses, or grapes. Tissue Culture: The technique of developing new plants from a cell or tissue in a nutrient medium under aseptic conditions. The cell or tissue is placed in a nutrient medium where it forms a mass of cells called callus. This callus is then transferred to another nutrient medium containing hormones where it differentiates and forms a plantlet. These are placed in soil to grow into mature plants. This technique is commonly used for ornamental plants to be grown in disease free conditions.
Sexual reproduction
The mode of reproduction that takes place with the involvement of two individuals of two different sexes i.e. male and female. During sexual reproduction, male organism having male sex organs produces male gametes i.e. sperms which are small and motile and the female organism having female sex organs produces ova/ egg which are generally large and store food. Male and female gametes fuse to form a zygote that grows into a new organism. Significance of Sexual Reproduction: Sexual reproduction involves DNA as well as cellular apparatus of two different organisms which promotes diversity of characters in the offspring. Since gametes are derived from two different organisms, it results in a new combination of genes which increases the chances of genetic variations. Sexual reproduction results in the origin of. new species. Sexual reproduction involves division in the sex organs that reduces the DNA matter to half so that the zygote formed after fusion has the same amount of DNA as the parents it maintains DNA in a species. Limitation of Sexual Reproduction: Sexual reproduction involves the process of combining DNA from two different organisms which may bring some undesirable features also. Sexual reproduction in flowering plants The reproductive parts are present in the flower. The parts of the flower are sepals, petals, stamens and carpels. Sepals are green structures that protect the inner parts when the flower is in bud stage.
Petals are colourful and attract the insects for pollination. Stamens are male reproductive parts and produce pollen grains that contain male gametes. Each stamen has two parts— Filament i.e. stalk and Anther i.e. swollen top part which has large number of pollen grains. The carpel is the female reproductive part present in the centre of the flower and produces ovules that contain female gametes. It has three parts—Stigma which is top sticky part and receives pollen grains during pollination. Style which is the middle long part and ovary which is the swollen part and contains ovules. Each ovule has an egg cell i.e. female gamete. Flower may be unisexual (papaya, watermelon) when it contains either stamens or carpels or bisexual (Hibiscus, mustard) when it contains both stamens and carpels. . Pollination: The process of transfer of pollen grains from an anther to the stigma of the flower is pollination. Two types of pollination are: (i) Self-pollination: The transfer of pollen grains from the anther to the stigma of the same flower or another flower of the same plant. (ii) Cross-pollination: The transfer of pollen grains from the anther to the stigma of another flower or another flower of a different plant of the same species. It generally takes place with the help of some agents like insects, birds,
Post-fertilisation changes: After fertilisation the following changes takes place in the flower. Zygote divides several times and forms an embryo inside the ovule. The ovule develops a tough coat and changes into the seed. The ovary grows rapidly and ripens to form a fruit. Petals, sepals, stamens, style and stigma shrivel and fall off. Seed and its parts: The advantage of seed is that it protects the future plant i.e. embryo. Embryo has two parts: plumule and radicle. Plumule develops into shoot and radicle develops into root. First the root develops as the plant takes nutrition from the soil and then the development of the shoot starts taking place. The process of development of a seedling from the embryo under appropriate conditions is known as germination.
Reproduction in human beings
Human beings show sexual reproduction. Male parent produces male gametes called sperms. Female parent produces female gametes called ova. Sperms have tail and are therefore, motile. They are produced in large numbers in the testes. Ovum is bigger, non-motile and only one ovary produces one ovum in one month. There is no food stored in the sperms whereas ova contain stored food. Both the gametes are microscopic unicellular and have half the number of chromosomes as compared to the body cells. Sperms (male reproductive gamete) Ova (female reproductive gametes) They have tail and are therefore motile. They are non -motile They do not contain any stored food. They contain stored food. They are produced large in numbers. It is produced only one from one ovary in one month. SIMILARITIES Microscopic gametes. Microscopic gametes Unicellular organism Unicellular organism Have half the number of chromosomes Has half the number of chromosomes
Human beings become reproductively active from the onset of puberty. Puberty is the period during adolescence when the rate of general body growth begins to slow down and reproductive tissues begin to mature. Onset of puberty in human males is between 11 to 13 yrs of age, while in human females is between 10 to 12 yrs. of age. Puberty is associated with many physical, mental, emotional and psychological changes in boys and girls which occur slowly over a period of time. These are called secondary sexual characters. For instance thick dark hair start growing in new parts of the body such as arm pits and genital area. Skin becomes oily and pimples may appear on the face. Individuals become more conscious of their bodies become more independent, more aggressive etc. In case of boys beard and mustache start appearing, voice begins to crack, reproductive organs develop and start producing releasing sperms. In case of girls, breast size begins to increase, menstruation starts. Male Reproductive System: Male reproductive system consists of the following components
- One pair of testes are present in a bag-like structure called scrotum which lies outside the abdominal cavity, hence they are extra abdominal in position. This is so because the testes have to be maintained at 1-3 degree lesser temperature than the body in order to produce functional sperms. Functions of testes To produce male gametes i.e. the sperms. To produce a male reproductive hormone called testosterone which is responsible for producing sperms as well as secondary sexual characteristics in males.
