 Somatic cells reproduce by mitosis
 The daughter cells have the same genetic makeup as the parent
 This is known as asexual reproduction
 If humans were to reproduce asexually then every child would be a duplicate of their parent
 Instead humans reproduce through sexual reproduction
 This involves the sperm and egg cell
 These are called gametes.

 When the sperm cell fuses with the egg cell (known as the oocyte) the
zygote is formed.
 This is known as fertilization

 Consider the following dilemma:
 If each person has 46 chromosomes, and both mother and father pass on their chromosomes, why don’t their children receive 92 chromosomes?

 This dilemma can be explained through the process of
meiosis.
 In meiosis, our cells take 46 chromosomes and produce 4 sex cells, known as gametes, with 23 chromosomes each.
Side note: When we say 46 When we say 23 chromosomes we are talking chromosomes we are talking about the diploid (2n) number about the haploid (n) number.

 This figure shows a
diploid parent cell divides to form two
haploid cells.
 The two haploid cells will then copy and divide to form 4 haploid cells.

 There are two stages of meiosis:
Meiosis 1 and Meiosis 2.
 Meiosis 1 produces two cells with
23 chromosomes.
 In meiosis 2, the two cells made in meiosis 1 will divide to form four cells with 23 chromosomes.
Let us take a look at Meiosis 1 first.

Interphase 1
During this stage the DNA is contained within the nucleus tightly packed. The DNA comes together to form structures known as
chromatin.
Prophase 1
At this stage the nuclear membrane begins to dissolve, the centriole
pairs begins to split and move to the poles of the cell. The homologous
chromosomes pair and exchange part of their DNA.

Metaphase 1
At this stage the homologous chromosomes pair up and attach themselves to the spindle and line up on the middle of the cell, known as the equatorial plate.
Anaphase 1
At this stage the homologous chromosomes move toward opposite poles in a process known as segregation.

 During telophase 1, a membrane begins to form around each nucleus on both poles of the cell.
 This causes a process known as cytokinesis to occur.

 The phases of meiosis 2 are similar to the phases of mitosis
 The key difference is that the cell that undergoes meiosis 2 is haploid instead of diploid
 In other words, there are 23 chromosomes (haploid) instead of
46 chromosomes (diploid)

 Use the link below to play a video that explains meiosis in depth. This video may have details we have not discussed in class. You are not responsible for this information
 Use video to help guide your learning
 http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter28/ani mation__how_meiosis_works.html
 Or try: http://highered.mcgrawhill.com/olc/dl/120074/bio19.swf

1. What is the difference between a gamete and a zygote?
2. If the diploid number of chromosomes for dogs is 78 then how many chromosomes are in the gamete cell of a dog?
Explain.
3. Compare the stages of mitosis and meiosis. What are their similarities? What are their differences?