From DNA to Protein
DNA directs the cell’s activities by telling it what proteins
to make and when. These proteins form structural elements in the
cell and regulate the production of other cell products. By controlling
protein synthesis, DNA is hugely important in directing life.
Protein synthesis is a two-step process. DNA resides in
the nucleus, but proteins are made in the cytoplasm. The cell copies
the information held in DNA onto RNA molecules in a process called
transcription. Proteins are synthesized at the ribosomes from the
codes in RNA in a process called translation.
Before getting into the way that the information on DNA
can be transcribed and then translated into protein, we have to
spend some time studying the major players in the process: DNA and
DNA and the Genetic Code
The sequence of nucleotides in DNA makes up a code that
controls the functions of the cell by telling it what proteins to
produce. Cells need to be able to produce 20
acids in order to produce all the proteins necessary to function.
DNA, however, has only four nitrogen bases. How can these
four bases code for the 20
amino acids? If adenine, thymine,
guanine, and cytosine each coded for one particular amino acid,
DNA would only be able to code for four amino acids. If two bases
were used to specify an amino acid, there would only be room to
code for 16
) different amino
In order to be able to code for 20 amino
acids, it is necessary to use three bases (which offer a total of 64 coding
combinations) to code for each amino acid. These triplets of nucleotides
that make up a single coding group are called codons or genes.
Two examples of codons are CAG, which codes for the amino acid glutamine,
and CGA, which codes for arginine.
Codons are always read in a non-overlapping sequence.
This means that any one nucleotide can only be a part of one codon.
Given the code AUGCA, AUG could be a codon for the amino acid methionine,
with CA starting a new codon. Alternatively, GCA could be a codon
specifying alanine, while the initial AU was the last two letters
of a previous codon. But AUG and GCA cannot both be codons at the
Degeneracy of the Genetic Code
There are 64 codons but only 20 amino
acids. What happens to the other 44 coding possibilities?
It happens that some of the different codons call for the same amino
acid. The genetic code is said to be degenerate because
of its redundancy.
Experiments have shown that there are also three stop
codons, which signal when a protein is fully formed, and
one start codon, which signals the beginning of an amino acid sequence.
Mutations of the Genetic Code
Since the sequence of nucleotides in DNA determines the
order of amino acids in proteins, a change or error in the DNA sequence
can affect a protein’s function. These errors or changes in the
DNA sequence are called mutations.
There are two basic types of mutations: substitution mutations
and frameshift mutations.
A substitution mutation occurs when a single nucleotide
is replaced by a different nucleotide. The effects of substitution
mutations can vary. Certain mutations might have no effect at all:
these are called silent mutations. For instance, because the genetic
code is degenerate, if the particular codon GAA becomes GAG, it
will still code for the amino acid glutamate and
the function of the cell will not change. Other substitution mutations
can drastically affect cellular and organismal function. Sickle-cell
anemia, which cripples human red blood cells, is caused by a substitution
mutation. A person will suffer from sickle-cell anemia if he has
the amino acid valine in his hemoglobin rather than glutamic acid.
The codon for valine is GUA or GUG, while the codon for glutamic
acid is GAA or GAG. A simple substitution of A for U results in
A frameshift mutation occurs when a nucleotide is wrongly
inserted or deleted from a codon. Both types of frameshifts usually
have debilitating or lethal effects. An insertion or deletion will
affect every codon in a particular genetic sequence
by throwing the entire three-by-three codon structure out of whack.
For example, if the A in the GAU were to be deleted, the code:
GAU GAC UCC GCU AGG
GUG ACU CCG CUA GG
and code for an entirely different set of amino acids
in translation. The results of such mutations on an organism are
The only sort of frameshift mutation that might not have
dire effects is one in which an entire codon is inserted or deleted.
This will result in the gain or loss of one amino acid but will
not affect surrounding codons.
Even the tiniest cells contain meters upon meters of DNA.
With the aid of special proteins called histones, this DNA is coiled
into an entangled fibrous mass called chromatin. When it comes time
for the cell to replicate (a process covered later in this chapter),
these masses gather into a number of discrete compact structures
In eukaryotes, the chromosomes are located in the nucleus
of the cell. Prokaryotes don’t have a nucleus: their DNA is located
in a single chromosome that is joined together in a ring. This ring
chromosome is found in the cytoplasm. In this chapter, when we talk about
chromosomes, we will be referring to eukaryotic chromosomes.
Different eukaryotes have varying numbers of chromosomes.
Humans, for example, have 46 chromosomes arranged in 23 pairs. (Dogs
have 78 chromosomes in 39 pairs. A larger number of chromosomes
is not a sign of greater biological sophistication.) The total number
of distinct chromosomes in a cell is the cell’s diploid number.
The cells in a human body that are not passed down to
offspring, called somatic cells, contain chromosomes
in two closely related sets—one set of 23 each from a person’s mother
and father—making up a total of 46 chromosomes. These sets pair
up, and the pairs are known as homologous chromosomes.
Each homologous pair consists of one maternal and one paternal chromosome.
The haploid number of a cell refers to half of the
total number of chromosomes in a cell (half the diploid number),
or the number of homologous pairs in somatic cells.
In humans and other higher animals, only the
sex cells that are passed on to offspring have the haploid number
of chromosomes. These sex cells are also called gametes.
Ribonucleic acid (RNA) helps DNA turn stored genetic messages
into proteins. As discussed in the Biochemistry chapter, RNA monomers
(nucleotides) are similar to those of DNA, but with three crucial
- DNA’s five-carbon sugar is deoxyribose.
RNA nucleotides contain a slightly different sugar, called ribose.
- RNA uses the nitrogenous base uracil in place of DNA’s
- The RNA molecule takes the form of a single helix—half
a spiral ladder—as compared with the double helix structure of DNA.
Two different types of RNA play important roles in protein
synthesis. During transcription, DNA is copied to make messenger
RNA (mRNA), which then leaves the nucleus to bring its still
encoded information to the ribosomes in the cytoplasm. In order
to use the information contained in the transcribed mRNA to make
a protein, a second type of RNA is used. Transfer RNA (tRNA) moves
amino acids to the site of protein synthesis at the ribosome according
to the code specified by the mRNA strand. There are many different tRNAs,
each of which bond to a different amino acid and the mRNA sequence
corresponding to that amino acid.