Nucleic Acids and Protein Synthesis
The Genetic Code
program that they seem to pass from one generation to another.
When a strain of information is passed to another, it is called transformation.
The Transforming Factor
DNA is the nucleic acid that stores and transmits the genetic information from
one generation of an organism to the next. DNA carries the genetic code.
Some virusus are known as bacteriophages which mean bacteria eaters.
the bacterium, the injected material begins to reproduce, making may copies of the
bacteriophage. Soon the bacterium bursts, and several hundred bacteriophages are
released to infect other cells. Because the material injected into the bacterium produces
new bacteriophages, it must contain the genetic code.
The Structure of DNA
DNA is a polymer formed from units called nucleotides. Each nucleotide is a
group, and a nitrogenous, or nitrogen-containing, base.
DNA contains four nitrogenous bases. Two of the nitrogenous bases, adenine and
guanine, belonging to a group of compounds known as purines. The remaining two,
cytosine and thymine, are known as pyrimidines.
Individual nucleotides are joined together to form a long chain.
Fibers that make up DNA are twisted, like the strands of a rope. Large groups of
molecules in the fiber are spaced out at regular intervals along the length of the
The Replcation of DNA
Because each of the two strands of the DNA double hehx has all the information,
by the mechanism of base pairing, to reconstruct the other half, the strands are said to
helix of DNA serves as a template, or pattern, against which a new strand is made.
enzymes separate, or “unzip,” the two strands of the double helix, insert the
appropriate bases, and produce covalent sugar-phosphate links to extend the growing
DNA chains. The enzymes even “proofread” the bases that have been inserted to ensure
that they are paired correctly.
broken and the two strands of the molecule unwind. Each of the separated strands
serves as a template for the attachment of complementary bases. Two DNA molecules
identical to each other and to the original molecule are made.
The double helix structure explains how DNA can be replicated, or copied.
information is put to good use. DNA contains a set of instructions that are coded in the
sequence, or order, of nucleotides. The first step in decoding that message is to copy
part of the sequence into RNA (ribonucleic acid). RNA is the nucleic acid that acts
as a messenger between DNA and the ribosomes and carries out the process by
which proteins are made from amino acids.
The Structure of RNA
RNA, like DNA, consists of a long chain of macromolecules made up of
nucleotides. Each nucleotide is made up of a 5-carbon sugar, a phosphate group, and
a nitrogenous base. The alternating sugars and phosphate groups form the backbone
of the RNA chain.
ribose, whereas the sugar in DNA is deoxyribose. Another difierence between RNA
and DNA is that RNA consists of a single strand of nucleotides, although it can form
double-stranded sections by folding back on itself in loops. DNA, as you will recall, is
double-stranded. Lastly, the nitrogenous bases found in DNA are adenine, thymine,
cytosine, and guanine. RNA also contains adenine, cytosine, and guanine, but uracil
(toom-uh-sihl) is present instead of thymine. Like DNA, RNA follows the base
pairing rules. Adenine bonds to uracil, and cytosine bonds to guanine. Although a cell
contains many different forms of RNA, there are three main types that are involved in
expressing the genetic code.
In its own way, an RNA molecule is a disposable copy of a segment of DNA.
The ability to copy a DNA base sequence into RNA makes it possible for a specific
place on the DNA molecule to produce hundreds or even thousands of RNA
moiecules with the same information as DNA.
Transcription: RNA Synthesis
molecule being synthesized turns out to be the same as the molecule being copied. In
RNA synthesis, the molecule being copied is just one of the two strands of a DNA
molecule. Thus the molecule being synthesized is different from the molecule being
copied. The term transcription is used to describe this process. Transcription is the
process by which a molecule of DNA is copied into a complementary strand of
DNA is found in the nucleus and ribosomes are located in the cytoplasm.
genetic information from the DNA in the nucleus out to the ribosomes in the
cytoplasm. The molecule that performs this function is messenger RNA (mRNA), one
of the three main types of RNA.
RNA strand. The messenger RNA strand is complementary to one of the DNA
strands. The base-pairing: mechanism ensures that the messenger RNA will be a
complementary copy of the DNA strand that serves as its template.
7-3 Protein Synthesis
The information that DNA transfers to messenger RNA is in the
form of a code. This code is determined by the way in which the four nitrogenous
bases are arranged in DNA.
The nitrogenous bases in DNA contain information that directs protein synthesis.
Because most enzymes are proteins, proteins control biochemical pathways within the
but they are also responsible for cell structure and cell movement. Together, DNA
and its assistant, RNA, are directly responsible for making proteins.
The Nature of the Genetic Code
DNA and RNA each contain different nitrogenous bases (DNA contains A, T,
C, G; RNA contains A, U, C, G); hence, diffexent nueleotides. In order to code for
the 20 different amino acids, more than one nucleotide must make up the code
word for each amino acid. The code word of the DNA nucleotides are copied onto a
strand of messenger RNA. Each combination of three nucieo- tides on the
messenger RNA is called a codon, or three- letter code word. Each codon specifies a
particular amino acid that is to be placed in the polypeptide chain. There is more
than onecodon for each amino acid.
The decoding of a messenger RNA message into a polypeptide chain (protein)
is known as translation. There is an elaborate mechanism that involves the two other
main types of RNA — transfer RNA (tRNA) and ribosomal RNA (rRNA) — and the
cytoplasmic organelle known as the ribosome.
are joined together to form polypeptides. Transfer RNA is a single strand of RNA that
loops back on it self. Ribosomal RNA makes up the major part of the ribosomes.
THE ROLE OF TRANSFER RNA
You will notice that there are three exposed bases on each transfer RNA
molecule. These nucleotides will base pair with a codon on messenger RNA. Because
the three nucleotides on transfer RNA are complementary to the three nucleotides
on messenger RNA, the three transfer RNA nucleotides are called the anticodon.
Attached to each transfer RNA molecule is the amino acid specihed by the
codon to which it base pairs. By matching the transfer RNA anticodon to the
messenger RNA codon, the correct amino acid is put into place. Each transfer RNA acts
like a tiny beacon for its specific amino acid.
THE ROLE OF THE RIBOSOME
Messenger RNA molecules do not automatically line up transfer RNA molecules
and link their amino acids together. Instead, this process of protein synthesis takes
place in organelles known as ribosomes. Ribosomes are made up of two subunits, a
large one and a smaller one. Each subunit consists of ribosomal RNA and proteins
(about 70 different types).
The first part of protein synthesis occurs when the two subunits of the ribosome
bind to a molecule of messenger RNA. Then the initiator codon: AUG binds to the first
anticodon of transfer RNA, signaiing the beginning of a polypeptide chain.
Soon the anticodon of another transfer RNA binds to the next messenger RNA
codon. T:his second transfer RNA carries the second amino acid that will be placed
into the chain of the polypeptide.
The polypeptide chain continues to grow until the ribosome reaches a stop
codon on the messenger RNA. A stop codon is a codon for which no transfer RNA
molecules exist. When the stop codon reaches the ribosome, the ribosome releases
the newly formed polypeptide and messenger RNA, completing the process of
As you can now see, the ribosome, in its own way, is at the center of the whole
business of making the genetic code work. In the nucleus, DNA directs the formation
of three different kinds of RNA: transfer RNA, ribosomal RNA, and messenger
RNA. They all leave the nucleus and then seem to go their separate ways.