Meiosis and Sexual Life Cycles Chapter 13 Meiosis and Sexual Life Cycles

Living organisms Are distinguished by their ability to reproduce their own kind

Heredity Is the transmission of traits from one generation to the next Variation Shows that offspring differ somewhat in appearance from parents and siblings Figure 13.1

Offspring acquire genes from parents by Genetics Is the scientific study of heredity and hereditary variation Offspring acquire genes from parents by inheriting chromosomes

Inheritance of Genes Genes Are the units of heredity Are segments of DNA Their locations along the length of the DNA are called: Loci “locus for singular”

Each gene in an organism’s DNA Has a specific locus on a certain chromosome We inherit One set of chromosomes from our mother, and One set from our father

Comparison of Asexual and Sexual Reproduction In asexual reproduction One parent produces genetically identical offspring by mitosis Figure 13.2 Parent Bud 0.5 mm

In sexual reproduction Two parents give rise to offspring Offspring have: Unique combinations of genes These genes are inherited from the two parents

Fertilization (diploid cells) and meiosis (haploid cells) alternate in sexual life cycles A life cycle Is the generation-to-generation sequence of stages in the reproductive history of an organism

Sets of Chromosomes in Human Cells In humans Each somatic (body) cell has 46 chromosomes, made up of two sets One set of chromosomes comes from each parent Chromosomes differ in: Size Positions of their centromeres Staining patterns (colored bands)

Distinguishing Chromosomes Using Staining A karyotype Is an ordered, visual representation of the chromosomes in a cell using staining. 5 µm Pair of homologous chromosomes Centromere Sister chromatids Figure 13.3

Homologous chromosomes Are the two chromosomes composing a pair Have the same characteristics (size, centromere position, staining pattern) Each member is contributed by one parent May also be called autosomes People normally have 22 pairs of autosomes

Sex chromosomes Are distinct from autosomes and from each other in their characteristics Are represented as X and Y Determine the sex of the individual, XX being female & XY being male Homologous in females (XX) Only partially homologous in males (XY)

A diploid cell Has two sets of each of its chromosomes a human has 46 chromosomes (2n = 46) All somatic cells are diploid cells

In a cell in which DNA synthesis has occurred: All chromosomes are duplicated Each chromosome consists of two identical sister chromatids Figure 13.4 Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

Unlike somatic (body) cells, reproductive cells or gametes (sperm and egg) are: Haploid cells containing only one set (n) of chromosomes Produced only in the gonads (ovaries & testes) At sexual maturity, The ovaries and testes produce haploid gametes by meiosis

During fertilization The zygote These gametes, sperm and ovum, fuse Fusion results in forming a diploid zygote The zygote Develops into an adult organism

The human life cycle Key Figure 13.5 Haploid gametes (n = 23) Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

The Variety of Sexual Life Cycles There are three main types of sexual life cycles They are all common in the alternation of meiosis and fertilization But differ in the timing of meiosis, and fertilization

Examples of types of Life Cycles: In animals (including humans) Meiosis occurs during gamete formation Gametes are the only haploid cells Gametes Figure 13.6 A Diploid multicellular organism Key MEIOSIS FERTILIZATION n 2n Zygote Haploid Mitosis (a) Animals

(b) Plants and some algae Exhibit a 2nd type of life cycle called alternation of generations This life cycle includes both diploid and haploid multicellular stages MEIOSIS FERTILIZATION n 2n Haploid multicellular organism (gametophyte) Mitosis Spores Gametes Zygote Diploid multicellular organism (sporophyte) (b) Plants and some algae Figure 13.6 B

In most fungi and some protists (3rd life cycle) Meiosis produces haploid cells that give rise to a haploid multicellular adult organism The haploid adult carries out mitosis, producing cells that will become gametes No multicellular diploid organism produced

(c) Most fungi and some protists MEIOSIS FERTILIZATION n 2n Haploid multicellular organism Mitosis Gametes Zygote (c) Most fungi and some protists Figure 13.6 C

Meiosis reduces the number of chromosome from diploid (2n) to haploid (n) Meiosis takes place in two sets of divisions meiosis I and meiosis II Meiosis I is also known as reductional division Like mitosis, meiosis is Preceded by chromosome replication

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The Stages of Meiosis Chromosome replication is followed by two consecutive cell divisions (meiosis I & meiosis II) These divisions result in 4 daughter cells (2 in mitosis) Each cell with half the parent-al number of chromosomes Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous separate Haploid cells with replicated chromosomes Sister chromatids Haploid cells with unreplicated chromosomes Meiosis I Meiosis II

Meiosis I Reduces the number of chromosomes from diploid to haploid, therefore: known as reductional division Meiosis II Produces four haploid daughter cells

Interphase and meiosis I Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Figure 13.8

MEIOSIS II: Separates sister chromatids Telophase I, cytokinesis, and meiosis II TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still double Figure 13.8

A Comparison of Mitosis and Meiosis Meiosis and mitosis can be distinguished from each other by: Three events that are unique to meiosis All of these events occur during meiosis l

Events Unique to Meiosis 1. Synapsis and crossing over During prophase I of meiosis, homologous chromosomes physically connect and exchange genetic information 2. Tetrads on the metaphase plate At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates

3. Separation of homologues At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell In anaphase II of meiosis, the sister chromatids separate

(before chromosome replication) A comparison of mitosis and meiosis Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n Daughter cells of mitosis 2n = 6

Origins of Genetic Variation Among Offspring In species that reproduce sexually The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation

Three mechanisms that contribute to genetic variation arising from sexual reproduction: Independent Assortment of chromosomes Crossing Over Random Fertilization

Independent Assortment of Chromosomes Homologous pairs of chromosomes Orient randomly at metaphase I of meiosis

In independent assortment Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs Key Maternal set of chromosomes Paternal set of Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Figure 13.10

Crossing Over Crossing over Produces recombinant chromosomes that carry genes derived from two different parents Figure 13.11 Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

Random Fertilization The fusion of gametes Will produce a zygote with any of about 64 trillion diploid combinations

Evolutionary Significance of Genetic Variation Within Populations Is the raw material for evolution by natural selection

Mutations Sexual reproduction Are the original source of genetic variation Sexual reproduction Produces new combinations of variant genes, adding more genetic diversity