BSC 1010C
General Biology I

Meiosis

Outline

1. Offspring acquire genes from parents by inheriting chromosomes
2. Like begets like, more or less: a comparison of asexual versus sexual reproduction
3. Fertilization and meiosis alternate in sexual life cycles
4. Meiosis reduces chromosome number from diploid to haploid
   1. The Stages of Meiotic Cell Division
   2. A Comparison of Mitosis and Meiosis
5. Sexual life cycles produce genetic variation among offspring
   1. Independent Assortment of Chromosomes
   2. Crossing Over
   3. Random Fertilization
6. Evolutionary adaptation depends on a population's genetic variation

Reproduction is an emergent property associated with life. The fact that organisms reproduce their own kind is a consequence of heredity.

Heredity = Continuity of biological traits from one generation to the next.

• Results from the transmission of hereditary units, or genes, from parents to offspring.

Because they share similar genes, offspring more closely resemble their parents or close relatives than unrelated individuals of the same species.

Variation = Inherited differences among individuals of the same species.

Though offspring resemble their parents and siblings, they also diverge somewhat as a consequence of inherited differences among them.

The development of genetics in this century has increased our understanding about the mechanisms of variation and heredity.

Genetics = The scientific study of heredity and variation.

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I. Offspring acquire genes from parents by inheriting chromosomes

DNA = Type of nucleic acid that is a polymer of four different kinds of nucleotides.

Genes = Units of hereditary information that are made of DNA and are located on chromosomes.

• Have specific sequences of nucleotides, the monomers of DNA.
• Most genes program cells to synthesize specific proteins; the action of these proteins produce an organism's inherited traits.

Inheritance is possible because:

• DNA is precisely replicated producing copies of genes that can be passed along from parents to offspring.
• Sperm and ova carrying each parent's genes are combined in the nucleus of the fertilized egg.

The actual transmission of genes from parents to offspring depends on the behavior of chromosomes.

Chromosomes = Threadlike structures in eukaryotic nuclei that are made of DNA and protein.

• Consist of a single long DNA molecule that is highly folded and coiled along with proteins.
• Contain genetic information arranged in a linear sequence.
• Contain hundreds or thousands of genes, each of which is a specific region of the DNA molecule, or locus.

Locus = Specific location on a chromosome that contains a gene.

• Each species has a characteristic chromosome number; humans have 46.

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II. Like begets like, more or less: a comparison of asexual versus sexual reproduction

Asexual Reproduction:

• Single individual is the sole parent.
• Single parent passes on all its genes to its offspring.
• Offspring are genetically identical to the parent.

Results in a clone, or genetically identical individual. Rarely, genetic differences occur as a result of notation, a change in DNA.

Sexual Reproduction:

• Two parents give rise to offspring.
• Each parent passes on half its genes to its offspring.
• Offspring have a unique combination of genes inherited from both parents.
• Results in greater genetic variation; offspring vary genetically from their siblings and parents.

What generates this genetic variation during sexual reproduction? The answer lies in the process of meiosis.

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III. Fertilization and meiosis alternate in sexual life cycles

The human life cycle follows the same basic pattern found in all sexually reproducing organisms; meiosis and fertilization result in alternation between the haploid and diploid condition.

Life cycle = Sequence of stages in an organism's reproductive history, from conception to production of its own offspring.

Somatic cell = Any cell other than a sperm or egg cell.

• Human somatic cells contain 46 chromosomes distinguishable by differences in size, position of the centromere, and staining or banding pattern.
• Using these criteria, chromosomes from a photomicrograph can be matched into homologous pairs and arranged in a standard sequence to produce a karyotype.

Karyotype = A display or photomicrograph of an individual's somatic-cell metaphase chromosomes that are arranged in a standard sequence.

• Human karyotypes are often made with lymphocytes.
• Can be used to screen for chromosomal abnormalities.

Homologous chromosomes (homologues) = A pair of chromosomes that have the same size, centromere position and staining pattern.

• With one exception, homologues carry the same genetic loci.
• Homologous autosomes carry the same genetic loci; however, human sex chromosomes carry different loci even though they pair during prophase of Meiosis I.

