What Does the Egg Cell Contribute to the Zygote?

A an animal begins as an embryo in the mother’s womb. But where do embryos come from? Embryos develop from a single cell called a zygote, which results from the fusion of a sperm and an egg. The egg contributes many important products to the zygote, which the sperm does not have. Each of these products is crucial for the zygote to develop into a normal embryo that knows its head from its tail.

The egg cell contributes most of the organelles, or organs within a cell, that are needed by the zygote. The sperm only provides its centrioles -- a structure that pulls dividing cells apart -- and a haploid, or half, nucleus. This means that the rest of organelles -- the machinery that makes cells tick -- necessary for the zygote’s survival come from the egg. These include the mitochondria, endoplasmic reticulum, and smaller vesicles. Additionally, the egg contributes its own haploid nucleus, which will fuse with the sperm’s haploid nucleus to give a diploid, or full, nucleus that contains all of the DNA needed for growth into a normal embryo.

The egg stores many gene products from the mother. These gene products are called mRNA, or messenger RNA. These mRNAs as passed on from the egg to the zygote. Within the zygote, the mRNAs are instructions for making specific proteins that tell the zygote how to divide into the many cells that make up the embryo. In mammals, which includes humans, two of these important instruction proteins are named beta-catenin or glycogen synthase kinase.

Humans have 23 pairs of chromosomes, or long strands of DNA that contain our genes. Half of each pair comes from the sperm, and the other half comes from the egg. One of the 23 pairs is called the sex chromosomes. Sex chromosomes are called either X or Y. Women have two Xs, and are thus called XX. Men have one of each, and are thus called XY. Daughters, since they are female and XX, get one X from mom and one X from dad. But sons, since they are XY, can only get their X chromosome from their mothers.

Mitochondria are organelles that make the energy molecules necessary for sperm or eggs to survive. However, after a sperm fuses with an egg to form a zygote, only the mitochondria from the egg are kept -- the sperm’s mitochondria are destroyed. Mitochondria contain their own DNA, called mtDNA, which is separate from the DNA in the chromosomes of the sperm and egg. Thus, we all have mtDNA that is similar to our mothers, grandmothers, great-grandmothers and so on. By comparing similarities in mtDNA, we can trace our maternal ancestry.