Difference between revisions of "Deoxyribonucleic acid"

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(Expanded and re-worded somewhat; I would appreciate advice on the talk page about how I can further clarify this article.)
(Modern understanding)
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==Modern understanding==
 
==Modern understanding==
 
[[Image:818.gif|right|thumb|Structure of DNA.]]
 
[[Image:818.gif|right|thumb|Structure of DNA.]]
Small lengths of DNA called [[gene]]s serve as the instructions for the body to carry out its functions and give rise to the physical traits of the organism. <ref>"Eye-color genes, through the proteins they encode, direct the amount and placement of melanin in the iris." [http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fgenetics%2Fans_044.html Ask A Scientist - Genes and eye color]</ref> DNA is packaged into [[chromosomes]]. Each individual human being has 23 pairs of [[chromosome]]s, where one set is inherited from his/her mother and the other set is inherited from his/her father. 22 of these chromosomes are referred to as [[autosome]]s, while the remaining chromosomes are the sex chromosomes.  Males possess a single X chromosome and a single Y chromosome; whereas females possess a pair of X chromosomes.  In total, it is estimated that there are roughly 23000 protein-coding genes in the human genome.
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Small lengths of DNA called [[gene]]s serve as the instructions for the body to carry out its functions and give rise to the physical traits of the organism. <ref>"Eye-color genes, through the proteins they encode, direct the amount and placement of melanin in the iris." [http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fgenetics%2Fans_044.html Ask A Scientist - Genes and eye color]</ref> DNA is packaged into [[chromosomes]]. Each individual human being has 23 pairs of [[chromosome]]s, where one set is inherited from his/her mother and the other set is inherited from his/her father. 22 of these chromosomes are referred to as [[autosome]]s, while the remaining chromosomes are the sex chromosomes.  Males possess a single X chromosome and a single Y chromosome; whereas females possess a pair of X chromosomes.  In total, it is estimated that there are roughly 20300 protein-coding genes in the human genome; however, due to mRNA splicing, it is estimated that these genes encode for over 1 million different protein products.
  
 
[[Prokaryotic DNA]] is circular (a closed loop).  Whereas, [[eukaryotic DNA|eukaryotic chromosomes]] are linear (with ends), with the notable exception of [[Mitochondrial DNA]] and plastid (chloroplast) DNA, which is separate from the DNA in the nucleus<ref>Mitochondria and chloroplasts are thought to have originated as prokaryotic cells living symbiotically inside of primitive eukaryotic cells, this is called the [[endosymbiotic hypothesis]]</ref>.  The ends of eukaryotic chromosomes are protected by [[telomere]]s, which are always tightly condensed except during S phase of [[mitosis]].<ref>Campbell, Neil A, et. al. ''Biology''. 6th ed. San Francisco: Benjamin Cummings, 2002. 299, 530-31.</ref>
 
[[Prokaryotic DNA]] is circular (a closed loop).  Whereas, [[eukaryotic DNA|eukaryotic chromosomes]] are linear (with ends), with the notable exception of [[Mitochondrial DNA]] and plastid (chloroplast) DNA, which is separate from the DNA in the nucleus<ref>Mitochondria and chloroplasts are thought to have originated as prokaryotic cells living symbiotically inside of primitive eukaryotic cells, this is called the [[endosymbiotic hypothesis]]</ref>.  The ends of eukaryotic chromosomes are protected by [[telomere]]s, which are always tightly condensed except during S phase of [[mitosis]].<ref>Campbell, Neil A, et. al. ''Biology''. 6th ed. San Francisco: Benjamin Cummings, 2002. 299, 530-31.</ref>
  
