Re: Why do we have a special type of cell division used for sexual reproduction

Good question Robby.

    Human beings are diploid organisms.  That means we have two 
sets of chromosomes; one donated by Mom (egg) and one donated 
by Dad (sperm).  All cells in the body have these two sets of 
chromosomes, except the gametes (egg or sperm).  The gametes 
only have one set of chromosomes.

     There are two mechanisms for cell division, mitosis and meiosis. 
Mitosis is more accurately called cell replication.  When a cell enters 
the cell cycle, everything in the cell is replicated (an identical copy is 
made), including the chromosomes.  Once that is complete, the cell 
undergoes mitosis.  The end result of mitosis is two identical 
daughter cells, each having two sets of chromosomes (the number  
with which the cell originally started).  
    During sexual reproduction the egg and the sperm combine to 
form a zygote (fertilized egg) that has two sets of chromosomes.  
Development would not occur normally if the fertilized egg had four 
sets of chromosomes, so the gametes (egg or sperm) must be 
formed by a process of cell (and chromosome) division.
    Meiosis is the process that germ cells (precursors to the gametes) 
undergo to become gametes.  It is more accurate to call this process 
cell replication/division because the end result is four daughter cells, 
each having one set of chromosomes.  In the 
first round of meiosis, the original germ cell divides to form 
gametes with half the amount of chromosomes as the original cell.  In the 
second round of meiosis, the daughter cells are replicated (much like in 
mitosis).  The gametes are haploid, meaning they have only one set of 
chromosomes.

    We have this special form of cell replication/division to be able to 
generate gametes (haploid cells).  If the germ cells merely 
underwent mitosis, the diploid gametes would not be able to 
combine to form a normal embryo.