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Deoxyribonucleic acid

2008-07-01T17:19:12Z

Digitalchameleon:

[[Image:799.gif|right|thumb|300px]]
'''Deoxyribonucleic acid''' (DNA) is an [[organic compound|organic]] [[chemical compound]] made up of [[molecule]]s shaped like a [[double helix]], a continuously twisted ladder. The individual monomeric building blocks of DNA (and other nucleic acids) are [[nucleotide]]s, which themselves consist of three principle moieties: A nitrogenous base, a sugar, and a phosphate group.

DNA carries the [[gene|genet]]ic coding for the [[protein]]s that form the bodies of living [[organism]]s. All organisms contain DNA, and the DNA of a sexually reproducing organism is a mix of its parents' DNA.

==History==
In the late 19th century [[Friedrich Miescher]], a Swiss [[biochemist]], discovered several different proteins and an unusual [[acid]] in the [[nucleus]] of cells. The acid was named deoxyribonucleic acid, or DNA. In 1944 two American [[biologist]]s, [[Alfred Hershey]] and [[Martha Chase]] used experiments with [[bacteria]] and [[bacteriophage]]s to show that DNA passed [[gene]]s 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 there was always the same amount of A as T and of G as C in cells, but they did not know the rules that controlled the arrangement.

British scientists [[Rosalind Franklin]] and [[Maurice Wilkins]] passed [[X-ray|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.

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 each pair of genes worked in a group of three making the code for amino acids, the building blocks of proteins. These groups are called [[codon|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]].

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 all the DNA they see, 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 [[chromosome|chromosomes]].

==Modern understanding==
[[Image:818.gif|right|thumb|Structure of DNA.]]
Small parts 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> Genes are 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 chromosome determines gender (sex chromosome).

[[Prokaryotic DNA]] is circular (a closed loop), while [[Eukaryotic DNA]] is linear (with ends) with the exception of [[Mitochondrial DNA]], which is circular too. DNA in [[prokaryote]]s usually consists only of one closed loop chromosome.<ref>Campbell, Neil A, et. al. ''Biology''. 6th ed. San Francisco: Benjamin Cummings, 2002. 299, 530-31.</ref>

DNA changes account for differences between individuals. However, they can also cause medical problems such as [[cancer]]

DNA is [[transcription|transcribed]] into [[mRNA]], which is in turn [[translation|translated]] into [[proteins]] built from [[amino acids]]. Additionally, DNA maybe transcribed into functional RNAs (ribosomal and transfer RNA) that do not undergo translation. [[RNA]] can also be reverse-transcribed back into DNA, which is the called [[Complementary DNA]] or [[cDNA]].

==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.

Every living thing on earth has DNA that uses the same basic chemicals as human DNA, but because their chemicals are in a different order, the DNA of a worm or a tree encodes for different proteins, and those proteins make different cells. Evolutionists believe that all these cells developed from the first single-cell creatures formed three thousand million years ago, evolving gradually into new species to adapt to their environment. Creationists believe that God created living things as distinct [[baraminology|"kinds"]], and point to [[information]] theory to show that the information on the DNA could not have arisen by chance.

==External links==
*[http://www.blackwellpublishing.com/trun/artwork/Animations/cloningexp/cloningexp.html Animated guide to DNA cloning]

== References ==
<references/>

[[Category:Genetics]]

Starlight problem

2008-07-01T17:10:43Z

Digitalchameleon:

The '''starlight problem''', or '''starlight travel-time problem''' is an apparent problem for the [[Young Earth Creationism|young-Earth creationist]] argument that the universe is only 6,000 years old, in which the age of the universe is based primarily on the [[Genesis]] narrative.
The problem is that if the universe is only 6,000 years old, one has to explain how light from stars more than 6,000 light years from Earth has reached us in the time available.

Despite this frequently being put as an argument against young-Earth creationism, light travel time is also a problem for the [[Big Bang theory]].

