Describe the structure of DNA. How does DNA store instructions for the cell?
Nucleotide: A nucleotide is a molecule made of many carbon, hydrogen, and oxygen atoms all connected
to each other to form a specific shape. The atoms that make up the nucleotide are connected to each
other with very strong bonds called covalent bonds. The strong covalent bond that holds the atoms of a
nucleotide together, ensure that the molecule won’t fall apart. Scientists wanted to determine the shape
of a nucleotide, what job it performed in the cell, where they are found in a cell and how many
nucleotides there are per cell. The image below shows a nucleotide with one of four different possible
nitrogen bases. A nucleotide is made up of a phosphate molecule, which is connected to a deoxyribose
sugar, which is connected to a nitrogen base (there are 4 different nitrogen bases). The four possible
nitrogen bases are adenine A, Thymine T, Guanine G, or Cytosine C.
Phosphate
Adenine A
Deoxyribose
Sugar
or
Notes:
or
or
A base pair is made up of two oppositely facing nucleotides that are held together by weak hydrogen
bonds that can be broken allowing the two nucleotides to separate from each other. The image above
shows a base pair form between thymine and adenine. The image below shows a base pair between
Guanine and Cytosine. Notice that A and T have two hydrogen bonds holding them together and G and C
have three hydrogen bonds holding them together. The Thymine molecule above would be part of a long
chain of nucleotides making up the left strand of this DNA molecule/chromosome and the Adenine
molecule would be part of the other strand of nucleotides making up the right side of this DNA
molecule/chromosome. Notice that all other bonds holding the atoms together are strong covalent bonds
except for the weak hydrogen bonds in the middle. G pairs with C and A pairs with T.
The image below shows an extremely short segment of a chromosome/DNA molecule. This segment is
only 4 base pairs long. What is the role of each molecule in determining the function of DNA? DNA stores
instructions for the cell and passes on that information to a new cell or a new offspring.
The image below shows a short segment of DNA with the nitrogen bases removed leaving only the
DNA backbone. Notice the DNA backbone is made up of the deoxyribose sugar molecules bonded to
the phosphate molecules. It is critically important that these molecules are all held together with
strong covalent bonds in order to prevent damage to the chromosome. If these bonds are broken,
the instructions are altered and the cell may no longer be able to function properly. The DNA
backbone provides structural support by holding the molecule together. The backbone is a repeating
sequence of deoxyribose sugar molecules attached to phosphate molecules. This order never
changes. For this reason, the backbone cannot store information and merely provides structural
support.
The weak hydrogen bonds are breakable which allow the two strands to separate. In order for DNA
to pass on information to the next generation of offspring or pass on information to a new cell, it must
be able to be unzipped. While the hydrogen bonds are weak and easily broken, the chromosome
does not spontaneously separate into two strands. A special molecule is needed to break the
hydrogen bonds and unzip the chromosome. The collective action of millions of hydrogen bonds
holding the two strands together prevent the chromosome from spontaneously unzipping.
The image below shows the hierarchical relationship between nucleotides and chromosomes. A
chromosome is an extremely long DNA molecule that is made up of millions of nucleotides. This
image shoes an extremely short piece of a chromosome that has been magnified to show 4 base
pairs. The full chromosome is a long DNA molecule that can be wound up tightly and packaged into
the structure shown on the left of this 2D model.
The data below shows a table displaying the length, in base pair units, of human chromosomes 1
through 22 and chromosome X and chromosome Y. Notice that chromosome 1 is the largest with
over 240 million base pairs and chromosome 21 is the smallest with over 46 million base pairs.
Scientists have determined the length of chromosomes belonging to other species as well including
plants, mammals, fish, and bacteria.
Chromosome
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
X
Y
Length (Base Pairs)
248,956,422
242,193,529
198,295,559
190,214,555
181,538,259
170,805,979
159,345,973
145,138,636
138,394,717
133,797,422
135,086,622
133,275,309
114,364,328
107,043,718
101,991,189
90,338,345
83,257,441
80,373,285
58,617,616
64,444,167
46,709,983
50,818,468
156,040,895
57,227,415