____________________, also called conception, occurs when the sperm and egg nuclei combine
When human sexual intercourse occurs, it takes about 5 minutes for sperm to reach the upper end of the fallopian tubes where conception usually takes place. Of the several hundred million sperm cells that enter a vagina, rarely do more than a few hundred successfully pass through the cervix and uterus. However, only one of these sperm can actually fertilize the single female egg cell (also called an ovum). The successful sperm cell must enter the uterus and swim up the fallopian tube to meet the egg cell. In the second phase, the cell membranes of the egg and sperm cling together in an intimate embrace, before fusing to allow the DNA to meet. Despite its importance, scientists know relatively little about the molecules that control fertilization. Fertilization. Photo by: Cappi Thompson. Fertilization is the process by which the nucleus of a sperm (a male reproductive cell) fuses (combines) with the nucleus of an egg (a female reproductive cell; also called an ovum). Fertilization occurs somewhat differently in plants and animals. One possible answer, says Snell, is that it helps organisms resolve two conflicting evolutionary demands: the need to maintain working machinery for membrane fusion, and the need to evolve species-specific sperm–egg interactions. Once the head of the sperm is inside the egg, the tail of the sperm falls off. The outside of the egg then thickens to prevent another sperm from entering. In humans, a number of variables affect whether or not fertilization occurs following intercourse. One factor is a woman's ovulatory cycle. However, the process varies with the kind of cell. Somatic, or body cells, such as those that make up skin, hair, and muscle, are duplicated by mitosis. The sex cells, sperm and ova, are produced by meiosis in special tissues of male testes and female ovaries. Gametes have a haploid number (23). When conception occurs, a human sperm and ovum combine their chromosomes to make a zygote (fertilized egg) with 46 chromosomes. This is the same number that the parents each had in their somatic cells. In doing this, nature is acting conservatively. Each generation inherits the same number of chromosomes. This is a diploid zygote. The cells from each parent that combine to form the zygote are called gametes. In humans, the male gamete is called sperm, and the female gamete is called an egg. When the gametes join they form a cell called a zygote. Human sperm and eggs contain 23 chromosomes. Human zygotes contain 46 chromosomes. When two become one: the mysterious process of fertilisation.Getty. Boy meets girl. Sperm meets egg. Now, scientists are a step closer to understanding the climax of this eternal love story: how sperm and egg merge to create a brand new individual.
Zygote