In this essay I will provide uses of DNA testing, explain the structure and location of DNA. I will discuss the replication sequence of DNA, mutations in gene sequence, and its molecular basis of heredity. I will summarize the function of, RNA, protein, chromosomes, and genes, as well as how genes replicate during sexual reproduction.
There are many applications for genetic testing besides the screening for genetic defects. Many people interested in their family ancestry can trace their ancestral pedigrees through a simple cheek cell smear, test are accessible for both home use, or an individual my want to procure the services of a professional genealogists. Genetic testing is also a useful resource for those who have a question about the paternity of a child or other family member.
A DNA molecule is a double stranded helix, each strand consist of one of four nucleotides, adenine which bonds with thymine and guanine which bonds with cytosine. The strand is twisted together through hydrogen bonds between the nucleotides. A phosphate and ribose backbone provides support and protection for the DNA molecule. DNA makes RNA, and RNA directs protein synthesis. The DNA molecule is located in the nucleus of cells.
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DNA molecules are synthesized through a process called replication. Replication starts by unzipping the DAN double helix with the enzyme helicase. The unzipping begins randomly along the DNA strand, forming the replication bubble, with the y-shaped ends of the bubble called the replication forks. The replication forks unwind the helix in both directions. The enzymes DNA polymerase catalyzes the synthesis of the new complementary strands adding nucleotides to the growing strand in the 5' to 3' direction. As the replication fork opens, the leading strand comprises a continuous strand of new nucleotides growing in the direction of the opening fork. The lagging strand grows away from the opening fork. DNA polymerase can add nucleotides only to the 5' end of a strand. The lagging strand is synthesized in fragments called, Okazaki fragment, which require an RNA primer to start the replication at the 3' end. DNA polymerase fills in the gaps with DNA nucleotides, the RNA primers are removed, and the fragments are joined together by ligase. Each of the parental strands has served as templates for the synthesis of the new complementary strands. After the complete DNA molecule has been divided and synthesized, the product is 2 identical copies of the original DNA molecule.
With all these copies, mutations will sometimes occur. As DNA is synthesized bases occasionally pair up improperly, this causes a potential mutation. But the enzyme DNA polymerase, can proofread the DNA strand to double check that it is the complete duplication of the original. This review assures a nearly identical duplication of DNA strand every time. A point mutation is a minor change to the DNA sequence. A point mutation changes a single nucleotide pair in a gene. Point mutations are often indistinct, and there for have a high probability of being passed on to the proceeding generations. There are three varieties of point mutations: base substitution, insertion, and deletion. In base substitution one base nucleotide is replaced by another. This is the most common mutation. The base substitution is broken down into one of several types of mutations: when the base substitution has no phenotypic effect it is known as a silent mutation. When the base substitution causes an amino acid to be exchanged for another it is know as a missense mutation. When the base substitution forms a stop codon, which results in a shortened protein that is typically nonfunctional it is know as a nonsense mutation. With Insertion and Deletion one or more nucleotide pairs are inserted or removed from the DNA molecule. If the number of nucleotides inserted or deleted is not a multiple of three, a frame shift mutation will occur. The result is an improper grouping of codons, which leads to a
different protein product, changing the meaning of the information. The resulting proteins are usually nonfunctional.
DNA can constantly repair itself, because of its most vital property: it stores its information redundantly. For each chain of DNA there is a complementary chain, which is used to double check information, providing a blueprint for DNA reconstruction.
