05.02 MEIOSIS AND GENETIC DIVERSITY

Because each pair of homologous chromosomes is positioned independently of the other pairs at metaphase I, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair. This is called independent assortment.

Each daughter cell represents one outcome of all possible combinations of maternal and paternal chromosomes. As shown in Figure 10.11, the number of combinations possible for daughter cells formed by meiosis of a diploid cell with two pairs of homologous chromosomes is four: two possible arrangements for the first pair times two possible arrangements for the second pair.

Learning Objective

Explain how the process of meiosis generates genetic diversity.

As a consequence of the independent assortment of chromosomes during meiosis, each of us produces a collection of gametes differing greatly in their combinations of the chromosomes we inherited from our two parents.

Figure 10.11 suggests that each chromosome in a gamete is exclusively maternal or paternal in origin. In fact, this is notthe case because crossing over produces recombinant chromosomes, individual chromosomes that carry genes (DNA) from two different parents.

Enduring Understanding

Heritable information provides for continuity of life.

Note that only two of the four combinations of daughter cells shown in the figure would result from meiosis of a singlediploid cell, because a single parent cell would have one of the two possible chromosomal arrangements at metaphase I, but not both.

However, the population of daughter cells resulting from meiosis of a large number of diploid cells contains all four types in approximately equal numbers. In the case of , eight combinations of chromosomes are possible for daughter cells. More generally, the number of possible combinations when chromosomes sort independently during meiosis is where n is the haploid number of the species.

Somatic Cells Are Diploid Cells

Image: An image of a cell that has divided by mitosis. Two diploid cells in telophase are shown. Somatic cells are any cells in the body that are not gametes. Somatic cells are diploid, meaning they contain two chromosome sets that are paired up together. One set was inherited from the mother and the other set was inherited from the father. Species can have different diploid numbers of chromosomes, abbreviated 2n. Human somatic cells contain 23 pairs of chromosomes, or 46 total chromosomes. So the human diploid number (2n) is 46 and the haploid number is 23.

As you'll learn more about in later chapters, mutations are the original source of genetic diversity. These changes in an organism's DNA create the different versions of genes known as alleles

Once these differences arise, reshuffling of the alleles during sexual reproduction produces the variation that results in each member of a sexually reproducing population having a unique combination of traits.

Essential Knowledge

Meiosis is a process that ensures the formation of haploid gamete cells in sexually-reproducing diploid organisms:Meiosis results in daughter cells with half the number of chromosomes of the parent cell.Meiosis involves two rounds of a sequential series of steps (meiosis I and meiosis II). Mitosis and meiosis are similar in the way chromosomes segregate but differ in the number of cells produced and the genetic content of the daughter cells.

Introduction

Meiosis plays an important role in genetic inheritance due to the replication and separation of chromosomes during meiosis. Remember that homologous pairs of chromosomes are found in any diploid cell. These chromosomes were initially brought together as a result of fertilization, with one chromosome in each set coming from the father and the other coming from the mother. Sister chromatids are formed during DNA replication, which occurs during interphase before meiosis begins. This means sister chromatids are genetically identical to each other, if no errors were made during the replication process. As the cell prepares for meiosis, the chromosomes duplicate in interphase. Now each chromosome has an exact copy, and these are referred to as sister chromatids. These sister chromatids pair up with another set of sister chromatids in prophase I of meiosis to form tetrads. The pairing occurs after the original homologous set of chromosomes reunite. A tetrad is a total of four sister chromatids, and this stage is the only time crossing over can occur. As you read about genetic inheritance, pay special attention to the roles chromosomes and meiosis play in genetic variation.

comparison

Type of ReproductionAsexual ReproductionSexual ReproductionNumber of ParentsOne parentTwo parentsGenetic DiversityIdentical offspringDiverse offspringCharacteristicsQuick, produces large number of offspringUsually slower, but genetic diversity helps ensure survival of the species

random fertilization

The random nature of fertilization adds to the genetic variation arising from meiosis. In humans, each male and female gamete represents one of about possible chromosome combinations due to independent assortment. The fusion of a male gamete with a female gamete during fertilization will produce a zygote with any of about 70 trillion diploid combinations. With the variation from crossing over, the number of possibilities is astronomical. You really are unique.

The published map distance for the spore color gene and the centromere is 26 map units. The data collected shows 29 map units. What could account for this difference?

The results show the total number of 50 asci. This sample is on a smaller scale and could account for the difference in map units.

In the case of humans , the number of possible combinations of maternal and paternal chromosomes in the resulting gametes is or about Each gamete that you produce in your lifetime contains one of roughly possible combinations of chromosomes.

This is an underestimate because it doesn't take into account crossing over.

In species that reproduce sexually, the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation.

Three mechanisms contribute to the genetic variation arising from sexual reproduction: independent assortment of chromosomes, crossing over, and random fertilization.

One aspect of sexual reproduction that generates genetic variation is the random orientation of pairs of homologous chromosomes at metaphase of meiosis I.

