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Basic Biology - Molecules of life - DNA
DNA is the main information carrier molecule in a cell
DNA may be single or double stranded. A single stranded DNA molecule, also called a polynucleotide, is a chain of small molecules, called nucleotides. There are four different nucleotides grouped into two types, purines: adenine and guanine and pyrimidines: cytosine and thymine. They are usually referred to as bases and denoted by their initial letters, A, C, G and T (not to be confused with amino acids!).
Different nucleotides can be linked together in any order to form a polynucleotide, for instance, like this
A-G-T-C-C-A-A-G-C-T-T
Polynucleotides can be of any length and can have any sequence. The two ends of this molecule are chemically different, i.e., the sequence has a directionality, like this
A->G->T->C->C->A->A->G->C->T->T->
The end of the polynucleotides are marked either 5' and 3' . By convention DNA is usually written with 5' left and 3' right, with the coding strand at top. Two such strands are termed complementary, if one can be obtained from the other by mutually exchanging A with T and C with G, and changing the direction of the molecule to the opposite. For instance,
<-T<-C<-A<-G<-G<-T<-T<-C<-G<-A<-A
is complementary to the polynucleotide given above.
Specific pairs of nucleotides can form weak bonds between them. A binds to T, C binds to G. Although such interactions are individually weak, when two longer complementary polynucleotide chains meet, they tend to stick together, like this
5' C-G-A-T-T-G-C-A-A-C-G-A-T-G-C 3'
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3' G-C-T-A-A-C-G-T-T-G-C-T-A-C-G 5'
Vertical lines between two strands represent the forces between them as shown above. The A-T and G-C pairs are called base-pairs (bp). The length of a DNA molecule is usually measured in base-pairs or nucleotides (nt), which in this context is the same thing.
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