Q.How many chromosomes will be there in the zygote?

The formation of male gametes in angiosperms is a two-stage process involving both meiosis and mitosis, starting from a diploid microspore mother cell (MMC). Understanding the sequence and outcome of these cell divisions is key to determining the final ratio.

Here's how the male gametes are formed from a microspore mother cell:

1. Microspore Mother Cell (MMC): We begin with a single diploid ($2n$) microspore mother cell, also known as a pollen mother cell, located within the microsporangium (pollen sac) of the anther.

2. Meiosis: The microspore mother cell undergoes meiosis. Meiosis is a reductional division that produces four haploid ($n$) cells from one diploid cell.

   Important

   Meiosis is crucial for reducing the chromosome number by half, ensuring that the gametes are haploid.

   [!FORMULA]

   $1 \text{ Microspore Mother Cell } (2n) \xrightarrow{\text{Meiosis}} 4 \text{ Microspores } (n)$

3. Microspore Development: These four haploid microspores are initially arranged in a cluster called a microspore tetrad. As the anther matures, these microspores separate and develop into pollen grains. Each pollen grain represents the immature male gametophyte.

4. First Mitotic Division (in each microspore): Inside each microspore (pollen grain), the haploid nucleus undergoes an unequal mitotic division. This division produces two cells:

   • A larger vegetative cell (or tube cell) with abundant food reserve and an irregularly shaped nucleus. This cell is responsible for forming the pollen tube.
   • A smaller generative cell which floats in the cytoplasm of the vegetative cell.
   Watch out

   This is a mitotic division, meaning the chromosome number remains haploid ($n$). The division is unequal, leading to cells of different sizes and functions. …

The female cockroach’s reproductive system is a compact, well-organized structure tucked inside the abdomen. To identify the parts, you need to know the flow: eggs are produced in the ovaries, travel through the oviducts, get fertilized by sperm stored in the spermathecae, and are then coated with a protective secretion from the colleterial glands before being deposited as an ootheca. The diagram shows these four key components in their natural arrangement.

Let’s walk through each label step by step.

1. Label A — the large, paired, grape-like structure

   This is the ovary. Each ovary consists of 8 ovarioles (the egg-producing tubes). In diagrams, the ovary is drawn as a cluster of finger-like or bead-like projections. Option (A) says “Oviduct, Ovariole…” — but A is not a single ovariole; it’s the whole ovary. Option (B) says “Ovariole” which is too specific (an ovariole is a subunit, not the whole structure shown). Option (C) says “Germarium” — that’s the tip of an ovariole, not the entire mass. So A is the ovary.

2. Label B — the tube leading from the ovary

   This is the oviduct. The two lateral oviducts (one from each ovary) join to form a common oviduct (vagina). In the diagram, B is clearly a duct emerging from the ovary. Options (A) and (D) both have “Oviduct” here, so we need to check the other labels to decide.

3. Label C — the small, paired, sac-like structures

   These are the spermathecae (singular: spermatheca). The female cockroach has two spermathecae that store sperm received during mating. They are drawn as small, rounded pouches attached to the genital chamber. Option (A) says “Spermathecae” for C, and option (D) also says “Spermathecae”. Option (B) says “Genital pouch” — that’s a larger cavity, not these small sacs. Option (C) says “Vagina” — the vagina is the common oviduct, not these pouches. So C is the spermathecae.

4. Label D — the branched, tubular glands …

The key here is understanding the precise definitions of breeding strategies in animal husbandry. Each option describes a different approach to controlling genetic traits, and the wording "within closely related line" is the critical clue.

1. Line breeding is the practice of mating animals that are related but not as closely as in inbreeding — for example, a sire with its granddaughters or a dam with its grandsons. The goal is to concentrate the genes of a particular ancestor while maintaining some genetic diversity. The phrase "within closely related line" directly matches this definition.

2. Close breeding (option A) is actually a synonym for inbreeding — mating very close relatives like siblings or parent-offspring. While it is "within a line," it is more extreme than what the question describes. The term "close breeding" is less standard in textbooks; "inbreeding" is the usual term.

3. Out breeding (option C) is the opposite — mating unrelated individuals within the same breed, often to introduce new traits or avoid inbreeding depression. This clearly does not fit "within closely related line." …

The key here is understanding the sequence of events when a sperm meets an egg. The sperm doesn't just burst through the zona pellucida in one go — it uses a two-step enzymatic strategy. First, it needs to get through the outer layer of cells (the corona radiata), and then it must penetrate the tough glycoprotein coat called the zona pellucida.

Hyaluronidase is the enzyme that helps the sperm push through the corona radiata by breaking down hyaluronic acid between those cells. But the zona pellucida is a different beast — it's made of specific glycoproteins (ZP1, ZP2, ZP3). To digest that layer, the sperm uses a different enzyme.

That enzyme is acrosin. It's stored in the acrosome (the cap-like structure at the sperm's head) and is released only after the sperm binds to the zona pellucida and undergoes the acrosome reaction. Acrosin is a serine protease that specifically cleaves the zona pellucida proteins, creating a path for the sperm to reach the egg's plasma membrane.

Let's walk through the reasoning step by step.

1. Identify the target structure. The question asks about dissolving the zona pellucida. This is the thick, translucent glycoprotein layer immediately surrounding the ovum's plasma membrane. It's not the outer cumulus layer (corona radiata).

2. Recall the sperm's enzymatic arsenal. The acrosome contains several enzymes: hyaluronidase, acrosin, and others like esterases and acid phosphatases. Each has a specific job.

3. Match the enzyme to the layer. Hyaluronidase acts on the corona radiata (the outer follicular cells), not the zona pellucida. Acrosin acts specifically on the zona pellucida. This is a classic distinction tested in exams. …

The question tests your understanding of the acrosome reaction — a critical step in mammalian fertilization. The sperm head is capped by the acrosome, a vesicle packed with enzymes. When the sperm reaches the egg, it must first penetrate two protective layers: the corona radiata (a loose layer of follicular cells) and then the zona pellucida (a glycoprotein shell surrounding the egg membrane). Different enzymes handle these two layers, and it's easy to mix them up.

1. Hyaluronidase is released first, as the sperm approaches the corona radiata. It breaks down hyaluronic acid, the "glue" holding the follicular cells together, allowing the sperm to swim through the outer cell layer. This is not the enzyme that dissolves the zona pellucida.

2. Acrosin is the key enzyme for the zona pellucida. After the sperm binds to the zona pellucida (via ZP3 receptors), the acrosome undergoes exocytosis — the acrosome reaction. Acrosin is released as a protease that digests the glycoproteins of the zona pellucida, creating a narrow tunnel for the sperm to pass through and reach the egg's plasma membrane.

3. Progesterone is a hormone secreted by the corpus luteum and placenta; it prepares the uterus for implantation but plays no direct role in dissolving the zona pellucida. …