Biology 12

254 MHR • Unit 3 Molecular Genetics

an investment that earns the cell a number of

advantages. Consider, for example, the enormous

amount of hereditary information that is contained

on even a single chromosome. If the entire

chromosome had to be transported out of the nucleus

to take part in protein synthesis, the cell would

expend a great deal of energy. The repeated transport

of DNA back and forth between the nucleus and

the cytoplasm would also place the hereditary

material at risk of damage. With the two-step

process, the cell copies a small portion of the DNA

molecule and then carries this copy out of the

nucleus while the DNA remains protected inside.

A single stretch of DNA can also be copied many

times over to produce a large number of RNA

molecules. Multiple copies help to speed up the

process of protein synthesis. Finally, the fact that

more than one step is involved in the process of

protein synthesis means that there are several

opportunities along the way to regulate gene

expression. As you will see later in this chapter,

different mechanisms operating at different stages

of the process allow for very sophisticated

regulatory functions.

The Genetic Code

and mRNA Codons

The next step for researchers was to determine

exactly which codons correlated with each amino

acid. Late in 1961, Marshall Nirenberg and Heinrich

Matthaei reported the first success in breaking the

genetic code. First they synthesized an artificial RNA

molecule made up only of uracil nucleotides. They

then cultured this molecule (which they called

poly-U) in a medium that contained the 20 amino

acids and the various substances required to catalyze

the formation of a protein. The polypeptide that

was produced in this medium was made up only of

the amino acid phenylalanine. As a result, Nirenberg

and Matthaei concluded that the codon UUU must

code for phenylalanine. Through a series of similar

experiments, researchers worked out the entire

genetic code by 1965. Table 8.1 shows the full set

of RNA codons and their corresponding amino

acids. Since information is passed from DNA to

RNA, codons are always written in the form of the

RNA transcript from the original DNA molecule.

By convention, the genetic code is always

presented in terms of the RNA codon rather than

the nucleotide sequence of the original DNA

strand. The RNA codons are written in the 5 ′to 3 ′

direction. To read the table, find the first letter of

the RNA codon in the column titled “First letter.”

Then read across the rows in the column titled

“Second letter” to find the second letter of the

codon. This will take you to a set of four possible

amino acids. Finally, read down the column titled

“Third letter” to find the last letter of the codon.

This will indicate the amino acid that corresponds

to that codon. For example, the RNA codon GAG

codes for glutamate. What amino acid corresponds

to the codon CAU?

Characteristics of the Code

The genetic code has a number of important

characteristics. Three key features are its continuity,

its redundancy, and its universality.

Continuity The genetic code reads as a long

series of three-letter codons that have no spaces or

punctuation and never overlap. This means that

knowing exactly where to start transcription and

translation is essential. Each sequence of nucleotides

has a correct reading frame, or grouping of codons.

Experiments such as Crick’s show that if the reading

frame on an RNA molecule is shifted by the insertion

or deletion of an additional nucleotide, there is no

Second letter

U CA G

First

letter

U

C

A

G

Third

letter

U C A G U C A G U C A G U C A G

phenylalanine

phenylalanine

leucine

leucine

leucine

leucine

leucine

leucine

isoleucine

isoleucine

isoleucine

start/

methionine

valine

valine

valine

valine

serine

serine

serine

serine

proline

proline

proline

proline

threonine

threonine

threonine

threonine

alanine

alanine

alanine

alanine

tyrosine

tyrosine

stop

stop

histidine

histidine

glutamine

glutamine

asparagine

asparagine

lysine

lysine

aspartate

aspartate

glutamate

glutamate

cysteine

cysteine

stop

tryptophan

arginine

arginine

arginine

arginine

serine

serine

arginine

arginine

glycine

glycine

glycine

glycine

Table 8.1

The genetic code