How chromosomes get together in one place?

In male animals, meiosis takes place in the testes and in females within the ovaries. In plants, meiosis occurs in the anthers to form pollen grains and within the ovary to form ovules. In meiosis a diploid cell divides in such a way so as to produce four haploid cells. These haploid cells are known as gametes. Each gamete is genetically different, as a result of both random alignment of the homologous pairs of chromosomes (independent assortment of the chromosomes) and crossing over between members of a homologous pair.

Meiotic division occurs in two stages, meiosis I and meiosis II, dividing the cells once at each stage. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated, so that each chromosome has two sister chromatids; a diploid organism now has a tetraploid DNA amount in the cell. The first stage of meiosis begins with a cell that has (if it is from a diploid organism) two copies of each type of chromosome, one from each of the mother and father, called homologous chromosomes, each of which has two sister chromatids. The homologous chromosomes pair up and may exchange genetic material with each other in a process called crossing over. Each pair then separates as two cells are formed, each with one chromosome (two chromatids) from every homologous pair.

In the second stage, each chromosome splits into two; each half, each sister chromatid, is separated into two new cells, which are haploid. This occurs in both of the cells formed in meiosis I. Therefore from each original cell, four genetically distinct haploid cells are produced. These cells can mature into gametes i.e sperm or ova in humans.

As long as a sperm cell is alive in the fallopian tube, it's capable of fertilizing an egg. If there's no egg in the fallopian tube, there's no chance of fertilization.

The fallopian tubes are about four inches (10 centimeters) long and transport the egg from the ovary to the uterus. They also provide sperm that make it that far with nutrients and a safe environment, the same kind the egg enjoys as it passes through. Of the millions of sperm cells that initially enter the cervix, there may be anywhere from one to a couple hundred that arrive at the fallopian tube

Eggs will survive about a day after they're released from the ovaries. If not fertilized, they'll break down. It's only during this day that a woman can become pregnant, though it may be a result of a sexual encounter days earlier, since sperm can survive in the fallopian tubes for a few days.

When an egg does pass through, the sperm have receptors that allow them to smell the eggs, which are surrounded by cells releasing the sweet scent: progesterone. Sperm cells most definitely become very active when an egg is present. In fact, progesterone makes sperm become so active that they slough off layers of proteins. Both the surge in activity and the loss of proteins enables sperm to pierce the egg. This process is called capacitation. Once this occurs, the sperm only have a few hours to live. Only a few -- perhaps half a dozen or less -- sperm cells will ever share proximity with the egg

The head of the sperm, once making contact with the exterior of the egg, will more or less "pop," releasing enzymes that allow it to cross through the barrier.

Once a sperm cell penetrates the exterior of the egg, fertilization occurs -- its DNA payload is delivered as the sperm is absorbed by the egg. The genetic blueprint of the child is now set in stone. Once a single sperm enters the egg, the egg's protective protein covering changes and doesn't allow other sperm to enter.

From one egg that has been fertilized by one sperm cell, encoded genetic information coupled with cell growth will eventually create an entire human being.

As the egg and sperm chromosomes combine, the egg is officially fertilized, creating a zygote. The combined chromosomes begin to divide into new cells, forming an embryo. This is sometimes also referred to as a blastocyst, a small ball of about 100 cells, until implantation.

A pregnancy isn't fully viable until the embryo created during fertilization implants in the uterine lining. In order for implantation to occur, the embryo must break back through the membrane to burrow into the uterine lining, a process called "embryo hatching." It can take the embryo anywhere from three to seven days to travel from the fallopian tube to the uterus and hatch before implanting into the uterus to form a pregnancy.

http://www.biology.arizona.edu/cell_bio/tutorials/...

Read more : http://www.ehow.com/info_8618534_happens-after-spe...

http://www.bbc.co.uk/schools/gcsebitesize/science/...

https://www.youtube.com/watch?v=BFrVmDgh4v4

reproduction , all cells have outer layer that holds extra cells in reproduction it the sperm and egg