Autosome = A chromosome that is not a sex chromosome.

Sex chromosome = Dissimilar chromosomes that determine an individual's sex.

• Females have a homologous pair of X chromosomes.
• Males have one X and one Y chromosome.
• Thus, humans have 22 pairs of autosomes and 1 pair of sex chromosomes.

Chromosomal pairs in the human karyotype are a result of our sexual origins.

• One homologue is inherited from each parent.
• Thus, the 46 somatic-cell chromosomes are actually two sets of 23 chromosomes; one a maternal set and the other a paternal set.
• Somatic cells in humans and most other animals are diploid.

Diploid = Condition in which cells contain two sets of chromosomes; abbreviated as 2n.

Haploid = Condition in which cells contain one set of chromosomes; it is the chromosome number of gametes and is abbreviated as n.

Gamete = A haploid reproductive cell.

Sperm cells and ova are gametes, and they differ from somatic cells in their chromosome number. Gametes only have one set of chromosomes.

Human gametes contain a single set of 22 autosomes and one sex chromosome (either an X or a Y).

• Thus, the haploid number of humans is 23.

The diploid number is restored when two haploid gametes unite in the process of fertilization. Sexual intercourse allows a haploid sperm cell from the father to reach and fuse with an ovum from the mother.

Fertilization = The union of two gametes to form a zygote.

• Contains the maternal and parental haploid sets of chromosomes from the gametes and is diploid (2n).
• As humans develop from a zygote to a sexually mature adult, the zygote's genetic information is passed with precision to all somatic cells by mitosis.

Gametes are the only cells in the body that are not produced by mitosis.

• Gametes are produced in the ovaries or testes by the process of meiosis.
• Meiosis is a special type of cell division that produces haploid cells and compensates for the doubling of chromosome number that occurs at fertilization.
• Meiosis in humans produces sperm cells and ova which contain 23 chromosomes.
• When fertilization occurs, the diploid condition (2n = 46) is restored in the zygote.

The Variety of Sexual Life Cycles

Alternation of meiosis and fertilization is common to all sexually reproducing organisms; however, the timing of these two events in the life cycle varies among species. There are three basic patterns of sexual life cycles:

Animal: In animals, including humans, gametes are the only haploid cells.

• Meiosis occurs during gamete production. The resulting gametes undergo no further cell division before fertilization.

• Fertilization produces a diploid zygote that divides by mitosis to produce a diploid multicellular animal.

Fungi and Some Protists: In many fungi and some protists, the only diploid stage is the zygote.

• Meiosis occurs immediately after the zygote forms.
• Resulting haploid cells divide by mitosis to produce a haploid multicellular organism.
• Gametes are produced by mitosis from the already haploid organism.

Plants and Some Algae: Plants and some species of algae alternate between multicellular haploid and diploid generations.

• This type of life cycle is callled an alternation of generations.
• The multicelitilar diploid stage is called a sporophyte, or spore-producing plant. Meiosis in this stage produces haploid cells called spores.
• Haploid spores divide mitotically to generate a multicellular hapiold stage called a gametophyte, or gamete-producing plant.
• Haploid gametophytes produce gametes by mitosis.
• Fertilization produces a diploid zygote which develops into the next sporophyte generation.

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IV. Meiosis reduces chromosome number from diploid to haploid

Meiosis and sexual reproduction significantly contribute to genetic variation among offspring. Meiosis includes steps that closely resemble corresponding steps in mitosis.

• Like mitosis, meiosis is preceded by replication of the chromosomes.
• Meiosis differs from mitosis in that this single replication is followed by two consecutive cell divisions: Meiosis I and Meiosis II.
• These cell divisions produce four daughter cells instead of two as in mitosis.
• The resulting daughter cells have half the number of chromosomes as the original cell; whereas, daughter cells of mitosis have the same number of chromosomes as the parent cell.

The Stages of Meiotic Cell Division

Interphase I: Interphase I precedes meiosis.

