The Questions of Developmental Biology

types of cells (Kleene and Humphreys 1977, 1985). Wold and her colleagues (1978) showed that
sequences present in sea urchin blastula messenger RNA, but absent in gastrula and adult tissue
mRNA, were nonetheless present in the nuclear RNA of the gastrula and adult tissues.
More genes are transcribed in the nucleus than are allowed to become mRNAs in the
cytoplasm. This censoring of RNA transcripts has been confirmed for specific messages by
probing for the introns and exons of specific genes. Gagnon and his colleagues (1992) performed
such an analysis on the transcripts from the CyIIIa genes of the sea urchin Strongylocentrotus
purpuratus. These genes encode calcium-binding and actin proteins, respectively, that are
expressed only in a particular part of the ectoderm of the sea urchin larva. Using probes that
bound to an exon (which is included in the mRNA) and to an intron (which is not included in the
mRNA), they found that these genes were being transcribed not only in the ectodermal cells, but
also in the mesoderm and endoderm. The analysis of the CyIIIa gene showed that the
concentration of introns was the same in both
the gastrula ectoderm and in the
mesoderm/endoderm samples, suggesting that
this gene was being transcribed at the same rate
in the nuclei of all cell types, but was made
into cytoplasmic mRNA only in ectodermal
cells (Figure 5.27). The unprocessed nRNA for
CyIIIa is degraded while still in the nuclei of
the endodermal and mesodermal cells.
Creating families of proteins through differential nRNA splicing
The average vertebrate nRNA consists of relatively short exons (averaging about 140
bases) separated by introns that are usually much longer. Most mammalian nRNAs contain
numerous exons. By splicing together different sets of exons, different cells can make different
types of mRNAs, and hence, different proteins. Whether a sequence of RNA is recognized as an
exon or as an intron is a crucial step in gene regulation. What is an intron in one cell's nucleus
may be an exon in another cell's nucleus.
Alternative nRNA splicing is based on
determining which sequences can be spliced out as
introns. This can occur in several ways (Figure 5.28).
Cells can differ in their ability to recognize the 5´ splice
site (at the beginning of the intron) or the 3´ splice site (at
the end of the intron). Or some cells could fail to
recognize a sequence as an intron at all, retaining it
within the message. The splicing of nRNA is mediated
through a complex called a spliceosome, made up of
small nuclear RNAs (snRNA) and proteins, that
assembles at a splice site. Whether a spliceosome
recognizes the splice sites depends on certain factors in
the nucleus that can interact with those sites and compete
or cooperate with the proteins that direct spliceosome