CHROMOSOMES As a eukaryotic cell prepares to divide the DNA of that cell is copied. The DNA molecule then coils into a structure called a chromosome. The two copies of DNA that make up each chromosome are called chromatids. The two chromatids of a chromosome are attached at a point called a centromere.

Sister Chromatids:

Chromosome Number & Structure Somatic Cells – body cells; any cell in the body other than a sperm or egg cell Each human somatic cell has 2 copies of 23 different chromosomes for a total of 46 chromosomes. Gamete Cells – reproductive cells; egg or sperm cells Each human gamete cell has 1 copy of 23 different chromosomes for a total of 23 chromosomes.

*somatic cells – are considered to be diploid because they contains 2 sets of chromosomes *are known as homologous chromosomes – they are chromosomes that are similar in size, shape, and genetic content (1 set comes from the mother & 1 set comes from the father) *gamete cells – are considered to be haploid because they contain only 1 set of chromosomes

Biologists use the symbol n to represent 1 set of chromosomes. Therefore, the haploid number in a human gamete can be written as n=23. The diploid number in a somatic cell can be written as 2n=46. Diploid Haploid 18 ? ? 3 24 ? ? 40 9 6 12 80 Somatic Cell Gamete Cell

Autosomes vs. Sex Chromosomes *autosomes – chromosomes that are not directly involved in determining the sex (gender) of an individual; of the 23 pairs of chromosomes in human somatic cells, 22 pairs are autosomes *sex chromosomes – 1 of the 23 pairs of chromosomes in humans determines the sex of the individual ex: X and Y chromosomes Humans: XX = female XY = male

Karyotypes: *A karyotype is a photograph of the chromosomes in a dividing somatic cell that show the chromosomes arranged by size. homologous chromosomes

Normal Female Karyotype Chromosomes #1-22 = Autosomes Chromosome #23 Sex Chromosomes Female

Normal Male Karyotype Chromosomes #1-22 = Autosomes Chromosome #23 Sex Chromosomes Male

Chromosomal Mutations 1. deletion – a piece breaks off 2.  duplication – a chromosome fragment attaches to its homologous chromosome which then has 2 copies of the gene in the fragment 3.  inversion – a chromosome piece reattaches to the chromosome but in reverse 4. translocation – a piece from another non-homologous chromosome reattaches

Chromosome Mutations