• Sexual reproduction involves the combining of two parent cells to
create a completely new cell, which becomes the offspring.
• Most cells in the human body contain 23 pairs of chromosomes (46
total). These cells are called a diploid cell.
• Since sexual reproduction involves the combining of two cells, sex
cells contain 23 unpaired chromosomes. These cells are called
haploid cells.
• Gametes are cells that contain half the usual number of
chromosomes—one chromosome from each pair.
• An egg is a gamete that forms in the female reproductive
organs.
• Sperm is a gamete that forms in the male reproductive organs
• Fertilization is the process that
takes place when a sperm and
an egg combine to form one
new cell.
• The fertilize egg (the newly
formed cell) is called a zygote.
• In order to produce gametes,
which are haploid, a different
type of cell division is necessary.
• Meiosis is a special type of cell
division that produces haploid
cells.
• During meiosis, a single cell goes
through two cell divisions—
meiosis I and meiosis II and takes
place only in the reproductive
organs.
• Meiosis I
1. Prophase I—Chromosome condense.
The threads of DNA coil up and
become visible.
2. Metaphase I—Chromosome pairs line
up. The pairs line up in the middle of
the cell.
3. Anaphase I—Chromosome pairs
separate. The pairs pull apart. This
step divides the alleles of each gene.
4. Telophase I and Cytokinesis—Two cells
form. The chromosome are in two
groups.
• Meiosis II
• Begins with the two cells from meiosis I.
• 1. Prophase II—Chromosomes are still
attached. Cell prepares to divide again.
Each cell has just one set of chromosome, but
each chromosome is still made of two copies
(chromatids) attached together.
• 2. Metaphase II—Chromosomes line up. The
unpaired chromosomes line up along the cell’s
center.
• 3. Anaphase II—Chromatids separate. The
two attached copies of each chromosome
separate and go to opposite ends.
• 4. Telophase II and Cytokinesis—Haploid cells
form. Each cell divides into two daughter cells,
producing a total of 4 cells. Each haploid
(daughter cell) has one copy of one unpaired
set of chromosomes.
• Only cells that are to become gametes go through meiosis. All
other cells divide by mitosis.
• Meiosis involves two cell divisions, but the chromosomes are not
copies before the second division, whereas in mitosis, the
chromosomes are always copied before division.
• Daughter cells produced by meiosis are haploid cells
• Daughter cells produced by mitosis contain exactly the same
genetic material as the parent cell.