DNA Structure and Replication
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.! 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.! 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 a monosaccharide called 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, and C.!! These
bases always bond in a certain way: A with T and C with G. 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
base pairs. (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. Find your copy of the DNA strand and color it according to the following color
scheme.
Color the thymines orange.! !Color the adenines green.! !
Color the guanines purple.! !Color the cytosines yellow.!
Note that that the bases attach to the sides of the ladder at the sugars, not the phosphate.
The combination of a single base, a deoxyribose sugar, and a phosphate make up a!nucleotide. DNA is actually a
molecule of repeating nucleotide monomers/subunits. Examine the nucleotides closer. 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. The two sides of the DNA ladder are held together loosely by!hydrogen bonds.!Color the dotted
lines between the base pairs representing the hydrogen bonding gray.
DNA: The Blueprint of Life
Every cell in your body has the same “blueprint;” the same DNA. Like the blueprints of a house tell the
builders how to construct a house, the DNA "blueprint" tells the cell how to build the organism. ! Yet, how can a
heart be so different from a brain if all the cells contain the same instructions? ! Although much work remains in
genetics, it has become apparent that a cell has the ability to turn off most genes and only work with the genes
necessary to do a particular job.! We also know that a lot of DNA does not code for anything at all.! These regions
of DNA that do not code for proteins are called “introns.” The sections of DNA that do actually code from proteins
are called “exons.”
DNA Replication
Each time a new cell is made, the cell must receive an exact copy of the parent cell DNA. The new cells
then receive the instructions and information needed to function. The process of copying DNA is
called!replication. Replication occurs in a unique way – instead of copying a complete new strand of DNA, the
process “saves” or conserves one of the original strand. For this reason, replication is called semi-conservative.
When the DNA is ready to copy, the molecule “unzips” itself and new nucleotides are added to each side.
