Reproduction

Reproduction refers to the process of replication of an individual organism from parent to child.

The tale of reproduction starts with cell division.

Cell division

Chromosomes contain information for the inheritance of features from parents to the next generation in the form of DNA (Deoxyribonucleic Acid) molecules.

DNA in the cell nucleus is the information source for making proteins. If information is changed, different proteins will be made, leading to altered body design. The 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 DNA in a reproducing cell, & need to be separated from each other. This copy requires a cellular structure for maintaining life processes.
• DNA copying is accompanied by the creation of additional cellular apparatus => DNA copies separate => Cell divides and gives rise to two cells.
• The two cells are similar: Since no biochemical reaction is absolutely reliable. => Variations each time. Some variations might be so drastic that the new DNA copy can’t work with the cellular apparatus. => Simply die.
• Many variations don’t lead to drastic outcomes. => surviving cells similar, but not identical.

Surviving cells are similar but subtly different from each other. This inbuilt tendency for variation during reproduction is the basis for evolution.

Importance of Variation:

• The population of organism fill well-defined places(niches) in the ecosystem, using their ability to reproduce.
• Consistency during reproduction is important for the maintenance of body design features that allow organisms to use that particular niche.
• Niches can change: Temperature change, water levels rise, meteorite bombardment etc.
• If the niche were drastically altered population would wipe off. If some variations are present, few individuals can survive the change.

Modes of Reproduction

Reproduction occurs in two main modes: sexual and asexual.

Asexual reproduction produces offspring from a single parent. In asexual reproduction, a single organism’s DNA replicates, producing identical offspring.

Sexual reproduction involves the fusion of genetic material from two organisms of the same species, resulting in offspring with genetic variation. In sexual reproduction, DNA from two parents combines, creating genetically diverse offspring. Typically we classify these parents as males and females depending on their function of birth giving and simply supplying the genetic material for the fusion.

Asexual Reproduction

Asexual reproduction, wherein the genetic material of a single organism is transferred to the next generation, can take place through the following different mechanisms:

1. Fission
2. Fragmentation
3. Regeneration
4. Budding
5. Vegetative Propagation
6. Spore Formation

Fission

Fission (or Cell division) is only possible for Unicellular organisms. For example, many bacteria and protozoa simply split into equal halves during cell division. In Amoeba, splitting can take place in any plane. We see many patterns of fission.

Some Unicellular organisms show somewhat more organisation of their bodies.

1. Fission in a definite orientation: In Leishmania (causes Kala-azar\whip-like structure at one end of cell) binary fission takes place in a definite orientation in relation to these structures.
2. Multiple offsprings through cell division: Plasmodium (Malarial Parasite) divides into many daughter cells simultaneously by multiple fission.
3. Yeast (unicellular fungi): Small buds that separate and grow further. [Remember Fungi can produce sexually/asexually]

Fragmentation:

In Multicellular organisms with relatively simple body organisation, simple reproductive methods can still work. For example, Spirogyra Breaks into smaller pieces upon maturation. Grow into new individuals. It is a random collection of cells and hence fragmentation is easy.

Most organisms can’t simply divide cell by cell. Different types of cells perform different types of functions.

Regeneration:

Many fully differentiated organisms have the ability to give rise to new individual organisms from their body parts. Many of the pieces grow into separate individuals. For example, Hydra and Planaria can be cut into any number of pieces & each piece grows into a complete organism.

• It is carried out by specialised cells: These proliferate and make a large number of cells. From this mass of cells, different cells change to become various cell types and tissues.
• It is not the same as reproduction.

