Designating Physical Chromosome Locations

The basic regions of a chromosome are illustrated in Figure 2.4. The center region of a chromosome, known as the centromere, controls the movement of the chromosome during cell division. On either side of the centromere are 'arms' that terminate with telomeres (Figure 2.4). The shorter arm is referred to as 'p' while the longer arm is designated 'q'.

Figure 2.4

Basic chromosome structure and nomenclature. The centromere is a distinctive feature of chromosomes and plays an important role during mitosis. On either side of the centromere are 'arms ' that extend to terminal regions, known as telom-eres. The short arm of a chromosome is designated as p ' while the long arm is referred to as 'q '. The band nomenclature refers to physical staining with a Giemsa dye (G-banded). Band localization is determined by G-banding the image of a metaphase spread during cell division. Bands are numbered outward from the centromere with the largest values near the telomeres.


(short arm)

Band 3



(long arm)



Human chromosomes are numbered based on their overall size with chromosome 1 being the largest and chromosome 22 the smallest. The complete sequence of chromosome 22 was reported in December 1999 to be over 33 million nucleotides in length. Since the Human Genome Project completed its monumental effort in April 2003, we now know the sequence and length of all 23 pairs of human chromosomes.

During most of a cell's life cycle, the chromosomes exist in an unraveled linear form. In this form, it can be transcribed to code for proteins. Regions of chromosomes that are transcriptionally active are known as euchromatin. The transcriptionally inactive portions of chromosomes, such as centromeres, are heterochromatin regions and are generally not sequenced due to complex repeat patterns found therein. Prior to cell division, during the metaphase step of mitosis, the chromosomes condense into a more compact form that can be observed under a microscope following chromosomal staining. Chromosomes are visualized under a light microscope as consisting of a continuous series of light and dark bands when stained with different dyes. The pattern of light and dark bands results because of different amounts of A and T versus G and C bases across the chromosomes.

A common method for staining chromosomes to obtain a banding pattern is the use of a Giemsa dye mixture that results in so-called 'G-bands' via the 'G-staining' method. These G-bands serve as signposts on the chromosome highway to help determine where a particular DNA sequence or gene is located compared to other DNA markers. The differences in chromosome size and banding patterns allow the 24 chromosomes (22 autosomes and X and Y) to be distinguished from one another, an analysis called a karotype.

A DNA or genetic marker is physically mapped to a chromosome location using banding patterns on the metaphase chromosomes. Bands are classified according to their relative positions on the short arm (p) or the long arm (q) of specific chromosomes (Figure 2.4). Thus, the chromosomal location 12p1 means band 1 on the short arm (p) of chromosome 12. The band numbers increase outward from the centromere to the telomere portion of the chromosome. Thus, band 3 is closer to the telomere than band 2. When a particular band is resolved further into multiple bands, its components are named p11, p12, etc. If additional sub-bands are seen as techniques are developed to improve resolution, then these are renamed p11.1, pi 1.11, etc. For DNA markers close to the terminal ends of the chromosome, the nomenclature 'ter' is often used as a suffix to the chromosome arm designation. The location of a DNA marker might therefore be listed as 15qter, meaning the terminus of the long arm of chromosome 15. Sometimes a DNA marker is not yet mapped with a high degree of accuracy in which case the chromosomal location would be listed as being in a particular range, i.e., 2p23-pter or somewhere between band 23 and the terminus of the short arm on chromosome 2.


A vast majority of our DNA molecules (over 99.7%) is the same between people. Only a small fraction of our DNA (0.3% or ~10 million nucleotides) differs between people and makes us unique individuals. These variable regions of DNA provide the capability of using DNA information for human identity purposes. Methods have been developed to locate and characterize this genetic variation at specific sites in the human genome.

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  • zewdi
    Where are chromosomes located forensics science?
    1 year ago

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