DNA

• DNA molecule


  • Twisted ladder or double helix structure
• Nucleotide


  Smaller molecule that makes up nucleic acids like DNA and RNA
• Components of a nucleotide


  • Sugar
  • Nitrogenous base
  • Phosphate group
• Nitrogenous bases in DNA


  • Adenine (A)
  • Thymine (T)
  • Cytosine (C)
  • Guanine (G)

  
• Base pairing in DNA

  Adenine pairs with Thymine, Cytosine pairs with Guanine

  
• DNA backbone


  Phosphates and sugar deoxyribose
• Gene


  Sequence of nucleotides on the DNA molecule that codes for an entire protein
• Chromosome


  • Made of 2 chromatids joined at the centromere
  • Different organisms have different number of chromosomes
  • Chromosomes occur in pairs (homologous)
• Somatic cells


  Have a diploid (2n) number of chromosomes
• Gametes

  Have a haploid (n) number of chromosomes
• Alleles

  Different forms of the same gene
• Homozygous alleles


  Both alleles are the same
• Heterozygous alleles

  Alleles are different
• Dominant allele


  Always shows in the person
• Recessive allele


  Only shows when it is on its own or when both alleles are the same (homozygous recessive)
• Mutation


  Change in the chemical structure of a gene or chromosome that changes the way an organism develops
• Causes of mutations

  • Radiation (alpha, beta, gamma, X-rays, UV light)
  • Chemicals (mutagens, carcinogens)
• Types of mutations

  • Chromosome mutations
  • Gene mutations
• Gene mutation


  Change in a single base in the DNA sequence
• Gene mutations


  • Albinism
  • Cystic fibrosis
• Chromosome mutation


  Mistake in the number of chromosomes, e.g. Down's syndrome
• Cell division

  Important for: The body to grow, Passing chromosomes from one cell to another

  
• Types of cell division

  • Meiosis
  • Mitosis

  
• Meiosis

  Occurs in the gametes (reproductive organs)
• Mitosis


  Occurs in all the other cells in the body (somatic cells)
• Mitosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase

  
• Prophase

  • Chromosomes become visible inside the nucleus, They shorten and thicken (Chromatids), Spindles (magnets) form at the sides of the nucleus
• Metaphase


  • The chromatids move to the middle of the nucleus
• Anaphase

  • The chromatids are drawn apart and become chromosomes which move towards the spindles
• Telophase


  • The chromosomes pull apart and the cell divides into two cells, each with 46 chromosomes, these become the daughter cells. The daughter cells are exact copies of the original cell.
• Mitosis occurs for growth of the body and as asexual reproduction in certain organisms such as the Amoeba
• Meiosis

  1. Prophase 1

  2. Metaphase 1

  3. Anaphase 1

  4. Telophase 1

  5. Metaphase 2

  6. Anaphase 2

  7. Telophase 2

  
• Prophase 1

  • There are 23 pairs of chromosomes at the start of meiosis, so 46 in total in the reproductive cells in the ovary or testis. One pair of each pair is from the mother and the other from the father. Chromosomes are photocopied.

  
• Metaphase 1


  • The chromosomes move towards the middle of the nucleus on top of each other
• Anaphase 1


  • The chromosomes divide and move away from each other due to the spindles (magnets) which form on the side of the nucleus
• Telophase 1


  • The chromosomes split into two cells. This is done randomly so each cell has a mixture of the father's and mother's chromosomes.
• Metaphase 2


  • The chromosomes line up in the middle of the nucleus
• Anaphase 2

  • The chromosomes split and move away from each other towards the spindles on the side of the nucleus
• Telophase 2


  • The nucleus splits into 2 with 23 chromosomes each (4 nuclei). Four daughter cells form, each with half the number of chromosomes of the parent cell. These are known as gametes.
• Differences between Mitosis and Meiosis


  • One division occurs in one cycle (Mitosis)
  • Two divisions occur in one cycle (Meiosis)
  • Two (2) daughter cells are produced in one cycle (Mitosis)
  • Four (4) daughter cells are produced in one cycle (Meiosis)
  • Daughter cells are genetically identical known as clones (Mitosis)
  • Daughter cells are produced genetically different from each other (Meiosis)
  • Daughter cells have same chromosome number as parents (46) (Mitosis)
  • Daughter cells have half the chromosomes of the parent cells (23) (Meiosis)
  • This only occurs in somatic cells (Mitosis)
  • This occurs in reproductive cells (sperms & eggs) (Meiosis)
  • Mitosis happens throughout the life cycle
  • Meiosis is completed after sexual maturity