Objective
🧬 Explain the reduction in chromosome number that occurs during meiosis.
🧬 Recognize and summarize the stages of meiosis.
🧬 Analyze the importance of meiosis in providing genetic variation.
Vocabulary
gene: functional unit that controls inherited trait expression that is passed on from one generation to another generation.
homologous chromosome: one of two paired chromosomes, one from each parent, that carries genes for a specific trait at the same location.
gamete: a haploid sex cell, formed during meiosis that can combine with another haploid sex cell and produce diploid fertilized egg.
haploid: cell with half the number of chromosomes as a diploid cell.
fertilization: process by which haploid gametes combine, forming a diploid cell with 2n chromosomes, with n chromosomes from the female parent and n chromosomes from the male parent.
diploid: having two copies of each chromosome
meiosis: reduction division process, occurring only n reproductive cells, in which one diploid cell produces four haploid cells that are not genetically identical.
crossing over: exchange of chromosomal segments between a pair of homologous chromosomes during prophase I of meiosis.
Chromosomes and Chromosome Number
Everyone has characteristics passed on to them by their parents, characteristics such as skin tone, hair color, or eye color is called a trait. Instructions and information for these traits are located in the chromosomes and each chromosome is made up of hundreds of genes that play a role in determining the characteristics and functions of the cell. Human body cells have 46 chromosomes. Chromosomes come in pairs. Each chromosome has about 1500 genes. You have 23 chromosomes from your father and 23 chromosomes from your mother, making 23 pairs of chromosomes.
What are homologous chromosomes?
The chromosomes that make up a pair, one from each parent, are called homologous chromosomes. Homologous chromosomes are the same length and have the centromere in the same place. They also carry genes for the same traits at the same place. Look at the picture below, and see if you can spot the homologous pair.
Homologous chromosomes are similar but not identical. For example, the gene for ear shape will be located at the same place on each homologous chromosome. Although these genes code for ear shape, the gene on one chromosome might code for one ear shape. The gene on the other chromosome might code for a different ear shape.
How is chromosome number maintained in a species?
The number of chromosomes does not change from generation to generation. You have the same number of chromosomes as your parents. Gametes, or sex cells with half the number of chromosomes, ensure the chromosome number stays the same.
The symbol n represents the number of chromosomes. In humans, n is equal to 23. A cell with n number of chromosomes is called a haploid cell. Gametes are haploid cells.
The process in which one haploid gamete joins with another haploid gamete is called fertilization. After fertilization, the cell has 2n chromosomes—n chromosomes from the female parent plus n chromosomes from the male parent. A cell with 2n chromosomes is called a diploid cell. Notice that n also represents the number of chromosome pairs in an organism.
Meiosis I
Recall that most cells are formed by mitosis. During mitosis the chromosome number stays the same. Because sex cells need half the number of chromosomes, a different process of cell division is needed. Gametes are formed during a process called meiosis. Meiosis is a kind of cell division that reduces the number of chromosomes by half through the separation of homologous chromosomes. Meiosis takes place in the reproductive organs of plants and animals. During meiosis, there are two cell divisions. They are called meiosis I and meiosis II.
What happens during interphase I?
Just as in mitosis, a cell goes through interphase before undergoing meiosis. A cell in interphase carries out a variety of metabolic functions, copies its DNA, and makes proteins.
What happens during prophase I?
Meiosis I begins with prophase I. During prophase I, replicated chromosomes, consisting of two sister chromatids, condense. When that happens, the chromosomes become visible under a light microscope.
As the homologous chromosomes condense, they begin to form homologous pairs in a process called synapsis. The homologous chromosomes are held tightly together along their lengths by a protein that acts like a zipper. Prophase I continues as the chromosomes move to opposite sides of the cell.
What is crossing over?
During synapsis, the chromosomes often swap pieces of DNA. Crossing over occurs when a section of one chromosome changes place with a section of its homologous chromosome. This is shown in the fi gure below. The centrioles figure move to the opposite poles of the cell. Spindle fibers form and bind to the sister chromatids at the centromere.
What happens during metaphase I?
The next phase is metaphase I. During metaphase I, the finished fiberspairs of homologous chromosomes line up in the center of the cell. The spindle fibers attach to the centromere of each homologous chromosome.
What happens during anaphase I?
Next is anaphase I. During anaphase I, each homologous chromosome is guided by the spindle fibers toward opposite poles of the cell. When this happens, the chromosome number is reduced from 2n to n. Notice that the sister chromatids do not split during meiosis I. Each homologous chromosome still has two sister chromatids.
What is the final stage of meiosis I?
The final stage of meiosis I is telophase I. During telophase I, the homologous chromosomes reach opposite poles of the cell. Each pole contains only one member of a pair of homologous chromosomes. The sister chromatids might not be identical because crossing over might have occurred during synapsis in prophase I. Crossing over is one way that meiosis leads to more genetic diversity. This diversity helps explain how species can change over time. At the end of telophase I, the cell undergoes cytokinesis, meaning it divides into two cells. The cells then might go into interphase again, but this time, the DNA is not copied during interphase. The events of meiosis I are shown below.
Visualizing Meiosis
Meiosis II
Meiosis is now half-finished. To complete meiosis, the cell must go through meiosis II. Meiosis II is similar to mitosis.
What events occur during meiosis II?
During prophase II, the spindle apparatus forms, and the chromosomes condense. During metaphase II, a haploid number of chromosomes lines up near the center of the cell by the spindle fibers. During anaphase II, the sister chromatids are pulled apart at the centromere by the spindle fibers, and the sister chromatids are pulled to the opposite poles of the cell. In telophase II, the chromosomes reach the poles, and the nuclear membrane and nuclei reform. Cytokinesis, or cell division, occurs. The result is four haploid cells, each with n number of chromosomes
The Importance of Meiosis
The figure below shows that meiosis and mitosis have similar steps, but they are different in important ways. An important difference is that mitosis produces two identical diploid daughter cells, while meiosis produces four different haploid daughter cells.
How does meiosis create genetic diversity?
The haploid daughter cells made by meiosis are not identical. Because the daughter cells are different, meiosis results in genetic variation.
One way that meiosis produces non-identical daughter cells occurs during prophase I. When pairs of homologous chromosomes line up at the center of the cell, they do so randomly. This means that each daughter cell gets a different, random assortment of chromosomes. The effect on genetic diversity is illustrated in the figure below.
The other way meiosis creates variation is through crossing over. Fertilization, when two haploid gametes combine, results in even more genetic variation.
Sexual Reproduction v. Asexual Reproduction
Asexual reproduction occurs when the organism inherits all of its chromosomes from one parent. The new organism is genetically identical to its parent. Asexual reproduction does not involve meiosis.
Bacteria reproduce by asexual reproduction. Plants and some simple animals can reproduce sexually or asexually. Complex animals only reproduce sexually.
Summary
🧬 DNA replication takes place only once during meiosis, and it results in four haploid gametes.
🧬 Meiosis consists of two sets of divisions.
🧬 Meiosis produces genetic variation in gametes.
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