Understanding Sexual Reproduction: What Just Happened?

Sexual Reproduction

If you are pregnant, congratulations are in order; you have successfully achieved sexual reproduction. What does that mean? Here is a very basic review of what you learned and probably forgot from Biology 101.

All living things need to reproduce to survive. There are basically two options, sexual or asexual. Over millions of years of evolution, sexual reproduction has been the big winner. Organisms like fungi and bacteria still reproduce asexually, but sexual reproduction is the way the vast majority of living things that you can see reproduce.

In sexual reproduction, two parents contribute their genetic material to create a new and different organism. In asexual reproduction, one parent gives birth to a new organism that is exactly the same as itself. Asexual reproduction is simple and kind of boring. It does not apply to us. Sexual reproduction is complicated and interesting. It is a bit miraculous.

What Happens During Human Sexual Reproduction?

One of the drawback of human sexual reproduction is that it takes two to tango. There are also no guarantees that it will work, and it takes a lot longer than asexual reproduction. One of the enduring mysteries of science is how and why sexual reproduction developed. When it does work, this is how it happens:

  • To achieve sexual reproduction, a miraculous transformation needs to occur in cells called gametes. Gametes are different from all other cells in the body. Other cells contain 46-paired chromosomes, called diploids. To form a gamete a diploid cell goes through a complicated process of getting all its chromosomes to cross over to one side. This type of cell is called a haploid. The process of making this change is called meiosis.
  • Through meiosis, a cell takes all the genetic material it has inherited and reduces it from 46 to 23 chromosomes. This process is so amazing that scientists are unable to explain how or why it ever happened in the first place.
  • Human male gametes become sperm cells. They are small little swimmers that don’t live long. The male needs to produce them constantly from the time of sexual maturity until the end of life.
  • The female gametes become ova (eggs). All ova are created at birth. There will not be any more. When the female ovulates, one precious ova at a time leaves the ovary for possible fertilization.
  • If mating takes place (you probably remember the mating part), male gamete meets female gamete somewhere along the fallopian tube. When the sperm penetrates the egg, two haploids become a single-cell diploid, called a zygote. This cell now has a pair of chromosomes for each trait from both mother and the father. This is the moment of conception. A new and unique human being has been created.
  • The zygote then travels down the fallopian tube to the uterus. As it travels, it begins to divide. Each division – called mitosis – doubles the number of cells. When the zygote reaches the uterus, it implants. It will keep dividing until it becomes an embryo and then a fetus. In humans, this gestation (pregnancy) takes about 38 weeks.

How Human Sexual Reproduction Compares to Other Species

Evolution has come up with many options when it comes to sexual reproduction. Mammal mating systems include monogamy (one male and one female), polygamy (one male and multiple females), and promiscuity (any male and any female). Monogamy is the rule in birds. Polygamy is the rule for most mammals. Humans have no rules; they can choose the mating system they want.

In most mammals, mating is restricted to certain times of the year or only when the female is ovulating. In humans, pregnancy only occurs around the time of ovulation, but mating can occur any time. When it comes to human mating, anything goes. Here are some ways that sexual reproduction as developed in other species:

  • Plants can be bisexual (male and female structure in one plant) or unisexual (male or female only plant).
  • Fertilization in many fish takes place outside the body without mating. Female fish release eggs and male fish release sperm into the water. The sperm and the eggs need to get together on their own.
  • Reptiles have openings called cloacae. The male reptile presses its cloaca to the female cloaca. Sperm transfers to the female. Fertilizations takes place inside the female. The female then passes eggs that mature outside the body.
  • Birds usually mate by touching cloacae together, called a cloacal kiss. It is a brief kiss. Mating may last less than a few seconds. Fertilization takes place inside the female and she lays eggs into a nest to mature.

Why Did Nature Choose Sexual Reproduction?

At first glance, it would seem that asexual reproduction would have won the evolutionary reproduction battle. Asexual reproduction is simple. All you need to do is break off a bud or a fragment. It works every time. Mating is not needed. Reproduction is quick. Since you don’t need to worry about gametes and meiosis, genetic mistakes (mutations) rarely occur. Asexual reproduction is simple and reliable, but not adaptable.

Sexual reproduction wins because it has a more important evolutionary advantage. Because sexual reproduction takes genes from two parents, there is always the ability to pick the best genes over time. Recombination and suppression of genes allows for a healthier and more adaptable species. There is less chance of passing on diseases and genetic defects. Mating is also more fun than budding.

Sexual reproduction has been called the greatest miracle in our universe. It took millions and millions of years for sexual reproduction to develop. Scientists are still trying to explain how and why sex developed. Could it all be coincidental, or was it a master plan? As Albert Einstein said, “God does not play dice with the universe.” I am going with a master plan. So what just happened when you became pregnant? You performed the miracle of the ages!

Christopher Iliades
Dr. Chris Iliades is a medical doctor with 20 years of experience in clinical medicine and clinical research. Chris has been a full time medical writer and journalist since 2004. His byline appears in over 1,000 articles online including EverydayHealth, The Clinical Advisor, and Healthgrades. He has also written for print media including Cruising World Magazine, MD News, and The Johns Hopkins Children's Center Magazine. Chris lives with his wife and close to his three children and four grandchildren in the Boston area.

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