[[Mutation]]s are simply variations in DNA sequence between individuals. Most mutations in DNA are termed "neutral" because they do not affect the [[phenotype|observable traits]] of the individual organism. Others can have beneficial effects and some can disrupt important functions.  It is mutations in DNA, giving rise to novel alleles ("versions" of a gene), which cause phenotypic variation between individuals of a particular species.  For instance, in humans, single-gene mutations are responsible for differences in [[ABO blood type]], eye color, hair color, and even the ability to taste certain molecules.  It is differential distribution of neutral mutations, resulting in individual to individual "restriction fragment length polymorphisms", that allows for DNA fingerprinting (see below).  Sadly, there are also thousands of mutations known to cause human disease; notably sickle-cell anemia, a few rare forms of autism, and (when the mutations spontaneously occur in a somatic cell) various types of cancer.  That said, most phenotypic traits and hereditary disease conditions arise as the net effect of several different genetic variations.
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[[Mutation]]s are simply variations in DNA sequence between individuals. The vast majority of spontaneous mutations in DNA are called "neutral" because they do not affect the [[phenotype|observable traits]] of the individual organism. Others can have beneficial effects and some can disrupt important functions Differential distribution of one type of neutral mutations, resulting in individual to individual "restriction fragment length polymorphisms", is the basis for modern DNA fingerprinting (see below).  It is mutations in DNA, giving rise to novel alleles ("versions" of a gene), which cause phenotypic variations between individuals of a particular species.  For instance, in humans, single-gene mutations are responsible for differences in [[ABO blood type]], eye color, hair color, and even the ability to taste certain molecules.  Sadly, there are also thousands of mutations known to cause human disease; notably sickle-cell anemia, a few rare forms of autism, and (when the mutations spontaneously occur in a somatic cell) various types of cancer.  That said, it is important to note that most phenotypic traits (e.g. height) and hereditary diseases arise as the net effect of several different genetic variations.
  
[http://www.ncbi.nlm.nih.gov/omim OMIM] (Online Mendelian Inheritance in Man), is an online database of genes known to be mutated in disease states.
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[http://www.ncbi.nlm.nih.gov/omim OMIM] (Online Mendelian Inheritance in Man), is an online database of genes known to be mutated in disease states.  [[Mendelian inheritance]] refers to the inheritance pattern observed in traits that are determined by a single gene.  This pattern was first discovered by [[Gregor Mendel]], an Austrian monk, during his work with pea plants from 1856-1863.  Although Mendel published his work in 1866, it's significance was largely ignored until it was rediscovered in the early 20th century; more than two decades after his death.
  
 
==DNA fingerprinting==
 
==DNA fingerprinting==

Revision as of 19:28, January 26, 2012

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Deoxyribonucleic acid (DNA) is a biomolecular polymer of deoxyribonucleotides. It exists most commonly as a double-stranded helix comprising two anti-parallel complimentary DNA molecules held together by hydrogen bonds, this is commonly annotated as "dsDNA" (double-stranded DNA)[1]. The nucleotides which are the "building blocks" of DNA consist of three principle parts: A nitrogenous base (the "letters" in the sequence), a sugar, and a phosphate group. When nucleotides are polymerized into DNA strands, they are bonded together in a chain, with the phosphate group of one nucleotide binding to the ribose of the next.

Complimentary DNA strands "compliment" each other by having compatible sequences, allowing the nitrogenous bases on one strand to form hydrogen bonds with the corresponding (complimentary) bases on the other strand. There are four nitrogenous bases, Adenonine (A), Cytosine (C), Guanine (G), and Thymine (T). Under the base pairing rules, A pairs with T and G pairs with C.

DNA contains genes that encode for proteins, which are produced by the DNA first being transcribed into mRNA and then translated into polypeptides (this process is called the central dogma of molecular biology). All organisms (plants, animals, bacteria, etc.) use DNA as their hereditary material. Offspring of sexually reproducing organisms contain DNA from both parents.

History

In the late 19th century Friedrich Miescher, a Swiss biochemist, discovered an unusual acid in the nuclei of cells. The acid was named deoxyribonucleic acid, or DNA. In 1944 the American biologists, Alfred Hershey, Thomas Gilmore and Martha Chase used experiments with bacteria and bacteriophages to show that DNA passed genes from one generation to the next.

At that time, it was unclear how this simple molecule could hold all the complex information controlling the development of humans, animals and plants. Scientists knew it was made of four chemical bases called adenine (A), thymine (T), guanine (G) and cytosine (C), plus phosphoric acid and a sugar. They also knew that the ratios of A and T as well as G and C were always the same, but they did not know the rules that controlled the arrangement.

British scientists Rosalind Franklin and Maurice Wilkins passed X-rays through DNA to study the patterns made when the crystals diffracted them. From studying photographs of patterns, Rosalind Franklin concluded that DNA must be be a helix. James Watson and Francis Crick, working in Cambridge, used this information to help them solve the puzzle of DNA structure. They built a model showing that if A always paired with T and G paired with C, DNA must be like a ladder made of two strands twisted together in a double helix. The sugar and phosphoric acid were the sides of the ladder, and the rungs were the paired bases that were held together through hydrogen bonding.

Watson and Crick suggested that DNA could unzip itself into two separate strands, and each strand could act as a pattern to grow a new strand. Crick showed later that areas of the DNA known as genes worked in groups of three to code for amino acids, the building blocks of proteins. These groups are called codons. They make about fifty thousand different types of protein, which make all the different types of cell in the body. Indian biochemist Har Gobind Khorana made all the possible codons and worked out which codons controlled which amino acid. Most codons are redundant and code for the same amino acids, these mostly are different in only the third base pair. This means that differences in genotype can build up in the third position (thereby changing the genotype) without changing the protein (keeps the same phenotype).

