BZYCT-133 Solved Assignment

1. a) Explain keratinization in terrestrial vertebrates.
   b) Give five features that you can use to distinguish between the skulls of frog and rabbit.
2. a) Explain the major differences between reptilian and avian digestive systems.
   b) Describe the structure of the respiratory system of cartilaginous fishes and state how does it differ from that of bony fishes.
3. a) Discuss lymphatic system in different vertebrates.
   b) Write short notes on:
   i) Blood filteration in kidney
   ii) Types of mammalian uteri
4. a) Which part of the brain is well developed in all vertebrates and why?
   b) What are pit organs in reptiles? How do vipers and boas locate prey?
5. Briefly write the functions of the following hormones secreted in mammals.
   a) Adrenocorticotropic hormone
   b) Parathormone
   c) Aldosterone
   d) Testosterone
   e) Progesterone
6. List at least three stages in gene expression that can be regulated to result in differentiated cell types? Explain any one of them with the help of an example.
7. Describe the mechanisms evolved by eggs to prevent polyspermy.
8. Make a flow chart to show the events in metamorphosis.
9. Discuss the process of development of extra embryonic membranes in chick.
10. a) Draw a flow chart to show how the three germinal layers are derived from the zygote.
    b) How do genetic and environmental defects cause problems in embryonic development?

Answer:

1. a) Explain keratinization in terrestrial vertebrates.

Answer:

1. Introduction to Keratinization in Terrestrial Vertebrates

2. The Structure and Function of Keratin

3. The Stages of Keratinization

• Basal Stage: In the basal layer of the epidermis, keratinocytes divide and differentiate. These stem cells are actively involved in cell division to replace older cells that migrate toward the surface. This stage is crucial for the generation of new keratinocytes.
• Spinous Stage: As cells move upward, they begin to synthesize keratin and other proteins such as filaggrin, which play a role in cell-cell adhesion. The keratinocytes become more flattened and start forming connections with each other through desmosomes, which are intercellular junctions.
• Granular Stage: In the granular layer, keratinocytes accumulate more keratin and other specialized proteins like keratohyalin, which helps in the aggregation of keratin filaments. These cells begin to lose their nuclei and other organelles, becoming more keratin-rich.
• Corneal Stage: In the outermost layer, the keratinocytes are fully keratinized, dead cells that form a tough, impermeable layer known as the stratum corneum. These cells are essentially flattened sacs filled with keratin fibers. The cells are no longer living and serve as the primary protective barrier.

4. The Role of Keratinization in Terrestrial Adaptation

• Desiccation (drying out): The skin of terrestrial vertebrates must prevent excessive water loss, which is where keratinization comes in. The keratinized layers, particularly the stratum corneum, act as an effective water-resistant barrier, reducing evaporation and preventing desiccation.
• Protection from physical damage: The hard, keratinized structures such as nails, claws, and scales provide mechanical protection. They protect the underlying tissues from abrasions, physical injuries, and punctures.
• Protection from pathogens: The keratinized skin layer serves as a physical barrier against invading microorganisms. Additionally, keratin itself has antimicrobial properties, further helping in defending against pathogens.
• Temperature regulation: In some vertebrates, keratinized structures like feathers or hair serve not only as protective barriers but also play a role in temperature regulation. Feathers, for example, trap air close to the body, providing insulation and helping to regulate body temperature.

5. Keratinization in Different Terrestrial Vertebrates

• Mammals: In mammals, keratinization is seen in the formation of skin, hair, and nails. The skin of mammals undergoes extensive keratinization, forming a tough, protective layer. Hair and nails are also composed of keratin, providing both structural support and protection.
• Reptiles: In reptiles, keratin is primarily found in the formation of scales, which provide protection from dehydration and mechanical damage. These keratinized scales help reptiles conserve water and regulate their body temperature.
• Birds: Birds have feathers and beaks made of keratin. Feathers are essential for flight, temperature regulation, and mating displays, while beaks, composed of hard keratin, are used for feeding and other activities.

6. Disorders Related to Keratinization

• Ichthyosis: This is a genetic disorder where keratinization is impaired, leading to the buildup of thick, scaly skin.
• Epidermolysis bullosa: This condition involves defective keratin production, making the skin highly fragile and prone to blisters.

Conclusion

1. b) Give five features that you can use to distinguish between the skulls of frog and rabbit.

Answer:

1. Introduction

2. Shape and Size of the Skull

3. Structure of the Jaw

4. Presence of Teeth

5. Foramen Magnum and Neck Vertebrae

6. Temporal Region and Jaw Musculature

Conclusion

2. a) Explain the major differences between reptilian and avian digestive systems.