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A PCR ampoule stores everything necessary for DNA duplication: a piece of DNA, large quantities of the four nucleotides, large quantities of the primer sequence, and DNA polymerase. The three part polymerase chain reaction is carried out in a single ampoule at different temperatures. Step one involves dividing the two DNA strands of the DNA helix by heating the ampoule to about 212 degrees Fahrenheit for 30 seconds. Step two is to bind the primer sequence to the newly separated strands. The primer sequence cannot bind to the DNA chains at temperatures as high as in step one, so the ampoule is cooled to about 113 degrees Fahrenheit. It is at this temperature, the primers bind to the ends of the DNA sections. Step two takes about 20 seconds. DNA polymerase works best at about 165 degrees Fahrenheit so for the third step the temperature is raised in the ampoule once again. Polymerase starts pairing nucleotides to the primer and subsequently develops a duplicate copy of the original template. This complete process takes only around two minutes, and can be duplicated several times in the same tube, making thousands of copies of the DNA sequence in less than an hour. PCR is a valuable research tool, allowing scientist to multiply unique regions of DNA, so they can be detected in large genomes. Researchers in the Human Genome Project are using PCR to look for markers in cloned DNA segments and to organize DNA fragments into libraries.
To the left you will see a drawing of a chromosome, which has been labeled with the individual parts of its make up. A chromatid is one of two identical copies of DNA making up a replicated chromosome, the pair of sister chromatids are joined at their centromeres. At the end of the chromatid are telomeres, a region of repetitive DNA which protects the end of the chromosome from deterioration. The p-arm is the short arm of the chromatid and the q-arm is the long arm of the chromatid. Loci are the specific gene location on the chromatid. A gene is the part of the chromatid that contains the section of DNA that controls both the coding sequence, which dictates what the gene will do, and the non-coding sequences, that controls how the gene will expressed.
DNA makes RNA, and RNA makes proteins…Transcription is the process of creating an equivalent RNA copy of a DNA sequence. A DNA sequence is read by RNA polymerase, which produces a complementary, antiparallel RNA strand. The gene's transcribed DNA sequence encodes for a specific protein from which is produced messenger RNA. In translation, messenger RNA is decoded to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein.
Fragile X syndrome is a genetic disorder caused by mutation of the Fragile X-mental retardation protein, FMR1 gene, on the X chromosome. Mutation of the FMR1 gene is found in about 1/2000 of the male population, and 1/260 of the females. Diagnoses for the complication are found in 1 out of about every 3600 males, and 1 in about every 6000 females. Fragile X syndrome has a wide range of phenotypical displays which vary from physical, emotional, behavioral, and intellectual, and range from minor in expression to sever symptoms. The syndrome is caused from the over expansion of a trinucleotide gene sequence (CGG) on the telomere of the X chromosome, which results in a inability to express the FMR1 protein required for normal neural development. There are three stages of the trinucleotide gene sequence (CGG) involved in Fragile X syndrome, which relate to the length of the repeated CGG sequence; Normal is 29-31(CGG gene sequence repeats), Permutation is 55-200 CGG gene sequence repeats) no phenotypical expression of the disorder, and Full Mutation, more than (200 CGG gene sequence repeats) Expression of the CGG gene sequence repeating codon to this degree mutes the expression of the FMR1 protein. This methylation of the FMR1 locus in chromosome band Xq27.3 is thought to express in the constriction of the telomere of the X chromosome which appears weakened at that point, it is this phenomenon that give the disorder its name.
Transmission of the fragile X chromosome is an X-linked dominant condition. Due to the fact that men acquire only one copy of the X chromosome, men with significant trinucleotide expansion at the FMR1 locus are symptomatic. Men with fragile X cannot transmit it to male offspring, because men donate the Y chromosome, opposed to an X chromosome, to their sons. Men do pass the trait to all of their female offspring, because men donate the X chromosome to their female offspring. Females have two X chromosomes, which improves the likely hood that a female will have a functioning FMR1 allele. Women possessing 1 - X chromosome with an extended FMR1 gene sequence might express some manifestation of the syndrome or be healthy, since the other X chromosome serves as a backup copy. Just 1- X chromosome is expressed in a cell because of X-inactivation. Women having one copy of the fragile X can pass the trait to their male or female offspring; either way, the child's probability of inheriting fragile X is about 50%.
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In summary, through this essay I have demonstrated uses of DNA testing, explain the structure and location of DNA. I have examined the replication sequence of DNA, mutations in gene sequence, and its molecular basis of heredity. I summarized the function of, RNA, protein, chromosomes, and genes, as well as how genes replicate during sexual reproduction.