7-2 Section Assessment Answers Biology

The dissimilar jail cell types in a multicellular organism differ dramatically in both structure and function. If we compare a mammalian neuron with a lymphocyte, for instance, the differences are and so extreme that it is hard to imagine that the two cells contain the aforementioned genome (Figure 7-1). For this reason, and because cell differentiation is often irreversible, biologists originally suspected that genes might be selectively lost when a prison cell differentiates. We at present know, however, that prison cell differentiation generally depends on changes in gene expression rather than on any changes in the nucleotide sequence of the jail cell's genome.

Effigy vii-1

A mammalian neuron and a lymphocyte. The long branches of this neuron from the retina enable it to receive electrical signals from many cells and comport those signals to many neighboring cells. The lymphocyte is a white blood cell involved in the immune (more...)

The Unlike Cell Types of a Multicellular Organism Comprise the Same DNA

The cell types in a multicellular organism get different from 1 another considering they synthesize and accumulate dissimilar sets of RNA and protein molecules. They generally do this without altering the sequence of their DNA. Evidence for the preservation of the genome during cell differentiation comes from a classic set of experiments in frogs. When the nucleus of a fully differentiated frog cell is injected into a frog egg whose nucleus has been removed, the injected donor nucleus is capable of directing the recipient egg to produce a normal tadpole (Figure 7-2A). Because the polliwog contains a total range of differentiated cells that derived their DNA sequences from the nucleus of the original donor cell, information technology follows that the differentiated donor cell cannot have lost any important Deoxyribonucleic acid sequences. A like conclusion has been reached in experiments performed with various plants. Here differentiated pieces of plant tissue are placed in civilization and then dissociated into single cells. Often, i of these individual cells can regenerate an entire adult plant (Figure 7-2B). Finally, this same principle has been recently demonstrated in mammals, including sheep, cattle, pigs, goats, and mice past introducing nuclei from somatic cells into enucleated eggs; when placed into surrogate mothers, some of these eggs (called reconstructed zygotes) develop into healthy animals (Figure 7-2C).

Effigy vii-2

Evidence that a differentiated cell contains all the genetic instructions necessary to direct the formation of a complete organism. (A) The nucleus of a peel cell from an adult frog transplanted into an enucleated egg tin can give ascent to an unabridged tadpole. (more...)

Further prove that big blocks of Dna are non lost or rearranged during vertebrate evolution comes from comparison the detailed banding patterns detectable in condensed chromosomes at mitosis (see Figure 4-xi). Past this benchmark the chromosome sets of all differentiated cells in the homo torso appear to be identical. Moreover, comparisons of the genomes of unlike cells based on recombinant Dna technology have shown, as a general rule, that the changes in gene expression that underlie the development of multicellular organisms are not accompanied by changes in the DNA sequences of the corresponding genes. There are, however, a few cases where DNA rearrangements of the genome take identify during the evolution of an organism—most notably, in generating the diversity of the immune system of mammals (discussed in Chapter 24).

Different Cell Types Synthesize Different Sets of Proteins

As a first step in understanding cell differentiation, we would like to know how many differences there are between any one cell type and some other. Although we still exercise not know the answer to this primal question, we tin brand certain general statements.

ane.: Many processes are common to all cells, and whatsoever 2 cells in a single organism therefore have many proteins in common. These include the structural proteins of chromosomes, RNA polymerases, DNA repair enzymes, ribosomal proteins, enzymes involved in the central reactions of metabolism, and many of the proteins that form the cytoskeleton.
2.: Some proteins are abundant in the specialized cells in which they function and cannot exist detected elsewhere, fifty-fifty by sensitive tests. Hemoglobin, for instance, can be detected only in red blood cells.
3.: Studies of the number of different mRNAs suggest that, at any one time, a typical man cell expresses approximately x,000–20,000 of its approximately 30,000 genes. When the patterns of mRNAs in a series of unlike human jail cell lines are compared, it is institute that the level of expression of well-nigh every active gene varies from i cell type to some other. A few of these differences are hitting, similar that of hemoglobin noted above but near are much more subtle. The patterns of mRNA abundance (determined using DNA microarrays, discussed in Affiliate 8) are so characteristic of cell type that they tin be used to type homo cancer cells of uncertain tissue origin (Figure 7-iii).
four.: Although the differences in mRNAs among specialized prison cell types are striking, they nonetheless underestimate the total range of differences in the pattern of poly peptide production. Equally we shall see in this chapter, there are many steps after transcription at which gene expression can exist regulated. In add-on, culling splicing tin produce a whole family of proteins from a single gene. Finally, proteins tin be covalently modified after they are synthesized. Therefore a better way of appreciating the radical differences in factor expression between cell types is through the use of ii-dimensional gel electrophoresis, where protein levels are directly measured and some of the most common posttranslational modifications are displayed (Figure vii-4).

