DNA
From CreationWiki, the encyclopedia of creation science
Deoxyribonucleic Acid (DNA) is the substance used to encode heritable molecular information in all living cells. It typically exists as two long chains that are bound together, and twisted into the shape of a helix (shown at right). The DNA in a single human cell would extend over 2 meters in length if attached end-to-end.
The individual nucleotides used to construct DNA or RNA are distinguished by the nitrogenous base that is attached to each. The four bases used in DNA are Adenine, Cytosine, Guanine, and Thymine. The corresponding names of the nucleotides are representative of the base it contains, such as Adenosine triphosphate contains Adenine. When diagramming the sequence of nucleotides within genes, they are typically abbreviated by the letters A,T,C, and G, which correspond to the first letter in the name of the various nucleotides shown in the following table.
| Nucleotide | Abbreviation |
| Adenosine triphosphate | ATP |
| Cytidine triphosphate | CTP |
| Guanosine triphosphate | GTP |
| Thymidine triphosphate | TTP |
Contents |
History
In the 1940’s Erwin Chargaff discovered what came to be known as Chargaff’s rule. He found that in DNA, the number of nucleotides possessing adenine and thymine nitrogenous bases was about equal. Also, the nucleotides with cystosine and guanine was about equal.[1]
After the discovery of Chargaff’s rule two scientists decided to try crystallize the DNA molecule. These scientists names are Rosalind Franklin and Maurice Wilkins. If these two could figure out how to get DNA to crystallize, then an x-ray pattern could be generated leading to an understanding of how DNA works. They were successful in their experimentation and the shape that they found was like a ladder between two strands that are side by side. The x-ray pattern also showed by an “X” shape that DNA had a helical shape.[2]
In 1953, two scientists wanted to put together a model of DNA. The two scientists came up with an accurate model from the picture of the x-ray that Rosalind Franklin and Maurice Wilkins came up with. The structure that Watson and Crick came up with was accurate enough that it has not been significantly modified. The double helix that was formed with little rungs that connect the two strands together. When Watson and Crick began studying how to begin their model of DNA they came across some problems. One of these problems was that how can you bond two bases when they were not the same size. They then discovered that because of Chargaff’s rule that would be able to pair adenine with thymine and guanine with cystosine. With this technique they would be able to make the DNA structure look stable and uniform. This structure would then compliment each other and look very uniform. [3]
Physical and Chemical Properties
DNA molecules are composed of two stranded molecules. The DNA strands are intertwined in the shape of a double helix. The strands are attached together by hydrogen bonds. Every strand of DNA is composed of adenine, cytosine, guanine, and thymine. The chains of DNA are the chemical building blocks. These are known as nucleotides.
DNA is based off of the four basic bases. One strand is composed of one of the four main bases and then is complimented by one of the bases. When two of the nucleotides that are paired together are called a base pair. Each of the nucleotides that are paired form hydrogen bonds. When the adenine and thymine are paired two hydrogen bonds are formed. When a cysteine and guanine are paired three hydrogen bonds are then formed. The reason a DNA molecule would be more strong would be because the content and the length of the DNA molecule affects it.
In some cases the DNA molecule is not a double helix it may appear as a non helical or single stranded form of DNA. This form of DNA is found in some viruses. A virus that contains a single stranded form of DNA would mutate frequently that it would at any other time, when it was in a double helix form. The species of virus that contain one stranded DNAs adapt faster so that they avoid becoming extinct. Viruses are the only organism that carry single stranded DNA.[4]
Strand Direction and Complementarity
DNA that makes up chromosomes and plasmids exists as two parallel strands. Each nucleotide in the strand it attached to the next through phosphate bonds. In addition, the two strands are also attached to each other at each nucleotide in the sequence through hydrogen bonding of the bases. In DNA, ATP always bonds to TTP in the adjacent strand, and CTP always pairs with GTP. This is known as base pairing, and the sequence of nucleotides on the parallel strands are said to be complementary.
Each DNA strand runs from 5’ to 3’ (5 prime to 3 prime). DNA is known as the double helix because it is composed of two strands. There are two types of strands, the template strand, or antisense strand, and the coding strand, or sense strand. The template strand contains the information for various genes within the DNA. It also contains anticodons. The coding strand also contains codons.[5]
The arrangement of the DNA strands is in a double helix structure, which reveals that the nucleotides are heading one way on one strand and heads the other way on the other strand. The strands are arranged in a way called the antiparallel place because if the strands had the same contents and not complementary the cell could only translate one of them. If the sequence is able to be translated or has already been translated then it is known as “sense”. The complement to the “sense” sequence would be the “antisense” sequence. [6]
DNA Sequencing
DNA sequencing is a technique used to determine the exact order of the building blocks (nucleotides) that comprise DNA. One such technique is reliant on the use of dideoxy nucleotides that will terminate the polymerization of the DNA strand at each type of nucleotide, and radioisotope-labeled nucleotides to visualize the pool of fragments.
Nucleotides are distinguished by the nitrogenous bases they contain and abbreviated by the letters A, C, G, and T (these can be easily remembered with the saying "God Authored The Code"). Therefore, DNA sequencing data is typically a long string of these letters (i.e. ACGCTCATCGTAA).
The Human Genome Project discovered that to improve the DNA sequencing they had to speed up the process of DNA sequencing. The standard method that was used in DNA sequencing is based on the separating of DNA fragments. The way the DNA separated into fragments is by gel electrophoresis. These methods were too labor intensive and cost too much money. To make things cheaper they decided to use gel-based sequencers, which are multiple tiny tubes run what they used to perform electrophoresis. The separations from this method are much faster because they have higher electric fields to complete the DNA sequencing in a shorter amount of time and this was also more cost effective.[7]
Gene Expression
- Main Article: Gene expression
The molecular information encoded in DNA exists as subunits called genes. A gene is basically a short stretch of DNA on a chromosome that is identified by specific sequences of nucleotides that designate the beginning and ending point of the gene. The nucleotides ATG signal the beginning of a gene, and either TAA, TAG, or TGA signal the end. Each human chromosome contains about 1000 genes.
These genes provides the cell with instructions on how to make specific proteins. The code is specified by the sequence or linear arrangement of the 4 different nucleotides that make-up DNA. An organelle in the cell called a ribosome reads this coded instruction and translates it into a sequence of amino acids that are used to make a protein. This process is known as gene expression.
Related References
- DNA History Library Thinkquest
- DNA Wikipedia
- Strand Direction University of Texas Health Center
- DNA Sequencing U.S. Department of Energy Office of Science, Office of Biological and Environmental Research, Human Genome Program
- Purves, William. Life: The Science of Biology. July 2005.
- Mythical, bloodsucking 'chupacabra' found in Texas just a plain old coyote The Associated Press
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