Animal: Lecture 2 Flashcards
What are model organisms?
Model organisms are those that are particularly easy to study, and also broadly representative.
- This is what we use to study development
- Their mechanisms tend to match up well with other species and apply to the concept at large.
Idea of model organisms for development?
Developmental genes and mechanisms are very similar across animal species
– E.g., Drosophila (fruit flies) and humans have similar genes that perform similar functions
Studying development in model organisms provides knowledge about development in general
– E.g., Sea urchins, frogs
Why do we use frogs or sea urchins to study development?
They are fairly easy to use for a few different reasons:
- Deposit a large amount of eggs in semi-aquatic/aquatic habitats
- Easy to obtain eggs and sperm and put them together
- Easy to see the development as the eggs and semi-transparent
Fertilization?
Haploid gametes join to form a diploid zygote.
Life cycle?
Meiosis ~> Haploid Gametes (sperm and egg) ~> Fertilization ~> Diploid Zygote ~> Mitosis ~> Multicellular Adult ~> Meiosis
What does fertilization in sea urchins look like?
Egg is much larger, but there is a huge quantity of sperm. One layer separates from the egg when the egg is fertilized.
Egg Cytoplasm?
Contains many proteins and mRNA involved in early development.
- Most of the information and proteins and such come from the mother.
Sperm and eggs in the sea urchins?
Sperm and egg are the haploid gametes.
Parts of the sperm?
Sperm tail - for motion
Sperm head - contains info
Nucleus - contains DNA
Acrosome - digestive enzymes
Job of the sperm?
Only job is to deliver the DNA to the egg.
Parts of the egg?
Jelly coat - surrounds the egg
Vitelline layer - contains receptors
Sperm-binding receptor - proteins where the sperm will attach
Cortical granules - released later in the process
Egg plasma membrane - membrane of the actual egg.
What happens when the jelly coat is contacted by the sperm?
Sperm contacts the egg’s jelly coat
- this triggers the acrosomal reaction
- acrosome releases hydrolytic enzymes to digest the jelly coat to try and bind with one of the sperm receptors.
What happens after the acrosomal reaction?
Surface proteins on acrosomal process bind to receptors on the egg cell membrane.
- Proteins extend and try to bind to receipts embedded in the vitelline layer and the plasma membrane of the cell.
- Ligands bind to the sperm-binding receptors on the egg.
What happens after the binding of the ligands?
Plasma membranes fuse together on both sides, triggering the fast block to polyspermy (prevents multiple sperm from coming through).
What is the fast bock to polyspermy?
Change in membrane charge from negative to positive: depolarization.
- Prevents binding of other sperm.
- Immediate but short-lived.
- Negative charge attracts sperm, so the change to positive repels them.
- Something else needs to happen long-term
What causes the slow block to polyspermy?
Sperm nucleus enters and cortical reaction causes the slow block to polyspermy.
What is the cortical reaction?
Cortical granules are released, pushing the vitelline layer away from the plasma membrane.
What is the slow block to polyspermy?
The vitelline layer is separated due to the cortical granule release, pushing the layer farther away.
- The space between the Vitelline layer and the membrane is called the perivitelline space.
- The fertilization envelope is the barrier to the sperm now, consists of the vitelline layer and the jelly coat.
Essentially, what is the difference between the two blocks?
Physical = slow block
Polarity = fast block
Final step of fertilization?
Fusion of sperm and egg nuclei form the diploid nucleus of the zygote.
What is cleavage?
The process by which the zygote becomes multicellular.
- Happens by rapid cell division with little growth of individual cells where the growth phases (G1 and G2) are skipped.
- Same amount/volume of space are occupied by more and more cells through this process.
Result of cleavage?
Cleavage results in many smaller cells called blastomeres.
- Cells getting smaller and smaller with each round of division
- No growth of cells between division
In frogs, zygote becomes…?
In frogs, the zygote becomes a blastula (hollow ball of cells).
Frog cleavage goes from 1 cell to a blastula through the process of cleavage.
What is a blastocoel?
The blastula is a hollow ball of cells with a fluid filled cavity called the blastocoel.
Animal pole vs. Vegetal pole?
Animal pole: smaller cells here, where the sperm likely entered.
