| chromosomal Mosaicism | HOME | |||
| TABLE OF CONTENTS | ||||
| SEARCH | ||||
| BACKGROUND | RESOURCES | GLOSSARY | ||
|
Background |
| |
|
WHAT ARE CHROMOSOMES? | ||
|
Chromosomes are the packaging for our genetic material, or DNA (deoxyribonucleic acid). DNA carries a specific code that gives instructions to our body on how to grow, develop and function. The instructions are organized into units called genes. Here's a picture of DNA. There are 46 chromosomes in most cells of the human body. The body is made up of many different cells that are the building blocks for the various tissues and organ systems in our body. If we were able to look inside most cells in a person’s body we would expect to see 46 chromosomes in each cell. | ||
|
The 46 chromosomes come in 23 matching pairs. We inherit one chromosome in each pair from our mother and one from our father. In the cytogenetics lab, scientists study chromosomes under a microscope. The 46 chromosomes can be organized into 23 pairs according to size, and the patterns of the dark and light bands. This arranged picture of the chromosomes is called a >karyotype. There are 22 pairs of autosomes and 1 pair of sex chromosomes The sex chromosomes determine whether an individual is male or female. Two X chromosomes make a female and one X and one Y make a male. The word karyotype is also used when describing a person's chromosomal make up. The normal female karyotype is 46, XX. The normal male karyotype is 46, XY. This is an illustration of a pair of chromosomes. The shorter, upper arm is called the p arm, for petit. The longer arm is called the q arm, because q is the next letter in the alphabet. The central part, joining the two arms, is called the centromere. Each numbered chromosome is unique and can be distinguished from one another by size, the location of the centromere and the pattern of dark and light bands. Show me a picture of all of the chromosomes.
| ||
| What happens to our chromosomes during reproduction? | ||
 Each person inherits one chromosome in each pair from their mother and one from their father. At fertilization or conception, an egg cell (from the mother) and a sperm cell (from the father) fuse to create a fertilized egg. The egg and the sperm each contain half of the total genetic material. When we make our egg or sperm there is an important process of dividing the 46 chromosomes in half, so that the egg contains 22 autosomes and one X chromosome and the sperm contains 22 autosomes and one "sex-determining" chromosome. The sex chromosome in the sperm determines the sex of the baby. If the sperm is carrying an X chromosome the baby will be female. If the sperm is carrying a Y chromosome the baby will be a male. When the 23 chromosomes in the egg fuse with the 23 chromosomes in the sperm the total number in the baby is restored to 46 chromosomes.
| ||
| How does a fertilized egg grow into a baby? | ||
 The fertilized egg starts as a single cell with 46 chromosomes, also called the zygote. All cells in the human body come from that very first cell. In order for the zygote to develop into a baby this single cell must divide. Before cells can divide the chromosomes must replicate themselves. So, in preparation for cell division, each chromosome makes an identical copy of itself. At cell division each chromosome and its identical copy pull apart into two separate daughter cells. Now the daughter cells have the same chromosomes as the original cell.
| ||