Differentiated Organism

Differentiated Organisms are multicellular organisms that have undergone cellular differentiation, i.e. a process that gives rise to cells with specialized functions. Tissues: When cells specialize to perform a particular function, they form tissues. In Animals, there are four basic types of tissues: Epithelial, connective, muscle, and nervous tissue. In Plants, there are two types of tissues: Meristematic Tissue (i.e. a dividing tissue or a growth tissue) and Permanent tissue. Organs: Groups of similar tissues work together to perform specific tasks and form organs, such as the heart, lungs, or kidneys. Organs with related functions combine to form organ systems. For example, the circulatory system includes the heart and blood vessels, working together to circulate blood throughout the body. Organism: An organism is a living thing that can function on its own i.e. it can Perform Essential Life Processes: grow, reproduce, respond to stimuli, and Maintain a consistent state called homeostasis. It can be a single-celled organism like an amoeba or a complex multicellular organism like a human—representing the highest level of biological organization.

Budding:

A bud develops as an outgrowth due to repeated cell division at one specific site. These develop into tiny individuals when fully mature, detach from their parent body and become new individuals. For example, Hydra.

Vegetative Propagation:

Many parts of plants: roots, stems and leaves develop into new plants under appropriate conditions. These are used in methods such as layering or grafting in many plants:

• Sugarcane, roses, or grapes for agricultural purposes.
• Bryophyllum: leaves have buds that fall on the soil & develop into new plants.

Advantages of Vegetative Propagation:

• Plants raised by vegetative propagation can bear flowers and fruits earlier than those produced from seeds. This is possible for Bananas, oranges, roses & Jasmine that have lost capacity to produce seeds.
• Plants produced have the same characteristics.

Spore formation:

In multi-cellular organisms, specific reproductive parts can be identified, referred to as the Sporangia.

• Hyphae are thread-like structures that develop on the bread are hyphae of the bread mould (Rhizopus). [Fungus is made of hyphae] These are not reproductive parts.
• Tinley bob-on-a-stick structure involved in reproduction ( Sporangia) contains cells (Spores) that eventually develop into new Rhizopus individuals. Spores are covered by thick walls that protect them until they come into contact with another moist surface & can begin to grow.

Tissue culture

In Tissue Culture, new plants are grown by removing tissue or separating cells from the growing tip of a plant. The cells are then placed in an artificial medium where they divide rapidly to form a small group of cells or calluses (soft tissue). The callus is transferred to another medium containing hormones for growth and differentiation. The plantlets are then placed in the soil so that they can grow into mature plants. Using this: many plants can be grown from one parent in disease-free conditions. This technique is commonly used for ornamental plants. Totipotency: the ability of a single cell to develop into a complete organism, including all of its differentiated cell types.

Reproduction in Plants

Asexual reproduction:  Without the production of seeds. Vegetative propagation: New plants are produced from roots, stems, leaves, & buds. They take less time to grow. For example, stem cutting of rose, potato plant sprouting from an eye, Bryophyllum with buds in leaf margin. Roots in Dahlia and Sweet potato. Budding: Microorganisms(single cellular) like yeast multiply in a few hours by a chain of budding. Demonstration: Pinch of yeast+ water + spoon full of sugar Shake & dissolve: Under microscope: budding. Fragmentation: Algae simply divide, when provided with water nutrients. Break into 2 or more fragments. If 2 fragments: Binary fission. Spore formation: Fungi have sporangium(head) on which spore formation takes place. These spores can travel long distances. Each spore is covered by a hard protective coat to withstand unfavourable conditions. It settles to germinate on a source of nutrition(a warm moist place). Plants such as moss & ferns also reproduce this way.

Sexual Reproduction:

The Sexual Reproduction depends on the involvement of two individuals. It allows more variation in the genetic pool assuring greater survivability of the organism.

Genetic Variation in Sexual Reproduction

The copying and fusion of DNA is very precise and errors are too few but enough to make genetic variations. Each new variation is an addition to the DNA copy that already has variations accumulated from previous generations. The combining variations from two or more individuals would thus create new combinations of variants.

Genetic Evolution:

These errors can create genetic diseases: If copying is less accurate, it will not work with cellular apparatus and will die. However, the errors might also help an organism to adapt even better to its surroundings.

Gametes or Germ Cells

A gamete is a reproductive cell of an animal or a plant. The female gametes are called the ova or egg cells, and the male gamete is known as the sperm.

