Key Concepts Meiosis is a type of nuclear division. It results in cells that have half as many chromosomes as the parent cell. In animals it leads to the formation of eggs and sperm. Each cell produced by meiosis receives a different combination of chromosomes. Because genes are located on chromosomes, each cell produced by meiosis receives a different complement of genes. Meiosis leads to offspring that are genetically distinct from each other and from their parents.

Key Concepts The leading hypothesis to explain meiosis is that genetically variable offspring are more likely to thrive in environments where parasites and disease are common. If mistakes occur during meiosis, the resulting egg and sperm cells may contain the wrong number of chromosomes. It is rare for offspring with an incorrect number of chromosomes to develop normally.

Introduction During sexual reproduction, a sperm and an egg unite to form a new individual. This process is called fertilization. Meiosis is nuclear division that precedes the formation of gametes (egg and sperm) and results in a halving of chromosome number.

Chromosomes Come in Distinct Types Each organism has a characteristic number of chromosomes. The karyotype is the number and types of chromosomes present in an organism. Sex chromosomes determine the sex of the individual; all other chromosomes are autosomes. Humans have 46 chromosomes in every cell except their gametes. 1 pair of sex chromosomes. 22 pairs of autosomes.

Homologous Chromosomes Have the Same Genes Chromosomes of the same type are called homologous chromosomes, or homologs. Chromosomes carry genes. A gene is a section of DNA that influences one or more hereditary traits in an individual. Different versions of a specific gene are called alleles. Homologs carry the same genes in the same locations, but each one may contain different alleles.

The Concept of Ploidy The haploid number n indicates the number of distinct types of chromosomes present. A cell’s ploidy (n, 2n, 3n, etc.) indicates the number of each type of chromosome present.

Ploidy Varies among Organisms Organisms whose cells contain just one of each type of chromosome are called haploid (n). Those whose cells contain two versions of each type of chromosome are termed diploid (2n). Diploid cells have one paternal chromosome and one maternal chromosome. Organisms with three or more versions of each type of chromosome are called polyploid (3n, 4n, etc.)

An Overview of Meiosis Meiosis reduces chromosome number by half. In diploid organisms, the products of meiosis are haploid. Just before meiosis begins, each chromosome in the diploid (2n) parent cell is replicated. When replication is complete, each chromosome consists of two identical sister chromatids attached at the centromere.

Meiosis Is Two Cell Divisions Meiosis consists of two cell divisions, called meiosis I and meiosis II. The two divisions occur consecutively but differ sharply.

An Overview of Meiosis I During meiosis I, the diploid (2n) parent cell produces two haploid (n) daughter cells. The homologs in each chromosome pair separate and go to different daughter cells. Although the daughter cells are haploid (n), each chromosome still consists of two identical sister chromatids.

An Overview of Meiosis II During meiosis II, the sister chromatids of each chromosome separate and go to different daughter cells. The four haploid daughter cells produced by meiosis II also have one of each type of chromosome, but now the chromosomes are unreplicated.

Meiosis Is a Reduction Division The outcome of meiosis is a reduction in chromosome number. For this reason, meiosis is known as a reduction division. In most plants and animals, the original cell is diploid and the four daughter cells are haploid. In animals, these daughter cells become gametes via a process called gametogenesis.

Fertilization Results in a Diploid Zygote When two haploid gametes fuse during fertilization, a full complement of chromosomes is restored. The cell that results from fertilization is diploid and is called a zygote. In this way, each diploid individual receives a haploid chromosome set from both its mother and its father. Homologs are therefore referred to as being either maternal chromosomes, from the mother, or paternal chromosomes, from the father.

The Life Cycle of a Sexual Organism An animal’s life cycle summarizes life from fertilization through offspring production. Meiosis in an adult produces haploid gametes that combine during fertilization to form a diploid zygote, which develops, through mitosis, into an adult of the next generation.

The Phases of Meiosis I Meiosis I is a continuous process with five distinct phases. These phases are as follows: Early prophase I Late prophase I Metaphase I Anaphase I Telophase I

The Phases of Meiosis I Early Prophase I: The homolog pairs come together in a pairing process called synapsis. The structure that results from synapsis is called a tetrad, consisting of two homologs. The chromatids of the homologs are called non-sister chromatids. Late Prophase I: These non-sister chromatids begin to separate. Exchange or crossing over between homologous non-sister chromatids occurs where chiasmata are formed during this stage. Metaphase I: The tetrads line up at the metaphase plate.

The Phases of Meiosis I Anaphase I: The paired homologs separate and begin to migrate to opposite ends of the cell. Telophase I: The homologs finish migrating to the poles of the cell. Then the cell divides in the process of cytokinesis.

The Result of Meiosis I The end result of meiosis I is that one chromosome of each homologous pair is distributed to a different daughter cell. A reduction division has occurred. The daughter cells of meiosis I are haploid and are still in the form of sister chromatids.

The Phases of Meiosis II Like meiosis I, meiosis II is a continuous process, but with four distinct phases: Prophase II Metaphase II Anaphase II Telophase II

The Phases of Meiosis II Prophase II: The spindle apparatus forms and one spindle fiber attaches to the centromere of each sister chromatid. Metaphase II: Replicated chromosomes line up at the metaphase plate. Anaphase II: Sister chromatids separate. The resulting daughter chromosomes begin moving to opposite sides of the cell. Telophase II: Chromosomes arrive at opposite sides of the cell. A nuclear envelope forms around each haploid set of chromosomes, and each cell undergoes cytokinesis.