- Attached to each testis is a highly coiled tube the sperm duct or the vas deferens that delivers the sperms formed in the testes. Each vas-deferens enters into the abdominal cavity. It unites with the duct coming from the urinary bladder to form a common duct called
Function of the uterus: The embryo formed in the fallopian tube comes down and gets attached to the endometrium (implantation) and develops for the next nine months till the baby is delivered. Vagina: The uterus opens into the vagina through the cervix. The vagina is a muscular tube through which the baby is delivered at the end of nine months. It also serves as the canal for receiving the sperms. The sperms travel upwards and reach the fallopian tube where one sperm fuses with the ovum to form the zygote. The zygote divides and redivides as it descends into the uterus and the implanted in the endometrium embryo gets. The endometrium thickens so as to receive the embryo. The embryo gets nutrition from the mother’s blood with the help of a special tissue called placenta, which is a disk-like structure embeded in the uterine wall. It contains finger-like villi on the embryo side, while on the mother’s side blood spaces surround the villi. Villi provides a large surface area for glucose and oxygen to pass from the mother to the developing embryo and the wastes to pass from the embryo to the mother through the placenta. When the embryo starts resembling a human is formed, it is termed as a foetus. The foetus continues to develop inside the uterus for almost nine months after which the baby is delivered as a result of rhythmic contractions of the uterine muscles. Menstruation: It is the loss of blood, mucous along with the unfertilized ovum and the ruptured cells and tissues of the endometrium through the vagina of the female. It is a 28-day cycle which occurs in every reproductively active female (from puberty). The flow of blood continues for 2 to 8 days. If the ovum does not get fertilized, then the endometrium starts sloughing off and there is loss of blood and mucous leading to menstruation. In case the ovum gets fertilized, then the endometrium becomes thick and spongy for nourishing the embryo and hence menstruation does not occur. A lady with a developing embryo in her womb is termed as pregnant. The beginning of menstruation at puberty is known as menarche. The stopage of menstruation when the woman is 45-55 yrs of age is called menopause. Reproductive Health: Sexually transmitted diseases and birth control. Several diseases occur as a result of sexual intercourse if one of the partners is infected. These are known as sexually transmitted diseases (STD’s). They can be caused by bacteria for example; syphilis, gonorrhoea; or caused by a virus for example; HIV-AIDS, warts etc. The transmission of these diseases can be avoided by using birth control measures such as wearing a condom during the sexual act. Birth control measures: They can be mechanical, IUCD devices,chemical and surgical. (i) Mechanical methods: Use of condoms: Condoms are used to prevent fertilization. It is not a sure short method. However it is the only method which can prevent Sexually Transmitted Disease (STD) as it prevents the exchange of
fluids between the two organisms. Also it has least side effects out of all methods. (ii)Intra Uterine Contraceptive Device (IUCD) or loop: It is inserted in the uterus and it prevents the implantation of the embryo in the uterine wall. Both methods (i) and (ii) cause side effects. (iii) Chemical methods: Oral contraceptive pills: These are hormonal pills which prevent ovulation but do not stop menstruation. (iv) Surgical methods Vasectomy: It involves cutting and ligating the vas deferens in males. Tubectomy: It involves cutting and ligating Reproductive organs the fallopian tubes in females. Out of all the above methods the surgical is a permanent one which has minimum side effects if the surgery is performed properly. Medical termination of pregnancy (MTP) or abortions is carried out to eliminate the developing embryo. This is to control the population and to control the STDs. This practice can, however, be misused to carry out female foeticide which involves the killing of the female foetus. It should be avoided at all cost as it disturbs the male-female ratio in a population.
