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CHAPTER 11 The Control of Gene Expression
Modules 11.1 – 11.11
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Researchers clone animals by nuclear transplantation
A nucleus of an egg cell is replaced with the nucleus of a somatic cell from an adult Thus far, attempts at human cloning have not succeeded in producing an embryo of more than 6 cells Embryonic development depends on the control of gene expression
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In reproductive cloning, the embryo is implanted in a surrogate mother
In therapeutic cloning, the idea is to produce a source of embryonic stem cells Stem cells can help patients with damaged tissues
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Donor cell Nucleus from donor cell Implant blastocyst in surrogate mother Clone of donor is born (REPRODUCTIVE cloning) Remove nucleus from egg cell Add somatic cell from adult donor Grow in culture to produce an early embryo (blastocyst) Remove embryonic stem cells from blastocyst and grow in culture Induce stem cells to form specialized cells for THERAPEUTIC use
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GENE REGULATION IN PROKARYOTES
11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes The process by which genetic information flows from genes to proteins is called gene expression Our earliest understanding of gene control came from the bacterium E. coli Figure 11.1A
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In prokaryotes, genes for related enzymes are often controlled together by being grouped into regulatory units called operons Regulatory proteins bind to control sequences in the DNA and turn operons on or off in response to environmental changes
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Enzymes for lactose utilization
The lac operon produces enzymes that break down lactose only when lactose is present OPERON Regulatory gene Promoter Operator Lactose-utilization genes DNA mRNA RNA polymerase cannot attach to promoter Protein Active repressor OPERON TURNED OFF (lactose absent) DNA RNA polymerase bound to promoter mRNA Protein Inactive repressor Lactose Enzymes for lactose utilization OPERON TURNED ON (lactose inactivates repressor) Figure 11.1B
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Two types of repressor-controlled operons
Promoter Operator Genes DNA Active repressor Active repressor Tryptophan Inactive repressor Inactive repressor Lactose lac OPERON trp OPERON Figure 11.1C
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CELLULAR DIFFERENTIATION AND THE CLONING OF EUKARYOTES
11.2 Differentiation yields a variety of cell types, each expressing a different combination of genes In multicellular eukaryotes, cells become specialized as a zygote develops into a mature organism Different types of cells make different kinds of proteins Different combinations of genes are active in each type
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Table 11.2
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11.3 Differentiated cells may retain all of their genetic potential
Most differentiated cells retain a complete set of genes In general, all somatic cells of a multicellular organism have the same genes
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Cell division in culture
So a carrot plant can be grown from a single carrot cell Root of carrot plant Plantlet Cell division in culture Single cell Adult plant Root cells cultured in nutrient medium Figure 11.3A
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Early experiments in animal nuclear transplantation were performed on frogs
The cloning of tadpoles showed that the nuclei of differentiated animal cells retain their full genetic potential Tadpole (frog larva) Frog egg cell Nucleus UV Intestinal cell Nucleus Transplantation of nucleus Nucleus destroyed Tadpole Eight-cell embryo Figure 11.3B
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The first mammalian clone, a sheep named Dolly, was produced in 1997
Dolly provided further evidence for the developmental potential of cell nuclei Figure 11.3C
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Scientists clone farm animals with specific sets of desirable traits
11.4 Connection: Reproductive cloning of nonhuman mammals has applications in basic research, agriculture, and medicine Scientists clone farm animals with specific sets of desirable traits Piglet clones might someday provide a source of organs for human transplant Figure 11.4
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11.5 Connection: Because stem cells can both perpetuate themselves and give rise to differentiated cells, they have great therapeutic potential Adult stem cells can also perpetuate themselves in culture and give rise to differentiated cells But they are harder to culture than embryonic stem cells They generally give rise to only a limited range of cell types, in contrast with embryonic stem cells
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Cultured embryonic stem cells
Differentiation of embryonic stem cells in culture Liver cells Cultured embryonic stem cells Nerve cells Heart muscle cells Different culture conditions Different types of differentiated cells Figure 11.5
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GENE REGULATION IN EUKARYOTES
11.6 DNA packing in eukaryotic chromosomes helps regulate gene expression A chromosome contains a DNA double helix wound around clusters of histone proteins DNA packing tends to block gene expression
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DNA double helix (2-nm diameter)
Histones “Beads on a string” Nucleosome (10-nm diameter) Tight helical fiber (30-nm diameter) Supercoil (200-nm diameter) 700 nm Figure 11.6 Metaphase chromosome
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11.7 In female mammals, one X chromosome is inactive in each cell
An extreme example of DNA packing in interphase cells is X chromosome inactivation EARLY EMBRYO TWO CELL POPULATIONS IN ADULT Cell division and X chromosome inactivation Active X Orange fur X chromosomes Inactive X Inactive X Allele for orange fur Black fur Active X Allele for black fur Figure 11.7
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11.8 Complex assemblies of proteins control eukaryotic transcription
A variety of regulatory proteins interact with DNA and each other These interactions turn the transcription of eukaryotic genes on or off Enhancers Promoter Gene DNA Activator proteins Transcription factors Other proteins RNA polymerase Bending of DNA Figure 11.8 Transcription
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11.9 Eukaryotic RNA may be spliced in more than one way
After transcription, alternative splicing may generate two or more types of mRNA from the same transcript Exons DNA RNA transcript RNA splicing or mRNA Figure 11.9
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Initial polypeptide (inactive) Folded polypeptide (inactive)
Translation and later stages of gene expression are also subject to regulation The lifetime of an mRNA molecule helps determine how much protein is made The protein may need to be activated in some way Folding of polypeptide and formation of S–S linkages Cleavage Initial polypeptide (inactive) Folded polypeptide (inactive) Active form of insulin Figure 11.10
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The most important control point is usually the start of transcription
Review: Multiple mechanisms regulate gene expression in eukaryotes Each stage of eukaryotic expression offers an opportunity for regulation The process can be turned on or off, speeded up, or slowed down The most important control point is usually the start of transcription
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Figure 11.11 Chromosome DNA unpacking Other changes to DNA GENE
TRANSCRIPTION GENE Exon RNA transcript Addition of cap and tail Intron Splicing Tail Cap mRNA in nucleus NUCLEUS Flow through nuclear envelope mRNA in cytoplasm CYTOPLASM Breakdown of mRNA Translation Broken-down mRNA Polypeptide Cleavage/modification/ activation ACTIVE PROTEIN Breakdown of protein Broken-down protein Figure 11.11
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