Nucleic acids

Nucleotides:

made up of three components -

• A pentose sugar
• A phosphate group
• A nitrogen containing organic base - cytosine, guanine, thymine, adenine, uracil
• Joined together by a condensation reaction
• form a single nucleotide

Two mononucleotides may be joined as the result of a condensation reaction between the deoxyribose sugar of one mononucleotide and the phosphate group on another.

The bond formed between them is a phosphodiester bond and the new structure is called a dionucleotide

Continued linking of mononucleotides forms polynucleotides

RNA:

Ribonucleic acid

• Polymer made up of nucleotides
• Single, relatively short polynucleotide chain
• Pentose sugar is always ribose
• Organic bases are A,G,C,U
• One type of RNA transfers genetic information from DNA to ribosomes (mRNA)
• The ribosomes themselves are made up of proteins and another type of RNA (rRNA)
• A third type is involved in protein synthesis (tRNA)

DNA:

Deoxyribosenucleic acid

• Pentose sugar is always deoxyribose
• Organic bases are A,T,C,G
• Made up of two strands on nucleotides
• Each of the two strands is extreamely long and are joined together by hydrogen bonds that form between certain bases
• C + G have 3 hydrogen bonds
• A + T have 2 hydrogen bonds
• Adenine and Guanine are purines
• Thymine, Uracil and Cytosine are pyrimidines

Base pairing -

• Adenine pairs with Thymine
• Guanine pairs with Cytosine
• Said to be complementary to each other
• Quantities of thymine and adenine are always the same and so are the quantities of cytosine and guanine
• However, the ratio of A+T and C+G varies in species

The double helix -

• Looks like a ladder-like arrangement with the polynucleotide chains being twisted
• The uprights of phosphate and deoxyribose wind around one another to form a double helix
• They form the structural backbone of the DNA molecule

Stability of DNA -

Is a stable molecule because:

• The phosphodiester backbone protects the more chemically reactive organic bases inside the double helix
• Hydrogen bonds link the organic base pairs forming bridges (rungs) between the phosphodiester uprights
• As there are three hydrogen bonds between C+G, the higher the proportion of C-G pairings, the more stable the DNA molecule
• There are other interactive forces between the base pairs that hold the molecule together (=base stacking)

Function of DNA -

• Heriditary material responsible for passing genetic information from cell to cell and generation to generation
• Around 3.2 billion base pairs in the DNA of a typical mammilian cell
• This vast number means that there is almost infinite variety of sequences of bases along the length of the DNA molecule
• It is this variety that provides the genetic diversity within living organisms

Adapted to carry out its functions because -

• It is a very stable structure which normally passes from generation to generation without change
• Only rarely does it mutate
• Its two separate strands are joined only with hydrogen bonds, which allow them to separate during DNA replication and protein synthesis
• It is an extremely large molecule and therefore carries an immense amount of genetic information
• By having the base pairs within the helical cylinder…