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.