Answer:

1. Introduction to Digestive Systems of Reptiles and Birds

2. General Structure of Reptilian Digestive System

• Mouth and Teeth: Reptiles generally have teeth that are adapted to their specific diet. For example, carnivorous reptiles like snakes have sharp, curved teeth to hold and tear prey, while herbivores like tortoises have flat teeth for grinding plants. The mouth is used primarily for ingestion and initial processing of food.
• Esophagus: The food passes through the esophagus, which is a simple muscular tube. In some reptiles, the esophagus can be very elongated, as seen in snakes, which swallow large prey whole.
• Stomach: Reptiles typically have a simple stomach where the majority of chemical digestion occurs. Some reptiles have a gizzard (like birds), but it is less developed and less important for mechanical digestion. The stomach of reptiles secretes gastric juices containing hydrochloric acid and digestive enzymes for breaking down proteins.
• Intestines: The small intestine in reptiles is where most of the nutrient absorption occurs. It is shorter compared to birds, reflecting the relatively low metabolic rate of reptiles. The large intestine absorbs water and salts, and the cloaca serves as the common exit for digestive, urinary, and reproductive products.
• Cloaca: In reptiles, the cloaca is the final chamber that serves multiple functions, including the elimination of waste and the expulsion of eggs or sperm.

3. General Structure of Avian Digestive System

• Beak: Birds do not have teeth. Instead, they use their beak to break down food mechanically. The shape of the beak is adapted to their feeding habits, whether they are seed eaters, insectivores, or carnivores.
• Esophagus and Crop: The esophagus in birds is similar to reptiles, but many birds have a specialized pouch called the crop. The crop stores food temporarily, allowing birds to regurgitate food when necessary or digest it in a controlled manner.
• Stomach: The avian stomach is divided into two parts: the proventriculus and the gizzard. The proventriculus is the glandular stomach that secretes digestive enzymes and hydrochloric acid to begin breaking down food. The gizzard, a muscular organ, serves as a mechanical digestive tool, grinding up food, especially in species that consume hard foods like seeds.
• Small Intestine: Like reptiles, the small intestine in birds is responsible for most of the nutrient absorption. It is longer than that of reptiles and has more surface area due to the presence of villi and microvilli. Birds have a more efficient absorption system, which is necessary to support their high metabolic rate.
• Ceca: Birds possess two ceca, which are blind pouches located at the junction of the small and large intestines. These structures are involved in the fermentation and breakdown of cellulose, which is particularly important for herbivorous birds. The ceca help to digest plant material that is otherwise difficult to break down.
• Large Intestine and Cloaca: The large intestine in birds is relatively short and primarily absorbs water and salts. The cloaca in birds functions similarly to reptiles, serving as the exit for waste, eggs, or sperm.

4. Major Differences Between Reptilian and Avian Digestive Systems

• Beak vs. Teeth: One of the most obvious differences is that birds lack teeth, relying on their beak for mechanical digestion, while reptiles have teeth suited to their specific diets. For example, carnivorous reptiles have sharp teeth for tearing prey, while herbivores have flat teeth for grinding plant matter.
• Crop and Gizzard: Birds possess a crop, which acts as a storage organ for food, and a gizzard, which is a specialized muscular organ used for grinding food, especially in birds that consume hard materials like seeds. In contrast, reptiles generally do not have a crop or gizzard, and their digestive processes are simpler.
• Stomach Structure: Birds have a two-chambered stomach (proventriculus and gizzard), with the proventriculus secreting digestive enzymes and acid, and the gizzard physically grinding food. Reptiles have a simpler, single-chambered stomach that performs both enzymatic digestion and some mechanical breakdown.
• Ceca: Birds have two ceca that play a role in the fermentation of plant material, especially in herbivorous species. Reptiles, on the other hand, usually do not have a functional cecum or have a less developed one. This difference is related to the fact that birds often consume fibrous plant material and need additional digestive processing for it.
• Digestive Efficiency: The avian digestive system is more efficient in terms of nutrient absorption, given the high metabolic rate required for flight. In contrast, the reptilian digestive system is slower and less complex due to the generally lower metabolic rate of reptiles.

5. Conclusion

2. b) Describe the structure of the respiratory system of cartilaginous fishes and state how does it differ from that of bony fishes.

