1.5-nucleic acids

• A nucleotide has three components, combined by a condensation reaction:


  A phosphate group, which has the same structure in all nucleotides

  A pentose sugar. This is either ribose(RNA) or deoxyribose(DNA)

  A nitrogenous base - Pyrimidines (single ring) Purine (double ring)
• Purines


  Adenine, Guanine and Cytosine
• Pyrimidines


  Thymine and Uracil
• Define ATP


  It is short of adenosine triphosphate and is sometimes referred to as the energy currency of the cell. It is synthesized when energy is made available such as in mitochondria and it is broken down when energy is needed such as in muscle contractions.
• What is the structure of DNA

  It is a double stranded polymer of nucleotide. Each polynucleotide may contain many million nucleotide units. It is in the form of a double helix, the shape of which is maintained by hydrogen bonds. The pentose sugar is always deoxyribose and DNA contains four organic bases (Adenine, Thymine, Guanine and Cytosine). Each strand is linked together by pairs of organic bases.
• What is the structure of RNA

  RNA is a single stranded polynucleotide

  RNA contains the pentose sugar ribose

  RNA contains the purine bases adenine and guanine and the pyrimidine bases cytosine and uracil, but not thymine.
• Messenger RNA


  long single stranded molecule. It is synthesised in the nucleus and carries the genetic code from the DNA to the ribosome in the cytoplasm.
• Ribosomal RNA


  Is found in the cytoplasm and comprises large, complex molecules. Ribosomes are made of ribosomal RNA and protein. They are the site of translation of the genetic code into protein.
• Transfer RNA


  Is a small single-stranded molecule which folds so that in places, there are base sequences forming complementary pairs. Its shape is described as a cloverleaf. Molecules of tRNA transport specific amino acids to the ribosomes in protein synthesis.
• DNA


  Pentose sugar - Deoxyribose

  Purine bases - Guanine, Adenine

  Pyrimidine bases - Cytosine, Thymine

  Strands - Double helix

  Length - long
• RNA


  Pentose sugar - Ribose

  Purine bases - Guanine, Adenine

  Pyrimidine bases - Cytosine, Uracil

  Strands - Single

  Length - Shorter than DNA
• Replication


  DNA comprises of two complementary strands, the base sequence of one strand determining the base sequence of the other. If two strands of a double helix are separated, two identical double helices can be formed , as each parent strand acts as a template for the synthesis of a new complementary strand.
• Protein synthesis


  The sequence of bases represents the information carried in DNA and determines the sequence of amino acids in proteins.
• Semi-conservative replication


  In which the parental double helix separates into two strands, each of which acts as a template for synthesis of a new strand.
• Genetic code


  Is the set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences) by living cells.
• Introns


  Non-coding nucleotide sequence in DNA and pre mRNA, that is removed from pre mRNA, to produce mature mRNA.
• Exon


  Nucleotide sequence in DNA and pre mRNA that remains present in the final mature mRNA, after introns have been removed.
• Transcription


  Process whereby part of the DNA acts as a template for the production of mRNA which carries information needed for protein synthesis from the nucleus to the cytoplasm.
• What are the steps in transcription


  DNA helicase breaks the hydrogen bonds between the bases in a specific region of the DNA molecule. This causes the two strands to separate and unwind.

  RNA polymerase binds to the template strand of DNA at the beginning of the sequence to be copied.

  Free RNA nucleotides align opposite complementary base pairs on the template strand.

  RNA polymerase moves along the DNA forming bonds that add RNA nucleotides. Behind this the DNA strand rewinds to reform the double helix.
• Translation


  In translation, messenger RNA (mRNA)—produced by transcription from DNA—is decoded by a ribosome to produce a specific amino acid chain, or polypeptide.
• What are the steps in translation

  Initiation - ribosome attaches to a "start" codon at one end of the mRNA.

  The three bases of the codon on the mRNA bond with three complementary bases of the anticodon on the tRNA with hydrogen bonds.

  Elongation - the two amino acids are close enough for a ribosomal enzyme to catalyse the formation of a peptide bond.

  The first tRNA leaves the ribosome, leaving its attachment site vacant.

  The ribosome moves one codon along the mRNA strand.

  The next tRNA binds.

  Termination - the sequence is repeated until a stop codon is reached.

  The ribosome-mRNA-polypeptide complex separates.
• Polypeptides can be chemically modified by combination with non-proteins such as carbohydrates


  To make glycoproteins
• Polypeptides can be chemically modified by combination with non-proteins such as lipids

  To make lipoproteins
• Polypeptides can be chemically modified by combination with non-proteins such as phosphate


  To make phospho-proteins