Human Physiology, 14th edition (2016)

66 Chapter 3

Precursor mRNA is much larger than the mRNA it forms.

Surprisingly, this large size of pre-mRNA is not due to excess

bases at the ends of the molecule that must be trimmed;

rather, the excess bases are located within the pre-mRNA.

The genetic code for a particular protein, in other words, is

split up by stretches of base pairs that do not contribute to

the code. These regions of noncoding DNA within a gene are

called introns; the coding regions are known as exons. Conse-

quently, pre-mRNA must be cut and spliced to make mRNA

( fig. 3.17 ).

When the human genome was sequenced, and it was dis-

covered that we have less than 25,000 genes and yet pro-

duce more than 100,000 different proteins, it became clear

that one gene could code for more than one protein. To a

large degree, this is accomplished by alternative splicing.

This is where stretches of DNA that serve as introns (non-

coding) sequences in the formation of one mRNA can serve

as exons (coding sequences) in the formation of a different

mRNA. Through such alternative splicing, a given gene can

produce several different mRNA molecules coding for dif-

ferent proteins.

An estimated 92% to 94% of human genes undergo alter-

native splicing of exons, with most of the variation occurring

between different tissues. The average gene contains eight exons,

specific proteins; (3) transfer RNA (tRNA), which is needed
for decoding the genetic message contained in mRNA; and
(4) ribosomal RNA (rRNA), which forms part of the struc-
ture of ribosomes. The DNA that codes for rRNA synthesis is
located in the part of the nucleus called the nucleolus. The DNA
that codes for pre-mRNA and tRNA synthesis is located else-
where in the nucleus.
In bacteria, where the molecular biology of the gene is
best understood, a gene that codes for one type of protein
produces an mRNA molecule that begins to direct protein
synthesis as soon as it is transcribed. This is not the case in
higher organisms, including humans. In higher cells, a pre-
mRNA is produced that must be modified within the nucleus
before it can enter the cytoplasm as mRNA and direct protein
synthesis.

Figure 3.16 RNA synthesis (transcription). Notice
that only one of the two DNA strands is used to form a single-
stranded molecule of RNA.

G C

DNA

RNA

A T

T

U

A

T

T

A

A

T A

A

G

T

A

G

C

A

A

U

U

U

A

C G

CG

U

C

G

G

C

C

A U

A

G

G

C

C

G

G

C

C

T

A

A

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G

U

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C

Figure 3.17 The processing of pre-mRNA into

mRNA. Noncoding regions of the genes, called introns, produce

excess bases within the pre-mRNA. These excess bases are

removed, and the coding regions of mRNA are spliced together.

Exons can be spliced together in different sequences to produce

different mRNAs, and thus different proteins.

Transcription

Introns

Pre-mRNA

DNA (gene)

Intron

mRNA

Exon Intron Exon

Exons spliced together

Exon