Answer:

1. Introduction to the Respiratory System of Fishes

2. Structure of the Respiratory System in Cartilaginous Fishes

• Gills: Cartilaginous fishes typically have five to seven pairs of gill slits located on the side of their head. These gills are responsible for the exchange of oxygen and carbon dioxide. Unlike bony fishes, cartilaginous fishes do not have gill covers (operculum); the gill slits are exposed, which makes them more vulnerable to physical damage but provides greater access to water for respiration.
• Spiracles: A distinctive feature of many cartilaginous fishes is the spiracle, which is a small opening located behind the eyes. The spiracle allows water to enter the gills even when the fish’s mouth is closed. This adaptation is particularly useful when the fish is feeding or resting on the seafloor, as it enables continuous water flow over the gills for respiration.
• Gill Rakers: The gills of cartilaginous fishes are often equipped with gill rakers, which filter out particulate matter from the water before it passes over the gill filaments. This ensures that debris and larger particles do not clog the delicate gill structures.
• Pharyngeal Slits: Water enters the fish’s mouth and flows over the gills, and the exchange of gases (oxygen and carbon dioxide) occurs through the gill filaments. As water exits the gill slits, it helps maintain the continuous flow of water necessary for gas exchange.
• Water Flow: Cartilaginous fishes do not rely on a suction pump mechanism for water intake, as many bony fishes do. Instead, they utilize ram ventilation, a process where water is forced over the gills by the fish’s forward movement through the water. As a result, many cartilaginous fishes must keep swimming to ensure a constant flow of water through their gills, although some species (like rays and skates) can also use spiracles to actively pump water when stationary.

3. Structure of the Respiratory System in Bony Fishes

• Gills and Gill Arches: Like cartilaginous fishes, bony fishes also have gills for respiration, typically organized into four pairs of gill arches. The gill arches support the gill filaments, which are lined with gill lamellae that facilitate the gas exchange process.
• Operculum: One of the key differences between bony fishes and cartilaginous fishes is the presence of the operculum, a bony flap that covers the gill slits. The operculum helps to regulate the flow of water over the gills by creating a pressure differential, allowing water to pass over the gill filaments even when the fish is not swimming. This is especially beneficial for fish that are less active or stationary in their environment.
• Mouth and Buccal Cavity: Water enters the mouth and is forced across the gills by the fish’s buccal pump mechanism. When the fish opens its mouth, the pressure in the buccal cavity drops, and water flows in. The operculum is then lifted to create a pressure difference, which forces the water out over the gills, facilitating gas exchange.
• Gill Filaments and Lamellae: Bony fishes have highly efficient gill structures that allow them to extract oxygen from water even when it has low oxygen content. The gill filaments are lined with gill lamellae, which increase the surface area for gas exchange. This structure is highly efficient in extracting oxygen and expelling carbon dioxide.
• Water Flow: Unlike cartilaginous fishes, bony fishes rely on a buccal-pump mechanism for water intake. This allows them to actively control water flow over their gills without needing to swim continuously, although many bony fishes also swim to enhance this process.

4. Key Differences Between the Respiratory Systems of Cartilaginous and Bony Fishes

• Gill Slits vs. Operculum: Cartilaginous fishes have exposed gill slits that are not covered by an operculum, while bony fishes possess an operculum that covers the gill slits, providing a more protected and regulated water flow.
• Spiracles: Cartilaginous fishes often possess spiracles, which allow water to enter the gills even when the fish’s mouth is closed. This is particularly useful for species like rays and skates, which feed on the seafloor. Bony fishes do not have spiracles, and water is typically taken in through the mouth.
• Water Flow Mechanism: Cartilaginous fishes rely on ram ventilation, where water is continuously pushed over the gills through the fish’s forward motion. In contrast, bony fishes use a buccal-pump mechanism, actively drawing water in through their mouths and forcing it out over the gills using the operculum.
• Gill Filaments and Efficiency: Bony fishes tend to have more efficient gill filaments and lamellae, which are more finely adapted to extract oxygen from water. Cartilaginous fishes, although having similar gill structures, often require continuous movement to maintain an efficient oxygen supply due to the lack of a buccal pump.
• Number of Gill Slits: Cartilaginous fishes typically have five to seven pairs of gill slits, whereas bony fishes usually have four pairs of gill slits.

5. Conclusion

3. a) Discuss lymphatic system in different vertebrates.

