Chromosomes

• We usually see chromosomes trapped at metaphase of mitosis
• Banded chromosomes make distinction possible
• Chromosomes of eukaryotes contain DNA and protein
• One long DNA double helix is present per chromatid
• DNA is highly organized in association with histones (protein)
• 5 histones are highly conserved from 1 species to another
• 4 histones (2 copies of each) make a ball
• 1 histone links adjacent balls together
• 146 base pairs of DNA wrap around each ball (2 loops per ball)
• Each ball is a nucleosome
• Nucleosomes are coiled into solenoids (6 nucleosomes per selenoid)
• Solenoids coil, forming a fiber
• Fibers coil to give a visible chromosome
• Chromosomes can best be seen when they are highly condensed at metaphase
• Stains such as Giemsa that bind to DNA make condensed chromosomes visible
• Adding colchicine to actively dividing cells destroys spindle fibers and traps the chromosomes at metaphase
• Individual chromosomes can be cut from a picture and aligned by pairs
• The largest chromosome pair is 1, next is 2, etc
• A picture of all chromosomes shows the karyotype of the individual
• karyotype: chromosome composition
• Humans have 23 pairs of chromosomes, 22 pairs of autosomes, and 1 pair of sex chromosomes
• Special techniques produce banding patterns that permit chromosomes of similar size and shape to be distinguished
• Fluorescent probes identify individual chromosomes based on DNA hybridization
• Dyes that result in different colors help in karyotyping
• Chromosomes vary in length of the arms and position of the centromere
• telocentric: the centromere is at the end of the chromosome
• metacentric: the centromere is near the middle
• submetacentric: centromere is slightly off-center
• acrocentric: centromere is very off center
• Centromeres provide the point of attachment for pulling daughter chromosomes to opposite poles in mitosis and meiosis
• Telomeres cannot replicate using DNA polymerase
• telomeres: short, highly repeated sequences that protect the ends of chromosomes from exonucleases
• Telomeres grow shorter with age
• Telomeres are maintained by telomerase enzyme in germline cells and some cancer cells (required for "immortality")
• Telomerase is a ribozyme (has an RNA component)
• Humans have 46 (23 pairs) chromosomes per diploid cell
• Dogs have 78 (39 pairs) of chromosomes
• Cats have 38 (19 pairs) of chromosomes
• Horses have 64 (32 pairs) of chromosomes
• Ducks have 80 chromosomes