The basic unit of heredity. A gene consists
of a long sequence of DNA nucleotides
which occupy a specific position (locus) on a chromosome.
Genes can carry the code for synthesizing proteins
and RNA molecules, and also regulate the transcription
of RNA sequences.
In humans, genes vary in size from a few hundred DNA bases to more than
2 million bases. The Human Genome Project has estimated that humans have
between 20,000 and 25,000 genes.
Every person has two copies of each gene, one inherited from each parent.
Most genes are the same in all people, but a small number of genes (less
than 1 percent of the total) are slightly different between people. Alleles
are forms of the same gene with small differences in their sequence of DNA
bases. These small differences contribute to each person's unique physical
Location of a gene
Geneticists use maps to describe the location of a particular gene on a
chromosome. One type of map uses the cytogenetic location to describe a
gene's position. The cytogenetic location is based on a distinctive pattern
of bands created when chromosomes are stained with certain chemicals. Another
type of map uses the molecular location, a precise description of a gene's
position on a chromosome. The molecular location is based on the sequence
of DNA building blocks (base pairs) that make up the chromosome.
|The CFTR gene is located on the long
arm of chromosome 7 at position 7q31.2.
Geneticists use a standardized way of describing a gene's cytogenetic location.
In most cases, the location describes the position of a particular band
on a stained chromosome:
It can also be written as a range of bands, if less is known about the exact
The combination of numbers and letters provide a gene's "address" on a chromosome.
This address is made up of several parts:
Sometimes, the abbreviations "cen" or "ter" are also used to describe a
gene's cytogenetic location. "Cen" indicates that the gene is very close
to the centromere. For example, 16pcen refers to the short arm of chromosome
16 near the centromere. "Ter" stands for terminus, which indicates that
the gene is very close to the end of the p or q arm. For example, 14qter
refers to the tip of the long arm of chromosome 14. ("Tel" is also sometimes
used to describe a gene's location. "Tel" stands for telomeres, which are
at the ends of each chromosome. The abbreviations "tel" and "ter" refer
to the same location.)
- The chromosome on which the gene can be found. The first number or
letter used to describe a gene's location represents the chromosome.
Chromosomes 1 through 22 (the autosomes)
are designated by their chromosome number. The sex
chromosomes are designated by X or Y.
- The arm of the chromosome. Each chromosome is divided into two sections
(arms) based on the location of a narrowing (constriction) called the
centromere. By convention, the shorter
arm is called p, and the longer arm is called q. The chromosome arm
is the second part of the gene's address. For example, 5q is the long
arm of chromosome 5, and Xp is the short arm of the X chromosome.
- The position of the gene on the p or q arm. The position of a gene
is based on a distinctive pattern of light and dark bands that appear
when the chromosome is stained in a certain way. The position is usually
designated by two digits (representing a region and a band), which are
sometimes followed by a decimal point and one or more additional digits
(representing sub-bands within a light or dark area). The number indicating
the gene position increases with distance from the centromere. For example:
14q21 represents position 21 on the long arm of chromosome 14. 14q21
is closer to the centromere than 14q22.
Types of gene
Various types of gene have been discovered. Structural genes
determine the biochemical makeup of the proteins. Regulator genes
control the rate of protein production. Architectural genes
are responsible for the integration of the protein into the structure of
the cell. Temporal genes control the time and place of
action of the other genes and largely control the differentiation of the
cells and tissues of the body.
Source: U.S. National
Library of Medicine