Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 13 Meiosis and Sexual Life Cycles

Overview: Variations on a Theme 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 Variation is demonstrated by the differences in appearance that offspring show from parents and siblings Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-1

Inheritance of Genes Genes are the units of heredity, and are made up of segments of DNA Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs) Gametes are produced from germ cells in gonads. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

A locus is the location of the gene on the chromosome. Alleles are different forms of the gene located at that locus.

Comparison of Asexual and Sexual Reproduction In asexual reproduction, one parent produces genetically identical offspring by mitosis A clone is a group of genetically identical individuals from the same parent In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Video: Hydra Budding Video: Hydra Budding Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-2a (a) Hydra 0.5 mm Bud Parent

Sets of Chromosomes in Human Cells Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes A karyotype is an ordered display of the pairs of chromosomes from a cell The two chromosomes in each pair are called homologous chromosomes, or homologs Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters

Fig. 13-3a APPLICATION

Fig. 13-3b TECHNIQUE Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome 5 µm

The sex chromosomes are called X and Y in most organisms. Human females have a homologous pair of X chromosomes (XX) Human males have one X and one Y chromosome The 22 pairs of chromosomes that do not determine sex are called autosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Sex Chromosomes

Each pair of homologous chromosomes includes one chromosome from each parent The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father A diploid cell (2n) has two sets of chromosomes For humans, the diploid number is 46 (2n = 46)

In a cell in which DNA synthesis has occurred, each chromosome is replicated Each replicated chromosome consists of two identical sister chromatids Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) 2n = 6 Centromere Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a single sex chromosome In an unfertilized egg (ovum), the sex chromosome is X In a sperm cell, the sex chromosome may be either X or Y

Gametes are produced from germ cells in gonads: ovaries in females, testes in males.

Fertilization is the union of gametes (the sperm and the egg) The fertilized egg is called a zygote and has one set of chromosomes from each parent The zygote produces somatic cells by mitosis and develops into an adult Behavior of Chromosome Sets in the Human Life Cycle Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

MEIOSIS A type of cell division usually used to make reproductive cells in which the chromosome number is HALVED! Gametes are the only types of human cells produced by meiosis, rather than mitosis Meiosis results in one set of chromosomes. Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Egg (n) Sperm (n) MEIOSISFERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

Why does meiosis have to occur? Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number.

Without meiosis….

The Variety of Sexual Life Cycles Meiosis does not occur at the same time in all life cycles. The three main types of sexual life cycles differ in the timing of meiosis and fertilization Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

In animals In most animals, meiosis produces gametes, which are the only haploid cells in animals. Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-6a Key Haploid (n) Diploid (2n) Gametes n n n 2n2n2n2n Zygote MEIOSISFERTILIZATION Mitosis Diploid multicellular organism (a) Animals

In plants and some algae Plants and some algae exhibit an alternation of generations This life cycle includes both a diploid and haploid stage The diploid organism, called the sporophyte, makes haploid spores by meiosis. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-6b Key Haploid (n) Diploid (2n) n n n n n 2n2n 2n2n Mitosis Zygote Spores Gametes MEIOSISFERTILIZATION Diploid multicellular organism (sporophyte) Haploid multi- cellular organism (gametophyte) (b) Plants and some algae

In a nutshell….. Sporophyte generation (2N) makes spores by meiosis Spores (1N) grow into the Gametophyte generation (1N) – produces gametes Gametes fuse to form the Sporophyte Generation and it starts all over again.

In fungi and some protists In 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-6c Key Haploid (n) Diploid (2n) Mitosis Gametes Zygote Haploid unicellular or multicellular organism MEIOSIS FERTILIZATION n n n n n 2n2n (c) Most fungi and some protists Their bodies are made of haploid cells.

Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis However, only diploid cells can undergo meiosis ! Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid Remember alleles are different forms of a gene and occupy the same gene locus.

Meiosis reduces the number of chromosome sets from diploid to haploid. Like mitosis, meiosis is preceded by the replication of chromosomes Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis Each daughter cell has only half as many chromosomes as the parent cell Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Stages of Meiosis Meiosis I (reductional division) results in two haploid daughter cells but the chromosomes still have two chromatids – DIVIDES THE CHROMOSOME NUMBER Meiosis II (equational division) - sister chromatids separate and results in four haploid daughter cells with unreplicated chromosomes – SEPARATES THE CHROMATIDS Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I Homologous chromosomes separate 1 Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes

Fig Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase II Metaphase IIAnaphase II Telophase II and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow Sister chromatids separate Haploid daughter cells forming

Division in meiosis I occurs in four phases: –Prophase I –Metaphase I –Anaphase I –Telophase I and cytokinesis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Metaphase I Fig. 13-8a Prophase IAnaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow

Prophase I Prophase I typically occupies more than 90% of the time required for meiosis During Prophase I, homologous chromosomes come together to form tetrads. This is called synapsis and the chromosomes are held together by the synaptonemal complex. Sometimes genetic material is swapped called crossing-over which are recognized by X-shaped regions called chiasmata.