• These are found in Multi-cellular organisms. These lineages of cells are produced and stored in specialised organs and have only half the number of chromosomes and half the amount of DNA as compared to the non-reproductive body cells.
• When these germ cells from two individuals combine during sexual reproduction to form a new individual, it results in the re-establishment of the number of chromosomes and the DNA content in the new generation.
• In very simple organisms, the two germ cells are not very different from one another, or may even be similar.

• In more complex organisms, the germ cells specialise: One germ cell is large and contains the food stores and the other is smaller and likely to be motile.
  • Conveniently, motile germ-cell = Male Gamete
  • Germ cell containing stored food = Female gamete.

Sexual Reproduction in Flowering Plants:

Angiosperms are those plants that have flowers and produce seeds enclosed within a carpel. Stamen and carpel are the Reproductive parts of the flowers that contain germ cells.

Flowers

• Stamen (male): It consists of Anther and Filament. Anther contains pollen grains which produce male gametes. Produces pollen grains, the Yellowish sticky powder, containing the genetic material.
• Pistil (female): Centre of flower; consists of:
  • Stigma: Pollen receptive surface, may be sticky,
  • Style: Middle elongated part, &
  • Ovary (protective part): Contains ovules; Each ovules have an egg cell. Fusion takes place in ovules. The ovary contains one or more ovules. A female gamete or egg is formed in the Ovule.

Types of Flowers

• Unisexual flowers: with only pistils. Both male & female unisexual parts can be on the same or different plants. Corn, Papaya & cucumber.
• Bi-sexual flower: These are flowers which have both stamens and Pistils. For example, Mustard, rose and petunia have bisexual flowers.
• Unisexual (either stamen or carpel): Papaya, watermelon;
• Bisexual (both stamen & carpel): Hibiscus, Mustard.

Geminosperms: These are non-flowering plants that produce seeds that are not enclosed. These seeds are exposed on the surface of cone scales or other structures, such as in conifers (pine, spruce, fir). Gymnosperms produce cones (male and female). The male cones release pollen, which is carried by wind to the female cones, where fertilization occurs.

Pollination:

Pollination is the process by which pollen is transferred to the female reproductive organs.

Generally, pollen grains have a tough protective coat, which prevents them from drying up, and they do not open up until pollination occurs. Pollens are light and are carried by wind or birds/ insects. Some pollen land on the stigma of the flower of the same kind and cause Pollination.

1. Self-pollination: Same flower.
2. Cross-pollination: different flowers.

Pollenating agents: Transfer of pollens done by them: wind, water, animals.

Fertilisation:

A cell that results after the fusion of gametes is called a Zygote. The process of fusion of male and female gamete is Zygote fertilisation. A zygote develops into an embryo which forms the fruit and the seed.

• After pollen lands on a suitable stigma, a tube grows out of the pollen grains and travels through the style to reach the ovary;
• Here it meets the female germ cell in a process called Fusion/Fertilization. During this process, a zygote forms which is capable of growing into a new plant.
• The zygote divides several times to form an Embryo within the ovule.
• The embryo develops into a seedling under appropriate conditions, this process is known as the

Fruit and seed formation:

• Ovule develops a tough coat and gradually converts into a seed, which contains future plants or embryos. The ovary grows rapidly and ripens to form a fruit.
• Petals, sepals, stamens, style and stigma shrivel and fall off.

Pollination → Fusion → Zygote → Embryo → seed → Germination.

• Ovary grows into fruit & other parts of the flower fall off. The seed contains an embryo enclosed in a protective seed coat.
• Seeds: In flower stamens are the male reproductive part & pistil is the female reproductive part.
• Seed dispersal: carried by wind have wings: Maple & drumstick, hairy seed: aak (madar), sunflower.
  • Dispersed by water: have floating ability, are spongy: coconut.
  • Dispersed by Animals: hook-like formation: Xanthium & Urena

Planting: The seed burst with a sudden jerk and scattered far from the parent plants: Castor and Balsam.

We shall study the reproduction in humans in the next chapter. This will give us an insight into the reproduction of animals in general.

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