CHAPTER-7 HOW DO ORGANISMS REPRODUCE?
NOTES
1. Different organisms appear similar because of the similarity in their design, size, and shape.
2. The DNA found in the chromosomes of the cell nucleus.
The molecules of DNA (Deoxyribonucleic Acid) carry genetic information that is passed from parent to offspring. The DNA in the nucleus of the cell contains instructions for protein synthesis.
3, The fundamental process of reproduction is the replication of DNA. Cells use various chemical reactions to replicate DNA.
4. In the reproductive cell, two copies of DNA are formed, and it is essential for them to separate from each other.
- It is not sufficient to keep one copy of DNA in the original cell and remove the other copy because the second copy will not have an organized cellular structure to maintain biological processes.
- Along with the replication of DNA, the formation of other cellular structures also takes place.
5. A single cell divides to form two cells. Although both cells are similar, no biological or biochemical process is entirely error-free. It is expected that some variations may occur during the DNA replication process. As a result, the replicated DNA copies will be similar but not identical.
- However, the replicated DNA will not be identical to the original DNA. Some variations may be so extreme that the new DNA copy may not be able to adapt to its cellular organization. Such offspring cells do not survive.
- On the other hand, many variations in DNA replication are not so extreme. Therefore, although offspring cells are similar, they still differ from each other in some way.
6. Importance of Variation- During reproduction, the accurate replication of DNA is crucial for maintaining the organism's physical structure and design, making it suitable for its specific habitat. Therefore, the stability of a species' population is linked to reproduction.
- If there is a population of bacteria found in temperate waters and the water temperature rises due to global warming, most individual bacteria will die. However, a few variants with heat-resistant ability will survive and continue to grow.
7. Modes of reproduction in unicellular organisms-
A) Fragmentation– In unicellular organisms, new organisms are produced through cell division or fragmentation. Some unicellular organisms have a more organized body structure.
- Multiple fission– A unicellular organism divides into many offspring cells simultaneously, which is known as multiple fission.
- On the other hand, in yeast, small buds emerge from the cell and separate, growing independently.
B) Rebuttal- In multicellular organisms with simple structures, a simple method of reproduction takes place. These fragments or pieces grow and develop into new individuals.
- This is not true for all multicellular organisms. They do not simply divide cell by cell.
- Specialized cells organize to form tissues for specific functions. These tissues then combine to form organs, which have a fixed position in the body.
- A common strategy used by multicellular organisms is that different types of cells become specialized for specific functions. In such organisms, there are specialized cells for reproduction.
- An organism must have certain cells with the ability to grow, divide, differentiate, and develop into specialized cells under suitable conditions.
C) Regeneration- Fully differentiated organisms have the ability to generate new individuals from their body parts. This means that if an organism is damaged or breaks into several pieces, each piece can grow and develop into a new organism. This process is called regeneration.
- Regeneration is carried out by specialized cells. Through the process of division and differentiation, these cells produce many new cells.
- During the transformation of a group of cells, different types of cells and tissues are formed. This transformation occurs in a highly organized and sequential manner, which is called development.
D) Budding- In Hydra, due to the regular division of cells, a bulge develops at a specific spot. This bulge (bud) grows into a small organism and, after full development, separates from the parent to become an independent organism.
E) Vegetative propagation-
- Unlike most animals, individual plants use this ability as a method of reproduction. Techniques like layering, cutting, or grafting in vegetative propagation are also used in agriculture.
- Plants grown through vegetative propagation are useful for growing plants like flowers and jasmine, which have lost the ability to produce seeds, compared to those grown from seeds.
- Another advantage of vegetative propagation is that all the plants produced by this method are genetically identical to the parent plant.
- Tissue Culture- In tissue culture technology, plant tissues or cells are separated from the growing region of the plant’s apex to grow new plants. These cells are placed in an artificial nutrient medium, where they divide and form a small mass of cells called a callus.
F) Spore Association-
- Thread-like structures develop on bread, which is the fungal network of Rhizopus. These are not reproductive structures, but the tiny spore-bearing structures on the upright filaments participate in reproduction.
- These clusters are sporangia. The spores inside them grow and develop into new Rhizopus organisms.
- Upon coming into contact with a moist surface, they begin to grow.
- The mode of reproduction in which a new generation is created by a single organism is called asexual reproduction.
8. Sexual Reproduction- In sexual reproduction, DNA from two different organisms is combined.
- Meiosis- The number of chromosomes in the somatic cells of an organism is reduced to half, and the amount of DNA is also halved. This is achieved through the process of cell division known as meiosis.
Dynamic reproduction- The cell is called the male gamete, while the reproductive cell in which food reserves are stored is called the female gamete.
9. Sexual reproduction in flowering plants-
- Different parts of a flower- Sepals, petals, stamens, and carpels. Stamens and carpels are the reproductive parts of a flower.
- Unisexual– When a flower has either stamens or carpels as the reproductive organ, it is called a unisexual flower.
- Bisexual– When a flower contains both stamens and carpels, it is called a bisexual flower. Example– Hibiscus, Mustard
- The stamen is the male reproductive organ, which produces pollen grains. Pollen grains are usually yellow in color.
