Chromosomes

Chromosomes

1. Bacterial chromosomes

   • Prokaryotes
   • single circular DNA molecule
   • one chromosome
     • all genes needed for life
   • not associated with proteins - naked DNA
   • single copy of gene except cell division

2. Plasmids

   • in some prokaryotes
   • Eukaryotes don't have
   • small extra DNA molecules
   • circular; naked
   • contains few genes not necessary for life
   • not replicated at the same time or rate
   • copies can be transferred
   • can cross species barrier
     • released at death and absorbed by new cell
     • natural
     • artificially possible

3. Differences between chromosomes

   • diff. chromosomes carry diff. genes
   • length
   • position of centromere
   • at least two different types
   • different gene sequnece

4. Eukaryote chromosomes

   • linear DNA molecules
   • histone proteins
     • globular in shape
     • wider than DNA
     • many with DNA wound around
     • adjacent separated by short DNA stretches
   • made of DNA and proteins

5. Homologous chromosomes

   • same gene sequence but not alleles
   • allows species to interbreed

6. Haploid nuclei

   • one chromosome of each pair
   • one full set of chromosomes
   • gametes have it

7. Diploid nuclei

   • pairs of homologous chromosomes
   • two full sets of chromosomes
   • zygote has it
   • all cells except gametes are diploid
   • two copies of each gene
   • dominant allele prevents effects of recessive mutation
   • more vigorous organisms

8. Chromosome numbers

   • characteristic feature of species members
   • only same numbers can interbreed
   • can change due to evolution
     • rare and unlikely

9. Sex determination

   • by sex chromosomes
     • X chromosome
       • large
       • centromere near middle
       • all humans have at least one
       • females have 2
       • one passed on in egg cell
     • Y chromosome
       • smaller
       • centromere near end
       • one in males
         • SRY or TDF
   • autosomes
     • chromosomes that don't determine sex
   • half sperm have X; half have Y
   • sex determined at fertilization

10. Karyograms

    • shows chromosomes of organisms
      • homologous pairs
      • decreasing lengths
    • visible in cell in mitosis
      • clearest: metaphase
    • some stains - distinctive banding on diff. chromosomes
    • arranged in pairs by length
      • done manually before
      • now digital
    • micrograph taken of dividing cells
    • types distinguished by:
      • position of centromere
      • banding pattern
      • size/length