The most important fact of mitosis is that each
daughter cell has the exact same genetic make-up
as the original cell.
Gregor Mendel – The Father of Genetics
- didn’t know where genes were located
- described in detail how genes must move in the formation
of gametes and subsequent fertilization
- each organism must inherit a single copy of every gene
from both of its parents
- each offspring therefore has two copies of each gene
- these two copies must be separated to form the
gametes of this organism
Chromosome Number

Normal body cells contain two copies of each chromosome




Diploid – term used to describe a cell with homologous
chromosomes
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

one received from each of the two parents
Homologous chromosomes
Same shape, size, and contain the same genes in same order
Symbol – 2N
Found in all normal body (somatic) cells
Haploid – term describing a cell with a single copy of each
chromosome


Symbol – N
Found in gametes (sex cells)
Phases of Meiosis
Meiosis is a process of reduction division in which the chromosome
number per cell is cut in half through the separation of homologous
chromosomes in a diploid cell. (Diploid  Haploid) (2N  N)
- requires two distinct divisions – Meiosis I and Meiosis II
- allows organisms to reproduce sexually and maintain the normal
diploid number in the offspring
Meiosis I = Reduction Division (Figure 11.15, pg 276)
- prior to meiosis I the chromosomes replicate
A. Prophase I
- nucleolus, nuclear membrane break down
- centrioles migrate to poles
- homologous chromosomes pair up to form tetrads – 4 chromatids
- crossing-over occurs
- homologous chromosomes exchange portions of themselves
which results in a mixing of genes between the two
chromosomes
Crossing-Over
Section 11-4
B. Metaphase I
- tetrads line-up at the equator of the cell
C. Anaphase I
- homologous pairs separate
- sister chromatids stay connected at their centromeres
D. Telophase I
- nuclear membranes reform around the chromosomes and
cytyokinesis takes place
- each cell is now haploid
** No DNA replication takes place before Meiosis II
2. Meiosis II (Figure 11.15, page 277)
- each cell’s chromosomes consist of two chromatids connected at the
centromere
A) Prophase II
- just like prophase in mitosis
B) Metaphase II
- chromosomes align at center of cell
C) Anaphase II
- sister chromatids are separated at the centromere and are pulled to
opposite poles
D) Telophase II
- new nuclear envelopes appear
- cytokinesis occurs
Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
Telophase II
Meiosis II results in four
haploid (N) daughter cells.
Results of Meiosis = Four haploid cells which are
genetically unique
Gamete formation (Figure 11.17, page 278)
- in male animals and the pollen grains of plants the haploid gametes are
called sperm cells
- in female animals generally only one of the cells formed by meiosis
develops into an egg
- in female animals uneven cytokinesis at the end of Meiosis I and
Meiosis II result in a large egg and 3 polar bodies