Powerpoint: Mitosis Meiosis
Learning Objectives
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- Identify the products of mitosis and meiosis
- Define haploid (N) and diploid (2N)
- Identify if a cell is diploid or haploid
- Identify the parts of the human body where mitosis and meiosis occurs
- Describe (with an analogy) the process of sexual reproduction, mitosis and meiosis
In the following weeks, we will be exploring the multicellular, eukaryotic kingdoms: Fungi, Plantae and Animalia. As multi-cellular organisms, cell division and sexual reproduction are critical life stages. As we will see, organisms have come up with some unique strategies for completing these processes.
To understand cell division and sexual reproduction in multicellular organisms it is important to first understand the processes of mitosis and meiosis. To familiarize ourselves with these terms, let’s take a look at human cell division and sexual reproduction and compare it with making airplanes (I’ll clarify in a minute).
Human Cell Division
Cell division in humans (and most organisms) occur via a process called mitosis.
Mitosis: process by which the nucleus of a cell is divided into two nuclei, each with the same number and kinds of chromosomes as the parent cell.
In mitosis, the cell is simply making a copy of itself. No new DNA combinations are being formed, nothing fancy. The product is two identical cells.
- The DNA replicates (makes an identical copy of all DNA in the cell)
- The DNA is pulled apart (each cell has one copy of all DNA)
- Identical daughter cells form
Building Airplanes Analogy: Now lets compare human cell division to airplanes. DNA is often called the “blueprint of life”, just like the blueprint of an airplane is used to build the airplane. So if we were to compare mitosis to airplane blueprints…
- Two airplane blueprints are replicated (copied onto another sheet of paper)
- Each of the two airplane blueprints now make one complete organism
Wait! You say. That doesn’t make any sense. Why would there be two airplane blueprints? Shouldn’t it just be one? As it turns out, in human cells, we also have two complete blueprints on how to make a human – one from the biological mother and one from the biological father. Because human beings (and most animals) have two complete “blueprints” in their cells, they are known as diploid organisms.
Diploid: organisms which have two copies of DNA, one inherited from each parent.
Haploid: cells or organisms which have only one copy of DNA. (e.g. human sperm and egg cells)
Interestingly, there are many organisms (particularly in the plant kingdom) that have more than two copies of DNA. For example, grocery bought strawberries can have eight copies of DNA! Our seedless bananas and watermelons have three copies of DNA.
Human Gametogenesis (creation of sex cells)
Humans, as with most if not all animals, most plants and some fungi, undergo sexual reproduction.
Sexual reproduction: process in which two cells, normally from different individuals, unite to produce the first cell of a new organism.
In order for sexual reproduction to happen, gametes must be created. Gametes are special cells that contain half the genetic information that most cells have. Females gametes are eggs, male gametes are sperm. Each of these have half the genetic information that a cell would have. When the two are combined, a zygote (the first cell of a multi-cellular organism is formed). Then mitosis occurs, the cells duplicate until the trillions of cells that make up your body is formed.
Because the gametes all have only one complete copy of DNA, they are referred to as N, or haploid. All other cells in your body other than sperm and egg however, have two complete copy of DNA. Therefore, we refer to them as 2N, or diploid.
Therefore, whereas Mitosis creates more of the same cells, meiosis creates cells with half the genetic information cells normally have. Combining the gametes of two different organisms produces an entirely new organism, which is the whole point of sexual reproduction: to create something new instead of more of the same.
