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17.2B: Physical Maps and Integration with Genetic Maps

  • Page ID
    13346
  • Physical maps display the physical distance between genes and can be constructed using cytogenetic, radiation hybrid, or sequence mapping.

    LEARNING OBJECTIVES

    Describe the methods used to physically map genes: cytogenetic mapping, radiation hybrid mapping, and sequence mapping

    KEY TAKEAWAYS

    Key Points

    • Physical maps provide specified detail about the number of bases and physical distance that exists between genetic markers.
    • Cytogenetic mapping is a method used to construct physical maps that uses stained sections of chromosomes to approximate the distance between genetic markers.
    • Radiation hybrid mapping is a method used to construct physical maps that uses radiation or x-rays to break DNA into fragments to determine the distance between genetic markers and their order on the chromosome.
    • Sequence mapping is a method used to construct physical maps that uses already-known locations of genetic markers to determine distances in number of base pairs.

    Key Terms

    • cytogenetic: of or pertaining to the origin and development of cells
    • physical map: a map showing how much DNA separates two genes and is measured in base pairs
    • expressed sequence tag: a short sub-sequence of a cDNA sequence that may be used to identify gene transcripts

    Physical Maps

    A physical map provides detail of the actual physical distance between genetic markers, as well as the number of nucleotides. There are three methods used to create a physical map: cytogenetic mapping, radiation hybrid mapping, and sequence mapping. Cytogenetic mapping uses information obtained by microscopic analysis of stained sections of the chromosome. It is possible to determine the approximate distance between genetic markers using cytogenetic mapping, but not the exact distance (number of base pairs). Radiation hybrid mapping uses radiation, such as x-rays, to break the DNA into fragments. The amount of radiation can be adjusted to create smaller or larger fragments. This technique overcomes the limitation of genetic mapping and is not affected by increased or decreased recombination frequency. Sequence mapping resulted from DNA sequencing technology that allowed for the creation of detailed physical maps with distances measured in terms of the number of base pairs. The creation of genomic libraries and complementary DNA (cDNA) libraries (collections of cloned sequences or all DNA from a genome ) has sped up the process of physical mapping. A genetic site used to generate a physical map with sequencing technology (a sequence-tagged site, or STS) is a unique sequence in the genome with a known exact chromosomal location. An expressed sequence tag (EST) and a single sequence length polymorphism (SSLP) are common STSs. An EST is a short STS that is identified with cDNA libraries, while SSLPs are obtained from known genetic markers and provide a link between genetic maps and physical maps.

    image

    Cytogenetic Map: A cytogenetic map shows the appearance of a chromosome after it is stained and examined under a microscope.

    Integration of Genetic and Physical Maps

    Genetic maps provide the outline and physical maps provide the details. It is easy to understand why both types of genome mapping techniques are important to show the big picture. Information obtained from each technique is used in combination to study the genome. Genomic mapping is being used with different model research organisms. Genome mapping is an-ongoing process; as better techniques are developed, more advances are expected. Genome mapping is similar to completing a complicated puzzle using every piece of available data. Mapping information generated in laboratories worldwide is entered into central databases, such as GenBank at the National Center for Biotechnology Information (NCBI). Efforts are being made to make the information more easily accessible to researchers and the general public. Just as we use global positioning systems instead of paper maps to navigate through roadways, NCBI has created a genome viewer tool to simplify the data-mining process.

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