• Chromosomes replicate as in mitosis.
• Each duplicated chromosome consists of two identical sister chromatids attached at their centromeres.
• Centriole pairs in animal cells also replicate into two pairs.

Meiosis I: This cell division segregates the two chromosomes of each homologous pair and reduces the chromosome number by one-half. It includes the following four phases:

Prophase I. This is a longer and more complex process than prophase of mitosis.

• Chromosomes condense.
• Synapsis occurs. During this process, homologous chromosomes come together as pairs.
• Chromosomes condense further until they are distinct structures that can be seen with a microscope. Since each chromosome has two chromatids, each homologous pair in synapsis appears as a complex of four chromatids or a tetrad.
• In each tetrad, sister chromatids of the same chromosome are attached at their centromeres. Nonsister chromatids are linked by X-shaped chiasmata, sites where homologous strand exchange or crossing-over occurs.
• Chromosomes thicken further and detach from the nuclear envelope.

As prophase I continues, the cell prepares for nuclear division.

• Centriole pairs move apart and spindle microtubules form between them. Nuclear envelope and nucleoli disperse.
• Chromosomes begin moving to the metaphase plate, midway between the two poles of the spindle apparatus.
• Prophase I typically occupies more than 90% of the time required for meiosis.

Metaphase 1. Tetrads are aligned on the metaphase plate.

• Each synaptic pair is aligned so that centromeres of homologues point towards opposite poles.
• Each homologue is thus attached to kinetochore microtubules emerging from the pole it faces, so that the two homologues are destined to separate in anaphase and move towards opposite poles.

Anaphase 1. Homologues separate and are moved towards the poles by the spindle apparatus.

• Sister chromatids remain attached at their centromeres and move as a unit towards the same pole, while the homologue moves towards the opposite pole.
• This differs from mitosis during which chromosomes line up individually on the metaphase plate (rather than in pairs) and sister chromatids are moved apart towards opposite poles of the cell.

Telophase I and Cytokinesis. The spindle apparatus continues to separate homologous chromosome pairs until the chromosomes reach the poles.

• Each pole now has a haploid set of chromosomes that are each still composed of two sister chromatids attached at the centromere.
• Usually, cytokinesis occurs simultaneously with Telophase 1, forming two haploid daughter cells. Cleavage furrows form in animal cells, and cell plates form in plant cells.
• In some species, nuclear membranes and nucleoli reappear, and the cell enters a period of interkinesis before meiosis II. In other species, the daughter cells immediately prepare for meiosis.
• Regardless of whether a cell enters interkinesis, no DNA replication occurs before meiosis I.

Meiosis II: This second meiotic division separates sister chromatids of each chromosome.

Prophase II.

• If the cell entered interkinesis, the nuclear envelope and nucleoli disperse.
• Spindle apparatus forms and chromosomes move towards the metaphase II plate.

Metaphase II. Chromosomes align singly on the metaphase plate.

• Kinetochores of sister chromatids point towards opposite poles.

Anaphase II. Sister chromatids separate.

• Centromeres of sister chromatids separate.
• Sister chromatids of each pair (now individual chromosomes) move toward opposite poles of the cell.

Telophase II and Cytokinesis.

• Nuclei form at opposite poles of the cell.
• Cytokinesis occurs producing four haploid daughter cells.

A Comparison of Mitosis and Meiosis

Though the processes of mitosis and meiosis are similar in some ways, there are some key differences:

• Meiosis is a reduction division. Cells produced by mitosis have the same number of chromosomes as the original cell, whereas cells produced by meiosis have half the number of chromosomes as the parent cell.
• Meiosis creates genetic variation. Mitosis produces two daughter cells genetically identical to the parent cell and to each other. Meiosis produces four daughter cells genetically different from the parent cell and from each other.
• Meiosis is two successive nuclear divisions. Mitosis, on the other hand, is characterized by just one nuclear division.