Figure vii-3

Differences in mRNA expression patterns among different types of human cancer cells. This figure summarizes a very large gear up of measurements in which the mRNA levels of 1800 selected genes (arranged top to bottom) were determined for 142 different homo (more...)

Figure vii-4

Differences in the proteins expressed by two human tissues. In each panel, the proteins take been displayed using 2-dimensional polyacrylamide gel electrophoresis (see pp. 485–487). The proteins have been separated by molecular weight (tiptop to (more...)

A Cell Can Change the Expression of Its Genes in Response to External Signals

About of the specialized cells in a multicellular organism are capable of altering their patterns of gene expression in response to extracellular cues. If a liver cell is exposed to a glucocorticoid hormone, for instance, the production of several specific proteins is dramatically increased. Glucocorticoids are released in the body during periods of starvation or intense exercise and indicate the liver to increase the production of glucose from amino acids and other small molecules; the set of proteins whose product is induced includes enzymes such as tyrosine aminotransferase, which helps to convert tyrosine to glucose. When the hormone is no longer nowadays, the product of these proteins drops to its normal level.

Other jail cell types respond to glucocorticoids differently. In fat cells, for example, the production of tyrosine aminotransferase is reduced, while some other cell types do not reply to glucocorticoids at all. These examples illustrate a general feature of jail cell specialization: different cell types oftentimes respond in different means to the same extracellular signal. Underlying such adjustments that occur in response to extracellular signals, in that location are features of the gene expression design that do not change and give each jail cell blazon its permanently distinctive grapheme.

Factor Expression Tin Be Regulated at Many of the Steps in the Pathway from Dna to RNA to Protein

If differences amongst the various cell types of an organism depend on the particular genes that the cells express, at what level is the command of cistron expression exercised? As nosotros saw in the last chapter, there are many steps in the pathway leading from Dna to protein, and all of them tin can in principle exist regulated. Thus a cell tin can control the proteins it makes past (1) controlling when and how often a given gene is transcribed (transcriptional control), (2) controlling how the RNA transcript is spliced or otherwise processed (RNA processing control), (3) selecting which completed mRNAs in the cell nucleus are exported to the cytosol and determining where in the cytosol they are localized (RNA transport and localization control), (4) selecting which mRNAs in the cytoplasm are translated past ribosomes (translational command), (v) selectively destabilizing certain mRNA molecules in the cytoplasm (mRNA degradation control), or (6) selectively activating, inactivating, degrading, or compartmentalizing specific poly peptide molecules after they have been made (protein activity control) (Figure seven-5).

Figure seven-5

Vi steps at which eucaryotic gene expression tin be controlled. Controls that operate at steps one through 5 are discussed in this chapter. Step half dozen, the regulation of protein activity, includes reversible activation or inactivation by protein phosphorylation (more...)

For nigh genes transcriptional controls are paramount. This makes sense because, of all the possible control points illustrated in Figure seven-5, merely transcriptional control ensures that the jail cell volition not synthesize superfluous intermediates. In the post-obit sections we talk over the DNA and protein components that perform this part by regulating the initiation of gene transcription. We shall return at the end of the chapter to the additional ways of regulating factor expression.

Summary

The genome of a cell contains in its DNA sequence the information to make many thousands of different poly peptide and RNA molecules. A cell typically expresses only a fraction of its genes, and the different types of cells in multicellular organisms arise because dissimilar sets of genes are expressed. Moreover, cells can change the pattern of genes they limited in response to changes in their surround, such as signals from other cells. Although all of the steps involved in expressing a gene tin can in principle be regulated, for most genes the initiation of RNA transcription is the near important indicate of command.

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