Introduction to Meiosis: Study with Video Lessons, Practice Problems & Examples

Meiosis consists of two rounds of cell division: meiosis I and meiosis II. Meiosis I, or reductional division, reduces a diploid germ cell (2n) into two haploid cells (n) by separating homologous chromosomes. Meiosis II, known as equational division, maintains haploidy by separating sister chromatids, resulting in four genetically diverse haploid gametes. This process is crucial for sexual reproduction, producing gametes like sperm and eggs, and ensures genetic variation through recombination and independent assortment.

Introduction to Meiosis

In this video, we're going to begin our introduction to meiosis. It's important to note that even before meiosis takes place, a diploid cell must first replicate its DNA and make proteins for cell division in a process known as interphase. Even before meiosis takes place, interphase still needs to occur. If we take a look at our image below, notice that the orange region represents interphase as we discussed in our previous lesson videos when we talked about the cell cycle. Even before meiosis takes place, interphase must occur first. Interphase is going to have a G₁ phase, an S phase, and a G₂ phase. Of course, the S phase is where the DNA is replicated. Notice that meiosis is indicated by this region of our image. When we look at the process of meiosis, it's actually broken down into 2 parts: meiosis 1 followed by cytokinesis, and meiosis 2 followed by cytokinesis. As we move forward in our course, we'll talk more about this process of meiosis and break it down much further. Meiosis is not going to be a cyclic process that regenerates the same types of cells like mitosis does. Instead of forming a full circle that starts and ends at the same place, meiosis creates a linear fashion that is not cyclic and does not regenerate the same types of cells. As we move forward in our course, we'll continue discussing meiosis. The biggest takeaway about this process is that it starts with a diploid germ cell and ends with 4 genetically diverse haploid cells or more specifically, haploid gametes. Recall that gametes are just the sex cells, either sperm or eggs. The germ cell, a diploid cell with 2 copies of every chromosome, acts as the precursor for making gametes. Meiosis starts with the germ cell and ends with making these gametes, either sperm or eggs. At the end of meiosis, we have 4 gamete cells that are haploid, represented by 𝑛, and they are genetically diverse. This cell would be genetically different from this one, which is different from this one and this one. Through this image and comparing it to the cell cycle we discussed in our previous lesson videos, you can get a better feel for how meiosis is similar yet different from the cell cycle. As we move forward in our course, we’ll discuss meiosis more thoroughly. So, I'll see you all in our next video.

Introduction to Meiosis Example 1

Alrighty. So here we have an example problem that wants us to complete the sentence using one of these 5 potential answer options down below. And it says that the process of meiosis produces, and option A notice says 2 diverse haploid gamete cells. Now of course from our last lesson video we know that meiosis is going to result in 4 cells, not 2 cells. So any option that says 2 cells, we know is going to be incorrect for meiosis. So we can eliminate both option A and option B, which both mention 2 cells. And another thing that we can note is that meiosis is going to create 4 genetically diverse cells that are not identical to each other. And so to say that it's going to create 4 identical cells is going to be incorrect. And so we can eliminate both answer option C and answer option E because they both say identical. And so it turns out that option D is going to be the correct answer just through that logic there. And so meiosis ends up producing 4 genetically diverse cells that are haploid gamete cells, meaning that they're going to be, haploid, meaning that they're going to have half the number of chromosomes or just one copy of every chromosome, and gametes, meaning that they're going to be sex cells either sperm or egg. And so answer option D is the correct answer to this example problem, and that concludes this example. So I'll see you all in our next video.

Which of the following steps must occur before Meiosis I in germ cells?

• A. The DNA of the haploid cell is replicated.
• B. The RNA of the diploid cell is replicated.
• C. The DNA of the diploid cells is replicated.
• D. The two cells need to be physically separated by cytokinesis.

Meiosis I & Meiosis II

So it turns out that meiosis is actually broken down into 2 rounds of cell division. The first round of cell division is meiosis 1, and the second round of cell division is meiosis 2. In the first round of cell division, meiosis 1, it's important to know that it's also sometimes referred to as reductional division. This is because meiosis 1 will reduce the ploidy of the cells, reducing a diploid cell into 2 haploid cells. It does this by separating homologous chromosomes from each other. You can see once again that in meiosis 1, a diploid or 2n germ cell is going to divide into 2 haploid daughter cells. The ploidy has been reduced because we've gone from a diploid cell into 2 haploid cells.

Now, the second round of cell division in meiosis is meiosis 2, which is sometimes referred to as equational division. That's because meiosis 2 maintains equal ploidy, taking 2 haploid cells and forming 4 haploid cells. You start with haploid cells and you end with haploid cells. That maintains equal ploidy, and that's why it's called equational, equa for equal. Meiosis 2 maintains equiploidy by separating sister chromatids instead of separating homologous chromosomes. This is a big difference between meiosis 1 and meiosis 2. Again, because meiosis 2 is equational division, it's going to maintain equal ploidy. You start with the 2 haploid cells from meiosis 1, and those 2 haploid cells from meiosis 1 each divide to produce a total of 4 genetically diverse yet still haploid gametes. You start with haploid cells and still end with haploid cells, and that's why meiosis 2 maintains equal ploidy.

