Meiosis is the process that all organisms go through in order to produce gametes, or sex cells. The process differs from mitosis, the process of somatic cell division, in a number of key ways. Always remember that the purpose of meiosis is to create variation within the population. As you'll see, there are many different mechanisms to create variation in the sex cells. Instead of creating an exact replica of its mother cell, in meiosis, the daughter cells look similar to its parent cells but not quite identical. Meiosis involves one round of DNA replication and two rounds of cellular division. The resulting cells are all genetically unique from one another and from the parent cell. The resulting cells are also haploid, meaning that they have half of the genetic content of a typical somatic cell.
A normal human being has 46 chromosomes, but if a gamete was produced with two diploid cells (meaning each sex cell contained 46 chromosomes), the resulting gamete would have 92 chromosomes! If we even have one extra chromosome, our body doesn't know how to deal with it, so imagine having twice as many chromosomes in your body! 💣💣
This is why during the process of meiosis, each sex cell only contains n amounts of chromosomes, so that the resulting gamete would be 2n, or a diploid cell. In the human's example, the sex cells would have 23 chromosomes each, so that the gamete has 46 chromosomes, which is the normal amount of chromosomes a human ought to have.
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The diagram below will walk through the steps that take place in meiosis and their importance to the creation of egg and sperm. Meiosis consists of two steps: Meiosis I and Meiosis II. Meiosis I consists of Prophase I, Metaphase I, Anaphase I and Telophase I. As you might guess, Meiosis II consists of Prophase II, Metaphase II, Anaphase II, and Telophase II.
Meiosis I
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Prophase I
Prophase is like regular prophase from mitosis. Like mitosis, DNA replicates and coils into nice chromosomes and the nuclear membrane disappears. But these chromosomes then go on to find their counterparts, or hom*ologous pairs. hom*ologous pairs are pairs of the chromosomes that contain and code for the same information. These hom*ologous pairs may also cross over, which is sharing information with each other. This creates more variation with the sex cells.
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Metaphase I
Metaphase I is also like good old metaphase from mitosis. But instead of lining up in the center individually, the chromosomes line up in hom*ologous pairs. For example, chromosomes that contain DNA coding for hair color would line up together as hom*ologous pairs while chromosomes containing information on eye color might line up together. Keep in mind, one of each within the hom*ologous pairs comes from the mother, while the other comes from the father's DNA. 🧬🧬
Also note that alignment is random, so the mother's DNA could end up on the right for one hom*ologous pair, while the father's DNA could end up also on the right for another hom*ologous pair. This also increases variation!
Anaphase I
Each hom*ologous pair is separated and moved to its respective poles, just like in regular anaphase. However, these chromosomes are stay intact, so the chromosomes are not split into chromatids like in anaphase.
Telophase I
Two daughter cells are created with 46 chromosomes! It almost might feel like mitosis because nothing really changed in chromosome numbers but keep in mind! The chromosomes now contain variation, which is the sole purpose of meiosis!
Meiosis II
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Prophase II
New spindles are formed and the chromosomes start coiling.
Metaphase II
The sister chromatids are lined up at the center of the cell.
Anaphase II
The chromatids are pulled apart.
Telophase II
The cell divides into two and voila! Four cells are created with different DNA from all four grandparent cells! This is what you call diversity! Now, the cells have n amount of chromosomes, or 23 chromosomes in each cell.
So basically, long story short, one cell duplicates its DNA, mixes the DNA and separates into normal-number-chromosome cells. Then, it divides once more to have half number of chromosomes in each of the four sex cells. You could then ask, why didn't the cell just divide into half? Well, remember, the whole point of meiosis is diversity, so mixing the DNA in meiosis I is crucial in serving its purpose!
A more simplified version (to understand the whole process) can be seen below!
1️⃣ Step 1
Step 1 shows the replication of DNA, as the cells now have the signature “X” formation of a duplicated chromosome.
2️⃣ Step 2
Step 2shows hom*ologous chromosomes pairing up. hom*ologous chromosomes are different versions of thesame chromosome. For example, humans have two versions of 23 chromosomes, for a total of 46 chromosomes. During this step of meiosis, our two different versions of our 23 chromosomes would pair up with one another in preparation for division.
3️⃣ Step 3
Step 3 shows the action of crossing over between hom*ologous chromosomes, a process that will be thoroughly discussed in the next section.
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4️⃣ Step 4
Step 4 shows the first round of cellular division, resulting in the splitting of hom*ologous chromosomes. The chromosomes that remain are referred to as sister chromatids.
5️⃣ Step 5
Step 5 shows the second round of cellular division, resulting in four genetically unique daughter cells. In comparison to the starting cell on the left hand side, the daughter cells have half the amount of chromosomes.
The AP exam does NOT require you to know the specific phases of cellular division (Prophase, Metaphase, Anaphase, and Telophase), so focus your energy on remembering how the chromosomes generally move and how the process contributes to genetic diversity.
Anaphase I: Anaphase I is a phase of meiosis where hom*ologous chromosomes separate and move towards opposite poles (ends) of the cell.
Anaphase II: Anaphase II is the stage in meiosis II where the sister chromatids, now individual chromosomes, separate and move towards opposite poles of the cell.
Cellular Division: Cellular division is the process by which a parent cell divides into two or more daughter cells.
Chromosomes: Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made up of protein and a single molecule of deoxyribonucleic acid (DNA).
Cross Over: Cross over is a process during meiosis where two chromosomes pair up and exchange segments of their genetic material.
Diploid Cell: A diploid cell is a cell that contains two complete sets of chromosomes, one from each parent.
DNA Replication: DNA replication is the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules.
Gametes: Gametes are mature sexual reproductive cells that come together during fertilization to produce offspring. In humans, these are sperm and egg cells.
Genetic Diversity: Genetic diversity refers to the total number of genetic characteristics in the genetic makeup of a species. It is distinguished by the variation at gene levels.
Genetically Unique: Being genetically unique means having a set of genes that is different from everyone else's.
Haploid: A haploid is a cell that contains one complete set of chromosomes.
hom*ologous Pairs: hom*ologous pairs are two chromosomes, one from each parent, that are similar in shape and size and carry the same type of genetic information.
Meiosis: Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.
Meiosis II: Meiosis II is the second part of meiosis, a type of cell division that produces four haploid cells from one diploid cell. This phase is similar to mitosis and includes prophase II, metaphase II, anaphase II, and telophase II.
Metaphase I: Metaphase I is a stage of meiosis where hom*ologous chromosomes (pairs of mother and father chromosomes) align at the equator of the cell.
Metaphase II: Metaphase II is the stage in meiosis II when chromosomes align at the equator or middle of the cell before they are pulled apart.
Mitosis: Mitosis is a part of the cell cycle where replicated chromosomes are separated into two new nuclei resulting in genetically identical cells with equal distribution of genetic material.
Prophase I: Prophase I is the first phase in meiosis I where chromosomes condense, hom*ologous chromosomes pair up to form tetrads, and crossing over occurs.
Prophase II: Prophase II is the initial stage of meiosis II where chromosomes condense and become visible while spindle fibers form and attach to these chromosomes.
Sister Chromatids: Sister chromatids are identical copies of a single chromosome that are connected by a centromere. They are created during the S phase of interphase and later separated during anaphase.
Telophase I: Telophase I is a stage in meiosis when divided genetic material gathers at both poles forming two new cells.
Telophase II: Telophase II is the final phase in meiosis where nuclear membranes form around each set of chromosomes, which decondense back into chromatin. The cytoplasm then divides resulting in four haploid daughter cells.
