Nucleic Acids: - Structure of RNA and DNA

- Structure of DNA and RNA -

Deoxyribonucleic acid and ribonucleic acid are important information-carrying molecules. In all living cells, DNA holds genetic information and RNA transfers genetic information from DNA to the ribosomes.

DNA molecule is a double helix with two polynucleotides chains held together by hydrogen bonds between specific complementary base pairs.

• Adenine is complementary to thymine
• Guanine is complementary to cytosine

Ribosomes are formed from RNA and protein.

Both DNA and RNA are polymers of nucleotides. Each nucleotides is formed from

• Pentose sugar
• Phosphate group
• Nitrogen-containing organic base

Nitrogen containing bases are:

• Adenine - A
• Thymine - T
• Guanine - G
• Cytosine - C
• Uracil - U ( RNA molecules only // replaces Thymine (T))

Pentose sugar, phosphate group and organic base are joined as result of condensation reactions, forming a mononucleotide.

2 mononucleotides may in turn be joined by the condensation reaction between the deoxyribose sugar of one mononucleotide and the phosphate group of another. The bond formed between them is called a phosphodiester bond.

RNA structure:

• RNA molecules is a relatively short polynucleotide chain
• The pentose sugar is ribose and the organic bases are adenine, guanine, cytosine, and uracil.

DNA structure // Double helix structure (DNA) ~

• The pentose sugar is deoxyribose and the organic bases are adenine, thymine, guanine and cytosine.
• Alternating phosphate and deoxyribose molecules make up the ‘uprights’ and pairs of organic bases comprise the ‘rungs’. The complementary base pairing ensures a standard ‘rung length’.

Stability of DNA ~

The phosphodiester backbone protects the more chemically reactive organic bases.

Hydrogen bonds link the organic base pairs forming bridges between the phosphodiester uprights

The higher the proportion of G-C pairings, with 3 hydrogen bonds, A-T pairings have 2 hydrogen bonds.

Function of DNA ~

- DNA is responsible for passing genetic information from cell to cell.

- Adaptations.

- Large molecule - carries an immense amount of genetic information.

- Base pairing leads to DNA being able to replicate and transfer into as mRNA.

- The base pairs are found in the helical cylinders therefore they are protected from outside chemical and physical forces.

- Mutations in DNA due to the stability of the molecule. // whether or not a mutation would occur.

- Separate strands are held by hydrogen bonds - easy to separate.

- The simplicity of DNA led to many scientists doubt that it carried the genetic code.