Figure 7–2 Differentiated cells contain all the genetic instructions necessary to direct the formation of a complete organism. (A) the nucleus of a skin cell from an adult frog transplanted into an enucleated egg can give rise to an entire tadpole. the broken arrow indicates that, to give the transplanted genome time to adjust to an embryonic environment, a further transfer step is required in which one of the nuclei is taken from an early embryo that begins to develop and is put back into a second enucleated egg. (B) in many types of plants, differentiated cells retain the ability to “de-differentiate,” so that a single cell can form a clone of progeny cells that later give rise to an entire plant. (C) A nucleus removed from a differentiated cell from an adult cow and introduced into an enucleated egg from a different cow can give rise to a calf. Different calves produced from the same differentiated cell donor are all clones of the donor and are therefore genetically identical. (A, modified from
J.B. Gurdon, Sci. Am. 219:24–35, 1968.) unfertilized MEIOTIC SPINDLE egg cell AND ASSOCIATED CHROMOSOMES REMOVED no. of reads no. of reads (B) start of transcription exons introns CELL LINE still do not have an exact answer to this fundamental question, we can make several general statements.
1.
Many processes are common to all cells, and any two cells in a single organism therefore have many gene products in common. These include the structural proteins of chromosomes, RNA and DNA polymerases, DNA repair enzymes, ribosomal proteins and RNAs, the enzymes that catalyze the central reactions of metabolism, and many of the proteins that form the cytoskeleton such as actin (Figure 7–3A).
2.
Some RNAs and proteins are abundant in the specialized cells in which they function and cannot be detected elsewhere, even by sensitive tests. Hemoglobin, for example, is expressed specifically in red blood cells, where it carries oxygen, and the enzyme tyrosine aminotransferase (which breaks down tyrosine in food) is expressed in liver but not in most other tissues (Figure 7–3B).
3.
Studies of the number of different RNAs suggest that, at any one time, a typical human cell expresses 30–60% of its approximately 30,000 genes at some level. There are about 21,000 protein-coding genes and a roughly estimated 9000 noncoding RNA genes in humans. When the patterns of RNA expression in different human cell lines are compared, the level of expression of almost every gene is found to vary from one cell type to another. A few of these differences are striking, like those of hemoglobin and tyrosine aminotransferase noted above, but most are much more subtle. But even those genes that are expressed in all cell types usually vary in their level of expression from one cell type to the next.
4.
Although there are striking differences in coding RNAs (mRNAs) in specialized cell types, they underestimate the full range of differences in the final pattern of protein production. As we discuss in this chapter, there are many steps after RNA production at which gene expression can be regulated. And, as we saw in Chapter 3, proteins are often covalently modified after they are synthesized. The radical differences in gene expression between cell types are therefore most fully revealed through methods that directly display the levels of proteins along with their post-translational modifications (Figure 7–4).