Tetrads – FOUR chromatids in 2 homologous chromosomes

Crossing Over Crossing over produces recombinant chromosomes, which combine genes inherited from each parent Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings chiasmata

Fig. 13-8b Prophase IMetaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore

Metaphase I In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole. Note: This is different from metaphase of mitosis! Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 homologs into daughter cells independently of the other pairs Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1Combination 2Combination 3Combination 4

Anaphase I In anaphase I, pairs of homologous chromosomes separate

Telophase I and Cytokinesis In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids Cytokinesis usually occurs simultaneously, forming two haploid daughter cells Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-8c Anaphase I Telophase I and Cytokinesis Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow

Division in meiosis II also occurs in four phases: –Prophase II –Metaphase II –Anaphase II –Telophase II and cytokinesis Meiosis II is very similar to mitosis! Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-8d Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming

Prophase II Fig. 13-8e In metaphase II, the sister chromatids are arranged at the metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical Metaphase II

Fig. 13-8f Anaphase II Telephase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming nc.com/webcontent/ animations/content/ meiosis.html In anaphase II, the sister chromatids separate Still haploid, but only one chromatid per chromosome.

Don’t forget the big picture! Meiosis I – halves chromosome no Meiosis II – divides out the chromatids

II T1

Comparison of mitosis and meiosis MitosisMeiosis Role in animal body Number of DNA replications Number of divisions Number of daughter Cells Chromosome number of Daughter cells

Fig. 13-9b SUMMARY Meiosis Mitosis Property DNA replication Number of divisions Occurs during interphase before mitosis begins One, including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Occurs during interphase before meiosis I begins Two, each including prophase, metaphase, anaphase, and telophase Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

Mitosis vs meiosis Unlike mitosis, meiosis results in FOUR cells that DIFFER genetically from each other. This is due to the formation of recombinant chromosomes during crossing-over and the orientation of chromosomes at metaphase called INDEPENDENT ASSORTMENT. No tetrads are formed in mitosis!

Fig. 13-9a MITOSIS MEIOSIS MEIOSIS I Prophase I Chiasma Chromosome replication Homologous chromosome pair Chromosome replication 2n = 6 Parent cell Prophase Replicated chromosome Metaphase Metaphase I Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I MEIOSIS II Daughter cells of meiosis II n n n n 2n2n 2n2n Daughter cells of mitosis Anaphase Telophase Notice difference in this step TETRADS Chromatids are split in mitosis, homologs in meiosis I

What is unique to Meiosis? Three events are unique to meiosis, and all three occur in meiosis l: –Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information –At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes –At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution Mutations (changes in an organism’s DNA) are the original source of genetic diversity in both eukaryotes and prokaryotes, Mutations create different versions of genes called alleles In sexually reproducing organisms, the reshuffling of alleles produces genetic variation. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Origins of Genetic Variation Among Offspring The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation in sexually reproducing organisms Three mechanisms contribute to genetic variation: – Independent assortment of chromosomes – Crossing over – Random fertilization

genetic diversity due to: Independent assortment of chromosomes in Metaphase I random fertilization

In independent assortment of chromosomes: The number of combinations possible when chromosomes assort independently into gametes is 2 n, where n is the haploid number For humans (n = 23), there are more than 8 million (2 23 ) possible combinations of chromosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) 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 And that does not include crossing-over! Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

And that makes you pretty unique!

Gametogenesis – formation of gametes Spermatogenesis – formation of sperm in gonads (testes) from germ cells Oogenesis – formation of eggs in gonads (ovaries) from germ cells

Gametogenesis unequal division of cytoplasm Polar bodies

Spermatogenesis All four sperm produced by meiosis are viable and same size. Sperm production starts in puberty and ends ?.

Egg Production - Oogenesis The first meiotic division begins before birth and is not completed until ovulation once a month during puberty. The second meiotic division is not completed until fertilization. The division of the cytoplasm is not equal. Only ONE of the four cells produced will be an egg. The other cells are called polar bodies and they disintegrate. Females stop producing eggs at menopause.

Oogenesis once a month

Gametogenesis unequal division of cytoplasm Polar bodies

Unequal division of cytoplasm Only one egg is formed