The floral homeotic genes of Arabidopsis belong to the MADS-box gene family, while in Drosophila, homeotic genes belong to the homeobox gene family. In both Arabidopsis and Drosophila, members of the Polycomb gene family control expression of these divergent homeotic genes. How do Polycomb genes control expression of two very different sets of homeotic genes?

IPSCs are nearly identical to human embryonic stemcells in terms of gene expression, but there may be otherways in which they are not equivalent. For example, thetelomeres of IPSCs often vary in length, with many IPSCscells having telomeres shorter than those of embryonic.How might shortened telomeres affect the life-span ofIPSCs or of differentiated cells derived from them?

Recall that the nuclear membrane disintegrates late in prophase of mitosis in most eukarvotic cells. Once the membrane is reformed in telophase in a daughter cell, several components of gene expression mignt therefore be "caught" out in the cytoplasm when they would otherwise onlv ever be found inside the nucleus. Consider where the following components of gene expression are made and where they runction. Which component is normally never found in the cytoplasm outside the nucleus? O A. Spliced intron • B. RNA polymerase O C. Histones • D. DNA polymerase

How do we know that the mutant Bar-eye phenotype in Drosophila is due to a duplicated gene region rather than to a change in the nucleotide sequence of a gene?

Recently, scientists constructed a transgene that expresses a mutant form of Drosophila histone H3 inwhich lysine 27 in the histone tail was changed to methionine (H3K27M). Expression of the H3K27Mtransgene results in aberrant development of fruit fliesbecause of inappropriate expression of many differentgenes. Explain this finding.

Discuss how the differential gene expression is possible when all somatic cells carry identical genes and genome.

It seems that developmental genetics boils down to a complex network of gene regulation. Try to draw a structure of this network for Drosophila. How many genes do you think are necessary to complete the developmental network for the fruit fly? How many genes do you think are needed for a network to specify one segment? Do you think it is more difficult to identify genes that are involved in the beginning, middle, or end of this network? Suppose you were trying to identify all of the genes needed for development in a chicken. Knowing what you know about Drosophila development, would you first try to identify genes necessary for early development, or would you begin by identifying genes involved in cell differentiation?

CD3 is a signaling protein that is typically found only in the plasma membrane of immune system T lymphocytes. CD3 is composed of several different polypeptides, including a gamma chain, CD3γ. Scientists analyzed the promoter of the CD3γ chain gene for regulatory sequences that might have positive or negative effects on expression of the gene. The scientists cloned fragments of the CD3γ gene that included the first transcribed nucleotides plus up to 789 nucleotides of upstream regulatory sequences into plasmids in which the gene for the firefly enzyme luciferase immediately follows the fragments. The plasmids were then introduced into a line of T lymphocytes (Figure 1), and the cells were allowed to grow for a short while. Because the regulatory sequences of the CD3γ gene immediately precede the luciferase gene in the plasmids, the activity, either positive or negative, of the regulatory sequences affected the amount of luciferase gene expression by the T lymphocytes. Luciferase…