The document outlines the molecular events that occur during fertilization. It discusses three stages of fertilization: sperm preparation, sperm-egg recognition and binding, and sperm-egg fusion. A key event is the increase in calcium levels in the egg cytoplasm, which prevents additional sperm from entering and signals the completion of meiosis. The fused egg and sperm each contribute a pronucleus, which then replicate their DNA and join during the first mitotic division to form a zygote with a combined genome. Maternal genes play an important role in early embryonic development by regulating processes like meiosis and the transition to embryonic gene expression.

1. Animal cell science and Technology
15. Molecular events in fertilization
Shailendra Singh Shera, Ph.D

2. Outlines
1. Molecular events in fertilization
2. Role of Maternal genes in early development of embryo
3. Practice questions
15. Molecular events in fertilization

3. Definition of fertilization
•Fertilization is the fusion of the male and female gamete. It is s a sequence of coordinated
molecular events involving the merging of the sperm with the egg, the fusion of the pronuclei and
the intermingling of the maternal and paternal chromosomes.
•It is the formation of a diploid zygote from a haploid egg and sperm
3-Stages of Fertilization
a. Sperm preparation: Capacitation and acrosome reaction
b. sperm-egg recognition and binding,
c. Sperm-egg binding and fusion

4. •A key signal resulting from the binding of a sperm to its receptor on the plasma membrane of the
egg is an increase in the level of Ca2+in the egg cytoplasm, probably as a consequence of
stimulation of the hydrolysis of phosphatidylinositol4,5-bisphosphate (PIP2).
•One effect of this elevation in intracellular Ca2+is the induction of surface alterations that
prevent additional sperm from entering the egg. Because eggs are usually exposed to large
numbers of sperm at one time, this is a critical event in ensuring the formation of a normal diploid
embryo.
•These surface alterations result from the Ca2+-induced exocytosis of secretory vesicles that are
present in large numbers beneath the egg plasma membrane. Release of the contents of these
vesicles alters the extracellular coat of the egg so as to block the entry of additional sperm.
The increase in cytosolic Ca2+following fertilization also signals the completion of meiosis.
•In eggs arrested at metaphase II, the metaphase to anaphase transition is triggered by
activation of the anaphase-promoting complex, resulting from Ca2+-dependent phosphorylation
and degradation of an inhibitory protein.

5. •The resultant degradation of cyclin B and condensin leads to completion of the second meiotic
division, with asymmetric cytokinesis (as in meiosis I) giving rise to a second small polar body.
Following completion of oocyte meiosis, the fertilized egg (now called a zygote) contains two
haploid nuclei (called pronuclei), one derived from each parent.
•In mammals, the two pronuclei then enter S phase and replicate their DNA as they migrate
toward each other. As they meet, the zygote enters M phase of its first mitotic division. The two
nuclear envelopes break down, and the condensed chromosomes of both paternal and maternal
origin align on a common spindle. Completion of mitosis then gives rise to two embryonic cells,
each containing a new diploid genome. These cells then commence the series of embryonic cell
divisions that eventually lead to the development of a new organism

6. Image source: Molecular Biology of
cell by Albert

7. •An embryo exclusively relies on maternal gene products, RNAs, and proteins for its early development
until activation of its own genome. The precise developmental time period and developmental processes
under maternal control when the embryo is largely transcriptionally silent vary among organisms. In some
animals, such as mice, humans, and nematodes (Caenorhabditis elegans), only the first or first couple of
cleavage cycles are accomplished before transcription of the embryonic genome is activated.
•Maternal genes are those expressed by cells of egg chamber and involved in early development of the
oocyte and syncytial blastoderm. The products of these genes such as, RNA or protein, are produced or
deposited in the oocyte or are present in the fertilized egg or embryo before expression of zygotic genes is
initiated. The proteins of these genes then stimulate other genes, which in turn stimulate yet other genes
in a cascade of control. As might be expected, most of the gene products in the cascade are regulatory
proteins, which bind to DNA and activate other genes These maternal gene products regulate meiosis,
oocyte development, and early development of the embryo including fertilization, transitions between
meiotic and mitotic cell cycles, and the switch from utilization of mRNAs and proteins provided by the
mother to the embryo’s own gene products during zygotic genome activation.
Role of Maternal gene in early embryonic development

8. Practice questions*
1. Fertilization may also be defined as
A. A sequence of coordinated molecular events involving the merging of the sperm with the egg.
B. The fusion of the pronuclei and the intermingling of the maternal and paternal chromosomes.
C. Both (a) and (b)
D. None of these
2. One effect of elevation in intracellular Ca2+ in egg cytoplasm during fertilization is
A. The induction of surface alterations that prevent additional sperm from entering the egg.
B. Migration of zygote from falopian tube to uterus
C. Fertilization of egg by multiple sperm simultaneously
D. Recycling of sperm receptor
3. Which of the following is the function of maternal gene?
A. Genes expressed by cells of sperm and involved in early development of the oocyte and syncytial
blastoderm.
B. Genes expressed by cells of egg chamber and involved in early development of the oocyte and syncytial
blastoderm.
C. Production of sperm receptor for attachment of sperm on to egg.
D. None of these
4.The term ‘zona pellucida’ is associated with
A. Sperm
B. Ovum or egg
C. Zygote
D. None of the above
5. The sperm binds to zona pellucida of egg through…………..
A. N-linked oligosaccharide on ZP3
B. O-linked oligosaccharide on ZP3
C. N-oligosaccharide on ZP1
D. O-oligosaccharide on ZP1

9. 1. http://www.vivo.colostate.edu/hbooks/pathphys/reprod/fert/fert.html
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5029953/
3. https://www.slideshare.net/kaushal2683/molecular-event-during-fertilization?qid=36f6c9e5-
6cc8-4569-9eed-b683ab7dff74&v=&b=&from_search=1
4. Molecular Biology of cells. Albert
MCQ Practice questions
1. Practice and Learn Animal cell Science and Technology: Multiple choice question for learning.
Author: Shailendra Singh Shera . Publisher: Amazon Kindle.
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References & Further reading