Answer:

1. Introduction to the Lymphatic System in Vertebrates

2. Lymphatic System in Fish

• Lymphatic Vessels: Fish have lymphatic vessels that run parallel to blood vessels, and they do not form a distinct network of larger lymph nodes. These vessels help circulate lymph and remove excess interstitial fluid.
• Lymphoid Tissue: While fish lack the complex lymphoid organs seen in higher vertebrates, they do possess structures such as the spleen and kidney that play roles in immune responses. The spleen, in particular, is involved in filtering blood and producing immune cells.
• Thymus: In fish, the thymus is involved in the production of T-cells, which are essential for the adaptive immune response.

3. Lymphatic System in Amphibians

• Lymphatic Vessels and Nodes: Amphibians have lymphatic vessels that are more prominent than those in fish. These vessels are responsible for transporting lymph and maintaining fluid balance. Some species have small lymph nodes along the lymphatic vessels, but they are not as developed as in mammals.
• Lymphoid Organs: Amphibians have specialized lymphoid tissues like the spleen and bone marrow. The spleen plays a critical role in immune defense by filtering blood and storing red blood cells. However, amphibians lack the thymus and the well-defined immune structures present in higher vertebrates.
• Immune Function: The immune system in amphibians relies heavily on the production of lymphocytes in the spleen and bone marrow. Amphibians are capable of mounting an immune response to various pathogens, but their lymphatic system is not as efficient as that of birds or mammals.

4. Lymphatic System in Reptiles

• Lymphatic Vessels and Nodes: Reptiles have a well-developed network of lymphatic vessels and lymph nodes. The nodes are scattered throughout the body and play a significant role in filtering lymph and aiding in the immune response.
• Spleen and Thymus: The spleen in reptiles functions similarly to that of mammals by filtering blood and producing immune cells. The thymus is also present and plays a key role in the development of T-cells, essential for adaptive immunity.
• Immune Response: The reptilian lymphatic system supports both innate and adaptive immunity. The spleen filters blood for pathogens, and the lymph nodes help activate the immune response by producing lymphocytes and antibodies.

5. Lymphatic System in Birds

• Lymph Nodes and Vessels: Birds have a network of lymph nodes that are larger and more developed than in amphibians or reptiles. These lymph nodes are distributed throughout the body and assist in filtering lymph and activating immune responses.
• Bursa of Fabricius: This specialized lymphoid organ is crucial in birds for B-cell maturation, which is a part of the adaptive immune system. The bursa produces antibodies and helps in defending against infections.
• Spleen and Thymus: The spleen in birds functions similarly to that in mammals, helping in blood filtration and immune cell production. The thymus also plays a vital role in the development of T-cells.

6. Lymphatic System in Mammals

• Lymph Nodes: Mammals have a large number of lymph nodes distributed throughout their body. These nodes filter lymph and house immune cells that help defend against pathogens.
• Thymus and Bone Marrow: The thymus in mammals is essential for the maturation of T-cells, while the bone marrow produces both red and white blood cells, including lymphocytes, which are crucial for immune responses.
• Spleen: The spleen in mammals filters blood, removes old red blood cells, and produces immune cells that help fight infections.
• Lymphatic Vessels: Mammals have a highly developed network of lymphatic vessels that transport lymph throughout the body, helping to maintain fluid balance and support the immune system.

7. Conclusion

3. b) Write short notes on:

Answer:

i) Blood Filtration in Kidney

• Structure of Nephron: Each nephron consists of a renal corpuscle (which includes the glomerulus and Bowman’s capsule) and a renal tubule (which includes the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting ducts).
• Filtration Process: Blood enters the glomerulus, a network of capillaries, under high pressure. The pressure forces plasma, water, ions, glucose, and small molecules from the blood into the Bowman’s capsule, forming filtrate. Larger molecules like proteins and blood cells are retained in the bloodstream due to the selective permeability of the glomerular membrane.
• Selective Reabsorption: The filtrate then flows through the renal tubule, where essential substances like water, glucose, and electrolytes are reabsorbed back into the bloodstream. This process occurs mainly in the proximal convoluted tubule and loop of Henle. The distal convoluted tubule fine-tunes the balance of electrolytes and pH.
• Secretion and Excretion: Excess waste products, such as urea, uric acid, and creatinine, are secreted into the filtrate in the distal tubule. The final filtrate, now called urine, is collected in the collecting ducts, transported to the renal pelvis, and excreted through the ureter.

