Nuclear parent cell dividing into two daughter cells
Nuclear division is a
process where a parent cell is divided into two or more daughter cells. There
are two types of cell division processes in multicellular organisms. The first
one is mitosis which is an asexual process. This is due to the parent cell dividing
into two daughter cells which contain the same number of chromosomes as the
parent cell (Lodish et al., 2003).
These cells are called as diploid cells as they contain the full set of
chromosomes (Lodish et al., 2003).
Meiosis is a sexual
process where four daughter cells are produced with half the number of
chromosomes. This type of cell is known as a haploid cell (Alberts et al., 2009). Genetic variation in
multicellular organisms takes place in the process of meiosis and not in
mitosis. In meiosis genetic variation is accomplished by two procedures-
crossing over and independent assortment (Weaver, 2012). The process of crossing
over takes place in the prophase I stage. Crossing over is the exchange of
genetic material between two homologous chromosomes (Weaver, 2012). A section
of the non-sister chromatids of the two homologous chromosomes crosses over
each other. The crossed over section is called a chiasma. At this point, the cohesion
proteins hold the sister chromatids along their entire length (Alberts et al., 2009). Many chiasmata can be
present indicating that many crossovers take place (Alberts et al., 2009). The crossing over process
is also important for proper segregation of the homologous pair (Alberts et al., 2009). The chiasma pulls the
sister chromatids in opposite direction of the spindle fiber contraction and
the cohesion proteins unbind allowing the homologous pair with newly formed
chromatid to separate (Alberts et al.,
2009). Crossing overplays a major role in genetic variation as it increases the
diversity of phenotypes formed.
The second process to
increase genetic variation is carried out according to the law of independent
assortment which was developed by Gregor Mendel. This process is carried during
the metaphase I stage. According to the law, the alleles for the traits are
separated when the gametes are formed. These separated alleles then once again
recombine randomly during fertilization presenting various combinations of
genotypes (Bailey, 2017). The random recombination is valuable as some
mutations that could take place would be canceled out allowing the offspring to
have a better survival chance than the parents (Weaver, 2012). Mendel arrived
at this conclusion by carrying out a cross-pollination experiment for different
colored pea plants (Bailey, 2017).
The crossing over,
independent assortment processes including mutation contribute greatly for the
genetic variation of multicellular organisms.