Biology chapter 2 Reproduction in living organisms

Biology - chapter 2| Reproduction in living organisms
Reproduction in living organisms
Importance of reproduction to living organisms:

* Reproduction is concerned with the continuity of the living organism on Earth. So that the importance of reproduction is not as in other vital activities.

* Living organism can live normally without reproduction, or even without sexual organs.

* If reproduction stops completely among certain species, extinction occurs.

The reproductive capacities among organisms
The rate of reproduction differ from one organism to another according to the following factors:

1. The surrounding environment (ambient):
- Aquatic organisms reproduce in a higher rate (more progeny) than terrestrial ones.
2. Dangers to which the organism is exposed:
- parasites reproduce at higher rates than others in order to compensate their loss..
3. Nature & duration of the organism’s life:
- primitive & short lived individuals reproduces at higher rates much more than higher or long- aged ones due to the provided care and protection.
- Species, which are found nowadays, indicate that their ancestors succeeded to survive, while others, like Dinosaurs, could not.


Asexual Reproduction:
- It is isolation of a body part as a spore cell or many cells or tissues and growth into a new organism that similar to the original part has been isolated.
- Therefore, the features of the following generations remain the same, even if the surrounding conditions change.
- At any change in the environment, most of the offsprings exposed to destruction unless their parents had been adapted for that change.
- This kind of reproduction is common in the plant kingdom & some primitive forms of the animal kingdom.
- Asexual reproduction depends on mitotic division of cells, where the produced cells have the chromosomal no of original cell.

Methods of reproduction in living organisms

1. Binary Fission under suitable conditions
- The nucleus of the cell divides by mitosis, into 2 cells, each become a new individual.
- Binary Fission occurs under the suitable conditions in Amoeba, Paramecium, simple algae and bacteria.
-Binary Fissionunder unsuitable conditions
- In unsuitable conditions, Amoeba secretes a chitinous coat around itself to form cyst to protect itself against unsuitable conditions.
- During improvement of the surrounding conditions it divides inside the coat several times by repeated binary fission to produce many young amoebae that liberate from the cyst.

2. Budding in unicellular
- Budding occurs in some unicellular & some multicellular ones.
- In unicellular organisms as yeast fungus, the bud produced from lateral protrusion of the original cell while, the nucleus divides mitotically into 2 nuclei one of them still in the mother cell while, the other moves towards the bud.
Budding in multicellular
- In multicellular as sponges and Hydra, the bud grows from interstitial cells, which divided from one side of the body to form protrusion.
- The protrusion grows to form an individual similar to its mother.
- It usually separates to start its independent live
- Sponges and Hydra reproduce also sexually
- besides their capacity for regeneration.


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Comparison between binary fission & budding

3. Regeneration
1- In higher vertebrates, regeneration never exceeds healing of wounds in skin, blood vessels and muscles.
2- In some crustaceans and amphibians it is limited to of the cut parts only Some living organisms are able to (restoration) regenerate the lost parts of their bodies
3- In Planaria, Sponges, Hydra & starfish If the body is cut into several parts, each part can grow to a new individual.
4. Regeneration
- In fresh water flatworm as Planaria regenerate takes place
if Planaria is cut into several transverse pieces or 2 longitudinal parts, Each will grow into a new individual.
- Hydra can regenerate if it is cut into several transverse pieces, each part will grow into a new individual.
- Breeders of pearl mussels tried to control starfish which feed on 10 of the pearl mussels daily.
- Breeders collect starfish and cut them into pieces and throw into the sea, But these pieces regenerated into new individuals by great number .
they were unintentionally helping their reproduction ,so they could control star fish by burning these pieces
- One of the star’s arms with a piece of its central disc can regenerate to a full star within a short time.
4. Sporogony
- The spore is a single cell formed of a nucleus surrounded little amount of cytoplasm, and a thick coat.
- When the spore matures it liberates from the mother plant spreads in air.
- When the spore reach a suitable medium for growth, its coat ruptures while it absorbs water and divide several times by mitosis and grow to a new individual.,
- Many fungi such as Bread mould, Mushroom , and plants as some Algae and Ferns reproduce by sporogony.



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Advantages of reproduction by spores
1- Quick propagation.
2- Tolerance to hard conditions.
3- Distribution to great distance.

5. Parthenogenesis
- It is the ability of the egg to develop to a new individual without being fertilized by a male gamete.
- It considered as a special kind of asexual reproduction because the offspring comes from the mother only

A) Natural Parthenogenesis
It occurs in a number of worms, crustaceans and insects.
- Honey bee is a known example, where the queen produces some eggs that develop without fertilization forming drones {haploid (N)} and some others that develop after fertilization to queen and workers {diploid (2N)}, according to the type of food provided later on.
- In aphid insect, the eggs result from a mitotic division where it develops to a diploid (2N) individuals from the beginning.

B) Artificial Parthenogenesis
It occurs when the frog eggs exposed to heat or electric shock, irradiation, some salts agitation or pricking with a needle.
- These stimuli lead to duplication of their chromosomes and developed without fertilization to individuals
- Ex. Frog, sea star, early embryonic stages of rabbits.

Comparison between Natural & Artificial parthenogenesis



6. Tissue culture
- Scientists carry out experiments of tissue culture on plants and animals
- In experiment, a scientist separated small pieces of carrot plant into conical flasks containing coconut milk, which contains the whole plant hormones and nutrient elements, the small pieces grow and develop into a full carrot plant

- Similarly, small pieces of a tobacco plant developed into a full plant.
- The somatic cell in the plant contains all genetic information (2N) as zygote, so the genetic information are translated into complete organism if cultivated in a proper nutrient medium containing plant hormones with certain ratio.


Importance of tissue culture
1- Propagating rare plants, desirable strains that resistance to diseases.
2- Solve food problem
3- to shorten the time needed for growing desirable crops

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