COMPARISON OF MEIOSIS I AND MITOSIS
		MEIOSIS I		MITOSIS
Prophase		Synapsis occurs to form tetrads. Chiasmata appear as evidence that crossing over has occurred.		Neither synapsis nor crossing over occurs.
Metaphase		Homologous pairs (tetrads) align on the metaphase plate.		Individual chromosomes align on the metaphase plate.
Anaphase		Meiosis separates pairs of individual divide and sister chromatids stay together. Sister chromatids of each chromosome move to the same pole of the cell; only the homologues separate.		Mitosis separates sister chromatids of chromosomes. Centromeres do not of individual chromosomes. Centromeres divide and sister chromatids move to opposite poles of the cell.

Meiosis II is virtually identical in mechanism to mitosis, separating sister chromatids.

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V. Sexual life cycles produce genetic variation among offspring

Meiosis and fertilization are the primary sources of genetic variation in sexually reproducing organisms. Sexual reproduction provides genetic variation by:

• independent assortment;
• crossing over during prophase I of meiosis;
• random fusion of gametes during fertilization.

Independent Assortment of Chromosomes

At Metaphase I, each homologous pair of chromosomes aliens on the metaphase plate. Each pair consists of one maternal and one paternal chromosome.

• The orientation of the homologous pair to the poles is random, so there is a fifty-fifty chance that a particular daughter cell produced by meiosis I will receive the maternal chromosome of a homologous pair, and a fifty-fifty chance that it will receive the paternal chromosome.
• Each homologous pair of chromosomes orients independently of the other pairs at metaphase I; thus, the first meiotic division results in independent assortment of maternal and paternal chromosomes.
• A gamete produced by meiosis contains just one of all the possible combinations of maternal and paternal chromosomes.

Independent assortment = The random distribution of maternal and paternal homologues to the gametes. (In a more specific sense, assortment refers to the random distribution of genes located on different chromosomes.)

• Since each homologous pair assorts independently from all the others, the process produces 2ⁿ possible combinations of maternal and paternal chromosomes in gametes, where n is the haploid number.
• In humans, the possible combinations would be 2²³ , or about eight million.
• Thus, each human gamete contains one of eight million possible assortments of chromosomes inherited from that person's mother and father.
• Genetic variation results from this reshuffling of chromosomes, because the maternal and paternal homologues will carry different genetic information at many of their corresponding loci.

Crossing Over

Another mechanism that increases genetic variation is the process of crossing over, during which homologous chromosomes exchange genes.

Crossing over = The exchange of genetic material between homologues; occurs during prophase of meiosis I. This process:

• Occurs when homologous portions of two nonsister chromatids trade places. During prophase I, X-shaped chiasmata become visible at places where this homologous strand exchange occurs.
• Produces chromosomes that contain genes from both parents.
• In humans, there is an average of two or three crossovers per chromosome pair.
• Synapsis during prophase I is precise, so that homologues align gene by gene. The exact mechanism of synapsis is still unknown, but involves the formation of the synaptonemal complex, a protein structure that brings the chromosomes into close association.

Random Fertilization

Random fertilization is another source of genetic variation in offspring. In humans, an egg cell that is one of eight million different possibilities will be fertilized by a sperm cell that is also one of eight million possibilities. Thus, the resulting zygote can have one of 64 trillion possible diploid combinations.

To summarize, there are three important sources of genetic variability in sexually reproducing organisms:

1. Independent assortment of homologous chromosome pairs during meiosis I.
2. Crossing over between homologous chromosomes during prophase of meiosis I.
3. Random fusion of gametes.

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VI. Evolutionary adaptation depends on a population's genetic variation

Inheritable variation is the basis for Charles Darwin's theory that natural selection is the mechanism for evolutionary change. Natural selection:

• increases the frequency of inheritable variations that favor the reproductive success of some individuals over others.
• Results in adaptation, the accumulation of inheritable variations that are favored by the environment.
• In the face of environmental change, genetic variation increases the likelihood that some individuals in a population will have inheritable variations that help them cope with the new conditions.

There are two sources of genetic variation:

1. Sexual reproduction: independent assortment in meiosis I, crossing over in prophase of meiosis I, and random fusion of gametes during fertilization.
2. Mutation, which is rare structural change in a gene.

Course Pages maintained by
Dr. Graeme Lindbeck .