Lecture'17'April'6,'2015''What'makes'cells'different?''Developmental'events'that'turn'genes'on'or'off'Reading:'Chapter'22'of'MBoC'pages'1305G1320'(same'as'lecture'16)''Synopsis:''It'is'well'established'that'differential*gene*expression'is'the'basis'for'differences'between'cells.''All'cells'have'the'same'DNA'sequence,'but'different'cell'types'express'different'subsets'of'genes.''Cells'also'remember'their'identity,'passing'it'on'to'their'progeny.''We'reviewed'the'two'components'for'understanding'differential'gene'expression:'1)'Molecular'processes'that'serve'as'“on”'and'“off”'switches'for'gene'expression:'transcriptional'and'postGtranscriptional'controls.''2)'Developmental'events'that'turn'different'sets'of'switches'to'the'“on”'or'“off”'position.''In'this'class'period'we'focused'on'developmental'events.'''Before'biologists'could'ask'questions'about'events'that'regulate'differences'in'cells,'they'needed'to'describe'development'in'some'detail.''Cell*fate'or'developmental*fate'refers'to'the'outcome'of'development'for'a'particular'cell'or'group'of'cells.'Developmental'fate'refers'not'only'to'the'terminal'differentiated'state'of'the'cell'(e.g.'muscle,'skin'etc.),'but'also'to'the'events'the'cell'experiences'during'development.''An'example'of'this'that'is'different'from'the'ones'used'in'class'is'the'fate'of'the'mesoderm'cells'of'the'sea'urchin.''The'terminal'fats'of'these'cells'is'to'produce'the'skeleton'of'the'larva'(among'other'things),'but'the'fate'of'the'mesoderm'is'also'to'move'into'the'blastocoel'cavity'during'gastrulation'(see'Figure'22G3'in'the'assigned'reading.)''''The'best'description'of'development'is'a'map'that'spells'out'the'developmental'fates'of'all'of'the'cells.''Researchers'obtain'this'detailed'description'using'two'major'techniques:''cell*lineage*tracing'and'fate*mapping.''Tracing'cell'lineages'requires'that'cells'be'transparent,'and'that'division'patterns'are'reproducible'from'animal'to'animal'(that'is,'cells'always'divide'the'same'number'of'times,'with'the'same'orientations'and'have'the'same'fates).''The'complete'cell'lineage'of'the'nematode'C.#elegans'is'known'and'provides'a'powerful'tool'for'developmental'analysis'in'this'animal.'''In'most'animals,'especially'vertebrates,'the'conditions'for'tracing'lineages'at'high'resolution'are'not'met.''In'these'cases,'the'method'of'fate*mapping'is'used.''In'fate'mapping,'single'cells'or'groups'of'cells'are'marked'at'a'particular'time'point'in'development'(we'used'the'late'blastula/early'gastrula'stage'of'the'frog'as'an'example),'and'at'later'time'points,'the'location'and'identity'of'the'labeled'cells'can'be'noted.'Slides'14'and'15'on'the'handout'provide'examples'of'fate'mapping'in'the'frog'(using'surface'dyes'and'injected'fluorescent'dyes'respectively)'a nd'