Exam 5: Lecture 4 Flashcards Preview

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Free Movement

-during early development mRNAs and proteins must be able to move freely throughout egg/oocyte
-maternal mRNAs being localized to anterior and posterior poles which requires microtubule tracks to be able to run across entire embryo unhindered
-also after maternal transcripts are translated, encoded proteins will diffuse away from mRNA transcript source
-for these events to happen early embryo can't be cellularized


Regulating Diffusion Gradients in Embryo

-proteins diffuse across the embryo nuclei bathed in different concentrations of each factor (bicoid, oskar, nanos)
-different concentrations must be maintained so transcription of zygotic Gap genes can be activated differently across embryo
-just prior to Gap gene transcription, embryo will undergo cellularization (process by which single cell embryo becomes multicellular organism)
-traps maternal proteins with each cell at a concentration appropriate for that cellular position along A/P axis


Example of Regulating Diffusion Gradients

-cells at most anterior pole now contain highest amounts of bicoid while those at midsection will have lower levels and cells at posterior pole will completely lack it
-differences lead to transcription of otd in anterior pole and Kruppel in midsection
-both genes remain off in posterior pole


Cellularization of embryo

-prevents proteins like bicoid, nanos, and oskar from diffusing any further across embryo
-if cellularization is delayed or prevented, these proteins will continue to diffuse and ultimately their concentration across the embryo will be equal in all cells
-slope of gradient will be a flat line


Anterior Half of Embryo

-there are high levels of Bicoid (bcd-activator) and Hunchback (Hb-genes that are necessary for the formation of the head segments
-one gene shut off in anterior section is Kruppel (Kr) which is required for the formation of the midsection
-even though Kr embryonic enhancer contains both Bcd and Hb binding sites the activity of the rpressor dominates at the higher concentrations


bcd Mutant

-in bcd mutant Kr expression is activated in the anterior section of the embryo
-this is due to loss of Hb repressor
-also indicates that other activators are also used to initiate Kr expression


Midsection of Embryo

-the levels of both proteins begin to taper off
-Kr expression is activated since Bcd activity dominates over Hb at lower levels
-thus anterior border of Kr expression is set b combined activities of Bcd and Hb
-posterior boundary of Kr expression is set by two repressors called Knirps (Kni) and Giant (Gt)
-expression of these tow genes is activated by gradients that are initiated by oskar and nanos at the posterior pole
-combined efforts of Bcd, Hb, Kni, and Gt restricts Kr expression to embryonic midsection
-Kni and Gt proteins also play important roles in regulating the fate of cells within posterior section of embryo


Gap Genes Examples

-hunchback, Kruppel, Knirps, and Giant are considered Gap genes since they are expressed in large domains and determine the fates of cells within these domains