One organic molecule that a cell
must have is DNA (deoxyribonucleic Acid). On this molecule all of
the information for the individual is stored. DNA is normally found
in a long thin, stringy threadlike state called Chromatin. It is
found in the nucleus of a cell that is in a part of the cell cycle called
interphase. The DNA code describes the kind of proteins the cells
must produce. Sections of DNA called genes code for different
proteins. The actual code for the genes depends on what kind of
nucleotide the DNA is made of AND the order in which the nucleotides are
arranged.If there is such a thing as a typical humans chromosome, it
would contain a half of a billion nucleotide bases, and would be about 2
inches long if it were laid out. DNA in an animal cell is double
stranded. DNA resembles a ladder that is twisted which is why this
nucleic acid is said to be a double helix. As DNA enters the m phase
of the cell cycle (mitosis) chromatin tightly coils around tiny spheres of
protein called histones. This action causes the DNA to "condense"
into a tight, organized form of DNA called a chromosome. The
chromosome structure allows the DNA to become more organized and allows
the cell to distribute the DNA into newly formed cells easily and
efficiently. It is like a spool of thread. While wound on a
spool, the thread is organized and easier to manipulate. If the
thread were taken off the spool and placed in a pile on the floor the
thread is disorganized and harder to work with.
All eukaryotic organisms have DNA in the form just described. in a
prokaryotic organism (bacteria) DNA is seen as a single piece of circular
DNA that is attached to the cell membrane of the bacteria.
The number of chromosomes in a cell depends on the type of organism and
the type of cell it is. If it is a somatic (body) cell it has twice
the number of chromosomes than a gamete (sex cell). The somatic
cells of a normal human contain 46 chromosomes while the sex cells have 23
chromosomes. Gametes are said to be haploid and are signified by the
letter n. in the human gamete n=23. Somatic cells are
diploid and contain two times the number of chromosomes in a haploid cell.
Diploid cells are signified by being 2n, which in human cells that
number is 46.
There are 23 pairs of chromosomes in the human cell. Half of the
pairs comes from each parent. Sexual reproduction requires two
parents to produce an offspring. The gender of that offspring in the
human depends on the sex chromosomes. A female has what is called a
pair of X chromosomes. The male gender has an X and a Y chromosome.
In Humans, the gender of the individual is determined by the presence of
the Y chromosome. The female shows female traits not because she has
2 X chromosomes, but because she lacks the Y chromosome. This is not
the case in all organisms. A quick example is the fruit fly
Drosophila melanogaster. The gender of this species is
determined by the presence of a second X chromosome. A male occurs
because he does not have a second X. In fact, even though a Y
chromosome is present in the genome of the fruitfly, a male is a male
because of the fact that he lacks a second X chromosome.