CHAPTER 13

CHAPTER 13

MEIOSIS AND SEXUAL LIFE CYCLES

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I. OVERVIEW•Living organisms are distinguished by their ability to reproduce their own kind•Genetics is the scientific study of heredity and variation•Heredity is the transmission of traits from one generation to the next via genes (DNA)•Variation is demonstrated by the differences in appearance that offspring show from parents and siblings

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I. OVERVIEW•Meiosis, sexual reproduction, and heredity are all aspects of the same process.•Meiosis is a modified type of cell division in sexually reproducing organism consisting of two rounds of cell division but only one round of DNA replication. It results in cells with half the number of chromosome sets as the original cell.

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II. Concept 13.1: Genes, DNA, and Chromosomes•Your genome is made up of the genes that you inherited from

your mother and fatherA. Inheritance of Genes

1.Genes are the units of heredity, and are made up of segments of DNA

• Gene- a unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA in some viruses).– A gene’s specific location along the chromosome is

called the gene’s locus.

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II. Concept 13.1: Genes, DNA, and Chromosomes•Your genome is made up of the genes that you inherited from

your mother and fatherA. Inheritance of Genes

1.Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs)

2.Each gene has a specific location called a locus on a certain chromosome

3.Most DNA is packaged into chromosomes4.One set of chromosomes is inherited from each parent

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6. Inheritance is possible because of DNA replication and sperm and egg which carry each parent’s genes which are combined at fertilization

B. Comparison of Asexual and Sexual Reproduction1. In asexual reproduction, one parent produces genetically

identical offspring by mitosis•Any genetic difference would be due to mutation•Does not involve the formation of gametes (sex cells)

2. A clone is a group of genetically identical individuals from the same parent

3. In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents•Involves the formation of gametes

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6. Inheritance is possible because of DNA replication and sperm and egg which carry each parent’s genes which are combined at fertilization

B. Comparison of Asexual and Sexual Reproduction1. In asexual reproduction, one parent produces genetically

identical offspring by mitosisAsexual Reproduction- a single individual is the sole parent and passes copies of all its genes to its offspring. As a result, the offspring are an exact copy of themselves (a clone).

•Any genetic difference would be due to mutation•Does not involve the formation of gametes (sex cells)

2. A clone is a group of genetically identical individuals from the same parent

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6. Inheritance is possible because of DNA replication and sperm and egg which carry each parent’s genes which are combined at fertilization

B. Comparison of Asexual and Sexual Reproduction3. In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents. Offspring of sexual reproduction vary genetically to their siblings and both parents.

•Involves the formation of gametes

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Asexual Reproduction

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III. Concept 13.2: Sexual Life Cycles•A life cycle is the sequence of stages in the reproductive history of an organism from conception to the production of its own offspring

A. Sets of Chromosomes in Human Cells1. Human somatic cells (any cell other than a gamete) have

23 pairs of chromosomes2. A karyotype is an ordered display of the pairs of

chromosomes from a cell (usually a wbc) in metaphase• Karyotype- a display of the chromosome pairs of a cell

arranged by size and shape.– 22 pairs of autosomes– 1 pair of sex chromosomes10

Karyotype

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Karyotype

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Female Karyotype

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Male Karyotype

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III. Concept 13.2: Sexual Life Cycles•A life cycle is the sequence of stages in the reproductive history of an organism from conception to the production of its own offspring

A. Sets of Chromosomes in Human Cells3. The two chromosomes in each pair are called homologous chromosomes, or homolog

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4. Homologous chromosomes have the same length, centromere position, staining pattern and carry genes controlling the same inherited characters

5. The sex chromosomes are X and Y and are also known as allosomes

6. Human females have a homologous pair of X chromosomes (XX)

7. Human males have one X and one Y chromosome8. The 22 pairs of chromosomes that do not determine sex

are called autosomes9. Each pair of homologous chromosomes includes one

chromosome from each parent

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10. The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father

