Changes in Genetic Information

The amount of genetic information held within a set of human chromosomes is enormous, equal to twenty sets of Encyclopaedia Britannica. Because each of the trillions of cells in an adult body results from mitosis (except for egg and sperm), genetic information had to be replicated many times and with a high degree of accuracy. DNA can peruse itself for errors and correct them, a process termed DNA repair. Still, occasionally a replication mistake occurs or DNA is damaged, altering the

Accuracy Dna Repair
original strand.

genetic information. Such a change in DNA is called a mutation (mu-ta'shun).

Some mutations can cause devastating medical conditions; occasionally, a mutation can confer an advantage. For example, up to one percent of the individuals of some populations have mutations that render their cells unable to become infected with HIV. These lucky people, thanks to their mutation, cannot contract AIDS. The vignette to chapter 3 (p. 65) describes how this mutation changes cells.

A type of genetic change that does not affect health is called a polymorphism. Researchers are currently identifying "single nucleotide polymorphisms" — called SNPs (pronounced "snips") — that are correlated to increased risk of developing certain disorders. SNP maps are helping researchers to extract meaningful medical information from human genome sequence data.

Gene Amplification

The polymerase chain reaction (PCR) is a procedure that borrows a cell's machinery for DNA replication, allowing researchers to make many copies of a gene of interest. Starting materials are

• two types of short DNA pieces known to bracket the gene of interest, called primers

• a large supply of DNA bases

• the enzymes that replicate DNA

A simple test procedure rapidly builds up copies of the gene. Here's how it works.

In the first step of PCR, heat is used to separate the two strands of the target DNA — such as bacterial DNA in a body fluid sample from a person who has symptoms of an infection. Next, the temperature is lowered and the two short DNA primers are added. The primers bind by complementary base pairing to the separated target strands. In the third step, DNA polymerase and bases are added. The DNA polymerase adds bases to the primers and builds a sequence complementary to the target sequence. The newly synthesized strands then act as templates in the next round of replication, which is immediately initiated by raising the temperature. All of this is done in an automated device called a thermal cycler that controls the key temperature changes.

The pieces of DNA accumulate geometrically. The number of amplified pieces of DNA equals 2n where n equals the number of temperature cycles. After just twenty cycles, one mil lion copies of the original sequence are in the test tube. Table 4A lists some diverse applications of PCR.

PCR's greatest strength is that it works on crude samples of rare and short DNA sequences, such as a bit of brain tissue on the bumper of a car, which in one criminal case led to identification of a missing person. PCR's greatest weakness, ironically, is its exquisite sensitivity. A blood sample submitted for diagnosis of an infection contaminated by leftover DNA from a previous run, or a stray eyelash dropped from the person running the reaction, can yield a false positive result. The technique is also limited in that a user must know the sequence to be amplified and that mutations can sometimes occur in the amplified DNA that are not present in the source DNA. ■

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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