1.6

• Chromosomes


  Consist of DNA and a protein called histone, and are only visible after they condense at the onset of cell division. Following DNA replication, a chromosome exists as two identical 'sister' chromatids joined by the centromere. Sister chromatids are genetically identical. Each chromosome contains genes that code for specific polypeptides.
• Codon


  A sequence of three nucleotides in DNA or RNA that specifies a particular amino acid
• Different species have different numbers of chromosomes e.g. humans have 46, whilst a potato has 48
• Homologous chromosomes


  Chromosomes that contain the same genes but may be different versions or alleles
• Diploid


  An organism with two complete sets of chromosomes
• Tetraploid


  An organism with four complete sets of chromosomes
• Haploid


  An organism with one complete set of chromosomes
• Cell cycle


  1. Interphase

  2. Mitosis
• Interphase


  The longest phase in the cell cycle, where DNA, protein and organelles are synthesised. The quantity of DNA doubles, though the chromosome number remains the same.
• Mitosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Mitosis


  • Results in the production of two genetically identical cells. It is important in growth and repair when differentiated cells replicate.
• Cytokinesis


  In animal cells, the membrane infolds via a cleavage furrow, until the two cells become separated. In plant cells, a cell plate develops from the centre outwards, until the cell is divided into two.
• Meiosis


  1. Interphase

  2. Meiosis I

  3. Meiosis II
• Meiosis I


  Homologous chromosomes separate. Crossing over can occur, increasing genetic variation. Bivalents align randomly at the equator, known as independent assortment, giving further genetic variation.
• Meiosis II


  Similar to mitosis, with no pairing of homologous chromosomes.
• Meiosis generates genetic variation through crossing over and independent assortment, and keeps the chromosome number constant by producing haploid gametes that recombine during fertilisation, restoring the diploid number in the zygote.