Name: _________________________________________________ Per: ________ Date: _________________________
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. How does it do this? The nucleus controls these activities by the chromosomes.
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.
Chromosomes are composed of genes. A gene 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. It stands for deoxyribonucleic acid. 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 actually
controls the cell's workings.
In 1953, James Watson and Francis Crick established the structure of DNA. The structure is a double helix, 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 deoxyriboses 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.) Note that
the bases attach to the sides of the ladder at the sugars and not the phosphate.
Color the thymines orange. Color the guanines purple.
Color the cytosines yellow. Color the adenines green.
The combination of a single base, a deoxyribose sugar, and a phosphate make up a nucleotide. DNA is actually a molecule or repeating
nucleotides. The two sides of the DNA ladder are held together loosely by hydrogen bonds. Color the hydrogen bonds gray.
The DNA helix is actually made of repeating units called nucleotides. Each nucleotide consists of three molecules: a sugar (deoxyribose),
a phosphate which links the sugars together, and then one of the four bases. Two of the bases are purines - adenine and guanine. The
pyrimidines are thymine and cytosine. Note that the pyrimidines are single ringed and the purines are double ringed. Color the nucleotides
using the same colors as you colored them in the double helix.
