CLASS 10 SCIENCE · CHAPTER 7 · STUDY NOTES
How do Organisms Reproduce?
An exploration of reproductive mechanisms in living organisms, covering DNA copying, asexual and sexual reproduction, plant germination, and human reproductive health.
EXPLORE
Pass on information—and a living cell
Explain why DNA copying must be accompanied by cellular organisation.LOOK · COMPARE · EXPLAIN
A DNA copy still needs a living cell.
What must accompany each new DNA copy?
DNA holds inherited information. A reproducing cell must copy this information and build cellular machinery.
- DNA
- The molecule that carries inherited information used by cells to make proteins.
Reproduction creates new individuals and helps a population continue across generations. It is not required for one individual to carry out nutrition, respiration or excretion.
A reproducing cell copies its DNA and builds additional cellular machinery. The copies are separated into organised cells; a loose DNA molecule is not a new organism.
Go deeper: Similar does not mean absolutely identical
DNA copying preserves much of the inherited information, so offspring generally resemble their parent or parents. Differences in copied information can affect proteins and features.
Copying can introduce variations. Some have little effect, some are harmful, and others may matter in a particular environment. Variation is not automatically an improvement.
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Question
A model makes a second DNA copy and pushes it outside the cell without any surrounding cellular structure. Why has it not yet modelled a new living individual?
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EXPLORE
Different variants. A changing environment.
Distinguish existing variation from a useful change created on demand.LOOK · COMPARE · EXPLAIN
The variants exist before the conditions change.
Predict which pre-existing variant remains after heat stress.
Variation already exists in a population. Its usefulness depends on the environment; it does not guarantee survival for every individual.
- Variation
- Differences among individuals in a population.
A population can contain inherited differences before its environment changes. A change in temperature may favour some variants over others.
In the model, heat-tolerant bacteria remain after heat stress. The example explains a possible advantage for the population; it does not guarantee that every individual survives.
Go deeper: The environment changes which variants succeed
A population suited to one niche can be affected if that niche changes. Existing differences may leave some individuals able to survive and reproduce under the new conditions.
The temperature change does not deliberately produce the needed DNA change. The illustrated counts are hypothetical, and a variant helpful in one environment need not be helpful in every environment.
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Question
A heat-tolerant bacterium was already present before a pond warmed. A caption says the heat invented its useful variant. How would you correct the caption?
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EXPLORE
Divide a cell—or separate a filament
Compare organism boundaries and the number of cells involved.LOOK · COMPARE · EXPLAIN
Count cells. Then identify the organism.
Does each starting individual have one cell or many?
Binary fission: one unicellular organism divides into two. DNA division precedes separation of the cell.
- Binary fission
- Division of one unicellular organism into two daughter individuals.
| Feature | Binary Fission | Multiple Fission |
|---|---|---|
| Number of offspring | Two daughter cells are produced. | Many daughter cells are produced simultaneously. |
| Example | Amoeba, Leishmania | Plasmodium (Malarial parasite) |
Amoeba can divide in different planes. Leishmania has an organised elongated body and divides in a definite orientation relative to its flagellum. In multiple fission, Plasmodium forms many daughter cells.
Spirogyra is a multicellular filament. Fragments contain cells that can grow into new filaments; fragmentation is not one cell dividing into two.
Go deeper: Body organisation changes the possibilities
Specialised tissues and organs must be arranged in the right places. A complex multicellular organism cannot generally reproduce by letting each of its cells independently split into a complete organism.
Nuclear division and distribution of cellular material must accompany fission. A long filament breaking into shorter multicellular pieces has a different starting structure and mechanism.
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Question
A sketch shows a filament of nine connected cells separating into three pieces of three cells. Why is calling this binary fission misleading?
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EXPLORE
Rebuild missing parts. Grow a new bud.
Separate organised regeneration from an outgrowth that becomes independent.LOOK · COMPARE · EXPLAIN
A piece rebuilds; a bud grows out.
Compare the structure present at the start.
Planaria can regenerate missing parts using specialised cells. Regeneration is not a universal reproductive method and is not the same as ordinary wound healing.
- Regeneration
- Rebuilding missing structures through cell proliferation and differentiation.
In the Planaria example, viable pieces can rebuild missing structures through specialised cells and organised development. This ability is not shared by all animals.
A Hydra bud grows through repeated cell division at a particular site. It develops into a small individual and later detaches. Yeast also reproduces by budding.
Go deeper: Repair is not always reproduction
Cell proliferation produces more cells; differentiation helps create the required cell types and tissues. Regeneration involves organisation, not merely increasing the size of a fragment.
Healing a wound does not normally create a separate individual. Most organisms do not depend on being broken into pieces to reproduce; the chapter uses particular examples rather than a universal rule.
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Question
One drawing shows a new Hydra growing from a small side outgrowth; another shows missing Planaria parts being rebuilt. What starting difference should the labels make clear?
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EXPLORE
New plants from parts. New fungi from spores.
Locate the structure that can establish a new individual.LOOK · COMPARE · EXPLAIN
Find the structure that starts new growth.
Look for a bud, node or spore—not just a detached piece.
A potato tuber is a stem. A piece with a bud can form a shoot and roots; a budless piece does not produce this shoot in the comparison.
- Vegetative propagation
- Production of a new plant from a root, stem or leaf rather than from a seed.
A potato tuber is a stem; its buds can produce shoots. Bryophyllum develops plantlets at leaf-margin notches. Stem cuttings, layering and grafting use suitable plant tissues, and a node matters in the money-plant comparison.
In Rhizopus, a sporangium holds spores above a network of hyphae. Protective spore walls help survival until suitable conditions allow germination; the stalk is not itself a spore.
Go deeper: Propagation depends on conditions and structures
Vegetative methods preserve many parental characteristics and can help propagate selected plants that produce few or no usable seeds. This does not mean every species, variety or detached piece behaves identically.
Tissue culture starts with selected tissue or cells. A medium supports cell division into a callus; different conditions and hormones support differentiation into plantlets that can later establish in soil.
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Question
A fungal sketch labels the supporting stalk as a spore and shows the rounded tip empty. Which structures should be relabelled to explain where new individuals can begin?
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