Genetics

• Life needs instructions
• Human body


  Made up of billions of tiny cells
• Nucleus


  Control centre of the cell
• Genes


  Messages written in a code that the cell is able to decipher, telling the body to make proteins
• Proteins


  Made according to the genetic instructions
• Chromosomes


  46 in the human body, containing genes
• Components of the body

  • Structural proteins (major component of muscle, skin and hair)
  • Enzymes (also proteins)
• DNA


  Hereditary material in humans and all other organisms, determines our characteristics
• Nucleotides


  Molecules that make up DNA, have a phosphate group, a deoxyribose sugar, and one of four nitrogen-rich bases: adenine (A), guanine (G), cytosine (C) and thymine (T)
• DNA structure


  • Double helix shape, with uprights made of alternating phosphate and sugar groups, and rungs made of pairs of nitrogen-rich bases
• Complementary base-pairing


  Adenine always pairs with thymine, guanine always pairs with cytosine
• Chromosomes


  Long, thin, threadlike structures found in the nucleus of cells, made of DNA wrapped around protein
• The cells in the human body each contain 46 chromosomes (in 23 pairs), except for sperm and egg cells which only contain 23 chromosomes (one of each pair), and red blood cells which have no nucleus and no chromosomes
• Genes


  Sections of DNA that code for proteins
• Genetic code


  The order of the four bases along the gene forms the language, with each three bases forming one genetic 'word' representing one amino acid
• mRNA (messenger RNA)


  A 'photocopy' of the gene, made during the process of transcription, that carries the genetic instructions from the nucleus to the ribosomes to produce proteins
• Transcription


  1. Initiation (RNA polymerase binds to the gene and unzips the DNA strands)

  2. Elongation (RNA polymerase moves along the gene, attaching complementary ribonucleotides to make an RNA strand)

  3. Termination (RNA polymerase detaches from the DNA when it reaches the termination site)
• RNA polymerase


  The enzyme that catalyses transcription, adding one RNA nucleotide at a time to the growing RNA strand
• Proteins


  Polymers made from monomers called amino acids, and are a vital part of all living things. They are involved with everything that happens in the cell.
• Amino acids


  • More than 50 exist, but only 20 are used to make proteins in the body
  • 9 are essential (must be obtained from diet)
  • 11 can be made by the body
• Amino acids


  1. Link together with peptide bonds to form a long chain (polypeptide)

  2. The chain bends, folds and twists into a 3D molecule (protein)

  3. The order of amino acids determines the shape and function of the protein
• DNA


  • The master blueprint for the production of proteins
  • A gene is a sequence of DNA that specifies the production of one particular protein
• In eukaryotic cells, the DNA is stored in the nucleus and proteins are made outside the nucleus


  DNA strands are too long to move via diffusion out of the nucleus
• Ribosomes


  • Where translation occurs
  • Can be free floating in cytoplasm or attached to rough endoplasmic reticulum
  • Made of protein and rRNA
• Types of RNA involved in protein production

  • mRNA (copy of gene)
  • rRNA (forms part of ribosome)
  • tRNA (brings amino acids to ribosome)
• mRNA


  Nucleotides pair like DNA but use Uracil (U) instead of Thymine (T)
• Translation


  1. Translates the message in mRNA into an amino acid sequence to make a protein

  2. Nucleotide 'words' called CODONS each correspond to one amino acid

  3. Initiation: mRNA attaches to ribosome and reads 'START' codon

  4. Elongation: Ribosome reads mRNA and brings specific tRNA with complementary anticodon to add amino acids

  5. Termination: Ribosome reads 'STOP' codon, ending translation and allowing polypeptide to fold
• tRNA


  • Brings the correct amino acid to the growing polypeptide chain
  • Made of RNA in a clover leaf shape with three loops
  • One loop contains the anticodon complementary to the mRNA codon
• Video resources available at https://www.youtube.com/watch?v=oefAI2x2CQM&t=467s from 3:55 onwards
• Life needs


  To pass on genes
• Inheritance


  Genetics is the study of inherited characteristics called traits
• It's hard to think of a time when we didn't know why offspring/children were similar to but not identical to their parents
• For many years people thought that tiny humans were in sperm and they combined with the woman's blood to grow into a baby
• Other cultures had their own theories but not much progressed until Mendel and his peas
• Mendel


  A monk in Austria in 1856 who carried out experiments on pea plants
• The results of Mendel's experiments led him to construct theories that became the basis for the study of modern genetics, and are still recognised and used today
• Mendel's Peas


  Mendel did lots of experiments on peas, one of which was on the flower colour
• Pure white flowers were flowers that only produced white-flower offspring
• Pure red flowers would only produce red offspring
• When cross breeding red and white flowered peas, only red-flowered offspring were produced