Brent Cornell

    
Eukaryotic chromosomes are linear molecules of DNA that are compacted during cell division (mitosis or meiosis)

Each chromosome has a constriction point called a centromere, which divides the chromosome into two sections (or ‘arms’)

• The shorter section is designated the p arm and the longer section is designated the q arm

Eukaryotic species possess multiple chromosomes that may differ in both their size and the position of their centromere

• Staining chromosomes with particular dyes (e.g. Giemsa stain) will additionally generate unique banding patterns

Each chromosome will carry specific genes and the position of a particular gene on a chromosome is called the locus

The region in which a locus is positioned can be identified via three points of reference:

• The first point of reference is a number (or letter) which denotes the chromosome (e.g. 7q31 refers to chromosome 7) 
• The second point of reference is a letter (p or q) to denote which arm the locus is positioned on (e.g. 7q31 is on the q arm)
• The third point of reference is a number corresponding to the G band location (e.g. 7q31 is at the longitudinal position 31)

Gene Locations as Shown by a Chromosome Map

    
The locus of a human gene and its polypeptide product can both be identified using a single online resource:

• GenBank – a genetic database that serves as an annotated collection of DNA sequences
  
  

Identifying Gene Loci

GenBank can be used to identify the specific location of a gene on any given chromosome

To identify a specific gene locus:

• Change the search parameter from nucleotide to gene and type in the name of the gene of interest
• Choose the species of interest (i.e. Homo sapiens) and click on the link (under ‘Name / Gene ID’)
• Scroll to the ‘Genomic context’ section to determine the specific position of the gene locus
• A visual profile can be generated by clicking on ‘Map Viewer’ link and looking at the Ideogram on the left side

Identifying Polypeptide Products:

GenBank can also be used to identify the polypeptide product of any given gene

To identify the polypeptide product of a gene:

• Change the search parameter from nucleotide to gene and type in the name of the gene of interest
• Choose the species of interest (i.e. Homo sapiens) and click on the link (under ‘Name / Gene ID’)
• The polypeptide product should be identified within the ‘Summary’ section

Below are examples of different genes that may be searched for:

• HBB  –  Haemoglobin beta gene
• DRD4  –  Dopamine receptor D4 gene
• F8  –  Coagulation factor VIII gene
• IGF1R  –  Insulin growth factor 1 receptor gene

Link:  Human Genome Online Chromosome Viewer