DevelopmentW2010.Lecture6notes

DevelopmentW2010.Lecture6notes - Development BIO 120 Winter...

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Development BIO 120 Winter 2010 Jeremy Lee Eva Murdock Patrick Yuh Lecture 6 Development of Multicellularity: Cleavage I. Cleavage a. Rapid mitotic divisions following fertilization (Fig. #1.) b. In cleavage, mitotic divisions occur rapidly, then slow at the switch to gastrulation (Fig. #2.) c. During cleavage, total volume of embryo does not change. Egg cytoplasm is divided sequentially into smaller cells. Cleavage results in a multicellular ball of cells commonly called a blastula. (Fig. #1) d. In most animals, cleavage is directed by proteins and mRNAs deposited by the mother into the oocyte (egg). The exception is mammals, in which maternal mRNAs and proteins are responsible only for very early cleavages; transcription of mRNAs and the synthesis of proteins that are required for most cleavage stages are activated from the zygotic genome. For example, the switch from maternal to zygotic control of cleavage occurs at the 2-cell stage in mice and goats. e. Cell cycle of dividing blastomeres (cells during cleavage) is different than cell cycle of somatic cells; often G1 and/or G2 phases are absent in cleavage cell cycle. (Fig. #3) II. Types of Cleavage a. Classification based on cleavage pattern (Fig. #4) 1. Holoblastic : cleavage furrows go completely through egg; usually found in eggs with little or moderate amounts of yolk. 2. Meroblastic: (includes discoidal cleavage and superficial cleavage ) cleavages are not completely through zygote. Occurs in eggs with a lot of yolk. b. Classification based on yolk types (Fig. #4) 1. Isolecithal : little yolk, distributed more or less evenly (echinoderms, most molluscs, annelids, flatworms, roundworms, mammals); have holoblastic cleavage. 2. Mesolecithal : moderate amount of yolk, distributed on one side of egg (amphibians); usually have holoblastic cleavage 3. Telolecithal: large amount of yolk deposited one side of egg (cephalopod molluscs, reptiles, fish, birds); have meroblastic cleavage 4. Centrolecithal: large amount of yolk deposited centrally (insects and most other arthropods); have meroblastic cleavage c. Classification based on cleavage symmetry 1. Radial : (Fig. #5) generates cells that are completely symmetrical around an axis from pole to pole of the egg. (echinoderms and amphibians) 2. Bilateral : (Fig. #5) cells oriented so there is only one plane of symmetry through the polar axis. (Ascidians, cephalopod molluscs) 3. Spiral : (Fig. #6) Cleavage planes are at oblique angles, causing daughter cells to be diagonally oriented to each other along polar axis. (most molluscs, annelids, flatworms, and roundworms)
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4. Discoidal: (Fig. #7). All cleavages, and resulting cells, are found in one small region on one end
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This note was uploaded on 03/30/2010 for the course BIOLOGY 125 taught by Professor Jeremylee during the Winter '10 term at UCSC.

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DevelopmentW2010.Lecture6notes - Development BIO 120 Winter...

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