In reference to Human reproduction, fertilisation leads Zygote and indeed it is the 1st living cell of a multicellular organisms.
What i feel that answer given below is up to the mark and needs no addition but indeed would like to tell
How zygote is formed in this unique process called fertilisation ???
when a sperm enters a female body(INSEMINATION) it needs a CAPACITATION just like compitance in bio-technology or just a Booster, sperm can be viable upto 24-48 hrs (some say 72 hrs).
Now Approximation of sperm and ovum begins; sperm travels at a rate of 1.5-3 mm/sec (last year's NCERT mentioned) reaches ampullary-isthmic junction part of ovary duct by the help of Prostaglandins and Mitochondrion energy
then Compatibility Reaction do take place with suitable protein fertilizin of female and anti fertilizin of male
now Acrosome Reaction; dissolves zona pellucida
Egg Reaction; fertilisation cone appears as Cone of Reception
Penetration of Sperm; CORTICAL REACTION happens and prevents the entry of another sperm
Activation of Ovum; ovum previously in 2ndry oocyte stage undergoes meiosis II and forms an actual female gamete
fusion of sperm and Egg Nuclei; leads to formation of synkaryon (by syngamy or karyogamy) and the proximal centriole brought be sperm leads to the formation of spindle for the the division of synkaryon(cleavage nucleus)
....... and forms actively dividing Zygote and goes immediately for cleavage.
Do focus on cleavage pattern of various animals in Embryology for your extra marks!!!