pre-mRNA (transcript RNA)exon Aexon Aexon Bexon BintronproteinsnRNA other
sliceosome proteinspre-mRNAmRNA260 MHR • Unit 3 Molecular Genetics
In eukaryotes, however, the processes of
transcription and translation are physically
separated by the double membrane that surrounds
the cell nucleus. These processes are also separated
by some additional reactions that modify the RNA
transcript before it leaves the cell nucleus.Processing of mRNA Transcript
In eukaryotes, the mRNA molecule that is released
when transcription ends is called precursor mRNA,
or pre-mRNA. Pre-mRNA undergoes several changes
before it is exported out of the nucleus as mRNA.Pre-mRNA Cap and Tail
The first changes to the pre-mRNA molecule occur
at the ends of the nucleotide strand. The 5 ′end is
capped with a modified form of the G nucleotide
known as a 5 ′cap. At the 3 ′end, an enzyme in the
nucleus adds a long series of A nucleotides, referred
to as a poly-A tail. The cap and tail have several
functions. They help to protect the finished mRNAmolecule from enzymes in the cytoplasm that are
designed to break down nucleic acid fragments into
their component nucleotides. In fact, the greater the
length of the poly-A tail, the greater the stability
of the finished mRNA. The cap and tail also serve
as signals that help to bind the molecules that
synthesize proteins. It also appears that the poly-A
tail plays a role in helping to transport the finished
mRNA molecule from the nucleus to the cytoplasm.mRNA Splicing
As you saw in the previous chapter, most eukaryote
genes contain both expressed nucleotide sequences
or exons (which form part of the instructions for
protein synthesis) and intervening non-coding
nucleotide sequences or introns. The RNA
polymerase does not distinguish between introns
and exons as it transcribes a gene. The initial
mRNA transcript therefore contains long stretches
of nucleotides that must be removed before the
transcript is used to construct a polypeptide.
The molecule that accomplishes this splicing act
is called a spliceosome. This large molecule is
formed from two components: proteins that are
joined to nucleic acids of a type of RNA called
small nuclear RNA(or snRNA) and other proteins.
The spliceosome cleaves the pre-mRNA at the ends
of each intron and then splices the remaining
exons. This process is illustrated in Figure 8.11.To view animation clips on DNA transcription, refer to your
Electronic Learning Partner.ELECTRONIC LEARNING PARTNER
A
B
Figure 8.11(A) During RNA splicing, the
nucleotide sequence on the snRNA molecules
in the spliceosome base-pairs with specific
nucleotide sequences at the ends of pre-
mRNA introns. (B) The spliceosome then
excises the introns, as shown by the closed
loops, and joins the exons. Studies show that
it is the RNA component of the spliceosome
that catalyzes the splicing. This is the first
known example of enzymatic activity in a
substance other than protein.