Genetics

D

Gel Electrophoresis

PCR and recombinant DNA techniques create large amounts of DNA segments. To study the structure of these segments, researchers use a process known as gel electrophoresis. This technique can be used to identify genes in humans that have previously been identified in other organisms, such as fruit flies. It can also be used to compare the DNA found from blood or hair samples at a crime scene with the DNA of a suspect in the crime. In gel electrophoresis, restriction enzymes break up the DNA under study into restriction fragments of varying lengths. Solutions containing these fragments are placed within a thick gel. An electric current is applied to the gel, causing one end of the gel to have a positive charge and the other to have a negative charge. All of the restriction fragments begin to move from the negative end of the gel toward the positive end. The smaller fragments move faster than the larger fragments. When the current shuts off, typically after several hours, the DNA fragments have spread out across the gel, with the smaller ones closer to the positive end. The dispersed fragments display a pattern resembling a bar code. Each bar in this pattern contains DNA fragments of a certain size. Scientists can identify specific restriction fragments by their location on the gel. A complementary sequence of DNA can be used as a probe to find a restriction fragment on the gel that has a particular nucleotide sequence. Scientists may use DNA found in blood at a crime scene as the probe to see if it pairs up with any of the DNA fragments in the gel electrophoresis. If pairing occurs, the DNA from the crime scene is from the same person who provided the DNA sample for the gel electrophoresis.

E

DNA Sequencing

Once an interesting piece of DNA has been isolated or identified, scientists often need to determine if the sequence of nucleotides in the fragment is related to known genes and to determine what kind of protein it might make. Scientists use DNA sequencing to detect genetic mutations linked to diseases such as cystic fibrosis. Scientists have also used this method to alter the sequence of a gene and study the function of the resulting protein. In DNA sequencing, scientists create many copies of a single-stranded DNA fragment that will be used to synthesize a new DNA strand. An equal number of copies of the fragment are placed into four different test tubes to act as the template for the synthesis of a new strand. The enzyme DNA polymerase and free nucleotides are added to each test tube. Each test tube also receives one type of dideoxy nucleotide—a nucleotide that closely resembles either adenine, guanine, thymine, or cytosine. These nucleotides can attach to the end of the new complementary DNA strand, but they cannot bind to anything else, thus they terminate the synthesis of the new DNA strand.

DNA polymerase uses the free nucleotides to build a complementary DNA strand. If the original DNA fragment contains guanine, DNA polymerase delivers a cytosine dideoxy nucleotide to pair with the guanine base on the original strand. The cytosine links with the growing chain of nucleotides on the complementary DNA strand, but it is unable to bind with any other nucleotide. The newly formed DNA fragment terminates with the cytosine dideoxy nucleotide at the end of the chain. The reactions in each of the four test tubes produce a series of DNA fragments in which the new strands terminate at a known base. Each test tube produces fragments that differ in length from the other test tubes. The newly formed fragments are sorted in an electrophoresis gel that can detect differences as small as one nucleotide in length. By analyzing these sorted fragments, scientists can determine the complementary base sequence for the original DNA fragment. This sequencing method has become a routine laboratory technique, automated with specialized machines and computers that can prepare DNA samples and read nucleotide sequences far faster and more accurately than people can.

F

Gene Chip

The gene chip, also known as a DNA chip or DNA microarray, is a thumbnail-sized chip of glass or silicon that carries DNA instead of electronic circuits. Gene chips can identify the genes that are active within a cell and help identify mutated genes. In one application, scientists take a single strand of DNA that contains a defective gene and use ultraviolet light to attach the strand onto a glass or silicon chip. A second DNA strand isolated from a patient is attached to fluorescent markers and deposited onto the chip. If the patient’s DNA strand bonds with the DNA already bonded to the chip, then the individual’s DNA contains the defective gene. When the DNA on the chip pairs with the fluorescent DNA, it develops a fluorescent glow that can be viewed with a microscope and interpreted by a computer. A diagnostic gene chip may soon be manufactured to hold the DNA sequences of all the known disease-causing genes, making diagnosis for genetic disorders fast, reliable, and inexpensive. Gene chips also distinguish between active DNA—DNA that is being transcribed to produce mRNA—and inactive DNA. Researchers use these chips to learn how the transcription of a group of genes is affected when cells are exposed to a drug.

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V

Human Genetics

Our understanding of human genetics builds on a foundation of information obtained from studying other organisms. Until the 1980s, genetic researchers focused their work on the fundamental genetic processes in simpler organisms, such as bacteria, plants, and fruit flies. Today an expanded array of tools available for the direct study of human genetics attracts scientists from around the world to collaborate to identify and study every human gene.

The genetic principles that Mendel first discovered in plants apply to humans as well. As in all other life forms, the DNA found in human cells encodes the proteins that are essential for reproduction, survival, and growth. The unique structure and behavior of DNA ensures that human traits are passed from generation to generation and accounts for why parents, children, and grandchildren often have similar facial features, hair color, height, and athletic or artistic abilities (see Heredity). Yet each of us inherits a unique genetic legacy from our parents and more distant ancestors. With the exception of identical twins, no two people have the exact same combination of alleles for the estimated 20,000 to 25,000 human genes.

Some human traits are controlled largely by a single gene. But most inheritable characteristics are influenced by a number of genes that interact in a complex fashion. Also, personal experiences and environmental factors combine with genetic influences to shape certain traits, including vulnerability to disease and characteristics such as intelligence, emotions, talents, and personality.

A

Human Genome

Human genes reside on 23 pairs of chromosomes found in the nucleus of every body cell except gamete cells. In each pair, one of the chromosomes is inherited from the mother and the other is passed down from the father. About 2 m (7 ft) of DNA is packaged into each chromosome. All of the genes carried on chromosomes form the human genome. A lesser amount of DNA can be found in mitochondria, cellular organelles responsible for creating the energy used in cell activities.

All but one of these 23 pairs are composed of chromosomes nearly identical in shape. Each of these 22 chromosome pairs, known as autosomes, contains the same genes (although they likely carry different alleles). The autosome pairs vary considerably in length, and scientists number them according to their relative size: Pair number 1 is the longest pair and pair number 22 is the shortest.

Rounding out the human genome is the 23rd pair of chromosomes, known as the sex chromosomes, which determine the sex of an individual. Females inherit two X chromosomes, a matched pair carrying the same genes. One X chromosome is inherited from the mother and one X chromosome is inherited from the father. Males inherit an X chromosome from their mother and a Y chromosome from their father. The Y chromosome is shorter than the X chromosome and bears far fewer genes. One gene on the Y chromosome causes an embryo to develop as a male rather than a female.

Humans produce gamete cells for sexual reproduction. These gametes contain a haploid number of chromosomes—23 chromosomes instead of the full complement of 46. Female gamete cells mature into eggs, with each egg containing chromosomes 1 through 22 and an X chromosome. Males produce gametes that mature into sperm, and each sperm cell has a single set of chromosomes 1 through 22 and either an X or a Y chromosome. During fertilization, an egg that joins with a sperm containing a Y chromosome develops into a male, and an egg fertilized by a sperm containing an X chromosome develops into a female.