Amphibians I

Question 1

What is the life cycle of Xenopus laevis?

Answer 1

1. Egg undergoes cleavage
2. Blastula undergoes gastrulation
3. Gastrula become a neurula
4. Neurula undergoes organogenesis
5. Tadpole undergoes metamorphosis

Question 2

What is oogenesis I?

Answer 2

1. Stem cell undergoes mitosis multiple times
2. Stem cell can become a primary oogonium
3. Primary oogonium undergoes more mitosis
4. “nest” of primary oocytes undergoes meiotic prophase I
5. Follicle cells surround oocyte, import of yolk
6. Primary oocyte grows larger and larger
7. primary oocyte is arrested in prophase I of meiosis I
8. hormonal stimulation (environment) induce oocyte maturation
9. primary oocyte undergoes asymmetric division where it produces a secondary oocyte and a small polar body
10. secondary oocyte is arrested in meiosis II until fertilization

Question 3

What are important features to know about the primary oocyte?

Answer 3

1. lampbrush chromosomes
2. many nucleoli
3. many yolk platelets
4. polarity
5. radially symmetric

Question 4

What is a lampbrush chromosome?

Answer 4

chromosome that is loosely packed, makes the DNA easy to access. This facilitates the transcription of mRNAs

Question 5

Why are nucleoli important?

Answer 5

Site for ribosome synthesis. Ribosome facilitate the translation of proteins.

Question 6

Where do yolk platelets come from? What do they provide?

Answer 6

Yolk platelets synthesized in liver, they provide nutrients for development

Question 7

What does oocyte polarity mean?

Answer 7

It has an animal pole (where embryo develops) and a vegetal pole (where nutrient comes from)

Question 8

Describe the steps of fertilization?

Answer 8

1. sperm makes contact with secondary oocyte
   2a. secondary oocyte undergoes meiosis II to produce 2 more polar bodies
   2b. cortex separates from the core
   3a. pronuclei from sperm and egg fuse in the cytoplasm
   3b. sperm gives centrioles which act on tubulins to polymerize microtubules
2. microtubules grow towards vegetal pole
3. microtubules grow along shear zone

Question 9

Describe cortical rotation?

Answer 9

1. microtubules facilitate movement of cortex with respect to the inner cytoplasm
2. rotate 30 degrees
3. clear cortical cytoplasm reveals diffuse black pigment
4. microtubules on shear zone degrade
5. cell is bilaterally symmetric

Question 10

What happens if you cut an egg so it doesn’t have a gray crescent?

Answer 10

it grows into a belly piece

Question 11

Why is grey crescent important?

Answer 11

it has dorsalizing capability

Question 12

What happens if you inhibit cortical rotation?

Answer 12

embryo is going to be featureless “ventralized”

Question 13

What happens if you get too much cortical rotation?

Answer 13

embryo structures are over emphasized “dorsalized”

Question 14

What happens if you impose a second rotation?

Answer 14

you will have a two headed embryo