Question

Arrange the following stages of fertilization and early development into a proper sequence
I. Sperm entry
II. Acrosomal reaction
III. Karyogamy
IV. Capacitation
V. Cortical reaction

A. I, III, II, IV ,V
B. III, V, I, IV ,II
C. IV, II, V, I ,III
D. V, I, IV ,II, III

Answer 1

When a sperm and an oocyte (egg) merge and their nuclei fuse, fertilisation happens. Since each of these reproductive cells is a haploid cell that contains half of the genetic material required to form a human being, a diploid cell is created by their combination. Half from the mother and half from the father, this new single cell, called a zygote, contains all the genetic material required to shape an individual.

Complete Answer:
Capacitation of Sperm: Freshly ejaculated sperm can not fertilise or is poorly capable of fertilising. Rather, they first have to undergo a sequence of changes collectively known as capacitation. Capacitation occurs while, as they typically do during gamete transport, sperm remain in the female reproductive tract for a period of time. The appropriate period of time varies with the species, but it typically takes several hours. Incubation of some fertilization media can also capacitate the sperm of many animals, including humans.

Sperm-Zona Pellucida Binding: A receptor-ligand interaction with a high degree of species specificity is the binding of sperm to the zona pellucida. The carbohydrate groups act as sperm receptors on the zona pellucida glycoproteins. There is no certain knowledge of the sperm molecule that binds this receptor, and indeed, there may be many proteins that may serve this purpose.

Acrosomal Reaction: The sperm must penetrate the two protective layers to enter the oocyte itself. The sperm first burrows into the corona radiata cells. The sperm then binds to the receptors in the zona pellucida upon contact with the zona pellucida. This initiates a mechanism called the acrosomal reaction in which the retained digestive enzymes are released by the enzyme-filled 'hat' of the sperm, called the acrosome.Via the zona pellucida, these enzymes clear a route that enables sperm to enter the oocyte. Finally, on the oocyte's plasma membrane, a single sperm makes contact with sperm-binding receptors.

The Zona Pellucida penetration: In conjunction with acrosomal enzymes, the continuous propulsive force from the flagellating tail of the sperm enables the sperm to form a tract through the zona pellucida. These two factors cause the sperm to pass through the zona pellucida-motility and zone-digesting enzymes.

Binding of Sperm-Oocyte: When the zona pellucida is breached by a sperm, it binds and fuses with the oocyte's plasma membrane. Binding occurs in the sperm head's posterior (post-acrosomal) area.

The Activation of Eggs and Cortical Reaction: The egg is in a quiescent state before fertilisation, arrested in the metaphase of the second meiotic division. The egg quickly undergoes a variety of metabolic and physical changes upon binding of a sperm, which are collectively called egg activation. Increased intracellular calcium concentration, completion of the second meiotic division and the so-called cortical reaction are prominent results.

The correct answer is option(A) I, III, II, IV, V.

Note: As both are part of sexual reproduction, occurring in eukaryotes, the evolution of fertilisation is related to the origin of meiosis. There are two contrasting hypotheses of how the meiosis-fertilization of the pair occurred. One being that, when eukaryotes originated from prokaryotes, they originated from prokaryotic sex (bacterial recombination). The other is that meiosis has arisen from mitosis.