Which of the following best defines
a haploid cell? (A) A cell that contains two copies
of each pair of homologous chromosomes. (B) A cell that has two copies of
each chromosome. (C) A cell that divides to form two
identical daughter cells. Or (D) a cell that only has one
copy of each chromosome.
As you may know, nearly every cell
in the body contains genetic material in its nucleus. This genetic material is carried by
a molecule called DNA. DNA is usually stored in a highly
coiled and condensed form that creates a densely packed structure known as a
chromosome. Human cells generally have 46
chromosomes, arranged into 23 pairs. The two chromosomes in each pair
are similar in size and appearance and carry the same genes at the same
position. These pairs are called homologous
pairs of chromosomes.
When a cell has its full set of
chromosomes, which would be 46 chromosomes in the case of human cells, it is called
a diploid cell, which is often also represented as 2n. It’s important to note that a
duplicated chromosome isn’t counted as two separate chromosomes. When cells divide to form new
cells, they must pass on copies of their chromosomes so that the newly formed cells
contain the genetic material they need to function.
There are two types of cell
division: mitosis and meiosis. Mitosis involves the division of a
cell into two identical daughter cells. During mitosis, the number of
chromosomes in a cell doesn’t change. The parent cell simply duplicates
each of its chromosomes and then splits into two, providing one copy of the
chromosomes to each new cell. Mitosis happens during growth and
development of an organism as well as to replace old or damaged cells.
Meiosis, on the other hand, is
important for the process of reproduction. This type of division only happens
in the gonads, which are the reproductive organs, in order to produce gametes, or
sex cells, that is, sperm or egg. In meiosis, a diploid cell divides
twice to form four haploid gametes. If the diploid cell had, say, just
two total chromosomes, or one pair, during the division the pair will be separated
so that only one chromosome of that pair ends up in the gametes.
In the case of human cells that
have 46 total chromosomes, or 23 pairs, this will mean that each haploid gamete has
only 23 total chromosomes, or half of the total chromosomal set of that
individual. These haploid cells are created so
that they can combine with the haploid cell of another individual through
fertilization to create a new offspring with the same number of chromosomes as the
parents, 46 in this case.
So what is the best definition of a
haploid cell? That would be (D). A haploid cell is a cell that only
has one copy of each chromosome.