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