If the DNA in one cell was stretched out, it would be about three feet long. Although DNA has a very simple structure, it can carry an enormous amount of information. Scientists do not yet understand the function of all DNA, but in 1991 a project called the Human Genome Project began to use computers to map the three billion base pairs which make up the 46 human chromosomes.

Modern understanding

Structure of DNA.

Small lengths of DNA called genes serve as the instructions for the body to carry out its functions and give rise to the physical traits of the organism. [2] DNA is packaged into chromosomes. Each individual human being has 23 pairs of chromosomes, where one set is inherited from his/her mother and the other set is inherited from his/her father. 22 of these chromosomes are referred to as autosomes, while the remaining chromosomes are the sex chromosomes. Males possess a single X chromosome and a single Y chromosome; whereas females possess a pair of X chromosomes. In total, it is estimated that there are roughly 20300 protein-coding genes in the human genome; however, due to mRNA splicing, it is estimated that these genes encode for over 1 million different protein products.

Prokaryotic DNA is circular (a closed loop). Whereas, eukaryotic chromosomes are linear (with ends), with the notable exception of Mitochondrial DNA and plastid (chloroplast) DNA, which is separate from the DNA in the nucleus[3]. The ends of eukaryotic chromosomes are protected by telomeres, which are always tightly condensed except during S phase of mitosis.[4]

Mutations are simply variations in DNA sequence between individuals. The vast majority of spontaneous mutations in DNA are called "neutral" because they do not affect the observable traits of the individual organism. Others can have beneficial effects and some can disrupt important functions Differential distribution of one type of neutral mutations, resulting in individual to individual "restriction fragment length polymorphisms", is the basis for modern DNA fingerprinting (see below). It is mutations in DNA, giving rise to novel alleles ("versions" of a gene), which cause phenotypic variations between individuals of a particular species. For instance, in humans, single-gene mutations are responsible for differences in ABO blood type, eye color, hair color, and even the ability to taste certain molecules. Sadly, there are also thousands of mutations known to cause human disease; notably sickle-cell anemia, a few rare forms of autism, and (when the mutations spontaneously occur in a somatic cell) various types of cancer. That said, it is important to note that most phenotypic traits (e.g. height) and hereditary diseases arise as the net effect of several different genetic variations.

OMIM (Online Mendelian Inheritance in Man), is an online database of genes known to be mutated in disease states. Mendelian inheritance refers to the inheritance pattern observed in traits that are determined by a single gene. This pattern was first discovered by Gregor Mendel, an Austrian monk, during his work with pea plants from 1856-1863. Although Mendel published his work in 1866, it's significance was largely ignored until it was rediscovered in the early 20th century; more than two decades after his death.

DNA fingerprinting

DNA can help police track down criminals if an attacker leaves something like hair or blood at a crime scene. Everybody’s DNA is unique unless they are identical twins, and the genetic ‘fingerprint’ of this material can be enough to confirm if a suspect was at the scene. Genetic fingerprinting can also show if somebody is closely related to somebody else. Archaeologists used samples from living relatives to identify whether bodies found buried in a forest in Russia were the remains of the tsar and his family, killed during the 1919 Russian Revolution.

As well as showing how different we all are, DNA shows how much we are all the same. Only one small part of one chromosome distinguishes a boy from a girl. Genes give us different colored eyes, hair and skin, but 99.5% of any human's DNA is in the same order as everybody else’s.

Human DNA is made up of the same components as the DNA of other organisms, with most organisms even sharing the same genes which code for the same proteins. This is why even fruit flies and mice can be used as model organisms in order to help better understand human genetic diseases.

External links

References

  1. Single-stranded DNAs (ssDNA) do exist in nature (notably, certain bacteriophages possess ssDNA genomes), however ssDNAs cannot be replicated without first synthesizing the complimentary strand to use as a template.
  2. "Eye-color genes, through the proteins they encode, direct the amount and placement of melanin in the iris." Ask A Scientist - Genes and eye color
  3. Mitochondria and chloroplasts are thought to have originated as prokaryotic cells living symbiotically inside of primitive eukaryotic cells, this is called the endosymbiotic hypothesis
  4. Campbell, Neil A, et. al. Biology. 6th ed. San Francisco: Benjamin Cummings, 2002. 299, 530-31.