ii) Types of Mammalian Uteri

1. Simple Uterus:
   • Found in species such as primates (including humans) and some bats.
   • The uterus is single, with no division, and the body of the uterus opens directly into the vagina.
   • This type of uterus is more suitable for a single offspring at a time.
2. Bicornuate Uterus:
   • Found in most mammals, including cows, dogs, and pigs.
   • The uterus has two horns (branches) that are partially or completely fused, forming a single uterine body. The horns are capable of supporting multiple fetuses during pregnancy.
   • The degree of fusion varies, and in some species, the horns may be well-developed, while in others, they are less pronounced.
3. Didelphic Uterus:
   • Found in marsupials (e.g., kangaroos, opossums).
   • This type has two completely separate uteri, each with its own cervix and vaginal opening.
   • Marsupials are unique in that they give birth to underdeveloped young, and the two uteri allow for the development of multiple embryos at different stages of gestation.
4. Duplex Uterus:
   • Found in rabbits and rodents.
   • The uterus has two completely separate horns and cervices, with no fusion between them. This allows for the development of two or more offspring at different stages of gestation.

4. a) Which part of the brain is well developed in all vertebrates and why?

Answer:

Introduction

The Brainstem and its Importance

1. Breathing: The medulla oblongata contains centers that control the rate and depth of breathing, which is essential for maintaining oxygen levels in the body.
2. Heartbeat: The medulla also regulates heart rate and blood pressure through the autonomic nervous system, ensuring that blood circulates effectively to supply oxygen and nutrients to tissues.
3. Movement Coordination: The cerebellum, located in the hindbrain, is responsible for motor control, balance, and coordination. It allows vertebrates to move efficiently and maintain posture.
4. Basic Reflexes: The brainstem controls many basic reflexes, including those for swallowing, vomiting, coughing, and sneezing, which are critical for an organism’s immediate survival.
5. Autonomic Functions: The brainstem is involved in the regulation of autonomic functions like digestion, temperature regulation, and sleep, all of which are essential for maintaining homeostasis in vertebrates.

Why is the Brainstem Well Developed in All Vertebrates?

• Evolutionary Necessity: From an evolutionary perspective, these basic functions are critical for survival. As vertebrates evolved from simpler organisms to more complex creatures, the brainstem remained the core structure in charge of managing life-supporting functions.
• Adaptation Across Environments: The brainstem allows vertebrates to adapt to diverse environments while maintaining essential bodily functions. Whether a vertebrate is an aquatic species that must regulate breathing underwater or a terrestrial species that needs precise movement coordination, the brainstem provides a universal foundation of regulatory control across species.

Conclusion

4. b) What are pit organs in reptiles? How do vipers and boas locate prey?

Answer:

1. Introduction to Pit Organs in Reptiles

2. Structure of Pit Organs

3. Function of Pit Organs

4. How Vipers Locate Prey Using Pit Organs

• Tracking Prey: Vipers use their pit organs to track the movements of warm-blooded animals like rodents, birds, and amphibians. When the prey comes within the snake’s thermal sensing range, the pit organs detect the heat and send signals to the snake’s brain, allowing it to follow the animal’s movements precisely.
• Striking Accuracy: Once the snake has located its prey, it can strike with accuracy, even if the prey is not visible. The pit organs guide the viper to its prey by detecting the heat, and the snake can strike rapidly with little visual input, relying purely on its thermal sensing.
• Nocturnal Hunting: Vipers are typically nocturnal, meaning they hunt primarily at night when visual cues are limited. The pit organs give them an advantage by allowing them to "see" in the dark using infrared radiation, making them effective hunters even in low-light conditions.

5. How Boas Locate Prey Using Pit Organs

• Ambush and Constriction: Unlike vipers, which often ambush their prey, boas may employ a combination of ambush and active hunting. The pit organs help them detect nearby prey when they are hiding in foliage or burrows, providing a way to locate prey in low-visibility environments.
• Prey Detection at a Distance: Similar to vipers, boas use their pit organs to detect the thermal signature of warm-blooded animals. This allows them to hunt effectively at night, or in dimly lit environments, without needing to see their prey directly.
• Hunting Adaptations: The pit organs in boas allow them to track the prey’s movement with precision. Once the prey is detected, the boa can strike and constrict with powerful muscles to capture and subdue it.

6. Differences in Prey Detection Between Vipers and Boas

• Vipers are more specialized for ambush hunting, where they remain still and wait for prey to come within range. Their pit organs are highly sensitive and provide fine-tuned thermal detection, allowing them to strike with pinpoint accuracy.
• Boas, on the other hand, are more active hunters and may use their pit organs for detecting prey in their environment, but they are also capable of tracking and constricting prey that is not in close proximity. While their pit organs are useful for locating nearby prey, they may also use their keen sense of smell and sight to assist in tracking and capturing prey.

