1.
Cleavage Patterns and Yolk Content
(Total 100 Marks)
a) Impact of Yolk on Cleavage Patterns (60
Marks)
•General Principle: The amount and distribution of
yolk in the egg greatly influence the pattern of
cleavage.
•Less Yolk, Complete Cleavage: In eggs with less
yolk (e.g., sea urchins, humans), cleavage is
holoblastic, dividing the entire egg.
•More Yolk, Incomplete Cleavage: In eggs with
more yolk (e.g., birds, reptiles), cleavage is
meroblastic, as the yolk-rich part does not divide.
•Uniform vs. Non-uniform Distribution: Uniform
yolk distribution (isolecithal eggs) leads to even
cleavage, while non-uniform distribution
(telolecithal eggs) leads to uneven cleavage.
• Examples:
• Sea Urchin: Isolecithal, undergoes radial
holoblastic cleavage.
• Frog: Moderate yolk, animal-vegetal polarity,
holoblastic but uneven cleavage.
• Bird: Telolecithal, undergoes discoidal
meroblastic cleavage.
b) Amphibian vs. Bird Cleavage (40 Marks)
• Amphibian Eggs:
• Moderate yolk concentration.
• Holoblastic cleavage, but uneven due to yolk
distribution.
• Forms a vegetal pole (yolk-rich) and an animal
pole.
• Bird Eggs:
• High yolk content.
• Meroblastic cleavage; only the blastodisc atop the
yolk cleaves.
• Does not divide completely due to the large yolk
volume.
2. Development of Higher Vertebrates
and Gastrulation (Total 100 Marks)
a) Development on Land (20 Marks)
•Amniotic Egg: Adaptation that enables development
on land, protecting the embryo within a fluid-filled
amnion.
•Shell and Membranes: Provide protection, prevent
desiccation, and allow gas exchange.
b) Gastrulation in Birds vs. Amphibians (80
Marks)
• Bird Gastrulation:
• Influenced by high yolk content.
• Formation of a primitive streak instead of a
blastopore.
• Epiblast cells migrate inward at the primitive
streak, forming germ layers.
• Amphibian Gastrulation:
• Less yolk, more uniform distribution.
• Formation of a dorsal lip and a blastopore.
• Involution of cells around the blastopore to form
germ layers.
•Comparison: The presence of a large yolk in birds
necessitates different mechanisms, like the
primitive streak, for layer formation.
3. Maternal Effect Genes in Drosophila
(100 Marks)
•Definition: Maternal effect genes are genes whose
products (mRNA, proteins) are deposited in the egg
by the mother and control early development.
•Establishment of Axes: These genes establish the
anterior-posterior and dorsal-ventral axes in the
embryo.
• Examples:
• Bicoid: Establishes the anterior end.
• Nanos: Involved in forming the posterior end.
• Dorsal: Determines the dorsal-ventral axis.
•Mechanism: The proteins or mRNAs from these
genes are localized in specific egg regions, guiding
the spatial development of the embryo.
•Mutations: Alterations in these genes can lead to
severe developmental defects, demonstrating their
crucial role in early embryogenesis.
4. Cell Adhesion and Axis
Determination (Total 100 Marks)
a) Importance of Cell Adhesion (36 Marks)
•Formation of Tissues: Essential for organizing cells
into tissues, enabling them to maintain structural
integrity.
• Examples:
• Cadherins: Mediate cell-cell adhesion in tissue
formation.
• Integrins: Connect cells to the extracellular
matrix.
• Tissue Morphogenesis: Adhesion molecules
guide the movement and arrangement of cells
during organ formation.
b) Left-Right Axis Determination in Chick Embryo
(64 Marks)
• Role of Specific Proteins:
• Nodal Flow: Cilia on the node create a leftward
flow, essential for left-right asymmetry.
• Sonic hedgehog (Shh): Involved in the
development of the left side.
• Pitx2: Activated by Nodal signaling, critical for
organ asymmetry.
Mechanism: These proteins interact to establish the
left-right axis, influencing the asymmetric placement of
organs.
