DNA - Its Structure (GCSE Biology)

DNA: Its Structure

DNA in More Detail

As we now know, DNA is a very important molecule in the body. It is also very complex. We need to have a simplified understanding:

• DNA is made from four different nucleotides. Each nucleotide of DNA is a monomer and these monomers are put together to form the polymer that is DNA.
• Nucleotides are made of different structures. A nucleotide is made up of a sugar, a phosphate group and a nitrogenous base. The sugar in DNA is deoxyribose. The deoxyribose then binds to two phosphate groups and one nitrogenous base. One of the phosphate groups will bind to another nucleotide. This is how the DNA forms a chain.
• There are four nitrogenous bases. The nitrogenous bases are Adenine (A), Thymine (T), Cytosine (C) and Guanine (G).
• Purines must bind with pyrimidines. The nitrogenous bases bind with one another to join nucleotides together. This is a stronger bond than the bond with the phosphate groups and the nucleotides. Adenine and guanine are purines, whereas thymine and cytosine are pyrimidines. This is a method of the classification of bases. Purines bind with pyrimidines, therefore Adenine must bind with Thymine and Cytosine must bind with Guanine. Any other types of binding cannot occur and if by an off chance they do, could have horrible physiological consequences

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The Triplet Code

• Amino acids are coded for by sequences of bases. Each amino acid is coded for by a sequence of 3 bases. There are 20 amino acids and each one has a few possibilities of sequences of bases that build it.
• The triplet code defines the amino acids. The code that defines which sequences of bases for which amino acids is known as the triplet code. This code has three important features.
  1. It is universal – all organisms use the same codons (sequence of three bases) to form proteins
  2. It is degenerate – this means that several codons can code for the same protein. This shields organisms from mutation to a certain extent.
  3. It is non-overlapping – this means that the code is read as a sequence of three bases at a time and then you move on to the next three bases.