Vegetal pole: larger cells, pretty much opposite the area where the sperm entered.
How can cells that possess the same genome look and behave differently?
This happens because of differential gene expression.
What is differential gene expression?
Cells express different genes depending on their location and the stage of development.
Expressing different genes leads to the production of different proteins or enzymes.
- Which in turn determine the structure and behaviour of the cell at any given time.
Differential gene expression in action?
Not every gene makes the same proteins.
- Not every gene in the genome gets expressed at the same time
- Some genes are activated while other aren’t. What activates them typically has something to do with a protein or enzyme or chemical.
How does a cell know what genes to express at any given time?
There are two mechanisms that determine the genes expressed:
1. Cytoplasmic determinants (signal comes from within the cell).
2. Inductive signals (signal comes from outside the cell).
What are cytoplasmic determinants?
Molecules within the cytoplasm that regulate gene expression.
- Can be differentially distributed to daughter cells resulting in differences in gene expression.
- Ex. Half of the dots on one side of the cell. Leads to a cell with the dots and one with nothing.
What are inductive signals?
The signal molecules that a cell is exposed to depend on its location within the embryo, and the state of development.
Ex. Stem cell near muscle cell will likely take on the traits and become the muscle cell. These traits were induced through chemicals surrounding it.
Preventing polyspermy ensures the zygote has the correct balance of maternal and paternal..?
a) proteins
b) chromosomes
c) mRNA
d) organelles
e) carbohydrates
CORRECT: B
True or false. A frog blastula is essentially the same size as the zygote from which it developed?
TRUE
True or false? Every somatic cell in a multicellular organism contains an identical genome?
TRUE AND FALSE
- True: essentially all cells
- False: no genome in red blood cells
If a cell that normally develops into a muscle cell develops into a skin cell when moved to a different part of the embryo, which mechanism is responsible for determining the cell’s fate?
a) cytoplasmic determinants
b) induction
CORRECT: B
What is morphogenesis?
The rearrangement of cells or sheets of cells (tissues) in the embryo to make structures/parts within your body.
Gastrulation?
Stage when the three germ layers are established, and the basic body plan is set up.
Organogenesis?
The formation of the organs .
Ex. Neurulaton = formation of the nervous system (brain)
What are the three layers resulting from gastrulation?
Ectoderm: skin cells
Mesoderm: bones, connective tissue
Endoderm: lining of the gut
What does frog gastrulation look like?
Begins at one end then flips. First entrance into the gut = early anus.
Steps of frog gastrulation?
- Cells in the vegetal hemisphere push inward.
- Outer cells (future endoderm and mesoderm) roll inwards.
- Blastocoel collapses and a new cavity called the archenteron is formed.
- Cells at the animal pole (future ectoderm) spread over the outer surface.
True or False? Endodermal (vegetal) cells express different genes than those expressed by the ectodermal (animal) cells?
TRUE, they have differential gene expression.
How is morphogenesis achieved?
Morphogenesis is achieved through changes in cell position, shape, and survival.
What is convergent extension?
Example of cells changing their position (location of part changes)
- Producing a longer, narrower structure.
Example of cells changing shape?
Ectodermal cells change shape during neural tube formation.
1. Microtubules extend, leading to the elongation of the cells.
2. Actin filaments shorten, leading to a triangular shape (curves)
3. More pinching in occurs
4. Neural tube splits off from the cell
Programmed cell death?
Programmed cell death also shapes embryos.
Ex. Fluorescing cells are undergoing apoptosis. This would be the removal of webbing between digits.
What causes changes in cell position, shape and survival?
A. The position of the cell within the embryo.
B. The cytoplasmic determinants within a cell.
C. Changes in gene expression within a cell
D. The proteins that are produced within a cell.
E. All of the above.
ANSWER: E
Fertilization?
Forms the unicellular diploid zygote
Embryonic stages?
- Cleavage converts the unicellular zygote into a multicellular embryo
- Blastula: hollow ball of cells
- Gastrulation: 3 germ layers are formed.
Differential gene expression?
The foundation of the diverse cell types and behaviours seen throughout development and throughout life.
Morphogenesis?
Occurs via changes in cell position, shape and survival & is responsible for forming organs.
