BIOL 303 Lecture Notes - Lecture 3: Cleavage Furrow, Spindle Apparatus, Sea Urchin
14 May 2018
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Biol 303 Feb 6th
General
• Topic sign-up today at 4 pm – limited spaces for each topic
• Darwin Day this Friday, tickets $20
Patterns of Development
Cleavage
• Happens immediately upon egg activation
• It is very fast → done by skipping G1 and G2 (the intermission
steps that happen between DNA synthesis and mitosis)
• Egg activation leads to a burst of translation of maternal RNAs
• Translation of cyclin B → cyclin B + CDK = MPF (Mitosis-Promoting
Factor)
• Rapid and synchronous mitotic divisions, egg cytoplasm divides
into blastomeres → becomes the blastula
• Biphasic cell cycle (after fertilization), controlled by maternally
provided cytoplasmic components → has only two phases
compared to the regular four-phase cell cycle
• Mid-blastula transition (MBT): oral ell le +G ad G,
asynchronous divisions controlled by the embryo genome
Cleavage Patterns
• Early cleavage patterns are distinct and invariable in different species
• The positioning of the mitotic spindle is maternally determined
• Two main types of signalling: Intrinsic cues (elements inherited) and extrinsic cues (from
other cells)
• What determines different cleavage patterns?
➢ Position of the mitotic spindle
➢ Amount and position of yolk – the yolk is a strong barrier for the cleavage furrow
Pop Quiz!
• Mutant Crayfish that clones itself
• A speciation event occurred and this new species now exists
Q1
• One reason why the crayfish may not take over the world is a concept called?
Answer: The ‘ed Quees ‘ae – all animals are identical clones so their pathogens may
eole faster tha the sie the at ehage alleles to eole etter ersios – one
lucky virus that can beat their genotype can wipe them all out
Q2
• Which of these models is correct regarding how disaggregated cells re-aggregate?
find more resources at oneclass.com
find more resources at oneclass.com
• Answer: Differential Adhesion Hypothesis → differet ell tpes ill e stikier tha
others and these sticky ones aggregate with one another
• Directed migration: there may be diffusing cues that are more highly concentrated in
the centre of the clumps
• Hooking proteins = cadherins → you need Calcium for them → catenins (especially
Beta-catenin) is necessary for them to hook one another
Q3
• You are attempting to fertilize sea urchin eggs in a solution that contains Na+ but no
Ca2+. What is the expected result and why?
• Answer: Normal fertilization → ou dot eed aliu i the solutio eause aliu
is produced in the ER so the slow block will still occur (and the fast block which requires
Na+)
Sea Urchin Development Overview
• 60-cell stage
• Gastrula stage
• Four-arm larva
• Eight-arm larva
• Adult sea urchin
Holoblastic radial cleavages lead to
1. Axis establishment: animal – vegetal
2. Morphogenesis (smaller and bigger cells)
Divisions 1-3 are equal
• You divide the cell in half – the first two are
equal, symmetrical divisions
• The third is an equal, equatorial division
This ^ results in 8 daughter cells of equal size
Division 4
• The fourth division happens unequally – this
sets up certain cell types
• In the animal half, there are equal meridian
divisions producing 8 mesomers
• In the vegetal half, there are unequal equatorial divisions that produce 4 macromeres
and 4 micromeres
• Macromeres are the biggest
• Micromeres are small and at the most vegetal part of the embryo
Fate Map at ~60 cell stage
• See slide image
• By the 60-cell stage, animal and vegetal polarity and fates are established
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
General: topic sign-up today at 4 pm limited spaces for each topic, darwin day this friday, tickets . Patterns of development: happens immediately upon egg activation. Amount and position of yolk the yolk is a strong barrier for the cleavage furrow. Pop quiz: mutant crayfish that clones itself, a speciation event occurred and this new species now exists. Beta-catenin) is necessary for them to hook one another. Q3: you are attempting to fertilize sea urchin eggs in a solution that contains na+ but no. What is the expected result and why: answer: normal fertilization (cid:455)ou do(cid:374)(cid:859)t (cid:374)eed (cid:272)al(cid:272)iu(cid:373) i(cid:374) the solutio(cid:374) (cid:271)e(cid:272)ause (cid:272)al(cid:272)iu(cid:373) is produced in the er so the slow block will still occur (and the fast block which requires. Sea urchin development overview: 60-cell stage, gastrula stage, four-arm larva, eight-arm larva, adult sea urchin. Holoblastic radial cleavages lead to: axis establishment: animal vegetal, morphogenesis (smaller and bigger cells)
