If the cell does not duplicate its DNA before
it divides, the daughter cells will be
haploid (one copy of each chromosome) instead
of diploid (two copies of each chromosome).
This is assuming the organism is diploid; some
animals such as certain species of frogs are
tetraploid, meaning there are 4 copies of
each chromosome in every cell!
DNA must be duplicated to ensure that each
cell gets the correct number of chromosomes.
Humans are diploid, meaning that we inherit one
set of chromosomes from mom and the other from
dad. Because of this, we always have two
alleles for every gene. This is why you could
have blue eyes even though your parents both have
brown eyes - if each parent gives you one copy of
a recessive "blue eye" allele, you will end up
with this recessive trait. If the cell only
gets one set of chromosomes, it will only express
the genes from that set. Most likely this cell
will not survive unless it is a gamete, which
must be haploid because two gametes will
eventually come together to form a diploid
Sister chromatids are joined so that they
can be aligned at the metaphase plate,
allowing them to be separated appropriately into
the two daughter cells. When the cell is not
undergoing mitosis, all the chromatin will
unravel and none of the chromosomes will be joined
- this is only something that happens to enable
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