A Review of Plant Life Cycles & Alternation of Generations

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  • 00:00 Plant Generations
  • 1:01 Alternation of Generations
  • 2:13 The Haploid Stage
  • 3:05 The Diploid Stage
  • 4:39 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Plants have a unique system of passing on genetic information. In this lesson we are going to check out the alternation of generations used by plants to keep their gene pools strong.

Plant Generations

We are often fascinated by the ways that certain things are passed on through generations. You got your great-grandmother's eyes, your uncle's double-jointed thumb, your grandfather's knee problems. Sorry about that one. But the point is that we can trace the lineage of many genetic traits because we know that every new generation is created from a combination of DNA from each parent. Half your traits come from one side, half come from the other.

Well, as much as we take this system for granted, it's not always true - at least not all of the time. Unlike animals, plants don't always pass down genetic information in the same way, but instead rely on an alternation of generations, an alternating life cycle that switches between different methods of passing on genes. Why do plants do this, and how does this work? Well, to understand this, we're going to have to take a look down this plant's family tree.

Alternation of Generations

Plants alternate between different methods of passing on genetic information. So, what are these different methods? The alternation of generations life cycle is a rotation of haploid and diploid stages between generations. These terms refer to the collection of chromosomes within the cell. A diploid cell contains two sets of chromosomes, one from the male parent and one from the female parent. For example, your cells contain chromosomes from both your mom and your dad, so they are diploid cells. A haploid cell, on the other hand, only contains one set of chromosomes.

The alternation of generation in plants is a rotation between these two systems of reproduction. One generation is haploid, which gives birth to a generation of plants with diploid cells, which creates a generation of haploid plants that give birth to a generation of diploid plants, on and on and on. The benefit of this complex system is that it allows for greater genetic control. The cloning of chromosomes in the haploid stage allows for the weeding-out of bad genes. The diploid stage creates genetic diversity.

The Haploid Stage

Let's follow this process a little more closely. This is a fern, a very old and relatively simple type of plant. We'll start with the haploid stage, with the spore. The spore only contains one set of chromosomes, and through mitosis, asexual reproduction, this spore will clone itself several times, creating a group of identical haploid cells called a gametophyte. Fern gametophytes tend to look kind of like a tiny heart-shaped leaf. The gametophyte starts to grow, and eventually it grows enough to produce gametes, sex cells, which are haploid but are capable of joining with other gametes. Gametes can be either female, generally called eggs, or male, generally called sperm.

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