The Replication of DNA: The DNA replication process and Genetic Disorders
Friedrich Miescher, a prominent physiological chemist from Switzerland, first discovered DNA in 1869 when he examined the remains of a used surgical bandage. He named his finding “nuclein” (“Who Discovered DNA”) since he knew its location was in the nucleus of every cell. The replication of DNA, an important process of life that can have drastic consequences if not carried out properly, involves numerous enzymes that play a critical role in the reproduction.
DNA Molecular Structure
The Structure of DNA
The structure of DNA is an intricate and sophisticated design that is unique to each individual. This molecule is found in all living organisms, and is responsible for cell formation, growth, and reproduction. (Irwin, “DNA”). The initials “DNA” stand for deoxyribose nucleic acid which is a long and thin molecule that is made of many subunits called nucleotides that are linked together in a chain by hydrogen bonds (Johnson and Raven 169). Every nucleotide is made of three parts: a phosphate group; a five-carbon sugar molecule called deoxyribose; and a nitrogen base. The sugar molecule and phosphate group are the same for each nucleotide on the DNA strand. The nitrogen base, however, can be one of four bases: adenine or guanine (the purines); or thymine and cytosine (the pyrimidines). Purines are large molecules each with a double ring of carbon and nitrogen. Pyrimidines have only one carbon and nitrogen ring (Johnson and Raven 169). Every living organism has the four bases, but the specific order they are arranged in is different in each organism (Irwin, “DNA”). Erwin Chargaff, an American biochemist working at Columbia University in New York, discovered that the amount of adenine always equals the amount of thymine, and the amount of guanine always equals the amount of cytosine. Today these rules are known as base-pairing rules, and they tell us that each DNA strand has its own specific sequence of bases (Johnson and Raven 170). “Because the sequence of bases in one chain is determined by the sequence of bases in the other, scientists say the two chains are complementary.” (Irwin, “DNA”). The sequence of bases make up a gene. A gene determines characteristics such as eye color, height, and skin and hair color (Irwin, “Genetics”). Watson and Crick, two scientists from Cambridge University, using a tin-and-wire model of molecules, discovered that, “the DNA molecule is in the shape of a double helix” (Johnson and Raven 170). The shape of a double helix is like a twisted rope ladder. The two strands of DNA are connected by alternating sugar and phosphate units. Attached to the sugar units on one side is either a purine or a pyrimidine. The same is true for the opposing side. Adenine will only be found bonding with thymine, and cytosine will only bond with guanine. For example, going from one strand to the other, the sequence of bases can be one of two: Sugar, thymine, adenine, sugar; or sugar, cytosine, guanine, sugar. These two examples confirm to the base-pairing rules. The sequence of adenine, thymine, cytosine and guanine on one strand will always determine the sequence of bases on the other. Watson and Crick first proposed that the complementary property of DNA is used as the basis for synthesizing a new strand of DNA (Johnson and Raven 170-172).
The Replication Process
In order for genes to be passed on and enable life to continue, an important process called DNA replication must take place inside every living cell (“DNA Replication”). “DNA replication is the process of making copies of strands of DNA. Existing DNA is used as a template for synthesizing the new strands” (“GOT It! ™ Terms A-Z”). Before replication begins, the two strands of DNA must be pulled apart so that the sequence of bases on each strand can be copied. An enzyme, helicase, runs along the strand of DNA, thus tearing it in two by breaking the hydrogen bonds that hold the double helix shape of DNA. Once the strands are separated, more enzymes keep the strands apart, to prevent them from twisting again. “The point at which the double helix separates is called the replication fork because of its “Y” shape” (Johnson and Raven 172). At the replication fork, an enzyme known as DNA polymerases moves along each of the DNA strands and adds nucleotides that correspond to the base-pairing rules. Once the DNA polymerases is finished adding nucleotides, it stays attached to the DNA strand until a signal is dispatched that tells it to detach. Although DNA polymerases also serve the role of making sure that nucleotides are paired correctly, sometimes an error is made. This is called a mutation, but it is a rare event that happens approximately once per one billion nucleotides (Johnson and Raven 172). When a gene is mutated the change is permanent, but it may or may not alter the organism in some way (Irwin, “Genetics”). Once the replication is completed an enzyme known as DNA ligase attaches the old strands of DNA to the new ones. “…the result is two DNA molecules, each composed of a template [original] strand and a new strand” (Cummings 167).
