Exam Answers - AS Level Biology

1) The nucleic acid is made of a phosphate group, a pentose sugar group and an organic base.
In RNA the pentose sugar is ribose, and in DNA it is Deoxyribose.
There are two main groups of organic bases pyrimidine and purine. Pyrimidine bases include Thymine, cytosine and uracil.
Cytosine and thymine are found in DNA. Uracil and cytosine are found in RNA. The purine bases are adenine and guanine.

2) The three main types of RNA are rRNA (ribosomal RNA), mRNA (messenger RNA) and tRNA (transfer RNA). mRNA is normally located in the nucleus and once transcription has been completed and the mRNA chain has been formed the mRNA exits via a nuclear pore and heads through the cytoplasm to the ribosome where it begins to get ready for translation. The tRNA molecules are normally located within the cytoplasm and have a role in protein synthesis, these bring the amino acids close to allow peptide links to form between them.

3) Between the bases there are hydrogen bonds. Cytosine and guanine have three hydrogen bonds whilst Adenine and thymine have two hydrogen bonds holding them together.

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