- The carpel is located at the center of the flower and is the female reproductive organ of the flower.
- The carpel consists of three parts – the swollen basal part is the ovary, the elongated middle part is the style, and the terminal part is the stigma, which is usually sticky.
- Self-pollination– The transfer of pollen grains from the stamen to the stigma is necessary. If this transfer occurs on the stigma of the same flower, it is called self-pollination.
- Cross-pollination– When the pollen grains from one flower are transferred to another flower, it is called cross-pollination.
- This transfer of pollen grains from one flower to another occurs through agents such as wind, water, or pollinators.
- After the pollen grains successfully reach the suitable stigma, the male gamete must reach the female gamete located in the ovary. A pollen tube develops from the pollen grain and travels through the style to reach the ovule.
- After fertilization, the zygote undergoes multiple divisions, and an embryo develops within the ovule. A hard covering forms around the ovule, which then transforms into a seed. The ovary grows rapidly and matures into a fruit.
- Germination– The seed contains the future plant or embryo, which develops into a seedling under suitable conditions. This process is called germination.
10. Sexual reproduction in humans-
- With age, certain changes occur in the human body. As a child progresses from Class 1 to Class 10, height and weight increase, and milk teeth fall out to be replaced by permanent teeth.
- During the early years of adolescence, physical appearance changes. Body proportions shift, new characteristics develop, and there are also changes in emotions and sensitivity.
- Some changes in boys and girls are similar. For example, the growth of hair clusters in the armpits and the genital area, which also darken in color. Fine hair also appears on the hands, feet, and face.
- Some changes differ between boys and girls. In girls, the breasts enlarge, and the skin around the nipples darkens. They also experience menstruation. In boys, facial hair such as a mustache and beard grows, and their voice deepens. Additionally, due to growth, boys may experience involuntary erections, often during sleep or daydreaming.
- In some individuals, these changes occur at a younger age and more rapidly, while in others, they happen at a slower pace.
- For the actual transfer of reproductive cells between two individuals, specific organs or structures are required.
- The male reproductive cell or sperm is produced in the testes.
- In addition to regulating sperm production, testosterone also controls the characteristics of puberty in boys.
- Sperm – Sperm are microscopic structures that primarily contain genetic material and have a long tail, which helps them swim toward the female reproductive cell.
B) Female Reproductive System– Female reproductive cells or ova (egg cells) are produced in the ovaries.
- At the time of a girl's birth, the ovaries contain thousands of immature eggs. At the onset of puberty, some of these begin to mature. Each month, one egg matures in either of the two ovaries.
- Both fallopian tubes merge to form a flexible, sac-like structure called the uterus. The uterus opens into the vagina through the cervix.
- During intercourse, sperm are deposited in the vaginal tract, from where they travel upward to reach the fallopian tube, where they may meet the egg. The fertilized egg then divides and forms a ball-like structure of cells, known as an embryo.
- The embryo gets implanted in the uterus, where it continues to divide, grow, and develop organs.
- The embryo receives nourishment from the mother's blood through a specialized structure called the placenta.
- Placenta– It is a disc-shaped structure embedded in the uterine wall. It provides a large surface area for the transfer of glucose, oxygen, and other essential substances from the mother to the embryo.
- It takes approximately nine months for the fetus to develop inside the mother's body. The baby is born through rhythmic contractions of the uterine muscles.
C) When fertilization does not occur?
- If the egg is not fertilized, it remains alive for about one day. Since the ovary releases one egg each month, the uterus also prepares itself every month for a possible fertilized egg. As a result, its inner lining becomes thick and spongy.
- This lining gradually breaks down and is expelled through the vaginal tract in the form of blood and mucus. This cycle takes about a month and is called menstruation or the menstrual cycle. Its duration typically ranges from 2 to 8 days.
D) Reproductive Health– Diseases can be transmitted from one person to another in various ways. Since sexual activity involves close physical contact, many diseases can also be sexually transmitted.
This includes bacterial infections such as gonorrhea and syphilis, as well as viral infections like genital warts and HIV-AIDS.
- The use of a protective covering, such as a condom, can help prevent the transmission of many of these diseases to some extent.
- The possibility of pregnancy always exists during sexual activity. There are various methods to prevent pregnancy.
- One method is mechanical barrier, which prevents sperm from reaching the egg. This includes the use of condoms that cover the penis or various devices placed inside the vagina.
- The second method involves altering the body's hormonal balance, preventing the release of an egg, thereby making fertilization impossible. These medications are usually taken in the form of pills. However, since they affect hormonal balance, they may also have some side effects.
- Other methods to prevent pregnancy include devices like loops or Copper-T, which are placed inside the uterus. However, uterine stimulation caused by these devices may also lead to some side effects.
- If the sperm ducts (vas deferens) in a male are blocked, the transfer of sperm will be stopped, preventing fertilization.
- If the fallopian tubes in a female are blocked, the egg will not be able to reach the uterus. In both cases (male and female sterilization), fertilization will not occur.
