Question

How could a single cell become such a big individual?

Answer 1

Hint: In a multicellular living organism, two types of divisions are observed namely mitosis and meiosis. Any single cell right from its creation to its birth undergoes such divisions and thus becomes a big individual.

Complete answer:
Any multicellular organism begins its life as a single-celled zygote. This zygote undergoes cleavage, gastrulation, germ layer formation, organogenesis and eventually reaches adulthood. Processes such as cell division by mitosis and meiosis and cell differentiation play a vital part in this journey of the single-celled zygote. Mitosis is a type of division where a cell is divided in such a manner that the daughter cells receive the exact number of chromosomes as the parent cell. Meiosis is different, in the manner, that the daughter cells receive half the number of chromosomes as their parents.

Additional Information:
After fertilization, the zygote undergoes a series of mitotic divisions collectively identified as the cleavage. The enormous volume of the zygote is distributed into smaller and numerous nucleated cells known as blastomeres. The blastomeres undergo repeated division without increasing much in size but increasing in numbers. They then arrange themselves into three(mostly) layers. Each layer is called a germ layer. This is referred to as gastrulation. Gastrulation is followed by organogenesis when the interaction between different germ layers occurs to produce distinct organs. The cells of the germ layer at this stage receive signals intrinsically i.e. encoded in its own DNA and also from extrinsically i.e. from surroundings about the final shape and function it will perform and execute for the rest of their lives. The organs which are involved in the same function work coordinatively and form an organ system.

Note: In humans, gastrulation results in the formation of three germ layers. These are ectoderm, mesoderm, and endoderm from outer to inward. The ectoderm forms the skin and the nervous system. The middle layer, mesoderm, forms the muscular tissue, connective tissue, excretory system, and gonads. The innermost layer, endoderm, gives birth to epithelial tissues of the gut.