genetics ch. 23

Variable gene activity hypothesis

differentiation accomplished by activating and inactivating genes at different times in different cells

what does the variable gene hypothesis assume?

cell contains entire genome and different transcription of selected genes controls development and differentiation of each cell

Maternal-effect genes

mRNA and proteins deposited in egg cytoplasm, products distributed in gradient or concentrated specific regions of cell, encode transcription factors and proteins, regulate gene expression, and products activate or repress expression of zygotic genome

Zygotic genes

transcribed in nuclei of developing embryo, transcribed in specific regions in response to distribution of maternal-effect proteins, deleterious recessive mutations in homozygotes lead to embryonic lethality

Segmentation genes

a group of developmental genes that define the segmented body plan of an embryo, organizing cells into repeated units along the anterior-posterior axis

 Gap proteins

act as transcription factors, activate pair-rule genes, whose products divide embryo into smaller regions two segments wide, anterior and posterior regions

hox genes

Specify the fate of each segment, action of maternal and segmentation genes defined by the action of homeotic (hox) genes

20 segmentation

identified and classified based on mutant phenotype

gap gene mutations

delete a group of adjacent segments and cause gaps in normal body plan, produce large gaps in embryo’s segmentation pattern, transcription of gap genes divides embryo into head, thorax, and abdomen

Pair-rule genes

Are expressed as series of seven narrow bands or stripes that extend around circumference of embryo

expression of pair-rule genes

Establishes boundaries of segments, and establishes the developmental fate of cells within each segment by controlling the expression of segment polarity genes

Segment polarity genes mutations

cause defects in segment portions

Expression of segment polarity genes

Controlled by transcription factors encoded by pair-rule genes, become active in single band of cells that extends around embryo’s circumference to divide embryo into 14 segments

why are gap genes activated?

to subdivide embryo into broad bands

what do gap genes activate?

pair-rule genes, which divide embryo into segments

what do segment polarity genes do?

divide each segment into anterior-posterior axes, which are then given identity by Hox genes

Homeotic selector (Hox) genes

action of maternal and segmentation genes defined by action of homeotic genes

Homeotic mutants

Structure formed by one segment is transformed into that formed by another segment

Homeotic genes (selector genes)

Activated as targets of zygotic genes, selector genes: Determine which adult structures will be formed by each body segment—antennae, mouth parts, legs, wings, thorax, and abdomen

Homeobox

hox genes contain a 180-b p domain known as homeobox, which encodes a DNA-binding sequence of 60 amino acids (homeodomain), expression of Hox genes is collinear with the anterior-to-posterior organization in the embryo

Binary switch genes

two alternative developmental fates for cell, initiate complete development or organ or tissue type

Gene-regulatory networks

Binary switch genes in conjunction with signaling pathways