fundamentals
Genes and Chromosomes
Chromosomes
A chromosome is made of a very long strand of DNA and contains many Genes (hundreds to thousands). The genes on each chromosome are arranged in a particular sequence, and each gene has a particular location on the chromosome (called its locus). The form of the gene that occupies the same locus on each chromosome of a pair (one inherited from the mother and one from the father) is called an allele. In addition to DNA, chromosomes contain other chemical components that influence gene function. Except for certain cells (for example, sperm and egg cells or red blood cells), the nucleus of every normal human cell contains 23 pairs of chromosomes, for a total of 46 chromosomes. Normally, each pair consists of one chromosome from the mother and one from the father. There are 22 pairs of nonsex (autosomal) chromosomes and one pair of sex chromosomes. Paired nonsex chromosomes are, for practical purposes, identical in size, shape, and position and number of genes. Because each member of a pair of nonsex chromosomes contains one of each corresponding gene, there is in a sense a backup for the genes on those chromosomes. The 23rd pair is the sex chromosomes (X and Y). The pair of sex chromosomes determines whether a fetus becomes male or female. Males have one X and one Y chromosome. A male’s X comes from his mother and the Y comes from his father. Females have two X chromosomes, one from the mother and one from the father. In certain ways, sex chromosomes function differently than nonsex chromosomes. The smaller Y chromosome carries the genes that determine male sex as well as a few other genes. The X chromosome contains many more genes than the Y chromosome, many of which have functions besides determining sex and have no counterpart on the Y chromosome. In males, because there is no second X chromosome, these extra genes on the X chromosome are not paired and virtually all of them are expressed. Genes on the X chromosome are referred to as sex-linked, or X-linked, genes. Normally, in the nonsex chromosomes, the genes on both of the pairs of chromosomes are capable of being fully expressed. However, in females, most of the genes on one of the two X chromosomes are turned off through a process called X inactivation (except in the eggs in the ovaries). X inactivation occurs early in the life of the fetus. In some cells, the X from the father becomes inactive, and in other cells, the X from the mother becomes inactive. Thus, one cell may express a gene from the person’s mother and another cell expresses the gene from the person’s father. Because of X inactivation, the absence of one X chromosome usually results in relatively minor abnormalities (such as Turner Syndrome ). Thus, missing an X chromosome is far less harmful than missing a nonsex chromosome (see If a female has a disorder in which she has more than two X chromosomes, the extra chromosomes tend to be inactive. Thus, having one or more extra X chromosomes causes far fewer developmental abnormalities than having one or more extra nonsex chromosomes. For example, women with three X chromosomes ( Trisomy X ) are often unaffected physically and mentally. Males who have more than one Y chromosome (see There are several types of Overview of Chromosome and Gene Disorders . A person may have an abnormal number of chromosomes or have abnormal areas on one or more chromosomes. Many such abnormalities can be diagnosed before birth (see Abnormal numbers of nonsex chromosomes usually result in severe abnormalities. For example, receiving an extra nonsex chromosome may be fatal to a fetus or lead to abnormalities such as Down Syndrome (Trisomy 21) , which commonly results from a person having three copies of chromosome 21. Absence of a nonsex chromosome is fatal to the fetus. Large areas on a chromosome may be abnormal, usually because a whole section was left out (called a deletion) or mistakenly placed in another chromosome (called translocation). For example, Chronic Myeloid Leukemia (CML) is sometimes caused by translocation of part of chromosome 9 onto chromosome 22. This abnormality can be inherited or be the result of a new are tiny structures inside cells that synthesize molecules used for energy. Each cell has 1000 to 2500 mitochondria. Unlike other structures inside cells, each mitochondrion contains its own circular chromosome. This chromosome contains DNA (mitochondrial DNA) containing 37 genes that code for 13 proteins, various RNAs, and several enzymes. Mitochondrial DNA usually comes only from the person’s mother because, in general, when an egg is fertilized, only mitochondria from the egg become part of the developing embryo. Mitochondria from the sperm usually do not become part of the developing embryo. 