If we look at our image down below, it's another representation of meiosis. Notice that before meiosis even begins, interphase is going to take place with the germ cell which basically takes unreplicated chromosomes and forms replicated chromosomes. This represents the germ cell, the diploid germ cell that, is going to begin meiosis. Meiosis is really broken up into 2 rounds of cell division. The first round of cell division is meiosis 1. In meiosis 1, it's also referred to as reductional division because it takes a diploid germ cell that is 2n and divides the diploid germ cell into 2 cells, daughter cells that are haploid or n. The second round of cell division is called meiosis 2. Meiosis 2 is also sometimes referred to as equational division because you start with haploid cells and you also end with cells that are still haploid. Each of these cells is going to undergo meiosis 2, a cell division to create a total of 4 haploid gametes. The gametes are going to be the sex cells, either sperm in males or eggs in females.

Each of these phases, these stages of meiosis, meiosis 1 and meiosis 2, consists of multiple phases. We're going to break down meiosis 1 and meiosis 2 in their own separate videos as we move forward in our course. But for now, this here concludes our introduction to meiosis and how meiosis is really broken down into 2 rounds of cell division in meiosis 1 and meiosis 2. We'll be able to get some practice as we move forward in our course, and I'll see you all in our next video.

In Meiosis I, cytokinesis usually occurs after telophase I and produces:

a) Four diploid cells.

b) Two haploid cells.

c) Four haploid cells.

d) Two diploid cells.

• A. Four diploid cells.
• B. Two haploid cells.
• C. Four haploid cells.
• D. Two diploid cells.

In Meiosis II, ________ cells are divided into 4 ___________ daughter cells.

a) Diploid; Haploid.

b) Haploid; Diploid.

c) Haploid; Haploid.

d) Diploid; Diploid.

• A. Diploid; Haploid.
• B. Haploid; Diploid.
• C. Haploid; Haploid.
• D. Diploid; Diploid.

Here’s what students ask on this topic:

What is the main difference between meiosis I and meiosis II?

The main difference between meiosis I and meiosis II lies in the type of chromosomes they separate. Meiosis I, also known as reductional division, reduces the chromosome number by separating homologous chromosomes, resulting in two haploid cells from one diploid cell. In contrast, meiosis II, known as equational division, maintains the chromosome number by separating sister chromatids, resulting in four haploid cells from the two haploid cells produced in meiosis I. This distinction is crucial for understanding how meiosis contributes to genetic diversity and the formation of gametes.

Why is meiosis important for sexual reproduction?

Meiosis is essential for sexual reproduction because it produces gametes—sperm and eggs—with half the chromosome number of the parent cell. This reduction in chromosome number is crucial for maintaining the species' chromosome count across generations. Additionally, meiosis introduces genetic diversity through recombination and independent assortment of chromosomes. This genetic variation is vital for evolution and adaptation, as it increases the likelihood of beneficial traits being passed on to offspring.

What are the stages of meiosis and their functions?

Meiosis consists of two main stages: meiosis I and meiosis II, each with its own sub-stages. Meiosis I includes prophase I (homologous chromosomes pair and exchange genetic material), metaphase I (homologous chromosomes align at the cell equator), anaphase I (homologous chromosomes separate), and telophase I (two haploid cells form). Meiosis II includes prophase II (chromosomes condense), metaphase II (chromosomes align at the cell equator), anaphase II (sister chromatids separate), and telophase II (four genetically diverse haploid cells form). Each stage plays a specific role in reducing chromosome number and ensuring genetic diversity.

How does meiosis contribute to genetic diversity?

Meiosis contributes to genetic diversity through two main mechanisms: recombination and independent assortment. During prophase I, homologous chromosomes undergo recombination, where they exchange genetic material, creating new allele combinations. Independent assortment occurs during metaphase I, where homologous chromosome pairs align randomly at the cell equator, leading to different combinations of maternal and paternal chromosomes in the gametes. These processes ensure that each gamete is genetically unique, increasing the genetic variation within a population.

What is the role of interphase in meiosis?

Interphase is a preparatory phase that occurs before meiosis begins. It consists of three stages: G1 (cell growth and protein synthesis), S (DNA replication), and G2 (further growth and preparation for division). During the S phase, the cell's DNA is replicated, resulting in duplicated chromosomes that are essential for the subsequent stages of meiosis. Interphase ensures that the cell has the necessary resources and genetic material to undergo the complex process of meiosis, ultimately leading to the formation of haploid gametes.