Name: _____________________________________
DNA - The Double Helix
Recall that the nucleus is a small spherical, dense body in a cell. It is often called the
"control center" because it controls all the activities of the cell including cell
reproduction, and heredity. Chromosomes are microscopic, threadlike strands composed
of the chemical DNA (short for deoxyribonucleic acid). In simple terms, DNA controls
the production of proteins within the cell. These proteins in turn, form the structural units
of cells and control all chemical processes within the cell. Think of proteins as the
building blocks for an organism, proteins make up your skin, your hair, parts of
individual cells. How you look is largely determined by the proteins that are made. The
proteins that are made is determined by the sequence of DNA in the nucleus.
Chromosomes are composed of genes, which is a segment of DNA that codes for a
particular protein which in turn codes for a trait. Hence you hear it commonly referred to
as the gene for baldness or the gene for blue eyes. Meanwhile, DNA is the chemical that
genes and chromosomes are made of. DNA is called a nucleic acid because it was first
found in the nucleus. We now know that DNA is also found in organelles, the
mitochondria and chloroplasts, though it is the DNA in the nucleus that controls the cell's
workings.
In 1953, James Watson and Francis Crick established the structure of DNA. The shape
of DNA is a double helix [color the title black] which is like a twisted ladder. The sides
of the ladder are made of alternating sugar and phosphate molecules. The sugar is
deoxyribose. Color all the phosphates pink [one is labeled with a "p"]. Color all the
deoxyribose blue [one is labeled with a "D"].
The rungs of the ladder are pairs of 4 types of nitrogen bases. Two of the bases are
purines – adenine and guanine. The pyrimidines are thymine and cytosine. The bases
are known by their coded letters A, G, T, C. These bases always bond in a certain way.
Adenine will only bond to thymine. Guanine will only bond with cytosine. This is
known as the "Base-Pair Rule". The bases can occur in any order along a strand of DNA.
The order of these bases is the code that contains the instructions. For instance,
ATGCACATA would code for a different gene than AATTACGGA. A strand of DNA
contains millions of bases. [For simplicity, the image only contains a few.]
Color the thymine orange.
Color the adenine green.
Color the guanine purple.
Color the cytosine yellow.
Note that that the bases attach to the sides of the ladder at the sugars and not the
phosphate.
