1. 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.

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

3. Chromosome Number & Structure
Somatic Cells – body cells; any cell in the body 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.

4. *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

5. 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 ? ? ? ? 9 6 12 80

6. 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

7. 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.

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

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

10. Nondisjunction What Should Happen Nondisjunction
The failure of homologous chromosomes or sister chromatids to separate properly during meiosis.
What Should Happen
Nondisjunction

11. 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

12. Chromosome Mutations