Creationists have proposed a number of solutions. The currently-favoured solutions involve time dilation, in which time passed slower on Earth than in other parts of the universe.
[[image:Crab composite cx.jpg|right|thumb|250px|The Crab nebula, a supernova remnant]]

== The size of the universe ==

Using parallax calculations, the [[Milky Way]] galaxy alone can be directly observed to have a diameter of approximately 80-100,000 light years. Calculations based on the observed brightness of [[supernova]]e of known energy output can determine the distance to more distant objects. The most distant object known is a star cluster approximately thirteen billion light-years away from Earth, observed by the [[Hubble Telescope]] using gravitational lensing.<ref>Amos, 2004</ref>

Although creationists have at times disputed the size of the [[universe]], virtually none propose that it is less than 12,000 light years across.<ref>Fangrad, Richard, 2007.</ref>

== Solutions to the problem ==

Several solutions have been proposed by creationists to explain how a young universe may be reconciled with these observations.

They also point out that there is a logical fallacy in the argument, as the observation that many stars are millions of light years away is one of ''distance'', not ''time''. No matter how reasonable, it is a ''deduction'', not an ''observation'', that the starlight from stars millions of light years away would have required millions of years to reach Earth.

=== Light created in transit ===
Some creationists have proposed that the light we see from stars more than 6,000 light years away was not emitted by those stars, but was created 'in transit' by God.
However, most creationists reject this explanation, as the light contains images of events that would therefore never have actually happened, including [[supernova]]e, meaning that we are seeing an image of a star exploding, even though the star never existed.
This would make the creator a deceiver.<ref>Humphreys, 1994, p.43-46.</ref>

=== Moon-Spencer theory ===
Some creationists promoted an idea by Parry Moon and Domina Spencer that light somehow takes a shortcut through "Riemannian Space", taking no more than 15 years to reach Earth from the outer limits of the universe.
However, this idea never really caught on and appears to no longer have adherents.<ref>Humphreys, 1994, p. 46.</ref>

=== Decrease in the speed of light ===

[[Image:Barry_setterfield.jpg‎|right|thumb|150px|[[Barry Setterfield]]]]
Creationist [[Barry Setterfield]] has proposed that the speed of light was faster in the past. <ref>Norman and Setterfield, 1986</ref>
This idea initially found wide acceptance by creationists, but is now widely rejected, although some still hold to the idea.

One criticism of it by anticreationists was that if the speed of light had changed, we should see the difference in the [[Fine Structure Constant]] as measured by nearby stars versus distant stars, but this was not observed.
Yet in 1999, John Webb, a professor at the University of New South Wales in [[Sydney]], Australia, and his colleagues reported astronomical observations suggesting that the value of the fine-structure constant may indeed have changed. They subsequently published this in ''[[Physical Review Letters]]''.<ref>Physical Review Letters 091301, 2001</ref><ref>Sarfati, 2001</ref>
However, although this showed that mainstream scientists are prepared to entertain the idea of a change in the speed of light when there is phsyical evidence to back it up, the observations did not provide any meaningful support for Setterfield's idea.

In addition, there were other problems with the theory, leading most creationists to drop the idea, although some credit it with stimulating further research.<ref>Humphreys, 1994, p.128</ref>

=== Humphreys' model ===
In 1994 Dr. [[Russell Humphreys]] proposed a new cosmology<ref>Batten et. al., 2007, p.90, Humphreys, 1994</ref> that includes a bounded universe with a center and an edge, that God had created 6,000 years ago as a much smaller body than today, then stretched it out, making it much larger.
In Humphreys' model, because the universe has a center and an edge (unlike the unbounded model of the Big Bang universe), the center of the universe is also the center of a [[gravity well]], meaning that gravity is stronger at the center of the universe than at the edge.

As gravity can affect the rate at which time passes, he calculated that while the six days of [[creation week]] were passing on Earth, billions of years' of time was passing at the edge of the universe.
According to this idea, the Biblical references to time are according to an observer (real or imaginary) on Earth, so ages are given in "Earth time".

This model receives cautious but wide support among creationists.

