Reproduction, growth and development

Every organism starts as a single cell called the zygote. The sequence of changes that an organism goes through from the time when its existence begin as a zygote till its death is called its life cycle. This is the process, by which the zygote forms, and growth and development, which are the changes that transform a zygote into an adult individual.

All living organisms reproduce, or give rise individuals’ o their own kind, for the survival of their species. In the process the parent’s traits, or genetic characters, pass on to the offspring which, in turn, pass on the traits to their won offspring.

There are two basic modes of reproduction-asexual and sexual. Asexual reproduction requires only one parent, any or a particular part of the parent’s body can give rise to an –individual. In sexual reproduction, on the other hand, two parents together- one bearing the male sex and the other bearing the female sex –produce the offspring. Each contributes a sex cell, or gamete. In animals, the male gamete, called the sperm, fuses with the female gamete, called the ovum, or egg cell, to form a zygote. The zygote goes through repeated division and develops in to an embryo. The embryo grows into a baby, which grows and becomes an adult.

The offspring produced by asexual reproduction have the same characters as thee parent, that is, the parent’s genetic traits are carried by the offspring generation after generation. Sexual reproduction, on the other hand, results in variation of character in the offspring as two parents are involved. The offspring thus receives some traits of both parents and in never exactly like either parent most organisms reproduce sexually.

Asexual reproduction

Asexual reproduction is common among lower organisms.

Binary fission

This is the simple division of a unicellular organism into two daughter cells. The parent cell’s nucleus divides into two and the middle o the cell gets constricted. This portion becomes more and narrower, until the cell splits into two daughter cells grow and then divide similarly. Reproduction by binary fission takes place among the simplest organisms, like Amoeba, Paramecium and bacteria.

In some organisms, like chlorella and Chlamydomonas one cell divides into four daughter cells, or under cells, or undergoes multiple fission.

Budding

In this process, which is common among some simple organisms like Hydra, the parent cell produces an outgrowth called a bud. The bud grows, and then gets detached from the parent’s body to lead its individual life. Budding is one of the ways by which yeast reproduces. It reproduces by binary fission too. It can produce buds in chains, i.e., the buds do not separate from the parent cells. The parent cell gives rise to one or more buds which cell gives rise to one or more buds which, in turn, produce their own buds, and so on.

Fragmentation

Organisms like spirogyra, which are made of filaments, or long, ribbon like structures, reproduce this way. The filaments grow as the cell which constitutes it grow and divide. Sometimes, the filament breaks into two or more parts and each such part grows part grows into an individual.

Regeneration

In the case of some organisms, the body breaks up into one several parts, and each part grows into an entire new individual. Hydra, r=earth warms and flatworms are a few organisms capable of such regeneration. There are others which can regenerate broken parts. For example, a lizard can grow back its tail and a grasshopper its legs, but they cannot give rise to complete new individuals.