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.

A gene is a segment of DNA that codes for a particular protein.

Chromosome Number & Structure Somatic Cells – body cells; any cell other than a sperm or egg cell Gamete Cells – reproductive cells; egg or sperm cells Each human somatic cell has 2 copies of 23 different chromosomes for a total of 46 chromosomes. Each of the 23 pairs of chromosomes are considered homologous chromosomes or homologues.

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

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

Changes in Chromosome Number Humans who are missing even 1 of the 46 chromosomes rarely survive. *trisomy – humans with more than 2 copies of a chromosome; they survive but do not develop properly ex: Down Syndrome – the individual has 3 copies of chromosome 21 *karyotype – a photo of the chromosomes in a dividing cell that show the chromosomes arranged by size; used to detect abnormalities in chromosome number.

Normal Karotype homologous chromosomes Chromosomes 1-22 = autosomes sex chromosomes = female

Karotype of Trisomy 21 Notice chromosome 21 has three chromosomes. sex chromosomes = male

MUTATIONS A mutation is any change in an organisms DNA (genetic information). Two Types of Mutations: Chromosomal Mutations – chromosomal mutations are changes in the number or structure of a chromosome. 2. Gene Mutations – gene mutations are changes in a single gene ( will discuss at a later date!)

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