Extra Chromosome 21
Genetic Mutations (Down Syndrome)
Genetic mutations may alter an organism in a beneficial way, but more often than not the mutation is fatal or causes some type of physical, mental, or emotional impairment. Trisomy 21, more commonly known as Down syndrome, is a genetic disorder caused by mutations in your genes that erroneously add an extra chromosome 21 (Cummings 304). A chromosome is a tightly coiled structure made of DNA that is found in all cells (“GOT It! ™ Terms A-Z”). Normally, an individual is given 23 pairs of chromosomes, each pair made of two chromosomes. The pairs are numbered 1-23, and whenever a pair accidentally “receives” an extra chromosome due to a DNA mutation, a genetic disorder develops. An individual with Down syndrome will exhibit some physical aberrations. One characteristic is, “a distinctive fold of skin, known as an epicanthic fold, in the corner of the eye” (Cummings 127). Other aberrations include a wider and flatter skull than normal, as well as an iris that contains spots, and a slightly protruding tongue, which causes the mouth to stay slightly open (Cummings 128). Down syndrome provides many disadvantages to the individuals who are born with it. It is a main cause of mental retardation as well as heart defects in the United States. All individuals with Down syndrome exhibit some type of mental retardation and 40 percent of them also have congenital heart defects. Children that have Down syndrome are 15 times more likely to develop respiratory infections and fall victim to leukemia. Individuals with a family history of Down syndrome are also more prone to developing Alzheimer disease and are more likely to develop the disease at an older age (Cummings 127-129). Besides random gene mutations, there are several other factors that may influence whether an individual will have Down syndrome. “Radiation, viral infections, hormone levels, and genetic predisposition…” (Cummings 128) are some factors that may contribute to an increased risk. The risk of giving birth to a child with Down syndrome increases as the mother ages. At an age of 20, the risk is 0.005 percent as opposed to three percent at age 45. The chance of an irregular pairing of chromosomes increase as a woman nears the end of her reproductive stage. Although Down syndrome is one of the most common chromosome defects in humans, it is only one of the thousands that may occur when a mutation occurs (Cummings 127).
Missing a Sex Chromosome
Genetic Mutations (Turner Syndrome)
Turner Syndrome is another common genetic disorder that is manifested when an individual is missing one or part of the 23rd pair of chromosomes, also known as the sex chromosomes. Every individual has two sex chromosomes. “XX” means the fetus will be a female and “XY” means it will be a male. Turner syndrome means that the individual is missing part of the “X” sex chromosome (“Turner Syndrome”). Like individuals with Down syndrome, those with Turner syndrome also exhibit some physical aberrations. These include a smaller jaw; a broader chest; and a low hairline in the back of the neck. There are numerous health consequences as a result of Turner syndrome. Women with this disorder may experience kidney abnormalities; an increased risk of heart defects; a lack of a menstruation cycle in 99% of women due to underdeveloped ovaries; infertility; and an imperfect sexual growth in up to 95% of women (“Turner Syndrome”). Although this disorder has many more disadvantages in a physical sense than Down syndrome; it does not usually affect the brain, and individuals with this disorder do not exhibit any type of mental retardation, and usually have a normal IQ. This is one genetic disorder that has baffled scientists. It does not occur as a result of increasing maternal age, and scientists believe that it only occurs by a random chance. If a couple has a baby with Turner syndrome, this is a very good indication that someone else in the family may be prone to a pregnancy in which the baby will exhibit signs of Turner syndrome. Like any genetic disorders, there is no cure for Turner syndrome because the mutations in the DNA keep repeating and they are very difficult to correct (“Turner Syndrome”).
DNA: A Brief Summary
The important discovery of DNA was done by Friedrich Miescher, a physiological chemist from Switzerland who examined a used surgical bandage in 1869 (“Who Discovered DNA). DNA is an important genetic material found in the nucleus of every cell. Genetic mutations may occur if the replication process is carried out erroneously. The replication process of DNA, a complicated process involving numerous enzymes with critical roles, is a vital process that must be carried in order for life to continue. Today, we know much about DNA and the role it plays in every living thing, yet there are still many things the biologists and other scientists have yet to discover.
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