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Chapter 6

Biology Main Page
Chapter 6 main
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.