2.3 nucleotides

• The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine.
• The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil
• Nucleotides join together by phosphodiester bonds formed in condensation reactions.
• A DNA molecule is a double helix composed of two polynucleotides joined together by hydrogen bonds between complementary bases whereas RNA is a relatively short single polynucleotide chain
• Adenosine triphosphate is a nucleotide derivative and consists of ribose, adenine and three phosphate groups.
• Energy is released when ATP is hydrolysed to form ADP and a phosphate molecule. This process is catalysed by ATP hydrolase
• • Condensation of ADP and inorganic phosphate catalysed by ATP synthase produces ATP during photosynthesis and respiration.
• DNA replication


  The semi-conservative replication of DNA ensures genetic continuity between generations of cells meaning that genetic information is passed on from one generation from the next.

  • The double helix unwinds and the hydrogen bonds between the complementary bases break using DNA helicase, separating the two strands of DNA

  • Both strands are used as templates and complementary base pairing occurs between the template strands and free nucleotides

  • Adjacent nucleotides are joined by phosphodiester bonds formed in condensation reactions using DNA polymerase
• Features of the genetic code:

  • The genetic code is non-overlapping meaning that each triplet is only read once and triplets don’t share any bases.

  • Genetic code is also degenerate meaning that more than one triplet codes for the same amino acids, this reduces the phenotypic effect of mutations . A change in the base sequence of DNA may alter the amino acid sequence and the protein. Some mutations are harmful.

  • The genetic code contains start and stop codons which either start or stop protein synthesis
• Transcription: protine synthisis

  • The hydrogen bonds between the complementary bases break and the DNA uncoils

  • One of the DNA strands is used as a template (antisense strand) by RNA polymerase to make the mRNA molecule.

  • Free nucleotides line up by complementary base pairing and adjacent nucleotides are joined by phosphodiester bonds made by RNA polymerase thus forming a single stranded molecule of mRNA

  • mRNA then moves out of the nucleus through a pore and attaches to a ribosome in the cytoplasm which is the site of next stage of protein synthesis called translation
• Translation: protein synthesis

  • mRNA attaches to a ribosome and transfer RNA collects amino acids from the cytoplasm and carries them to the ribosome.

  • tRNA attaches itself to mRNA by complementary base pairing – two molecules attach to mRNA at a time

  • The amino acids attached to two tRNA molecules join by a peptide bond and then tRNA molecules detach themselves from the amino acids, leaving them behind

  • This process is repeated thus leading to the formation of a polypeptide chain until a stop codon is reached on mRNA and ends the process of protein synthesis
• in protine sythisis tRNA is a single stranded molecule with a binding site at one end thus it can only carry one type of amino acid, and a triplet of bases at the other