Even though many plants seem simple, they often have very complex life cycles. We will look at how plants alternate between different life stages as well as the terms used to describe these unique points in their life cycle.
Alternation of Generations
Illustration of diploid and haploid cells with common abbreviations
Imagine if you looked exactly like your grandmother but nothing at all like your mother. In fact, your mother would only have half of the genetic information that you have. While this seems very unrealistic, it's actually how many plants reproduce by using an alternating life cycle.
This alternation of generations is a life cycle that includes both diploid and haploid multicellular stages. Most of the definition is probably unfamiliar, so let's review the terms before talking about what the cycle actually looks like. You may remember some of these terms from genetics. 'Diploid' and 'haploid' both refer to the number of copies of chromosomes. Chromosomes carry genetic information.
'Diploid' means 'two sets of chromosomes.' This is commonly abbreviated as 2n because the n stands for chromosomes and diploid cells have 2 copies. In diploid cells, one copy of the chromosome comes from each parent. For example, in humans, you get one copy of chromosomes from your mom and one copy of chromosomes from your dad. The same idea is found in plants. Each diploid cell contains one copy of chromosomes from the male parent and one copy of chromosomes from the female parent. 'Haploid' means 'one set of chromosomes.' This is commonly abbreviated as n because there's only one copy of the chromosomes.
The last vocab word we need to look at before moving on is 'multicellular.' 'Multicellular' means that it contains more than one cell. This is different from many life cycles in other organisms - such as humans - because our haploid cells are unicellular, meaning 'only one cell.' In plants, part of the life cycle is completed by multicellular haploid cells.
Let's now look at what this life cycle entails. In the future, we will look at how specific types of plants - such as ferns, gymnosperms and angiosperms - go through this alternation of generations, but it is important to first understand the basics of this life cycle pattern. First, let's look at a diagram and use this in order to go through the steps. We can see in this diagram that the life cycle is broken into n on the top and 2n on the bottom. Remember that n refers to haploid cells that only contain one copy of chromosomes and that 2n refers to diploid cells that contain two copies of chromosomes.
Because the oddity of this cycle may not be clear using n and 2n, let's look at what it might look like with a different organism. Take the Garblinx. We see here that the diploid, or 2n, organism looks like this. However, when it moves into the haploid stage, the Garblinx looks completely different! Two of these haploid organisms will get together and mate in order to produce a new 2n organism that looks similar to our first Garblinx. This Garblinx will eventually produce a new haploid organism, and so on.
You may ask why plants even bother with such an odd and complicated life cycle. First, going through a haploid-only stage allows for the weeding out or removal of bad genes. Second, going through a diploid stage allows for genetic variation. Both of these help improve the overall survival of the plant species.
The Haploid Stage
There are other terms on the diagram that are probably not familiar. Let's start with the terms found in the top section. Spores are unicellular haploid cells that divide to become multicellular. Spores are the first part of the haploid life cycle in plants. Spores start out as one cell and then go through mitosis in order to become multicellular. You may remember that mitosis is asexual reproduction that produces cells that are identical. The spores divide repeatedly, creating many identical cells that are all haploid.
Alternation of generations diagram
Once the cells have divided and created a multicellular structure, it is now called a gametophyte. We can see on our diagram that the gametophyte contains many of the same cells that are all haploid. A gametophyte is the multicellular haploid stage. This structure will look different depending on the type of plant, but it is always made up of many cells containing only one set of chromosomes. The gametophyte makes gametes.
Gametes are haploid cells that will unite during sexual reproduction to create a diploid cell. Gametes are often called sex cells. There are two types of gametes: egg and sperm. Eggs are the female gametes, and sperm are the male gametes. It is the same in plants and in humans. Gametophytes will produce both male and female gametes through mitosis. These sperm and egg will create the next part of the plant life cycle.
The Diploid Stage
The gametes created by the gametophyte fuse to become a diploid zygote. A zygote is simply the product of the union of haploid gametes, and it is often referred to as a fertilized egg. We can see on our diagram that the two gametes - the egg and the sperm - unite to form the first part of the 2n life cycle. While our diagram shows only one gametophyte, it is important to note that in order to improve genetic diversity, it is best if the egg and sperm come from different plants. This will allow different genes to be passed down to the next generation.
Much like the spores went through mitosis in order to become the gametophyte, the zygote will go through asexual reproduction to create a multicellular structure. The zygote creates identical cells that are all diploid, making the sporophyte. The sporophyte is the multicellular diploid stage. We can see on our diagram that the sporophyte is made up of similar cells and that they are all diploid, as it is in the 2n section of our diagram.
The sporophyte will go through meiosis in order to produce spores. While mitosis is asexual reproduction that creates two new identical cells, meiosis is considered sexual reproduction. Meiosis, as you may remember from genetics, creates four new cells that are all genetically different and contain half the number of chromosomes found in the parent. Let's look at this a bit more to understand meiosis.
Diagram of meiosis
Meiosis starts out with a diploid cell. Before dividing, the cell will copy the chromosomes. It then goes through the first division, creating two new cells that have a diploid number of chromosomes. However, before it divides again, the cell does not copy the chromosomes. This results in four cells that only have one copy of chromosomes. Remember that cells containing only one copy of each chromosome are called haploid. The sporophyte will go through meiosis in order to produce spores. Spores are the first part of the haploid stage that we looked at, completing the life cycle.
Plants go through a complex life cycle known as alternation of generations. This cycle involves both diploid and haploid multicellular stages. 'Haploid' means that cells contain only one set of chromosomes, while 'diploid' means that cells contain two sets of chromosomes. Remember that haploid is abbreviated as n and that diploid is abbreviated as 2n, indicating the number of copies of chromosomes.
We first looked at the haploid stage in the life cycle. This stage starts with the spore. The spore will then go through mitosis in order to create the gametophyte, which is the multicellular haploid stage. The gametophyte will then produce gametes, which are unicellular haploid cells. Gametes can either be male - sperm - or female - eggs.
The egg and sperm will fuse in order to create the first part of the diploid life stages. The zygote is a unicellular diploid structure that will divide in order to create the sporophyte. The sporophyte is the multicellular diploid stage of the plant life cycle. This structure will eventually create the haploid spores by the process of meiosis.
Meiosis - also called sexual reproduction - creates four cells that are genetically different and haploid. These spores start the haploid stage of the plant life cycle. While different types of plants have variations of this alternation of generations, the same basic structures - such as the gametophyte and the sporophyte - will be found in these cycles.