Halving the Chromosome Number Meiosis Special type of

Halving the Chromosome Number • Meiosis § Special type of cell division § Used only for sexual reproduction § Halves the chromosome number prior to fertilization • Parents are diploid (2 n) • Meiosis produces haploid (n) gametes • Gametes fuse in fertilization to form a diploid (2 n) zygote • The zygote becomes the next diploid (2 n) generation 1

Halving the Chromosome Number • In diploid body cells, chromosomes occur in pairs • Humans have 23 different types of chromosomes • Diploid (2 n) cells have two chromosomes of each type • Chromosomes of the same type are said to be homologous chromosomes (homologues) § They have the same length § Their centromeres are positioned in the same place § One came from the father (the paternal homolog) the other from the mother (the maternal homolog) § When stained, they show similar banding patterns 2

Homologous Chromosomes Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Sister chromatids a. duplication nonsister chromatids duplication kinetochore centromere chromosome homologous pair paternal chromosome b. chromosome maternal chromosome a: © L. Willatt/Photo Researchers, Inc. 3

Halving the Chromosome Number • Homologous chromosomes have genes controlling the same trait at the same position § Each gene occurs in duplicate § A maternal copy from the mother § A paternal copy from the father • Many genes exist in several variant forms in a large population • Homologous copies of a gene may encode identical or different genetic information • The variants that exist for a gene are called alleles • An individual may have: § Identical alleles for a specific gene on both homologs (homozygous for the trait), or § A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait) 4

Halving the Chromosome Number • Overview of Meiosis § Meiosis I • Chromosomes are replicated prior to meiosis I – Each chromosome consists of two identical sister chromatids • Homologous chromosomes pair up – synapsis • Homologous pairs align themselves against each other side by side at the metaphase plate • The two members of a homologous pair separate • Each daughter cell receives one duplicated chromosome from each pair § Meiosis II • • • DNA is not replicated between meiosis I and meiosis II Sister chromatids separate and move to opposite poles The four daughter cells contain one daughter chromosome from each pair Each daughter chromosome consists of a single chromatid The daughter cells are haploid 5

Overview of Meiosis Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction

Overview of Meiosis Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. centrioles nucleolus centromere chromosome duplication 2 n = 4 MEIOSISI Homologous pairs synapse and then separate. 8

Overview of Meiosis Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. centrioles sister chromatids nucleolus centromere synapsis chromosome duplication 2 n = 4 MEIOSISI Homologous pairs synapse and then separate. 9

Overview of Meiosis Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. First division centrioles nucleolus centromere sister chromatids synapsis chromosome duplication 2 n = 4 MEIOSISI Homologous pairs synapse and then separate. n=2 10

Overview of Meiosis Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. First division centrioles nucleolus centromere Second division sister chromatids synapsis chromosome duplication 2 n = 4 MEIOSISI Homologous pairs synapse and then separate. n=2 11

Overview of Meiosis Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. First division centrioles nucleolus centromere Second division sister chromatids synapsis chromosome duplication 2 n = 4 MEIOSISI Homologous pairs synapse and then separate. n=2 MEIOSISII Sister chromatids separate, becoming daughter chromosomes. 12

Overview of Meiosis Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. First division centrioles nucleolus centromere Second division Four haploid daughter cells sister chromatids synapsis chromosome duplication 2 n = 4 MEIOSISI Homologous pairs synapse and then separate. n=2 MEIOSISII Sister chromatids separate, becoming daughter chromosomes. 13

Genetic Variation • Meiosis brings about genetic variation in two key ways: § Crossing-over between homologous chromosomes, and § Independent assortment of homologous chromosomes • Crossing Over: § Exchange of genetic material between non-sister chromatids during meiosis I § At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues • Holds homologues together • Aligns DNA of non-sister chromatids • Allows crossing-over to occur § Then homologues separate and are distributed to different daughter cells 14

Crossing Over Occurs During Meiosis I Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. nucleoprotein lattice sister chromatids of its homologue sister chromatids of a chromosome A A a a B B b b chiasmata of nonsister chromatids c 1 and 3 C C c D d d A a B b C c D D d 12 34 Bivalent forms a. b. 1 2 3 4 Crossing-over has occurred c. a: Courtesy Dr. D. Von Wettstein 1 2 3 4 Daughter chromosomes d. 15

Genetic Variation • Independent assortment § When homologous chromosome pairs align at the metaphase plate: • They separate in a random manner • The maternal or paternal homologue may be oriented toward either pole of mother cell § Causes random mixing of blocks of alleles into gametes 16

