What Controls the Synthesis of Proteins?
Nucleic acids contain the information, in a coded form, which specifies the sequence in which amino
acids will be put together to form a protein. The monomer of a nucleic acid is a nucleotide, which has
three parts itself: a 5-carbon sugar (either ribose, in RNA, or deoxyribose, in DNA), a base (there are
5, which we'll call A,T,C,G, and U), and a phosphate group.
Stick model of a nucleotide with the base A
Simplified diagram of the same nucleotide
Guess what kind of reaction combines nucleotides to form a polynucleotide?
As usual, synthesis is used, breaking an -OH and an -H from the phosphate of one nucleotide and the
sugar of another, and forming a bond between them. When many nucleotides are connected
together by synthesis reactions, they form a polynucleotide strand of DNA.
The noteworthy thing about
nucleic acids is that the
bases are attracted to each
other (due to opposite
charges arranged in
matching patterns). A is
attracted to T or U; C is
attracted to G. These pairs
are called complementary
bases. A DNA molecule
consists of two
polynucleotide strands
bound together by the
bonds between the
complementary bases.
The sequence of the bases in a strand can be any combination of the bases A,T,C and G (U is used
instead of T in RNA); the other strand, though, has to have complementary bases. The order of the
bases in a strand contains the information that specifies the order of the amino acids in a protein.
Suppose you have a DNA molecule, and one strand in it has the sequence AAGGCATGT. What will
be the sequence of the other strand in the DNA molecule?
How many different 10-nucleotide DNA strands could be made? Here are two possibilities:
AAAAAAAATA, AAAAAAAATG. How many different sequences are possible (the two ends are
chemically different, so mirror images are different sequences)?
Let's start with an easier question.
How many 2-nucleotide strands are possible? List all the two-nucleotide combinations and count
them. Here are a few to get you started. Finish the list, and write down the total number of many you
get.
AA, AC, AG, AT,
Total number of possible 2-nucleotide strands = ___________________
Okay, now back to the original question. How many 10-nucleotide DNA strands could be made,
then? How would you calculate the answer?
As you can see, that's a lot. The human genome (or that of most any mammal, for that matter)
contains about 3 billion nucleotides. Imagine how many different sequences of 3 billion nucleotides
are possible. This means that there's room in your genome for a whole lot of information, specifying a
lot of different proteins.
Protein synthesis requires two steps: transcription, in which an RNA molecule complementary to one
strand of a DNA molecule is synthesized; and translation, in which coded information in an RNA
molecule is used to direct the synthesis of a polypeptide (protein).