Life cycles Notes

What Stages Occur in All Sexual Life Cycles?

1. Every species has one main goal: survive long enough to reproduce.
2. The way a species achieves this determines its life cycle.
3. Life cycles vary because different organisms face different challenges: predators, food availability, climate, habitat, competition.
4. These challenges shape how fast they grow, how much they change, and how many offspring they produce.
5. These three stages are the universal foundation of every sexual life cycle:
   1. Gametogenesis
   2. Fertilization
   3. Meiosis

Gametogenesis: How Are Gametes Produced?

1. All sexual life cycles begin with the formation of gametes (sex cells).These gametes are produced in specialised organs:
   1. In animals: testes produce sperm; ovaries produce ova (egg cells).
   2. In flowering plants: pollen grains contain male gametes; ovules contain female gametes.
2. Gametes are always haploid (n)meaning they contain half the usual number of chromosomes.
3. This reduction happens to prevent chromosome doubling across generations.

Fertilization: How Are Gametes Combined?

1. During fertilization, the nuclei of the two gametes merge.
2. This restores the normal chromosome number:
   1. Two haploid gametes (n + n) → one diploid zygote (2n).
3. In humans, sperm (23 chromosomes) fuses with an egg (23 chromosomes) to form a zygote containing 46 chromosomes.
4. Fertilization marks the beginning of a new life cycle, with genetic material from both parents.

Meiosis: Why Must Chromosome Number Be Halved?

1. Without meiosis, gametes would have the full diploid chromosome number.
2. Fertilization would then double the chromosome number every generation, which is not sustainable.
3. Meiosis prevents this by producing haploid cells (n).
4. After fertilization, the diploid number (2n) is restored, and the cycle continues.

Why Do Some Animals Change Shape as They Grow While Others Do Not?

1. Some animals keep the same basic body shape as they grow, while others look completely different at each stage.
   1. A human infant grows into an adult gradually, the basic body plan stays the same.
   2. A caterpillar, tadpole, or maggot, however, doesn't resemble its adult form at all.

1. These differences exist because some animals undergo metamorphosis, while others follow direct development.

How Do Plant Life Cycles Compare?

1. Plant life cycles follow a different structure:
   1. Seed
   2. Germination
   3. Growth of roots, stems, leaves
   4. Flowering
   5. Seed production
2. Plants rely heavily on receptors (light, gravity, moisture) to guide these transitions.
3. Light receptors help seedlings grow toward sunlight.
4. Gravity receptors help roots grow downward for stability and water.

What Triggers Organisms to Move From One Stage to Another?

1. Life cycle transitions do not happen automatically.
2. Organisms need a signal telling them the time is right.These signals come from:
   1. Inside the body (hormones)
   2. Outside the body (temperature, day length, food availability)
3. Receptors detect these signals and activate developmental pathways.