6.2

• Chromosomes


  Thread-like structures located inside the nucleus of animal and plant cells
• Chromosomes


  • Each chromosome is made of protein (histone) and a single molecule of deoxyribonucleic acid (DNA)
  • Chromosomes can be seen through a microscope when the nucleus dissolves during cell division
• Haploid


  • Refers to one set of chromosomes (n)
  • In humans, (n) = 23 chromosomes
  • Only gametes (sperm and ova) are haploid
• Diploid


  • Refers to pair of chromosomes (2n), having one from each parent
  • In humans, (2n) = 46 chromosomes
  • Only somatic (body) cells are diploid
• Chromosomes


  • Linear in eukaryotes
  • Circular in prokaryotes
  • The number of chromosomes is constant in each species
  • Each chromosomes carries a unique set of genes
  • Chromosomes are passed on to daughter cells during mitosis and to germ cells during meiosis
• Chromosomal coiling and structure

  1. DNA (about 2m in every human cell) is wound and histones forming a nucleosome

  2. The DNA is then coiled and super coiled to form the chromosome structure we see during mitosis
• Chromosome structure


  • Each chromosome (linear) consists of: Centromere, Chromosome arms
  • The centromere rarely in the exact middle and hence the short arm
• Chromosome 1


  Longest chromosome with 200 genes
• Y chromosome


  Shortest chromosome with just over 200 genes
• Homologous chromosomes


  • Chromosomes that contain the same genes at the same location (loci)
  • Are usually in pairs in diploid cells
  • Pair up during metaphase 1 in meiosis
  • Alleles can be different
• Chromatids


  • Identical DNA molecules
  • Formed during S phase in the cell cycle
  • Contain identical alleles
• Types of chromosomes


  • Autosomes
  • X chromosomes
  • Y chromosomes
• Sex determination


  1. Male gives X and Y chromosomes

  2. Female gives X and X chromosomes

  3. Gender is determined by male
• Species with different chromosomes

  • Birds, butterfly and strawberries have Z and W chromosomes
• Polyploidy


  • More than 2 sets of chromosomes in a genome. E.g., 3n, 4n, 6n...
  • Can occur through errors in meiosis
  • In humans, polyploidy zygotes don't survive
  • Can also occur through mitosis producing somatic polyploidy cells, but these may not affect health
• Polyploidy can result in larger and more vigorous plants, but it can result in low fertility rates
• Colchicine can be used to artificially introduce polyploidy to plants
• Mitochondrial DNA (mtDNA)


  Contains the code for the proteins that make up the mitochondria. There are 37 genes in human mtDNA
• Chloroplast DNA (cpDNA)


  Contains about 100 genes which code for the proteins involved in photosynthesis