2B

• enzyme - protein that acts as a biological catalyst for a specific reaction
• catalyst - substance that increases the rate of reaction without changing the substances produced or being changed itself
• Enzymes are globular proteins which are produced during protein synthesis. They have a specific shape due to their primary, secondary, tertiary and quaternary structure, so they catalyse specific reactions.
• specificity - characteristics of enzymes that means that each enzyme will only catalyse a specific reaction or group of reactions, due to their specific shapes that come from protein structure
• Changes in temperature and pH affect the efficiency of an enzyme because they affect the intramolecular bonds that are responsible for the specific shape.
• anabolic reaction - reactions that synthesise (builds) new molecules in a cell
• catabolic reaction - reactions that break down substances in a cell
• metabolism = anabolic + catabolic
• metabolic pathway - series of linked reactions in the metabolism of a cell
• Enzymes can be intracellular (catalyse in cell) or extracellular (catalyse outside the cell they were made in).
• Enzymes have different names:

  • recommended name, which is informative, +ase
  • longer systematic name, which describes the reaction, +ase
  • classification number
• Enzymes lower the activation energy of a reaction. They form the enzyme-substrate complex with the substrate. Once the products are released, the enzyme forms another complex with another substrate.
• Lock and key hypothesis

  • active site is specific to one substrate
  • bonds in substrate are weakened (catabolic)
  • reactants are closer so easier to form bonds (anabolic)
  • the products do not fit so they are released
• Induced fit hypothesis

  • active site has a flexible specific shape
  • shape of site changes to form complex
  • products are released so active site returns to inactive shape
• molecular activity - number of substrates transformed per minute by an enzyme
• Few enzymes are needed due to their high molecular activity. If all the enzymes are being used in a reaction, the rate of reaction cannot increase further. Enzyme controlled reactions are affected by concentration of enzymes. The number of substrates affects the rate of reaction, but only until the enzymes are saturated.
• Temperature increases the rate of reaction because the number of successful collisions increase. Once the temperature is past optimum, the enzyme starts to denature because hydrogen and Van der Waal's bonds break as they have more kinetic energy and vibrate faster.
• Denaturing is the loss of the specific shape of a protein due to changes in the tertiary and quaternary structure.
• pH affects enzyme activity by changing its shape. Different enzymes have different pH optimums, depending on their function. Changes in pH affect hydrogen and disulfide bonds, which can change the protein structures.
• Mononucleotides have 3 parts, joined by condensation reactions:

  • 5 - carbon pentose sugar
  • nitrogenous base
  • phosphate group
• Nitrogenous bases are either purine or pyrimidine. Purines have 2 rings, and are adenine and guanine. Pyrimidines have 1 ring, and are cytosine, thymine and uracil.
• The phosphate group (PO4-3) makes the nucleotide acidic and negative.
• Nucleic acids (polynucleotides) are information molecules. They are polymers composed of many mono nucleotides.
• In eukaryotic cells, DNA is stored in chromosomes in the nucleus. In prokaryotes, DNA floats freely in the cytoplasm.
• Nucleic acids are chains of nucleotides joined together by condensation reactions that form phosphodiester bonds between the sugar of one nucleotide and the phosphate group of another. This forms then phosphate - sugar backbone found in DNA and RNA.
• DNA is made up of chains containing guanine, cytosine, thymine, and adenine.
• RNA is made up of chains containing guanine, cytosine, uracil, and adenine.
• RNA form single polynucleotide strands that can fold into complex shapes or thread-like molecules.
• DNA molecules are made of 2 polynucleotide strands that wrap around each other, forming a double helix. The two sugar-phosphate backbones are held together by complimentary base pairing between T and A, and C and G. There are 10 base pairs for each complete twist of the helix. The bases are in a specific sequence.
• Nitrogenous bases bond together with hydrogen bonds that form between the amino and carbonyl groups on the purine and pyrimidine. There are 3 bonds between C and G, and 2 bonds between A and T.
• DNA replicates using semi-conservative replication. Each new double helix has 1 strand from the original DNA, and 1 strand made of new material. This was proved by the Meselson and Stahl experiment which used heavy and light nitrogen to show that the mass of strands changed over generations of replication.
• Replication

  1. DNA helicase "unzips" the 2 polynucleotide chains by breaking the hydrogen bonds between base pairs
  2. The exposed bases attract free DNA nucleotides, which are lined up by DNA polymerase to form complimentary base pairs and phosphodiester bonds
  3. DNA ligase catalyses the formation of phosphodiester bonds between the new nucleotides in Okazaki fragments
• In the DNA double helix, base sequence varies. Every 3 bases, a triplet code, codes for 1 of 20 amino acids.
• gene - sequence of bases on a DNA molecule which codes for a sequence of amino acids in a polypeptide chain which affects a characteristic in the phenotype of an organism
• The gene is made up of triplet codes that code for amino acids, or signify the beginning or end of an amino acid sequence.
• Codons are identified on mRNA and then the complimentary base pair is recorded for DNA., since DNA is too big to research. This research was used to compile a dictionary of genetic code, where combinations of bases are linked to the amino acid they make.
• 98% of DNA is non-coding, but researchers don't know why.
• Non-overlapping code means that a codon can only code for 1 amino acid, so none of the bases go towards other amino acids. This limits the effects of substitution mutations to 1 amino acid.
• Degenerate code contains more information than needed (e.g. TGA and TGG for threonine) in order to avoid repercussions in case of mutations.
• RNA is similar to DNA, but it has a different pentose sugar, uracil instead of thymine, and a single helix.