DNA
DNA, short for deoxyribonucleic acid, is the molecule that contains the genetic code of organisms. This includes animals, plants, protists, archaea and bacteria.
DNA is in each cell in the organism and tells cells what proteins to make. A cell's proteins determine its function. DNA is inherited by children from their parents. This is why children share traits with their parents, such as skin, hair and eye color. The DNA in a person is a combination of the DNA from each of their parents.
Viruses use either DNA or RNA to infect organisms.[1] The genome replication of most DNA viruses takes place in the cell's nucleus, whereas RNA viruses usually replicate in the cytoplasm.
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[change] Structure of DNA
DNA has a double helix shape, which is like a ladder twisted into a spiral. Each step of the ladder is a pair of nucleotides.
[change] Nucleotides
A nucleotide is a molecule made up of:
- deoxyribose, a kind of sugar with 5 carbon atoms,
- a phosphate group made of phosphorus and oxygen, and
- nitrogenous base
DNA is made of four types of nucleotide:
The 'rungs' of the DNA ladder are each made of two bases, one base coming from each leg. The bases connect in the middle: 'A' pairs with 'T', and 'C' pairs with 'G'. The bases are held together by hydrogen bonds.
Adenine (A) and thymine (T) can pair up because they make two hydrogen bonds, and cytosine (C) and guanine (G) pair up to make three hydrogen bonds. Although the bases are always in fixed pairs, the pairs can come in any order. This way, DNA can write 'codes' out of the 'letters' that are the bases. These codes contain the message that tells the cell what to do.
[change] Chromatin
On chromosomes the DNA is bound up with proteins called histones to form chromatin. This association takes part in epigenetics and gene regulation. Genes are switched on and off during development and cell activity, and this regulation is the basis of most of the activity which takes place in cells.
[change] Copying DNA
When DNA is copied this is called DNA replication. Briefly, the hydrogen bonds holding together paired bases are broken and the molecule is split in half: the legs of the ladder are separated. This gives two single strands. New strands are formed by matching the bases (A with T and G with C) to make the missing strands.
First, an enzyme called DNA helicase splits the DNA down the middle by breaking the hydrogen bonds. Then after the DNA molecule is in two separate pieces, another molecule called DNA polymerase makes a new strand that matches each of the strands of the split DNA molecule. Each copy of a DNA molecule is made of half of the original (starting) molecule and half of new bases.
[change] Mutations
When DNA is copied, mistakes are sometimes made – these are called mutations. There are three main types of mutations:
- Deletion, where one or more bases are left out.
- Substitution, where one or more bases are substituted for another base in the sequence.
- Insertion, where one or more extra base is put in.
- Duplication, where a sequence of bases pairs are repeated.
Mutations may also be classified by their effect on the structure and function of proteins, or their effect on fitness. Mutations may be bad for the organism, or neutral, or of benefit. Sometimes mutations are fatal for the organism – the protein made by the new DNA does not work at all, and this causes the embryo to die. On the other hand, evolution is moved forward by mutations, when the new version of the protein works better for the organism.
[change] Protein synthesis
DNA is what tells the cell how to make particular proteins. Proteins form structures, and also form enzymes. The enzymes do most of the work in cells. Proteins are made out of smaller polypeptides, which are formed of amino acids. To make a protein to do a particular job, the correct amino acids have to be joined up in the correct order. A piece of DNA that contains instructions to make a protein is called a gene.
Proteins are made by machines in the cell called ribosomes. Ribosomes read codons, 'words' made of three base pairs that tell the ribosome which amino acid to add. Ribosomes are in the main body of the cell, but DNA is only in the nucleus of the cell. The codon is part of the DNA, but DNA never leaves the nucleus. Because DNA cannot leave the nucleus, the cell makes a copy of the DNA sequence in a similar single-strand molecule called RNA.This is smaller and can get through the holes – pores – in the membrane of the nucleus and out into the cell.
This one-stranded copy of DNA is called mRNA, for messenger RNA. The ribosome scans along an mRNA, reading the code while it makes protein. Another RNA called tRNA helps match the right amino acid to each codon.
[change] History
DNA was first isolated – extracted from cells – by Swiss physician Friedrich Miescher in 1869, when he was working on bacteria from the pus in surgical bandages. The molecule was found in the nucleus of the cells and so he called it nuclein.[2]
In 1919 this discovery was followed by the discovery by Phoebus Levene of the base, sugar and phosphate nucleotide unit.[3] In the 1950's, Erwin Chargaff [4] found that the amount of thymine (T) present in a molecule of DNA was roughly equal to the amount of adenine (A) present. He found that the same applies to guanine (G) and cytosine (C).
A few years after Chargaff's discovery, a British scientist named Rosalind Franklin studied crystals of DNA and how they diffract beams of X-rays. She found that an "X" pattern was produced, showing that the crystal was probably helix shaped.[5] Francis Crick and James Watson were also working on the structure of DNA, and with Franklin's results and models of the bases, they worked out the shape of the molecule.[6] How Watson and Crick got Franklin's results has been much debated. Crick, Watson and Maurice Wilkins were awarded the Nobel Prize in 1962 for their work on DNA – Rosalind Franklin had died in 1958.
[change] Other pages
[change] References
- ↑ Van Etten JL, Lane LC, Dunigan DD (2010). "DNA viruses: the really big ones (giruses)". Annual Review of Microbiology 64: 83–99. doi:10.1146/annurev.micro.112408.134338. PMC 2936810. PMID 20690825.
- ↑ Dahm R (2005). "Friedrich Miescher and the discovery of DNA". Dev Biol 278 (2): 274–88. PMID 15680349.
- ↑ Levene P, (1919). "The structure of yeast nucleic acid". J Biol Chem 40 (2): 415–24. http://www.jbc.org/cgi/reprint/40/2/415.
- ↑ Vischer E. and Chargaff E. (1948). "The separation and quantitative estimation of purines and pyrimidines in minute amounts". J. Biol. Chem. 176: 703–714. http://www.jbc.org/cgi/content/full/280/24/e21.
- ↑ Franklin R.E. & Gosling R.G. (1953). "Molecular configuration in sodium thymonucleate". Nature 171: 740–741.
- ↑ Watson J.D. & Crick F.H.C. (1953). "A structure for deoxyribose nucleic acid". Nature 171: 737–738. http://www.exploratorium.edu/origins/coldspring/printit.html.
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