Watson-Crick base pairing

2012-10-24T20:48:50Z

110027483:

[[Image:Watson n crick.jpg|left|James Watson and Francis Crick]] [[Image:BASE PAIRS.png|right|199x253px|Figure 1.1 A-T and G-C base pairs]]

DNA consists of two types of bases, namely; [[Purine|purines]] and [[Pyrimidine|pyrimidines]]. There are two types of purines: [[Adenine|adenine]] and [[Guanine|guanine]], as well as two types of Pyrimidines: [[Cytosine|cytosine]] and [[Thymine|thymine]]. In the Watson-Crick DNA base pairing model a purine always binds with a pyrimidine, however, each purine binds to one particular type of pyrimidine.

Adenine (A) binds to thymine (T) whilst, guanine (G) binds to cytosine (C). However, in RNA unracil (U) is substituted for thymine (T). This base pairing is referred to as complementary base pairing, hence the base pairs are called complementary [[Base pairs|base pairs]] <ref>Hartl D., Ruvolo M. (2012) Genetics; Analysis of Genes and Genomes, 8th edition, Burlington: Jones and Barlett.</ref>.  The base pairs are bound together by [[Hydrogen bonds|hydrogen bonds]], although the number of H-bonds differs between base pairs. G-C base pairs are bound by three (3) hydrogen bonds whilst, A-T base pairs are bound by two (2) hydrogen bonds as illustrated in the figure on the right <ref>Becker., Kleinsmith., Hardin and Bertoni (2009) The World of the Cell, Seventh Edition, San Francisco: Pearson Education.</ref>.

Importance    

Watson-Crick base pairing is of very great importance as it is a deciding factor in [[Semi-conservative replication|DNA Replication]] <ref name="null">Genetic Science Learning Center (2011) Build a DNA Molecule. Learn.Genetics. Available at: http://learn.genetics.utah.edu/content/begin/dna/builddna/. [Last assessed: 26/11/2011] University of Utah</ref>. It ensures that pairs form between complementary bases only. The formation of base pairs between two non-complementary bases results in [[Mutations|gene mutations]] which can be detrimental to development of an organism.                      

=== Background ===

In 1953, [[James watson|James D Watson]] and [[Francis Crick|Francis Crick]] discovered the structure of DNA using X ray Crystallography. They worked out that [[DNA|DNA]] was a double helix using Rosalind Franklin's X ray diffraction pattern <ref>BBC NEWS. Science/Nature Available: http://news.bbc.co.uk/1/hi/sci/tech/2804545.stmfckLR[accessed 2 December 2011]</ref>. At first, it was thought that [[DNA|DNA]] was made up of many chemicals, which proved too difficult to analyse, but the researchers persisitence led to the discovery of complementary base pairing <ref>DNA tutorial Available :http://www.dnatutorial.com/BasePairing.shtml [accessed 2 December 2011]</ref>.

=== References ===

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Polycistronic

2012-10-24T20:44:13Z

110027483:

Polycistronic mRNA is the term given to a molecule that comprises coding sequences for several different polypeptide chains. A cistron is a nucleotide sequence, which contains coding sequences for one polypeptide chain only. In addition, in polycistronic mRNA, each region that contains a sequence for a different polypeptide is preceded by a ribosome binding site, and also an AUG start codon. Often, the different coding sequences in a polycistronic molecule encode for the different proteins that occur in metabolic pathways.<br>

Polycistronic

2011-12-02T11:33:52Z

110027483: Created page with "Polycistronic mRNA is the term give to a molecule that comprises coding sequences for many different polypeptide chains (cistron is the term given to a nucleotide seque..."

Polycistronic mRNA is the term give to a molecule that comprises coding sequences for many different polypeptide chains (cistron is the term given to a nucleotide sequence that only contains conding sequences for one polypeptide chain). Furthermore, in polycistronic mRNA each region that contains a sequence for a different polypeptide is preceded by a ribosome binding site, and also an AUG start codon. Often, the different coding sequences in a polycistronic molecule encode for the different proteins that occur in metabolic pathways.

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Watson-Crick base pairing

Polycistronic

Polycistronic