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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Most differentiated (specialized) cells retain a complete set of genes –In general,

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Presentation on theme: "Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Most differentiated (specialized) cells retain a complete set of genes –In general,"— Presentation transcript:

1 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Most differentiated (specialized) cells retain a complete set of genes –In general, all somatic cells of a multicellular organism have the same genes whether it is a liver cell, heart cell, muscle cell etc Differentiated cells may retain all of their genetic potential

2 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Table 11.2

3 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 11.2 Differentiation yields a variety of cell types, each expressing a different combination of genes CELLULAR DIFFERENTIATION AND THE CLONING OF EUKARYOTES

4 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A chromosome contains a DNA double helix wound around clusters of histone proteins DNA packing tends to block gene expression 11.6 DNA packing in eukaryotic chromosomes helps regulate gene expression GENE REGULATION IN EUKARYOTES

5 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 11.6 DNA double helix (2-nm diameter) Metaphase chromosome 700 nm Tight helical fiber (30-nm diameter) Nucleosome (10-nm diameter) Histones “Beads on a string” Supercoil (200-nm diameter)

6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings An extreme example of DNA packing in interphase cells is X chromosome inactivation 11.7 In female mammals, one X chromosome is inactive in each cell Figure 11.7 EARLY EMBRYO Cell division and X chromosome inactivation X chromosomes Allele for orange fur Allele for black fur TWO CELL POPULATIONS IN ADULT Active X Inactive X Orange fur Inactive X Active X Black fur

7 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A variety of regulatory proteins interact with DNA and each other –These interactions turn the transcription of eukaryotic genes on or off 11.8 Complex assemblies of proteins control eukaryotic transcription Enhancers DNA Activator proteins Other proteins Transcription factors RNA polymerase Bending of DNA Transcription Promoter Gene Figure 11.8

8 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Exons DNA RNA splicing or RNA transcript mRNA After transcription, alternative splicing may generate two or more types of mRNA from the same transcript 11.9 Eukaryotic RNA may be spliced in more than one way Figure 11.9

9 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The lifetime of an mRNA molecule helps determine how much protein is made –The protein may need to be activated in some way Translation and later stages of gene expression are also subject to regulation Figure Folding of polypeptide and formation of S–S linkages Initial polypeptide (inactive) Folded polypeptide (inactive) Cleavage Active form of insulin

10 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 Review: Multiple mechanisms regulate gene expression in eukaryotes

11 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chromosome GENE RNA transcript mRNA in nucleus mRNA in cytoplasm Polypeptide ACTIVE PROTEIN GENE Exon Intron Tail Cap NUCLEUS Flow through nuclear envelope CYTOPLASM Breakdown of mRNA TranslationBroken- down mRNA Broken- down protein Cleavage/modification/ activation Breakdown of protein DNA unpacking Other changes to DNA TRANSCRIPTION Addition of cap and tail Splicing Figure 11.11

12 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –Can differentiated cells reverse become dedifferentiated? This is common in plants –So a carrot plant can be grown from a single carrot cell Figure 11.3A Root of carrot plant Adult plant Root cells cultured in nutrient medium Cell division in culture Single cell Plantlet

13 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 Figure 11.3B Tadpole (frog larva) Intestinal cell Frog egg cell Nucleus UV Nucleus destroyed Transplantation of nucleus Eight-cell embryo Tadpole

14 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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

15 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Remove nucleus from egg cell Donor cell Add somatic cell from adult donor Grow in culture to produce an early embryo (blastocyst) Nucleus from donor cell Implant blastocyst in surrogate mother Remove embryonic stem cells from blastocyst and grow in culture Clone of donor is born (REPRODUCTIVE cloning) Induce stem cells to form specialized cells for THERAPEUTIC use

16 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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

17 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Scientists clone farm animals with specific sets of desirable traits Piglet clones might someday provide a source of organs for human transplant 11.4 Connection: Reproductive cloning of nonhuman mammals has applications in basic research, agriculture, and medicine Figure 11.4

18 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 11.5 Connection: Because stem cells can both perpetuate themselves and give rise to differentiated cells, they have great therapeutic potential

19 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Differentiation of embryonic stem cells in culture Figure 11.5 Cultured embryonic stem cells Different culture conditions Different types of differentiated cells Heart muscle cells Nerve cells Liver cells

20 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes GENE REGULATION IN PROKARYOTES Figure 11.1A

21 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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

22 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The lac operon produces enzymes that break down lactose only when lactose is present Figure 11.1B DNA mRNA Protein Regulatory gene PromoterOperatorLactose-utilization genes OPERON RNA polymerase cannot attach to promoter Active repressor OPERON TURNED OFF (lactose absent) DNA mRNA Protein OPERON TURNED ON (lactose inactivates repressor) Lactose Inactive repressor RNA polymerase bound to promoter Enzymes for lactose utilization

23 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Two types of repressor-controlled operons Figure 11.1C Tryptophan DNA PromoterOperatorGenes Active repressor Inactive repressor lac OPERONtrp OPERON Lactose


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