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Flashcards in Lecture 3 Deck (27):
1

What is the difference between fate, determination and specififcation?

Fate is what is likely to happen to a cell based on its location
Determination is what will happen to a cell as it has reached a point where it can no longer change its fate
Specification is when a cell will turn into a specific cell even if ti recieves no more signalling but its fate can still be changed by new signalling

2

What are the series of steps that begin in a developing embryo after the fusion of the pronuclei?

It first enters the phase of cleavage where it undergos rapid mitotic division
The embryo will then undergo gastrulation which is a dramatic rearrangement of the cells in the embryo to establish the three germ layers
During these phases the major axes of the body are formed and cell acquire their fates
(axial specification can begin earlier however cleavage ALWAYS proceeds gastrulation

3

What occurs in cleavage?

A series of rapid mitotic divisions in which the egg is divided into smaller and smaller cells known as blasotmeres and there is no overall increase in the embryos mass

4

How is the synchrony of cell division during cleavage controlled?

In most organisms both this and the placement of blastomeres relative to each other is under the control of proteins and mRNAs stoed in the oocyte as only later does this process come under control of the zygotic genome

5

How can the mitotic divisions during cleavage occur so much more rapidly than typical cell divisions?

The gap periods G1 and G2 re abolished from the cell cycle resulting in a biphasic cell cycle of M and S phase controlled by MPF

6

What is MPF?

Mitosis promoting factor which controls cleavage, it is a complex of cyclin dependent kinase 2 and cyclin B
MPF is high in the M phase and undectable in the S phase
When cdc2 is bound to cyclin B it is active and phosphorylates key target proteins including histones, nuclear envelope lamin proteins and the regulatory unit of cytoplasmic myosin

7

What controls the levels of cyclin B in the cell during cleavage?

It is controlled by proteins in the egg cytoplasm which regulate its periodic synthesis and degradation
Eventually the cytoplamsic regulators will be used up and the cell will need to begin to produce its own allowing it to enter the mid-blastula transition

8

What occurs in the mid-blastula transition?

the cell cycle lengthens by gaining the G1 and G2 phases and synchrony of division is lost
New mRNAs are transcribed resulting in new proteins being synthesized many of which will initiate gastrulation and specify cell fate

9

What occurs in the mid-blastula transition?

the cell cycle lengthens by gaining the G1 and G2 phases and synchrony of division is lost
New mRNAs are transcribed resulting in new proteins being synthesized many of which will initiate gastrulation and specify cell fate

10

What are the two co-ordinated processes that control cleavage?

karyokinesis which is the mitotic division of the nucleus
cytokinesis which is the division of the cytoplasm of the cell

11

What is the mechanical agent for karyokinesis?

the microtubular mitotic spindle

12

What is the plane of cleavage in spiral cleavage?

There are successive division planes at slight angles to each other resulting in a spiral pattern of blastomeres

13

What are the two parameters that control the cleavage pattern?

The amount and distribution of yolk which inhibits cleavage and time of formation and positioning of the mitotic spindle

14

What is the pattern of cleavage in isolecithal eggs?

Isolechithal eggs have even yolk distribution and cleavage can be radial, bilateral or rotational
it is also is typically holoblastic cleavage

15

What is the patterno of embryone cleavage in mesolechital eggs?

These eggs have a moderate yolk and undergo cleavage which is holoblastic but displaced

16

What is the pattern of cleavage in telolecithal eggs?

These eggs have a dense yolk and undergo meroblastic cleavage

17

What is the pattern of cleavage in centrolecithal eggs?

centrolecithal eggs have a central yolk and cleavage is meroblastic

18

What is the difference between holoblastic and meroblastic cleavage?

Holoblastic is total or entire cleavage while meroblastic is only partial cleavage

19

What is the plane of cleavage in radial cleavage?

The cleavage occurs at right angles to the cell surface giving tiers of blastomeres

20

What is the plane of cleavage in spiral cleavage?

There are successive division planes at slgith angles to each other resulting in a spiral pattern of blastomeres

21

How does the mitotic spindle function during cell division?

During the S phase the centrosome of the cell (a microtubular organizing centre) is composed to two centrioles at right angles this strucure duplicates and forms asters by moving to opposite sides of the nucleus
Spindle fibers then attach to the kinetocore on the chromosome to pull it apart during cell division

22

How is the blastocoel formed?

Fluid accumlates in the blastocoel due to the Na/K pump located on one side of the embryo this drices osmotic water movement to the blastocoel through aquaporin channels
tight junctions on the other side of the cell prevent the water from escaping causing hydrostatic pressure to build resluting in the cells stretching to form the fluid filled cavity known as the blastocoel

23

What are the early cleavage events that go on in the sea urchin?

The first and second cleavages are meridional thrugh the animal and vegetable poles at right angles to each other
the third cleavage is equilateral seperating the animal and vegetable hemispheres
The fourth cleavage results in the animal half cells being divided meridonally into eight equal balstomeres and the vegetable half divides equatorially but unequally to give four large macromeres and four small micromeres

24

How does gastrulation begin in the sea urchin?

With an epithelial to mesenchymal transition with most vegetable cells becomging motile and mesenchymal in form they migrate into the blastocoel as single primary mesenchymal cells and later lay down the spicules of the skeleton.

25

What are the requirements for cellular internalization?

loss of cell-cell adhesion and is associated with repression of cadherin expression and its removal from the cell surface by endocytosis, and the loss of alpha and beta catenins.

26

What follows Internalization of the primary mesenchyme in the sea urchin?

invagination and extension of the endoderm to form the embryonic gut (archenteron)

27

How is the archenteron formed in the sea urchin?

Cells at the tip of the invaginating endoderm (which later detach as the secondary mesenchyme and form pigment and muscle cells and immunocytes) extend long filopodia that adhere to the blastocoel wall and then contract to pull the elongating gut across the blastocoel until it comes into contact and fuses with the mouth region (deuterostome)
Extension is aided by rearrangement of cells in the endodermal sheet, leading to the formation of a tube