At metaphase I, the homologous pairs, each consisting of one maternal and one paternal chromosome, are situated at the metaphase plate. (Note that the terms maternal and paternal refer, respectively, to whether the chromosome in question was contributed by the mother or the father of the individual whose cells are undergoing meiosis.) Each pair may orient with either its maternal or paternal homolog closer to a given pole—its orientation is as random as the flip of a coin. Thus, there is a 50% chance that a given daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a 50% chance that it will get the paternal chromosome.

In meiosis in humans, an average of one to three crossover events occurs per chromosome pair, depending on the size of the chromosomes and the position of their centromeres. As you learned in Figure 10.9, crossing over produces chromosomes with new combinations of maternal and paternal alleles.

At metaphase II, chromosomes that contain one or more recombinant chromatids can be oriented in two nonequivalent ways with respect to other chromosomes because their sister chromatids are no longer identical. The possible arrangements of nonidentical sister chromatids during meiosis II increase the number of genetic types of daughter cells that can result from meiosis. Figure 10.12 provides an overview of how crossing over increases genetic variation by producing recombinant chromosomes that include DNA from two parents

How do you explain the differences between the recombinant asci and the parental types?

Crossing over occurred in the recombinant asci during meiosis, which resulted in different patterns of black and tan cells within the asci, making the pattern different from the parental types.

How does crossing over increase genetic diversity?

During crossing over, sister chromatids exchange portions of DNA. The number of genes on each chromatid stays the same, but the genes will be different, increasing genetic diversity in the daughter cells.

Homologous Chromosomes

Image: Coloured scanning electron micrograph (SEM) of chromosomes during anaphase (I) of meiosis (gamete formation). Chromosomes consist of deoxyribonucleic acid (DNA) and proteins. During meiosis, four daughter nuclei are formed from one parent nucleus after two stages of nuclear division. Meiosis occurs only in the sex cells (gametes) of the testes and ovaries. At anaphase (I), pairs of homologous chromosomes are separated and pulled to opposite poles of the cell by spindles (not seen). This results in two cells with half the usual number of chromosomes. The full complement is restored when two gametes fuse during fertilization. Magnification: x4,500 when printed 10 centimetres wide. Homologous chromosomes are paired chromosomes found in somatic cells (cells that are not gametes). Each chromosome in the pair carries genes for the same characteristics, but one of the chromosomes originally came from the father and the other came from the mother during fertilization. During mitosis, these chromosomes are replicated so that each resulting daughter cell has the same sets of chromosomes as the original cell. It is through mitosis that an organism grows from a zygote to a multicellular organism.

Gametes Are Haploid Cells

Image: Computer artwork of a human sperm (centre right) penetrating an egg (yellow) during the process of fertilization. Other sperm are seen at left. Sperm have an oval head and a hair-like tail, which beats with a whiplash motion to swim. The human female usually produces a single large egg from the ovary, while the male releases some 300 million much smaller sperm. The sperm travel through the uterus (womb) and up the fallopian tubes to reach the egg. The sperm must penetrate the egg; this penetration is aided by enzymes contained in the sperm's head. Only one sperm can fuse with the egg nucleus. Fertilization enables male and female genetic material to be shared. Sperm and ova (eggs) are gametes. They are haploid cells, meaning they contain a single set of un-paired chromosomes. They contain half the number of chromosomes found in somatic cells (body cells that are not gametes) so that when gametes come together during fertilization, the resulting zygote will contain one full set of paired chromosomes. In humans, the gametes contain a haploid number of 23 unpaired chromosomes-22 autosomes plus a single sex chromosome (X or Y). The haploid number in any organism is symbolized by n.

Independent Assortment of Chromosomes

Image: Illustration of sex cells being produced during meiosis In metaphase I of meiosis, the homologous pairs are randomly oriented along the metaphase plate. Each homologous pair may orient with its maternal or paternal chromosome closer to a given pole. This means that there is a 50 percent chance that a particular daughter cell will get the maternal chromosome of a certain homologous pair and a 50 percent chance that it will receive the paternal chromosome. Because each homologous pair orients independent of the other pairs during metaphase I, the first meiotic division results in a mixture of maternal and paternal chromosomes in each daughter cell. This means that there are a great number of different chromosome combinations available for the haploid daughter cells formed during meiosis. In the case of humans, the number of possible combinations of maternal and paternal chromosomes in a resulting gamete is 223, or 8.4 million.

