How Do Sperm Cells Transmit DNA to The Embryo?

What is DNA?

Every human being has a unique set of chemical blueprints that determine their physical and genetic characteristics like body looks and functions. These blueprints are called genes that are contained in DNA. This DNA is packed into chromosomes present in every cell (in the nucleus) of the human body. DNA (deoxyribonucleic acid) is a long spiral-shaped double-stranded molecule. The segments of DNA contain instructions in the form of genes for making specific body proteins. Scientists believe that there are 25,000 protein-coding genes, each one of which is specific for either a physical trait or bodily function.

“Along with the DNA segments, genes are packed into chromosomes. There are 46 chromosomes in every cell arranged as 23 pairs. Half of the chromosomes come from one parent and a half from another. Among these 23 pairs, 22 pairs are non-sex chromosomes known as autosomes, and one pair is of sex chromosome (sex-determining chromosomal pair). ”

Along with the DNA segments, genes are packed into chromosomes. There are 46 chromosomes in every cell arranged as 23 pairs. Half of the chromosomes come from one parent and a half from another. Among these 23 pairs, 22 pairs are non-sex chromosomes known as autosomes, and one pair is of sex chromosome (sex-determining chromosomal pair). The sex chromosome can be XX or XY. The mother always gives X while the father can give X or Y. If a father passes on X, it’ll make XX, which is biologically female, and if he provides Y making XY, the baby will be male. Other than determining sex, these chromosomes also have genes for controlling other body functions. X chromosomes have more genes as compared to the Y chromosome.

Structure of Sperm

Sperm is microscopic with four parts: a head, neck, middle piece, and a tail. The head contains a nucleus in which the haploid set (23) of the chromosome is present. The anterior part of this nucleus is protected by a cap-like structure known as acrosome. This acrosome contains enzymes that are used to fertilize with the ovum or egg. Middle pieces contain mitochondria, which provide energy to the sperm for movement.

Transmission of Genetic Material By Sperm

Human development starts when an egg from a mother and a sperm from a father, each carrying a single set of chromosomes combine to form a single cell. During ejaculation, millions of sperms are released into the female reproductive tract. Only a few thousand of these move up to reach the oocyte (egg) in the uterine tubes, the rest of them die due to an acidic vaginal environment. In the tube where oocyte is released, and sperm reaches there fertilization or fusion of two cells occur. Commonly only a single sperm can fuse with the egg. Fusion is not a single step process, but many processes occur before sperm reaches, and many occur while and after fusing. The eggs are covered with a layer known as corona radiata, and the sperm must penetrate this layer to reach the outer layer of the egg known as zona pellucida. A single sperm binds to the receptor on the outer layer of egg acrosome releases digestive enzymes, which enables sperm to burrow 

into the layer. The first sperm making this contact will fertilize the egg. When this fusion occurs, the egg changes the outer surface to prevent polyspermy (fusion of more than one sperm). The egg then creates a change in the membrane outside to prevent other sperms from attaching. 

Once penetrated inside the egg, the packed genetic material in the sperm’s nucleus is released, and a new membrane forms around male genetic material, forming male pronucleus. The female genetic material also reformed into a female pronucleus. Both of these contain 23 chromosomes each. Both these pronuclei fuse resulting in a single cell with 46 chromosomes (23 from each parent) resulting in a unique genetic code. This cell is called a zygote. It is then moved towards the uterus for implantation where it grows and matures for the next nine months into a complete human being.

References

1. Quora. (2017, August 15). Male DNA Is Often Found In Women’s Brains. Where Does It Come From? Retrieved May 25, 2020, from https://www.forbes.com/sites/quora/2017/08/15/male-dna-is-often-found-in-womens-brains-where-does-it-come-from/
2. Praderio, C. (2018, March 29). If you’re a woman, you might have male DNA inside your body – here’s why. Retrieved May 25, 2020, from https://www.insider.com/why-women-retain-male-dna-2018-3
3. Marchesi, D. E., & Feng, H. L. (2007). Sperm DNA Integrity From Sperm to Egg. Journal of Andrology, 28(4), 481-489. doi:10.2164/jandrol.106.002105 
4. DeLuca SZ, O’Farrell PH. Barriers to male transmission of mitochondrial DNA in sperm development. Developmental cell. 2012 Mar 13;22(3):660-8.