7. Conclusion

Briefly write the functions of the following hormones secreted in mammals.

Answer:

a) Adrenocorticotropic Hormone (ACTH)

b) Parathormone (PTH)

c) Aldosterone

d) Testosterone

e) Progesterone

List at least three stages in gene expression that can be regulated to result in differentiated cell types? Explain any one of them with the help of an example.

Answer:

Stages in Gene Expression Regulation

1. Transcriptional Regulation: Control of gene expression at the level of transcription involves regulation of the RNA polymerase enzyme and transcription factors, which determine whether a gene is transcribed into mRNA. This regulation ensures that the appropriate genes are turned on or off during different developmental stages and in specific tissues.
2. Post-Transcriptional Regulation: After transcription, the mRNA transcript can be regulated by processes like splicing, mRNA stability, and RNA editing. These modifications can alter the final protein product, allowing for differential expression of genes in various cell types.
3. Translational and Post-Translational Regulation: Once mRNA is translated into protein, further regulation can occur at the translational level (e.g., through initiation factors or microRNA activity) and post-translational modifications (such as phosphorylation or acetylation), affecting protein activity, localization, and function.

Example: Transcriptional Regulation – The Role of Transcription Factors in Muscle Differentiation

• MyoD, a key transcription factor, plays a central role in the initiation of muscle cell differentiation. When myoblasts are exposed to specific signaling cues, MyoD is activated and binds to the promoter regions of muscle-specific genes, such as those encoding actin and myosin, which are critical for muscle contraction.
• MyoD’s activation leads to the transcription of these genes, pushing the cell towards a muscle fate. In addition to MyoD, other transcription factors like Myf5 and MyoG are involved in the progression of muscle differentiation, while transcriptional repressors can prevent the expression of genes not required for muscle function.

Conclusion

Describe the mechanisms evolved by eggs to prevent polyspermy.

Answer:

1. Introduction to Polyspermy Prevention

2. Mechanisms to Prevent Polyspermy

Fast Block to Polyspermy

• Membrane Depolarization: When the first sperm fuses with the egg’s plasma membrane, it causes a sudden change in the membrane potential, known as membrane depolarization. This depolarization prevents other sperm from fusing with the egg by altering the electrical charge across the membrane. As a result, other sperm are unable to attach to the egg and undergo fertilization.
• Time Limitation: The depolarization is a temporary response that lasts only for a short time (a few seconds to minutes). This gives the egg time to initiate the slower, more permanent mechanism of blocking polyspermy, which is the slow block.

Slow Block to Polyspermy

• Cortical Reaction: One of the key events in the slow block to polyspermy is the cortical reaction. Upon fusion of the sperm with the egg, the egg’s cortical granules (small vesicles located just beneath the egg membrane) release their contents into the perivitelline space (the space between the egg membrane and the surrounding zona pellucida or vitelline layer). The contents of these granules include enzymes that modify the egg’s outer matrix.
• Formation of the Fertilization Envelope: The enzymes released from the cortical granules cause the modification and hardening of the egg’s extracellular matrix, particularly the zona pellucida (in mammals) or the vitelline membrane (in other species). This forms a fertilization envelope, a physical barrier that prevents further sperm from entering the egg. The fertilization envelope remains intact and blocks any additional sperm from attaching to the egg.
• Sperm-binding Sites are Blocked: In addition to the formation of the fertilization envelope, the cortical granules also release enzymes that destroy any sperm-binding receptors on the egg membrane, preventing additional sperm from binding to the egg.

3. Other Mechanisms Involved in Preventing Polyspermy

• Sperm Competition: In some species, there is a form of sperm competition where only the sperm with the best motility or genetic compatibility can successfully fertilize the egg. Other sperm are either discarded or neutralized by the egg’s immune responses or chemical environment.
• Zonal Inhibition: In some species, including humans, there is a localized reaction within the zona pellucida (in mammals) or the vitelline membrane that becomes resistant to sperm penetration after the first sperm enters. This change makes the membrane less permeable to additional sperm.
• Polyspermy Proteins: Some eggs produce specific proteins that are capable of actively blocking sperm entry once fertilization has occurred. These proteins either interact directly with sperm or alter the egg’s environment to create a hostile environment for other sperm.

4. Example of Polyspermy Prevention in Humans

5. Conclusion

Make a flow chart to show the events in metamorphosis.