=== Time dilation field ===

Dr. [[John Hartnett]], a creationist physicist, spurred by Humphreys' model, has proposed an alternative [[time dilation]] model, by theorizing the Earth was in a time-dilation field during the first few days of creation, from Earth's point of view, while billions of years passed for the rest of the universe.
According to the Bible, God "stretched out"<ref>For example, {{Bible ref|Isaiah|40:22}}</ref> the heavens (space), and this movement during [[creation week]] caused time to travel faster for those objects, in accordance with [[Albert Einstein|Einstein's]] [[Special Theory of Relativity]], adding to the time dilation caused by [[gravity]], per Humphreys, in accordance with Einstein's [[General Theory of Relativity]].<ref>Hartnett, 2003, Hartnett 2007</ref>

==Starlight Problem for the Big Bang Theory==

Despite the use of the Starlight Problem in arguing against young-Earth creationism, the [[Big Bang theory]] has its own light travel problem, known as the horizon problem.

The Big Bang model proposes that the temperature of the [[Cosmic Microwave Background]] (CMB) would have varied considerably from place to place early in the universe, yet because the speed at which this radiation can disperse from hotter to colder parts of the universe is limited by the speed of light, there has not been enough time for the radiation to even out, yet observations today show it to be extremely uniform<ref name="Lisle">Lisle, 2003</ref> (with fluctuations at the part-per-million level<ref>[http://map.gsfc.nasa.gov/m_uni/uni_101Flucts.html Fluctuations in the Cosmic Microwave Background]</ref>).
{{QuoteBox|The problem is this: even assuming the big bang timescale, there has not been enough time for light to travel between widely separated regions of space. So, how can the different regions of the current CMB have such precisely uniform temperatures if they have never communicated with each other? This is a light-travel–time problem.<ref name="Lisle" />}}

A number of solutions to this problem have been proposed, including several versions of an "inflationary model", in which space itself expanded faster than the speed of light early in the Big Bang, but ''after'' different areas exchanged radiation to even out the temperature.
However, there is no consensus on which explanation is correct, and each of the proposed solutions have their own problems.<ref name="Lisle" /> Of course, all solutions which could be used to explain a creationist model could be used in a big bang model as well.

== Bibliography ==
* [http://www.nature.com/physics/highlights/6849-3.html#ref1 Inconstant constant?] ''Physical Review Letters 091301'' (9th August 2001)</ref>
* Amos, Jonathan, [http://news.bbc.co.uk/2/hi/science/nature/3490657.stm Hubble sees 'most distant object'], BBC News, 15th February, 2004.
* Batten, Don, Catchpoole, David, Sarfati, Jonathan, Wieland, Carl, [http://www.creationontheweb.com/images/pdfs/cabook/chapter5.pdf How can we see distant stars in a young universe?], chapter 5 of The Creation Answers Book, 2007.
* Fangrad, Richard, [http://www.creationontheweb.com/content/view/5425 The media spin on the creationists] (Creation Ministries International), 14th November, 2007.
* Hartnett, John C., A new cosmology: solution to the starlight travel time problem, ''Journal of Creation'' 17(2):98–102, August 2003. [http://www.creationontheweb.com/content/view/1616/ HTML] [http://www.creationontheweb.com/images/pdfs/tj/j17_2/j17_2_98-102.pdf PDF]
* Hartnett, John, Starlight, Time and the New Physics, Creation Book Publishers, 2007 ISBN 978-0-949906-68-7
* Humphreys, D. Russell, Starlight and Time, Master Books, 1994, ISBN 0-89051-202-7.
* Laird, Jeff, [http://swordofthemind.blogspot.com/2008/05/starlight-time-and-new-physics-review.html "Starlight, Time, and the New Physics" (Review)], Gladio Mentis, 26th May, 2008.
* Lisle, Jason, [http://www.creationontheweb.com/content/view/167/ Light-travel time: a problem for the big bang], ''Creation'' 25(4):48–49, September 2003.
* Norman, T.G. and Setterfield, B., ''The Atomic Constants, Light and Time'', SRI International Invited Research Report, Menlo Park, 1986.
* Sarfati, Jonathan, [http://www.creationontheweb.com/content/view/2430 Have fundamental constants changed, and what would it prove?] 22nd August, 2001 (Creation Ministries International).
* Wieland, Carl, [http://www.creationontheweb.com/content/view/2551/ Speed of light slowing down after all?], 9th August 2002 (Creation Ministries International).

==Notes==
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[[Category:Science]]
[[Category:Astronomy]]