Unit 1 - higher biology

• DNA


  Deoxyribonucleic acid found in the nucleus of cells that stores genetic information in its sequence of bases which determines the order of amino acids and structure of proteins

  
• DNA molecule


  • Consists of two strands of repeating units called nucleotides
  • Each nucleotide consists of a molecule of deoxyribose sugar, a phosphate group, and a base

  
• Nucleotides


  Linked by chemical bonds between the 3' sugar of one and the 5' phosphate of the next, resulting in a 5' end and 3' end of each strand
• DNA strands


  Run antiparallel to each other in opposite directions
• Base pairing


  Adenine pairs with thymine, guanine pairs with cytosine
• Double helix


  DNA forms a twisted coil due to the alignment of complementary base pairs
• DNA organisation in prokaryotes

  • Single circular chromosomes
  • Small circular plasmids

  
• DNA organisation in eukaryotes

  • Linear chromosomes in the nucleus
  • Circular chromosomes in mitochondria and chloroplasts
  • Circular plasmids in yeast

  
• DNA replication


  Semi-conservative - each new DNA molecule consists of one original parent strand

  
• DNA replication enzymes


  • DNA polymerase and Ligase

  
• DNA replication


  1. Unwind double helix

  2. Break hydrogen bonds between complementary bases

  3. Expose bases at replication fork

  4. Primers bind to 3' end of template strand

  5. DNA polymerase adds complementary nucleotides to 3' end of new strand

  6. Ligase seals fragments on lagging strand

  
• Requirements for DNA replication

  • DNA template
  • Primers
  • DNA nucleotides
  • Enzymes (DNA polymerase and ligase)
  • Supply of ATP

  
• Polymerase chain reaction (PCR)


  Technique used in the laboratory to amplify DNA or create many copies of DNA

  
• Stages of PCR


  1. Heat DNA to 92-98°C to separate strands

  2. Cool to 50-65°C to allow primers to bind

  3. Heat to 70-80°C to allow DNA polymerase to replicate region

  
• After each PCR cycle, the number of copies of DNA is doubled
• Negative control in PCR

  Contains all requirements for PCR except sample DNA
• Gel electrophoresis


  Separates amplified DNA fragments by size as they migrate through an agarose gel in an electrical field
• Requirements of PCR


  • DNA template
  • Primers
  • Nucleotides
  • pH buffer
  • Heat-tolerant DNA polymerase

  
• Practical applications of PCR


  • Solving crimes
  • Settling paternity suits
  • Diagnosing genetic disorders
• Genotype


  Determined by sequences of DNA bases in genes

  
• Phenotype


  Determined by the proteins that are made when genes are expressed, influenced by environmental factors
• Differences between DNA and RNA

  • DNA is double-stranded, RNA is single-stranded
  • DNA has deoxyribose sugar, RNA has ribose sugar
  • DNA has adenine, thymine, guanine and cytosine, RNA has adenine, guanine, cytosine and uracil
  • DNA is found in the nucleus, RNA is found in the nucleus and cytoplasm

  
• Types of RNA


  • mRNA (messenger)
  • rRNA (ribosomal)
  • tRNA (transfer)

  
• Transcription of DNA into mRNA

  1. RNA polymerase binds to promotor

  2. Unwinds double helix

  3. Breaks hydrogen bonds

  4. Synthesises primary transcript

  
• RNA splicing


  Introns (non-coding regions) are removed, exons (coding regions) are joined to form mature transcript

  
• Alternative RNA splicing


  Different combinations of exons are spliced to form alternative mature transcripts from one gene

  
• Translation


  Synthesis of protein using the mRNA instruction

  
• Genetic code


  Made up of a series of base triplets called codons
• tRNA


  Single-stranded RNA molecule with an anticodon (triplet of bases) that binds to mRNA codon and carries a specific amino acid

  
• Translation


  1. Begins at start codon, ends at stop codon

  2. tRNA anticodons bind to mRNA codons by complementary base pairing

  3. Amino acids are joined by peptide bonds to form polypeptide

  
• Proteins


  Polypeptide chains made of amino acids held together by peptide bonds, folded into 3D shape

  
• Cellular differentiation


  Process by which a cell expresses certain genes to produce proteins characteristic for that cell type, allowing specialised functions

  
• Meristems in plants


  • Regions of unspecialised cells that can divide and/or differentiate into specialised plant cells
  • Found at shoot and root tips, and in side shoots
• Stem cells in animals

  • Unspecialised cells that can divide and/or differentiate
  • Embryonic stem cells are pluripotent, tissue (adult) stem cells are multipotent
• Therapeutic uses of stem cells

  • Bone marrow transplants
  • Corneal damage treatment
  • Temporary skin grafts
• Research uses of stem cells


  • Model cells to study disease development and drug testing
  • Provide information on cell processes like growth, differentiation and gene regulation
• Ethical issues with stem cell use

  • Destruction of embryos for embryonic stem cells
  • Sourcing from IVF embryos or amniotic fluid
  • Use of induced pluripotent stem cells
• p or can be used in drug testing
• Stem cell research provides information on how cell processes such as cell growth, differentiation and gene regulation work
• Ethics


  Moral values and rules that apply to human conduct