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Chromosomes
(Eukaryotic DNA Structure)
• When the cell is not dividing, DNA is long and stringy and called chromatin.
• When the cell is dividing, DNA is tightly coiled around proteins and is called chromosomes.
Chromosome Numbers
• Each species has a specific number of chromosomes in the nucleus in its cells.
• For example, humans have 46 chromosomes in each body cell, dogs have 78 in each body cell.
Chromosome Numbers
• There is a tongue fern that has 1262 in each body cell!
• The number of chromosomes that an organism has is NOT related to the complexity of the organism!
Chromosome Numbers
• In body cells, chromosomes come in pairs.• Sex chromosomes are the pair of chromosomes
that determine the sex of the organism. • In humans, the sex chromosomes are X and Y. • Normal females have 2 X chromosomes, and
normal males have 1 X and 1 Y.
Chromosome Numbers
• All of the other chromosomes are called autosomes.
• So, in humans:– 22 pairs or 44
chromosomes are autosomes
– 1 pair or 2 chromosomes are sex chromosomes
There are certain disorders associated with having too little or too many chromosomes.
Chromosome Abnormalities
1. Down syndrome – 3 copies of chromosome #21 (trisomy 21)
Chromosomes Abnormalities
2. Turner syndrome – female with only 1X chromosome (XO) (monosomy X)
Chromosome Abnormalities
3. Klinefelter syndrome – male with XXY
Chromosome Abnormalities
4. Patau syndrome – 3 copies of # 13
• Doctors can detect these conditions before birth:– Amniocentesis – removes amniotic fluid from
around baby …
– Chromosomes are photographed and organized onto a chart called a karyotype.
• Chromosomes come in pairs because you inherited one copy of the chromosome from your mother and on copy of the chromosome from your father.
• The two chromosomes in each pair are called homologous chromosomes because they have the same size and shape and carry the same genes.
• Most cells in your body are diploid (2n), which means they have 2 sets of chromosomes (one from each parent). The diploid number represents how many chromosomes are in these cells.
• Sex cells (a.k.a. gametes, a.k.a. sperm and egg) are haploid (n), which means they only have 1 set of chromosomes. The haploid number will always by half the diploid number.
Organism # of chromosomes in body cells (diploid number)
# of chromosomes in sex cells (haploid number)
Human 46
Fruit fly 4
Dog 78
Sand dollar 26
Cat 16
Carrot 18
horse 64