The Result of Meiosis II Meiosis II results in four haploid cells, each with one of each type of chromosome. Thus, one diploid cell with replicated chromosomes gives rise to four haploid cells with unreplicated chromosomes.

Comparison of Meiosis and Mitosis The key difference between the two processes is that homologs pair in meiosis, but do not in mitosis. Because homologs pair in prophase of meiosis I, they can migrate to the metaphase plate together and then separate during anaphase of meiosis I, resulting in a reduction division. Meiosis thus produces four daughter cells with half the genetic material of the parents, while mitosis produces two daughter cells that are genetically identical to the parent cells.

Where Does Crossing Over Occur? After replication, sister chromatids stay tightly joined along their entire length. When homologs synapse, two pairs of non-sister chromatids are brought close together and are held there by a network of proteins called the synaptonemal complex. Crossing over occurs when chromosomal segments are swapped between adjacent homologs.

A Closer Look at Crossing Over At each point where crossing over occurs, the non-sister chromatids from each homolog get physically broken at the same point and attached to each other. As a result, segments of maternal and paternal chromosomes are swapped. Crossing over can occur at many locations along the length of such synapsed homologs. Both sets of non-sister chromatids may undergo crossing over, resulting in the swapping of segments between maternal and paternal chromosomes.

The Consequences of Meiosis Independent shuffling of maternal and paternal chromosomes and crossing over during meiosis I result in four gametes with a chromosome composition different from that of the parent cells. The changes in chromosomes produced by meiosis and fertilization are significant because chromosomes contain the cell’s hereditary material.

Sexual Reproduction Leads to Greater Variation Offspring produced during asexual reproduction are clones that are genetically identical to one another as well as to the parent. In contrast, offspring produced by sexual reproduction, the fusion of gametes, have a chromosome makeup different from that of one another and from that of either parent. Genetic variation in sexual reproduction results from independent assortment and crossing over.

Independent Assortment Produces Genetic Variation Separation and distribution of homologous chromosomes during meiosis I can result in a variety of combinations of maternal and paternal chromosomes. Each daughter cell gets a random assortment of maternal and paternal chromosomes, and thus genes, which generates a great deal of genetic diversity in the subsequent gametes. Humans, with a haploid chromosome number of 23, can produce 223 (~ 8.4 million) different combinations of chromosomes in gametes.

The Role of Crossing Over Crossing over produces new combinations of alleles on the same chromosome, combinations that did not exist in each parent. Crossing over is a form of genetic recombination that increases the genetic variability of gametes produced by meiosis beyond that produced by random assortment of chromosomes.

How Does Fertilization Affect Genetic Variation? Crossing over and the random mixing of maternal and paternal chromosomes ensure that each gamete is genetically unique. The genetic variation introduced during meiosis ensures that even in self-fertilization, where gametes from the same individual combine, the offspring will be genetically different from the parent.

Outcrossing Further Increases Genetic Variation In many sexually reproducing species, gametes from different individuals combine to form offspring, a process called outcrossing. Outcrossing increases the genetic diversity of the offspring because chromosomes from two different parents are combined. In humans, this means that two parents can potentially produce 8.4 million  8.4 million = 70.6  1012 genetically distinct offspring. This does not even take additional variation from crossing over into consideration!

Why Does Meiosis Exist? Sexual reproduction is common among multicellular organisms, but organisms in most lineages of the tree of life undergo asexual reproduction.

The Paradox of Sex The mathematical model of John Maynard Smith predicts that asexually reproducing organisms should reproduce faster and outcompete similar organisms that invest in sexual reproduction. Asexual reproduction is much more efficient than sexual reproduction because no males are produced.

The Purifying Selection Hypothesis In asexual reproduction, a damaged gene will be inherited by all of that individual’s offspring. On the other hand, sexually reproducing individuals are likely to have offspring that lack deleterious alleles present in the parent. Natural selection against deleterious alleles is called purifying selection. Over time, purifying selection should steadily reduce the numerical advantage of asexual reproduction.

Mistakes in Meiosis If a mistake occurs during meiosis I and the chromosomes from the parent cells are not properly distributed to each daughter cell, the resulting gametes will contain an abnormal set of chromosomes.

How Do Mistakes Occur? For a gamete to get one complete set of chromosomes, each pair of homologous chromosomes must separate from each other during the first meiotic division, and the sister chromatids must separate from each other and move to opposite poles during meiosis II. If both homologs or both sister chromatids move to the same pole of the parent cell, the products of meiosis will be abnormal. This sort of meiotic error is referred to as nondisjunction.

Types of Nondisjunction If nondisjunction occurs in meiosis I, two gametes will have an extra copy of a chromosome (causing a condition called trisomy), and two gametes will lack that chromosome (monosomy). An example of trisomy is Down syndrome, which is caused by an extra copy of chromosome 21.

Frequency of Nondisjunction Nondisjunction may occur in as many 10 percent of meiotic divisions. However, aneuploid zygotes (those with too few or too many chromosomes) typically do not survive to produce viable offspring. Mistakes in meiosis are the leading cause of spontaneous abortion (miscarriage) in humans.

Why Do Mistakes Occur? Meiotic errors appear to be accidental, with no genetic predisposition. Maternal age is an important factor in the frequency of trisomy.

Most Common Aneuploidy Disorders Most instances of aneuploidy in humans involve Down Syndrome (chromosome 21) or the sex chromosomes. Sex chromosome aneuploidy can occur in many different forms: Klinefelter syndrome develops in XXY males. Trisomy X (karyotype XXX). Females with Turner Syndrome have monosomy – their karyotype is XO (they are lacking a second X chromosome) and are usually sterile.