B1.2 - What happens in cells?

• DNA

  Contains all of an organism's genetic material, the chemical instructions it needs to grow and develop
• Chromosomes

  • Long molecules of coiled up DNA
  • DNA is divided up into short sections called genes
• DNA

  • A double helix (a double-stranded spiral)
  • Each DNA strand is made up of lots of nucleotides joined together in a long chain, making DNA a polymer
• DNA bases

  • A (adenine)
  • C (cytosine)
  • G (guanine)
  • T (thymine)
• Base pairing

  A always pairs up with T, and C always pairs up with G (complementary base-pairing)
• Nucleotides

  • Each DNA nucleotide has the same sugar and a phosphate group. The base on each nucleotide is the only part of the molecule that varies (i.e. it's either A, C, G or T).
  • The base is attached to the sugar.
• Polymer and Monomer

  • Polymer - Large, complex molecules composed of long chains of monomers joined together
  • Monomers - Small, basic molecular units

  DNA is a polymer made up of nucleotide monomers
• DNA

  Controls the production of proteins (protein synthesis) in a cell
• Gene

  • A section of DNA that codes for a particular protein
  • Each gene contains a different sequence of bases which is what allows it to code for a particular protein.
• Proteins

  • Made up of chains of molecules called amino acids
  • Each protein has its own particular number and order of amino acids
  • This gives each protein a different shape, which means each protein can have a different function
  • It's the order of bases in a gene that decides the order of amino acids in a protein
• Triplet code


  • Each amino acid is coded for by a sequence of three bases in the gene
  • The amino acids are joined together to make proteins , following the order of the bases in the gene
• Protein synthesis- Transcription


  • DNA is found in the cell nucleus and can't move out of it because it's too big. The cell needs to get the information from the DNA to the cell cytoplasm where proteins are synthesised. This is done using a molecule called mRNA, which is similar to DNA, but it's shorter and only a single strand:

  1. The DNA contains the gene coding for the protein.
  2. In the nucleus, the two DNA strands unzip around the gene. The DNA is used as a template to make the mRNA. Base pairing ensures it's complementary (it matches the opposite strand). This step is called transcription
• Cell metabolism

  • Thousands of different chemical reactions going on inside cells all the time
  • These reactions need to be carefully controlled to get the right amounts of substances and keep the organism working properly
  • Raising the temperature will make a reaction happen quicker. This would speed up the useful reactions but also the unwanted ones too. There's also a limit to how far you can raise the temperature inside a living creature before its cells start getting damaged.
• Enzymes

  • Biological catalysts that speed up reactions without being changed or used up in the reaction itself
  • Every different biological reaction has its own enzyme, which is a protein coded for by a different gene and has a unique shape
  • Enzymes reduce the need for high temperatures and we only have enzymes to speed up the useful chemical reactions in the body.
• Enzymes

  • Chemical reactions usually involve things either being split apart or joined together.
  • The substrate is the molecule changed in the reaction.
  • Every enzyme has an active site - the part where it joins on to its substrate to catalyse the reaction. Enzymes usually only work with one substrate so they have a high specificity for their substrate. This is because, for the enzyme to work, the substrate has to fit into the active site. If the substrate's shape doesn't match the active site's shape, then the reaction won't be catalysed. This is called the 'lock and key' hypothesis.
• Enzymes -Temperature

  • Changing the temperature changes the rate of an enzyme-controlled reaction
  • Higher temperature increases the rate at first, the enzymes and the substrate move about more, so they're more likely to meet up and react. But if it gets too hot, some of the bonds holding the enzyme together break so the enzyme becomes denatured - it loses its shape and the substrate doesn't fit the active site any more so the enzyme can't catalyse the reaction anymore and the reaction stops.
  • The enzyme is denatured irreversibly - it won't go back to its normal shape if things cool down again.
• Optimum temperature


  • The temperature when the reaction goes fastest, just before it gets too hot and starts to denature. The optimum temperature for the most important human enzymes is about 37 °C - the same temperature as our bodies.
• Enzymes - pH

  • If pH is too high or too low, it interferes with the bonds holding the enzyme together
  • This changes the shape of the active site and can irreversibly denature the enzyme.
• Optimum pH


  • The pH at which an enzyme works best, often neutral pH 7 but not always.
  • For example, pepsin is an enzyme used to break down proteins in the stomach. It works best at pH 2, which means it's well-suited to the acidic conditions in the stomach
• Enzyme concentration

  • The more enzyme molecules, the more likely a substrate molecule will meet up with one and join with it, increasing the rate of reaction
  • But, if the amount of substrate is limited, there comes a point when there are more than enough enzyme molecules to deal with all the available substrate, so adding more enzyme has no further effect.
• Substrate concentration

  • The higher the substrate concentration, the faster the reaction - it's more likely the enzyme will meet up and react with a substrate molecule
  • But after that, there are so many substrate molecules that the enzymes have about as much as they can cope with (all the active sites are full), and adding more makes no difference.
• Investigating enzyme activity

  • To investigate the effect of pH, add a buffer solution with a different pH level to a series of different tubes containing the enzyme-substrate mixture.
  • Vary the initial concentrations of the substrate to investigate the effect of substrate concentration.
  • Vary the initial concentrations of the enzyme to investigate the effect of enzyme concentration.
• Protein synthesis - Translation


  1.The mRNA molecule then moves out of the nucleus and into the cytoplasm in the ribosome.

  2.Amino acids that match the triplet codes on the mRNA are joined together in the correct order. This makes the protein coded for by the gene. This step is called translation.
• Protein synthesis:

  A) DNA

  B) amino acids

  C) protein

  
• mRNA:

  • Single strand copy of DNA
  • There is no Thymine (T) in mRNA, so a base called Uracil (U) binds with Adenine (A).
• Protein synthesis:

  A) mRNA

  B) template

  C) mRNA

  D) protein

  E) amino acids

  
• Lock and Key Hypnosis:

  A) enzyme

  B) active site

  C) substrate

  D) products

  E) unchanged

  
• Enzyme - Temperature:

  A) optimum

  B) active

  
• Enzyme - pH

  A) optimum

  
• Substrate concentration:

  A) active sites

  B) substrate

  
• Enzyme concentration:

  A) substrate

  B) active

  C) sites