11. A diploid cell (2n) has two sets of chromosomes12. For humans, the diploid number is 46 (2n = 46)13. In a cell in which DNA synthesis has occurred, each

chromosome is replicated14. Each replicated chromosome consists of two identical

sister chromatids

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15. A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n)

16. For humans, the haploid number is 23 (n = 23)17. Each set of 23 consists of 22 autosomes and a single sex

chromosome18. In an unfertilized egg (ovum), the sex chromosome is X19. In a sperm cell, the sex chromosome may be either X or Y

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B. Behavior of Chromosome Sets in the Human Life Cycle

1. Fertilization is the union of gametes (the sperm and the egg)2. The fertilized egg is called a zygote and has one set of

chromosomes from each parent 3. The zygote produces somatic cells by mitosis and develops

into an adult4. At sexual maturity, the ovaries and testes produce haploid

gametes5. Gametes are the only types of human cells produced by

meiosis, rather than mitosis

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6. Meiosis results in one set of chromosomes in each gamete

7. Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number

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Human Life Cycle

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C. The Variety of Sexual Life Cycles The alternation of meiosis and fertilization is common to

all organisms that reproduce sexually The three main types of sexual life cycles differ in the

timing of meiosis and fertilization: animals, plants and algae, and fungi and protists

Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis

However, only diploid cells can undergo meiosis In all three life cycles, the halving and doubling of

chromosomes contributes to genetic variation in offspring

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1. Animals• In animals, meiosis produces gametes, which undergo

no further cell division before fertilization• Gametes are the only haploid cells in animals• Gametes fuse to form a diploid zygote that divides by

mitosis to develop into a multicellular organism

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Life Cycle of Animals

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2. Plants and some Algae• Plants and some algae exhibit an alternation of

generations• This life cycle includes both a diploid and haploid

multicellular stage• The diploid organism, called the sporophyte, makes

haploid spores by meiosis• Each spore grows by mitosis into a haploid organism

called a gametophyte• A gametophyte makes haploid gametes by mitosis• Fertilization of gametes results in a diploid sporophyte

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Life Cycle of Plants and some Algae

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3. Most Fungi and Some Protists• The only diploid stage is the single-celled zygote; there

is no multicellular diploid stage• The zygote produces haploid cells by meiosis•Each haploid cell grows by mitosis into a haploid

multicellular organism• The haploid adult produces gametes by mitosis

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Life Cycle of Most Fungi and Some Protists

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IV. Concept 13.3: Meiosis—Reduction Division

A. Overview1. Like mitosis, meiosis is preceded by the replication of

chromosomes2. Meiosis takes place in two sets of cell divisions, called

meiosis I and meiosis II3. The two cell divisions result in four daughter cells, rather

than the two daughter cells in mitosis4. Each daughter cell has only half as many chromosomes

as the parent cell•See page 256 (Be familiar with the ways that mitosis and meiosis are alike and different)

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B. Stages of Meiosis

1. In the first cell division (meiosis I), homologous chromosomes separate

2. Meiosis I results in two haploid daughter cells with replicated chromosomes; it is called the reduction division

3. In the second cell division (meiosis II), sister chromatids separate

4. Meiosis II results in four haploid daughter cells with unreplicated chromosomes

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5. Meiosis I is preceded by interphase, in which chromosomes are replicated to form sister chromatids

6. The sister chromatids are genetically identical and joined at the centromere

7. The single centrosome replicates, forming two centrosomes

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8. Phases of Meiosis I and Meiosis IIMeiosis I

Prophase IMetaphase IAnaphase ITelophase I

Meiosis IIProphase IIMetaphase IIAnaphase IITelophase II

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C. Interphase

•Precedes meiosis•Chromosomes replicate

as in mitosis• In animal cells, centriole

pairs duplicate

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D. Meiosis IReduces chromosome number by one-halfFour phases:

1. Prophase I -Chromosomes become visible as long, thin,

single threads-Chromosomes begin to contract which

continues throughout Prophase I

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--homologous chromosomes undergo synapsis (pairing gene by gene)--involves formation of synaptonemal complex (protein structure that aids in the chromosomal pairing)