Answer:

Start
  |
  v
Egg Stage
  |
  v
Larva Stage (Caterpillar)
  |
  v
Pupa Stage (Chrysalis)
  |
  v
Adult Stage (Butterfly)
  |
  v
End

Explanation of Each Stage:

1. Egg Stage: The life cycle begins when the female insect lays eggs. These eggs are often laid on or near a food source for the larvae.
2. Larva Stage (Caterpillar): After hatching, the larvae (commonly known as caterpillars in the case of butterflies) emerge and begin to feed. This stage is primarily focused on growth, and the larvae will molt several times as they grow.
3. Pupa Stage (Chrysalis): Once the larva has reached a sufficient size, it enters the pupal stage. During this time, the caterpillar undergoes significant transformation inside a protective casing (the chrysalis).
4. Adult Stage (Butterfly): Finally, the adult insect emerges from the chrysalis. It will undergo a period of expansion and hardening of its wings before it can fly and reproduce, completing the life cycle.

Discuss the process of development of extra embryonic membranes in chick.

Answer:

1. Introduction to Extra-Embryonic Membranes in Chick Development

2. Overview of the Extra-Embryonic Membranes

• Amnion: A protective membrane that surrounds the embryo, providing a cushioning fluid-filled environment.
• Chorion: Plays a key role in gas exchange and contributes to the formation of the placenta in some species.
• Allantois: Involved in waste storage and gas exchange, it grows rapidly and merges with the chorion.
• Yolk Sac: Provides nutrients to the embryo, as it is connected to the yolk, the primary nutrient source.

3. Formation of Extra-Embryonic Membranes in Chick

• Formation of the Amnion: The amnion begins as a fold of the ectoderm (the outermost layer of the embryo) during the gastrulation process. This fold grows and eventually surrounds the embryo, creating the amniotic cavity. The amnion is filled with amniotic fluid, which serves as a shock absorber and prevents dehydration of the embryo.
• Formation of the Chorion: The chorion originates from the mesoderm (middle layer of the embryo) and the ectoderm. During development, the chorion forms as a membrane that surrounds both the embryo and the other extra-embryonic membranes. It eventually fuses with the allantois, forming the chorioallantoic membrane. The chorion is involved in gas exchange, providing oxygen to the embryo and removing carbon dioxide.
• Formation of the Allantois: The allantois originates as an outpouching of the hindgut and grows rapidly, extending toward the chorion. It serves as an important organ for waste storage and gas exchange. In chicks, the allantois becomes highly vascularized and plays a role in connecting the embryo to the placenta. As the allantois grows, it fuses with the chorion, contributing to the chorioallantoic membrane.
• Formation of the Yolk Sac: The yolk sac is the first extra-embryonic membrane to form. It develops from the hypoblast and is responsible for providing nutrients to the developing embryo. The yolk sac is connected to the embryo by a stalk and envelops the yolk, facilitating nutrient transfer. In early stages of development, it is also involved in the initial stages of blood cell formation.

4. Functional Role of Each Extra-Embryonic Membrane

• Amnion: The primary function of the amnion is protection. The fluid-filled sac ensures that the developing embryo is cushioned, protecting it from mechanical shocks, dehydration, and temperature fluctuations. The amnion also maintains a constant internal environment for the embryo’s growth.
• Chorion: The chorion is responsible for gas exchange. In the chick, it is richly vascularized, allowing the transfer of oxygen from the surrounding environment into the embryo, and removing carbon dioxide. The chorion also facilitates some nutrient exchange and contributes to the formation of the placental interface in certain species.
• Allantois: The allantois plays two main roles: waste disposal and gas exchange. As the embryo produces waste products such as uric acid, these are stored in the allantoic cavity. Additionally, the allantois is involved in respiratory exchange, as it provides a large surface area for the exchange of gases between the embryo and the external environment.
• Yolk Sac: The yolk sac’s primary function is nutrient transfer. It absorbs nutrients from the yolk and delivers them to the developing embryo through the bloodstream. Early on, the yolk sac also contributes to the formation of blood cells before the embryo’s circulatory system becomes fully functional.

5. Interactions Between Extra-Embryonic Membranes and the Embryo

• Circulatory System: The allantois, with its dense network of blood vessels, becomes part of the embryonic circulatory system, carrying oxygen and nutrients from the yolk sac to the developing embryo. This network also helps with the removal of metabolic waste.
• Regulation of Development: The amnion helps regulate water balance by preventing excessive loss of fluids. The chorion’s role in gas exchange ensures that the embryo has the necessary oxygen levels for growth and development. The yolk sac plays a major role in the nutrient distribution, which supports the embryo’s metabolic needs.

