Egg cells have adaptations in formation, structure and genetic makeup that enable them to function. Egg cells have similar genetic composition to sperm, but their physical structure and initial formation are unique.
Like sperm, eggs are haploid cells. Haploid cells have one full set of chromosomes. This means that when a sperm fertilizes an egg, the resulting offspring has a set of chromosomes from each parent. Every sex cell, egg or sperm, comes from a parent cell with two full sets of chromosomes. The chromosome number halves during meiosis. This adaptation ensures that the chromosome number does not perpetually double with each generation of offspring.
Egg cells differ greatly from sperm in their structure. Sperm are adapted for mobility, while eggs are adapted for fertilization and the development of a new organism. Egg cells are much larger than sperm and contain a greater volume of cytoplasm. The cytoplasm of an egg cell contains ribosomes, transfer RNA and messenger RNA. Messenger RNA provides the instructions for protein assembly while transfer RNA delivers protein components to ribosomes to be built. The cytoplasm also provides the nutrients for the developing organism.
Egg cells also form differently than sperm cells. A single parent cell results in the formation of four sperm cells. Egg cells, however, must have more cytoplasm to function. Rather than dividing cytoplasm between four egg cells, the parent cell divides into a single egg and three structures called polar bodies. The polar bodies, produced so that appropriate chromosome numbers are maintained, soon degrade and are reabsorbed by the body. The egg cell now has the requisite number of chromosomes as well as all the cytoplasm it will need if fertilized.