--doubled chromosomes appear as tetrads (refers to doubled chromosomes in Prophase I)

--crossing over occurs

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--pairing between homologs becomes less tight and they appear to repel each other

--chiasmata (visible points where crossovers occurred earlier) appear

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PROPHASE I

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MEIOSIS

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Prophase I (continued) Centriole pairs move apart

and spindle microtubules form between them

Nuclear envelope and nucleoli disappear

Chromosomes begin moving to the metaphase plate

Occupies more than 90% of time required for meiosis

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2. Metaphase ITetrads are aligned on

the metaphase plate

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3. Anaphase IHomologous chromosomes

separateChromosomes move

toward the poles by the spindle apparatus

Chromosomes now referred to as dyads (half of a tetrad, refers to the chromosomes in Anaphase I)

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4. Telophase I and CytokinesisSpindle apparatus continues to separate homologous

chromosome pairs (dyads) until the chromosomes reach the poles.

Cytokinesis occurs as in mitosis. In some species, the nuclear envelope and nucleoli

reappear, and the daughter cells enter a period of interkinesis before Meiosis II. In other species, the daughter cells immediately prepare for Meiosis II.

NO DNA replication occurs before Meiosis II

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TELOPHASE I and CYTOKINESIS

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E. Meiosis II1. Prophase II If cells entered

interkinesis, nuclear envelope and nucleoli disappear.

Spindle apparatus formsChromosomes (dyads)

begin to move towards the Metaphase II plate

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2. Metaphase IIChromosomes (dyads)

align on the metaphase plate

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3. Anaphase IICentromeres of sister

chromatids separateMonads (half of a dyad,

refers to chromosomes in Anaphase II) move toward opposite poles

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4. Telophase IINuclei form at

opposite poles of the cell

Cytokinesis occurs producing four haploid daughter cells

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Animal Meiosis

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F. Comparison of Meiosis I and MitosisStage 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 doubled chromosomes align on metaphase plate

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Stage Meiosis MitosisAnaphase •Pairs of chromosomes

separate•Centromeres do not divide and sister chromatids stay together•Sister chromatids of each chromosome move to the same pole of the cell

•Sister chromatids of individual doubled chromosomes separate•Centromers divide •Sister chromatids move to opposite poles of the cell

F. Meiosis II is virtually identical to mitosis.56

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V. Concept 13.4: Genetic Variation Contributes to Evolution

Mutations (changes in an organism’s DNA) are the original source of genetic diversity

Mutations create different versions of genes called alleles Reshuffling of alleles during sexual reproduction produces

genetic variation

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A. Origins of Genetic Variation Among Offspring

1. The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation

that arises in each generation2. Three mechanisms contribute to genetic variation:

– Independent assortment of chromosomes– Crossing over– Random fertilization

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3. Independent Assortment of Chromosomes•Homologous pairs of chromosomes orient randomly at Metaphase I of Meiosis•In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs•The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number•For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

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Independent Assortment

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4. Crossing Over•Crossing over produces recombinant chromosomes, which combine genes inherited from each parent•Crossing over begins in Prophase I, as homologous chromosomes pair up gene by gene (synapsis)•Synapsis involves the formation of synaptonemal complex (a protein structure that brings chromosomes into close association)•In crossing over, homologous portions of two nonsister chromatids trade places•Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

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Crossing Over

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5. Random Fertilization

•Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)

• Fertilization- the union of haploid gametes to produce a zygote.– Meiosis- gametes (sex cells) reproduce by meiosis

• The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations•Each zygote has a unique genetic identity

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B. Genetic Variation and Evolution•Natural selection (Darwin’s theory of evolution) results in the accumulation of genetic variations favored by the environment•Sexual reproduction contributes to the genetic variation in a population, which originates from mutations

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You should now be able to:1. Distinguish between the following terms: somatic cell and

gamete; autosome and sex chromosomes; haploid and diploid

2. Describe the events that characterize each phase of meiosis

3. Describe three events that occur during meiosis I but not mitosis

4. Name and explain the three events that contribute to genetic variation in sexually reproducing organisms

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