Topic 2

• DNA nucleotide


  Contains a deoxyribose sugar, a phosphate group, and one of the four nitrogenous bases (adenine (A)/cytosine (C)/guanine (G)/thymine (T))

  
• DNA nucleotides in each DNA strand

  Joined together by phosphodiester bonds

  
• DNA


  • Double-stranded (double helix)
  • Two DNA strands are antiparallel (run in opposite directions)
  • Two DNA strands are complementary (complementary base pairing, hydrogen bonds between adenine (A) and thymine (T); cytosine (C) and guanine (G))

  
• DNA replication


  Semiconservative
• DNA replication steps


  1. DNA helicase unzips the DNA - DNA strands separate

  2. DNA nucleotides line up along the DNA strands through complementary base pairing (hydrogen bonds)

  3. DNA polymerase catalyses the formation of phosphodiester bonds between adjacent DNA nucleotides

  
• DNA replication is semiconservative – Meselson and Stahl experiment
• RNA nucleotide


  Contains a ribose sugar, a phosphate group, and one of the four nitrogenous bases (adenine (A)/cytosine (C)/ guanine (G)/ uracil (U))

  
• RNA nucleotides in each RNA strand

  Joined together by phosphodiester bonds

  
• RNA


  • Single-stranded
  • mRNA is linear
  • tRNA has a cloverleaf structure
• Transcription (nucleus) from DNA to RNA

  1. DNA helicase unzips the DNA - DNA strands separate

  2. RNA nucleotides line up along one DNA strand (the template / antisense strand) through complementary base pairing (hydrogen bonds)

  3. RNA polymerase catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides

  4. RNA detaches from the DNA

  
• Translation (cytoplasm) from RNA to protein


  1. Ribosome attaches to mRNA

  2. Each tRNA is attached to one specific amino acid

  3. tRNA brings specific amino acid to ribosome/mRNA

  4. Anticodons on tRNA line up along codons on mRNA through complementary base pairing (hydrogen bonds)

  5. Formation of peptide bonds between adjacent amino acids in a condensation reaction

  6. tRNA released from the mRNA/ribosome

  7. Ribosome moves along mRNA from start codon (AUG) to stop codon – stop codon signals the end of the polypeptide chain
• Genetic code


  • Triplet code – three nucleotides (codon) code for one amino acid
  • Nonoverlapping code – adjacent codons do not overlap; each nucleotide is read only once
  • Degenerate code – more than one codon can code for one specific amino acid, this is important as one nucleotide substitution may result in a codon that codes for the same amino acid, no effect on polypeptide structure (silent mutation)
• The sequence of codons determines the sequence of the amino acids/ primary structure of the protein
• Cell membrane


  Semipermeable phospholipid bilayer

  
• Phospholipids in cell membrane

  • Hydrophobic fatty acid tails interact with each other to form a hydrophobic core/interior
  • Hydrophilic phosphate head groups interact with aqueous environment on the outside and inside of the cell (cytoplasm)
• Semipermeable


  Only small, nonpolar or uncharged molecules can pass by simple diffusion (for example, oxygen)
• Cell membrane


  Fluid mosaic

  
• Fluid mosaic


  • Movement of phospholipids in the plane of the membrane
  • Proteins are embedded in the phospholipid bilayer, forming a mosaic of components; some proteins are fixed, but some can move

  
• Simple diffusion


  1. Small, nonpolar or uncharged molecules

  2. From high concentration to low concentration

  3. Down the concentration gradient

  4. No proteins
• Osmosis


  1. Diffusion of free water molecules

  2. From a high concentration of free water molecules to a low a concentration of free water molecules
• Facilitated diffusion


  1. Large, polar or charged molecules

  2. From high concentration to low concentration

  3. Down the concentration gradient

  4. Channel proteins (CFTR, open or close) or carrier proteins (change shape upon ligand binding)
• Active transport


  1. Large, polar or uncharged molecules

  2. From low concentration to high concentration

  3. Against the concentration gradient

  4. Uses ATP

  5. Carrier proteins (pumps, change shape upon molecule binding)
• Prenatal testing/screening


  Procedures to detect genetic diseases in a fetus
• Advantages of prenatal testing/screening


  • Prepares parents for child with genetic disease
  • Gives choice of abortion
• Disadvantages of prenatal testing/screening

  • Increased risk of miscarriage due to the procedure
  • Test may be inaccurate
  • False positive, healthy fetus may be aborted
  • False negative, parents not prepared for child with genetic disease
  • Killing of fetus is wrong/unethical
• Chorionic villus sampling (CVS)


  Prenatal test where fetal cells/DNA is obtained from placental tissue
• Amniocentesis


  Prenatal test where fetal cells/DNA is obtained from amniotic fluid
• Chorionic villus sampling (CVS)

  • Uses a needle
  • Greater risk of miscarriage, between 1 and 2%
  • Performed early, between 10-14 weeks of pregnancy; allows early decision to abort/termination less physically traumatic
  • Results available sooner; first results after 2-3 days, full results after 2 weeks
  • Cannot detect gene problems on X chromosome because they are inactivated in fetal placental cells
• Amniocentesis


  • Uses a needle
  • Lower risk of miscarriage, 1%
  • Performed later, between 14-20 weeks of pregnancy; late decision to abort/termination more physically traumatic
  • Results not available until 2-3 weeks after test
• Primary structure


  Linear sequence (order) of amino acids; determines the folding of the protein
• Amino acid structure


  • Consists of amine group, carboxyl group, hydrogen, variable R group (side chain)
• Amino acids connected by peptide (amide) bonds in a condensation reaction


  Form a peptide

  
• Secondary structure


  Hydrogen bonds between carbonyl (C=O) and amino groups (N-H) of peptide bonds (backbone); two main types: α-helices and β-pleated sheets
• Tertiary structure (folding)


  Interactions between functional groups of side chains; can form two main types: covalent bonds (disulfide bonds/bridges) and non-covalent bonds (ionic bonds, hydrophobic interactions, hydrogen bonds)
• Quaternary structure


  Interactions between more than one polypeptide chain; each polypeptide chain represents a subunit
• Two main types of proteins

  • Globular proteins
  • Fibrous proteins

  
• Globular proteins


  • Functional role, compact/roughly spherical shape, water-soluble, rich tertiary structure

  
• Globular proteins


  • Transport (haemoglobin), storage (myoglobin), enzymes (lysozyme), antibodies
• Fibrous proteins


  • Structural role, long strands, water insoluble, little or no tertiary structure

  
• Fibrous proteins


  • Collagen in arteries, keratin in skin