Nucleic acids are polymers of nucleotides. Examples of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The genetic information is stored in DNA of living organisms expect RNA viruses. A nucleotide is made up of three components: a five carbon pentose sugar either deoxyribose or ribose, a nitrogenous base and a phosphate group.
The base is attached to the carbon atom 1 and the phosphate group to carbon atom 5 of the pentose sugar.
The base is attached to the carbon atom 1 and the phosphate group to carbon atom 5 of the pentose sugar.
Phosphate group
Phosphoric acid is joined by condensation to the pentose sugar. It gives nucleic acids their acidic property. |
A five-carbon sugar (pentose)
Ribose is pentose sugar with a hydroxyl group attached to carbon atom 2 whereas deoxyribose has a hydrogen atom attached to carbon atom 2. |
Nitrogen-containing bases
A base is a ring structure containing nitrogen.
A base is a ring structure containing nitrogen.
- In DNA, there are four different bases. There are the two purines (double ring structure), adenine (A) and guanine (G) and the two pyrimidines (single ring), cytosine (C) and thymine (T).
- RNA contains the four bases adenine, guanine, cytosine and uracil (U) in place of thymine.
formation of nucleotides and nucleic acid
In formation of a nucleotide a nitrogenous base is first linked to pentose by condensation reaction to form nucleoside. Phosphate group is then added to the nucleoside to form a nucleotide. Nucleic acids are polynucleotides. Polynucleotides are formed by linking nucleotides together. Two nucleotides are linked together through condensation reactions to form dinucleotides. Further additions of nucleotides to dinucleotides in polynucleotides are linked together in phosphodiester bonds. A nucleotide chain has one 5' end and a 3' end. The 5' end of the polynucleotide chain is the end with the free phosphate group.
|
deoxyribose nucleic acid (DNA)
In 1953, James Watson and Francis Crick deduced the double-helix model of DNA structure. Watson and Crick proposed that DNA is a polymer consisting of two polynucleotide stands. The two polynucleotide strands are coiled in a right-handed spiral to form a double helix. The two chains are held together by hydrogen bonds between complimentary bases. (T and A are complementary bases; G and C bases are complementary bases).
The polynucleotide chains of DNA molecule are anti-parallel (the 5' end of one chain lies next to the 3' end of the other chain). The polynucleotide chain is made up of deoxyribonucleotides that are lined together by phosphodiester bond.
The polynucleotide chains of DNA molecule are anti-parallel (the 5' end of one chain lies next to the 3' end of the other chain). The polynucleotide chain is made up of deoxyribonucleotides that are lined together by phosphodiester bond.
Ribose nucleic acid
RNA consists of a single polynucleotide strand. Each RNA nucleotide contains a ribose sugar, a phosphate and one of the four nitrogen bases, adenine, uracil, cytosine and guanine. The pyrimidine thymine in DNA in replaced by uracil.
There are three types of RNA in cells:
There are three types of RNA in cells:
- ribosomal RNA (rRNA)
- messenger RNA (mRNA)
- transfer RNA (rRNA)
rRNA
More than 80% of RNA in the cell is rRNA. rRNA is found in ribosome. A ribosome is constructed from rRNA (50%) and protein (50%). Functions of rRNA:
|
mRNA
mRNA is the longest RNA molecule, and it contains 70 to 3000 nucleotides. mRNA molecules are long uncoiled molecules. Function of mRNA:
|
tRNA
tRNA are the shortest RNA molecules tRNA makes up about 10% to 15% of RNA cells. The polynucleotide chain is folded to form a clover-leaf. The anticodon of tRNA contains 3 bases which are complimentary to the codon for the amino acid it carries. Function of tRNA:
|