6. Conclusion

10. a) Draw a flow chart to show how the three germinal layers are derived from the zygote.

Answer:

10. b) How do genetic and environmental defects cause problems in embryonic development?

Answer:

1. Introduction to Embryonic Development and Defects

2. Genetic Defects in Embryonic Development

• Types of Genetic Defects: Genetic defects can be broadly classified into chromosomal abnormalities, single-gene mutations, and multifactorial disorders.
  • Chromosomal Abnormalities: These occur when there is a structural or numerical change in chromosomes. For example, Down syndrome (Trisomy 21) occurs when there is an extra copy of chromosome 21. This leads to developmental delays, intellectual disabilities, and physical malformations.
  • Single-Gene Mutations: These mutations affect a single gene and can lead to specific developmental defects. For example, cystic fibrosis is caused by a mutation in the CFTR gene, which leads to thick mucus production, respiratory issues, and digestive problems.
  • Multifactorial Disorders: These disorders arise from a combination of genetic and environmental factors. For example, cleft lip and palate are influenced by genetic mutations and environmental exposures, resulting in facial malformations that affect feeding, speech, and hearing.
• Impact of Genetic Defects: Genetic defects can cause malformations in organs and systems, developmental delays, intellectual disabilities, and can even be fatal. Some defects, like neural tube defects (spina bifida), occur during early embryonic development and affect the nervous system. Congenital heart defects can also result from mutations in genes that control heart formation.
• Inherited Genetic Defects: Genetic defects can be inherited from one or both parents, leading to the expression of certain inherited diseases. Autosomal dominant diseases like Huntington’s disease are caused by mutations in a single copy of a gene, whereas autosomal recessive diseases like sickle cell anemia require mutations in both gene copies.

3. Environmental Defects in Embryonic Development

• Teratogens and Their Effects: Teratogens are external agents that can cause birth defects by disrupting the normal development of the embryo. These can be physical agents, chemicals, infections, or even maternal health conditions.
  • Chemical Teratogens: Certain chemicals, such as alcohol, tobacco smoke, prescription drugs, and illegal drugs, can interfere with embryonic development. For example, fetal alcohol syndrome (FAS) is a result of alcohol consumption during pregnancy, leading to developmental delays, facial abnormalities, and neurological impairments.
  • Infections: Maternal infections, such as rubella, cytomegalovirus, or toxoplasmosis, can be teratogenic. Rubella infection during the first trimester can cause heart defects, deafness, and blindness in the developing fetus.
  • Environmental Pollutants: Exposure to pollutants like pesticides, heavy metals, and radiation can also disrupt development. For instance, exposure to mercury during pregnancy can lead to neurological impairments and birth defects.
  • Maternal Health Conditions: Chronic maternal conditions such as diabetes, obesity, and hypertension can also negatively affect embryonic development. For example, gestational diabetes can lead to macrosomia (excessive fetal growth), while maternal obesity increases the risk of neural tube defects and heart defects.
• Critical Periods of Vulnerability: The effects of environmental factors are especially significant during the critical windows of embryonic development, which occur during the early stages of organ formation. These critical periods vary for different organs, and exposure to teratogens during these times can lead to severe malformations.

4. Interaction Between Genetic and Environmental Defects

• Gene-Environment Interaction: Certain genetic mutations may make an embryo more susceptible to the effects of environmental toxins. For example, an embryo with a genetic predisposition for cleft lip and palate may be more likely to develop the condition if the mother is exposed to teratogens like tobacco smoke or certain medications.
• Multifactorial Conditions: Many conditions, such as congenital heart defects and neural tube defects, result from a combination of genetic susceptibility and environmental influences. Genetic mutations may predispose an embryo to these conditions, but the likelihood of developing them can increase with exposure to environmental teratogens during pregnancy.

5. Prevention and Mitigation of Developmental Defects

• Prenatal Care: Regular prenatal care and screening can help detect potential issues early in the pregnancy, allowing for better management of risks.
• Avoidance of Teratogens: Pregnant women should avoid harmful substances, such as alcohol, tobacco, drugs, and exposure to environmental toxins. Vaccinations, such as the rubella vaccine, can also prevent infections that could harm the embryo.
• Genetic Counseling: For couples with a family history of genetic disorders, genetic counseling can help assess the risk of inherited conditions and inform decisions about family planning.
• Folic Acid Supplementation: Taking folic acid before and during early pregnancy significantly reduces the risk of neural tube defects, a common developmental defect.

6. Conclusion