In every cell, DNA is organized into chromosomes. These physical structures of condensed genetic material organize themselves each time cell division occurs, to ensure the replication and passing on of all the cell's DNA. Chromosomal abnormalities are responsible for some of the most common birth defects identified by doctors. Understanding chromosomes' function and structure can help scientists prevent and treat these devastating diseases.
46 Chromosomes of the Human Genome
Every human has 46 chromosomes - 23 pairs - of which one in each pair is inherited from the mother and one from the father. Of these 23 pairs, 22 are the autosomal chromosomes, which have the same structure and genes in both men and women. The final pair is made up of two sex chromosomes, which can be different in males and females. These are the X and Y chromosomes. In females, there are two X chromosomes, while males have one X and one Y.
Chromosome Location – DNA in the Cell Nucleus
Chromosomes are made of coiled, condensed DNA and are found in the nucleus of all cells. When DNA needs to be accessed to make proteins or provide instructions for cell activity, the chromosomes uncoil and are not visible as solid structures. Just before the cell divides, the chromosomes condense again in preparation for replicating all of the genes they contain.
Karotyping – What Chromosomes Look Like
A karotype is the full complement of all chromosomes in a human cell. Scientists can analyze the chromosomes in a karotype by freezing the chromosomes chemically during cell division and taking a picture. From these pictures of karotypes, we know that most chromosomes look like striped bands. When they duplicate themselves in preparation for mitosis or meiosis, they form two duplicate bands joined at the center, which causes them to resemble the shape of the letter X. At the center of the X is a structure called the centromere.
Chromosomal Abnormalities – Downs's Syndrome, Trisomy and Other Birth Defects
Down's syndrome is probably the most famous disease caused by a chromosomal abnormality. Also known as trisomy 21, Down's syndrome is the result of poor sorting during meiosis, the process where chromosomes are divided between future egg or sperm cells to be passed along to offspring.
Trisomy occurs when three copies of a chromosome get passed to offspring instead of the usual two. Trisomy 21 is when this occurs in chromosome 21. Other trisomies sometimes seen in infants include trisomy 18 and trisomy 13. Trisomies of the sex chromosomes can also occur, resulting in individuals with XXX, XXY or XYY in their karotype. Most trisomies are fatal in embryos and will not develop at all, but instead will fail to fertilize or miscarry early in a pregnancy.
Another chromosomal abnormality is when one of the two copies is missing, so the offspring only inherits one copy of a particular chromosome. This is less frequent than trisomy and is usually fatal to an embryo.
Chromosomes are an important structure within the cell nucleus and their proper functioning during cell division helps organize the vast amount of genetic material each cell contains. To learn more about DNA and the genetic code held within chromosomes, see the article Structure and Function of DNA: Understanding the Basics of the Genetic Code.
Sources
Griffiths, Anthony JF et al. " Chromosome Mutation II: Changes in Chromosome Number ". An Introduction to Genetic Analysis.New York: W. H. Freeman. 2000.
Lodish et al., Molecular Cell Biology. W.H. Freeman and Company. 2004
Bridget Coila - I'm a cell and molecular biologist, freelance writer and photographer currently living in Beijing, China. I'm fascinated by science, ...
Join the Conversation