IQ3: DNA and Polypeptide Synthesis

• Similarities between DNA in Eukaryotes and Prokaryotes

  The genetic code is universal i.e. the same nucleotide base pairing is used in both prokaryotes and eukaryotes to instruct protein synthesis. 
• DNA in Prokaryotes

  • Contain single chromosomes in the form of a circular strand of DNA
  • Not a helix but two circles of single-stranded DNA twisted around each other
  • Since prokaryotes do not have a nucleus, the DNA floats in a dense region called the nucleoid
• DNA in Eukaryotes

  • DNA is located in the nucleus
  • DNA is linear
  • Non nuclear DNA- mitochondrial DNA (mtDNA), circular, inherited from the mother, helpful in determining how a person has a certain disorder in a family.
• Introns: DNA that is not used to make proteins or RNA
• Exons: DNA that contains sequences that code for proteins or RNA
• Enzymes - Help to speed up reactions in living cells by binding to active sites
• Antibodies - Antibodies are proteins that are produced by the immune system to fight infection and remove foreign viruses or bacteria. Substances that trigger the release of antibodies are called antigens
• Structural Proteins - Structural proteins provide support for our bodies and cells. The most prominent structural proteins are the keratins, which form skin, fur, hair, wool, claws, nails, hooves, horns, scales, beaks and feathers.
• Transport Proteins - The last type of protein we are going to look at are transport proteins. These proteins move molecules around our bodies. E.g haemoglobin
• Genes carry the information that determines your traits. Genes are portions of DNA that make proteins
• Gene expression- when the end product of the gene has been made by the cell
• • Coded instructions for the production of a particular protein are said to be ‘switched on’ in the DNA of that cell
  • This ensures that each cell develops the necessary structures, in keeping with the type of tissue to which it belongs. 
  • For example, in skin tissue, genes for the pigment protein melanin and the protein keratin are switched on in each cell, ensuring that the cells become skin cells. 
• mRNA- single stranded molecule that carries information from the DNA in the nucleus to the ribosomes in the cytoplasm
• • tRNA- 75 nucleotide long, twisted in a clover shape. One end has 3 unpaired bases (anticodon), the other has an amino acid. Each tRNA molecule only attaches to one particular amino acid. Link between mRNA and amino acids
• rRNA- forms a structural part of ribosomes
• Transcription within Polypeptide synthesis

  1. RNA polymerase binds to a part of DNA (promoter).
  2. The section containing the gene to be used unzips. Only one strand of DNA has the genetic information (sense strand) 
  3. The sense strand is the template for the RNA nucleotides, which assemble the complementary molecules 
  4. mRNA moves out of the nucleus and into the cytoplasm where it encounters many ribosomes
• Translation in polypeptide synthesis

  1. Ribosomes move along the mRNA molecule, attaching the tRNA molecules to the mRNA by pairing the anticodons with the codons. 
  2. The amino acids from the tail end of each tRNA are linked to one another by an enzyme to form a polypeptide chain.
  3. Each amino acid is then spliced off its tRNA carrier.
  4. The tRNAs move away from the mRNA, leaving the growing chain of amino acids. 
• Polypeptide synthesis is important for creating proteins that carry out different functions in our bodies including structural support, biochemical catalysts, hormones and enzymes. Proteins are essential for the correct functioning of cells and to produce an overall phenotype in organisms that is free of disorders
• • The phenotype of an organism is the structure, behaviour and physiology of an organism.
• • Gene expression is the translation of genes into their protein end products. 
• • These products determine the physical and chemical features typical of each cell type and the overall phenotype of an organism.
• • Gene expression is regulated by proteins which means that genes must produce the proteins that regulate their own expression.
• If an individual with a given genotype develops in one environment, its phenotype may be different than if it had developed in a different environment.
• examples of environmental effects on gene expression and phenotype includes:

  • Flower colour varies with soil pH
  • Feather colour varies of flamingos with different levels of carotenoids in their diet 
• Proteins speed up chemical reactions, play a role in cell-cell recognition and cellular communication, movement, storage, cell membrane functions and even structural support.
• • Proteins are made up of amino acids. 
  • There are 20 types of amino acids which are joined together in a specific order to form polypeptide chains. 
• • Enzymatic proteins
  • Acts as catalysts in cellular reactions
  • Are biological catalysts: living proteins which speed up chemical reactions occurring in cells
  • They act on a specific molecule (substrate) to either, break it down into a simple substance of synthesise it
  • E.g. lipase and amylase catalyse the breakdown of bonds
• • Immunological proteins (antibodies)
  • Protects against disease by recognised foreign bodies and microbes; activate immune cells
  • E.g. antibodies, complement proteins
• • Structural proteins
  • Provide support as it maintains cell shape and make connective tissue
  • It forms the structural components of cells and organs 
  • E.g. collagen, keratin, cell membranes 
• • Hormonal proteins
  • Coordinate an organism’s activities by triggering responses 
  • are proteins that are secreted into the body by glands (endocrine cells) --> Travel to target tissues, where they cause a change in activity
  • E.g. insulin, glucagon