Asexual reproduction

In asexual reproduction, one individual (parent) passes copies of all its genes to its offspring. There is no fertilization or fusion of gametes involved. Mitosis plays an important role in asexual reproduction, so any genetic difference between parent and offspring is due to mutations or errors in the replication of the DNA. Asexual reproduction is just one function of mitosis; it also occurs in all organisms for growth and repair. There are many different types of asexual reproduction. Examples include: Binary Fission: Single-celled organisms can reproduce asexually through cell division. Most bacteria use this type of reproduction. Budding: A new mass of living cells divides and grows on the side of the parent. The mass develops into a miniature version of the parent, called a bud, which eventually detaches to take on a life of its own. Hydras, a relative of the jellyfish, and yeast often reproduce this way. Gemmules (Internal Buds): A parent releases a specialized mass of cells that can develop into offspring. Sponges exhibit this type of reproduction. Asexual reproduction can be very advantageous to certain organisms. Organisms that remain in one place and are unable to look for mates may need to reproduce asexually. Another advantage of asexual reproduction is that numerous offspring can be produced in a short amount of time and with little energy. A disadvantage of this type of reproduction is the lack of genetic variation. All of the organisms are genetically identical and, therefore, share the same weaknesses.

sexual reproduction

In sexual reproduction, two individuals (parents) contribute genes to the offspring. Sexually-reproducing organisms have different sets of genes for every trait, called alleles. Each parent contributes one allele for each trait to the offspring. Sexual reproduction results in a greater variation of offspring than asexual reproduction. As you saw in the introduction, two parents can give rise to offspring that vary genetically from their siblings and their parents. Genetic variation is important for the survival of a species, especially if there are changes in the environment.

Meiosis and Crossing Over

In species that reproduce sexually, the shuffling of chromosomes during meiosis and fertilization is responsible for the variation that arises each generation. Three main mechanisms responsible for the greatest genetic variation are: crossing over arrangements of chromosomes in metaphase I random fertilization During the crossing over that occurs in prophase I, sister chromatids within each tetrad attach and swap portions of adjacent DNA molecules. Neither chromatid gains or loses any genes, but this crossing over increases the genetic variation in the sex cells. For example, a human sex cell has 23 chromosomes. If only one cross-over event occurs in each tetrad, more than 64 trillion combinations are possible for a given zygote. There are usually two or three cross-over events in each tetrad. In metaphase I, the orientation of homologous pairs is random as they line up along the center of the cell. Each homologous pair can line up in two different ways. Differences in the homologous pairs' orientation in metaphase I lead to variation in how the chromosomes are separated and grouped together in anaphase I.Changes in the orientation of tetrads along the center of the cell vary the combination of chromosomes in the resulting daughter cells. The fusion of gametes during fertilization adds to the genetic variation arising from meiosis. For example, a human egg cell representing one of eight million possible chromosome combinations from a female will be fertilized by a human sperm cell that represents one of eight million different possible chromosome combinations from a male. Any two parents can produce a zygote with any of about 64 trillion diploid combinations. No wonder brothers and sisters can look so different!

Ascospores

In this experiment, a cross was made between wild type (black) and tan strains. The resulting zygote produced will be either parental asci of four black and four tan spores in a row (4:4), or a recombinant asci without this pattern. This image shows the various patterns produced from meiosis. Some asci contain the 4:4 pattern, while others show that crossing over occurred and the arrangement of the colored cells are different.

sexual reproduction greatly increases the genetic variation present in a population. Although Darwin realized that heritable variation makes evolution possible, he could not explain why offspring resemble—but are not identical to—their parents.

Ironically, Gregor Mendel, a contemporary of Darwin, published a theory of inheritance that helps explain genetic variation,

A population evolves through the differential reproductive success of its members. On average, those best suited to the environment leave more offspring, thereby transmitting their genes.

Thus, natural selection leads to an increase in genetic variations favored by the environment. As the environment changes, the population may survive if some members can cope with the new conditions. Mutations, the original source of different alleles, are mixed and matched during meiosis and sexual reproduction. New combinations of alleles may work better than those that previously prevailed.

Meiosis and Recombinant Asci; Sordario Fimicola

The fungus Sordario fimicola is an organism with haploid cells for most of its life cycle. During the fusion of two mycelia, it becomes a diploid cell. To return to its haploid state, it must undergo meiosis. This image shows an ascus, which is a sac containing eight haploid ascospores that are all black in color. This is the wild type. The fungus has one gene that controls the color of the spores, either black or tan.

Sister Chromatids

mage: Fluorescence micrograph of anaphase during mitosis (nuclear division). During mitosis, two daughter nuclei are formed from one parent nucleus. At anaphase, sister chromatids are moved to separate poles of the cell by microtubules (red). Chromatids consist of deoxyribonucleic acid (DNA, white). Two identical chromatids make up one chromosome. Microtubules, part of the cell's cytoskeleton, grow from either pole of the cell and attach to centromeres (blue) at the centre of each chromosome. The microtubules then contract, pulling the chromatids to opposite poles. The cell goes on to divide in half, with each new cell retaining a copy of the parent cell's genetic information. When a chromosome duplicates in preparation for mitosis or meiosis, pairs of identical sister chromatids are formed. The sister chromatids are connected along their entire lengths by sister chromatid cohesion, eventually being separated by a combination of molecular and mechanical processes. In mitosis, the sister chromatid are separated during anaphase to eventually form two diploid daughter cells. In meiosis, the sister chromatids stay together until anaphase II, where they are pulled apart to eventually form four haploid daughter cells.