Independent Assortment Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Combination 1 Combination 2 Combination 7 Combination 5 Combination 3 Combination 4 Combination 6 Combination 8 17

Genetic Variation • Fertilization – union of male and female gametes § Chromosomes donated by the parents are combined § In humans, (223)2 = 70, 368, 744, 000 chromosomally different zygotes are possible • If crossing-over occurs only once § (423)2, or 4, 951, 760, 200, 000, 000 genetically different zygotes are possible • Crossing-over may occur several times in each chromosome 18

Genetic Variation • Significance of genetic variation: § Asexual reproduction produces genetically identical clones § Sexual reproduction causes genetic recombinations among members of a population § Asexual reproduction is advantageous when the environment is stable § However, if the environment changes, genetic variability introduced by sexual reproduction may be advantageous • Some offspring may have a better chance of survival 19

The Phases of Meiosis • Meiosis I: § Prophase I • • A spindle forms The nuclear envelope fragments The nucleolus disappears Each chromosome is duplicated (consists of two identical sister chromatids) • Homologous chromosomes pair up and physically align themselves against each other side by side (synapsis) • Synapsed homologs are referred to as a bivalent (two homologues) or a tetrad (four chromatids) § Metaphase I • Homologous pairs are arranged at the metaphase plate • Bivalents are aligned independently of one another 20

The Phases of Meiosis • Meiosis I § Anaphase I • Homologous chromosomes of each bivalent separate from one another • Homologues move towards opposite poles • Sister chromatids do not separate • Each is still a duplicated chromosome with two chromatids § Telophase I • Daughter cells have one duplicated chromosome (n) from each homologous pair 21

The Phases of Meiosis • Interkinesis § Two haploid (n) daughter cells, each with one duplicated chromosome of each type § Interkinesis is similar to mitotic interphase except • It is usually shorter • DNA replication does not occur 22

The Phases of Meiosis • Meiosis II (mitosis of two haploid cells) § Prophase II – Chromosomes condense § Metaphase II – Chromosomes align at metaphase plate § Anaphase II • Centromere dissolves • Sister chromatids separate and become daughter chromosomes § Telophase II and cytokinesis • Four haploid (n) cells all genetically unique 23

Meiosis I in Plant and Animal Cells

Meiosis II in Plant and Animal Cells

Meiosis Compared to Mitosis • Meiosis § Requires two nuclear divisions § Chromosomes synapse and cross over § Centromeres survive Anaphase I § Halves chromosome number § Produces four daughter nuclei § Produces daughter cells genetically different from parent and each other § Used only for sexual reproduction • Mitosis § Requires one nuclear division § Chromosomes do not synapse nor cross over § Centromeres dissolve in mitotic anaphase § Preserves chromosome number § Produces two daughter nuclei § Produces daughter cells genetically identical to parent and to each other § Used for asexual reproduction and growth 28

Meiosis Compared to Mitosis 29

Meiosis Compared to Mitosis 30

Meiosis Compared to Mitosis Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 2 n = 4 Prophase I Synapsis and crossing-over occur. Metaphase. I Homologous pairs align independently at the metaphase plate. Anaphase I Homologous chromosomes separate and move toward the poles. MEIOSISI 2 n = 4 Prophase MITOSIS Metaphase Chromosomes align at the metaphase plate. Anaphase Sister chromatids separate and become daughter chromosomes.

Meiosis Compared to Mitosis Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Telophase I Daughter cells are forming and will go on to divide again. n=2 Daughter cells Sister chromatids separate and become daughter chromosomes. n=2 MEIOSIS I cont'd Four haploid daughter cells. Their nuclei are genetically different from the parent cell. MEIOSIS II Daughter cells Telophase Daughter cells are forming. MITOSIS cont'd Two diploid daughter cells. Their nuclei are genetically identical to the parent cell. n=2

The Cycle of Life • A life cycle is all the reproductive events that occur from one generation to the next similar generation • In plants, haploid multicellular “individuals” alternate with diploid multicellular “individuals” (alternation of generations) • The haploid individual: § Is called the gametophyte § May be larger or smaller than the diploid individual • The diploid individual: § Is called the sporophyte § May be larger or smaller than the haploid individual • • Mosses are haploid for most of their life cycle In fungi and most algae, only the zygote is diploid Ferns & higher plants are diploid for most of their life cycles In plants, algae, and fungi gametes are produced by haploid individuals 33

The Cycle of Life • In animals: § § “Individuals” are diploid and produce haploid gametes The only haploid part of the life cycle is the gametes The products of meiosis are always gametes Meiosis occurs only during gametogenesis • Production of sperm – Spermatogenesis – All four cells become sperm • Production of eggs – Oogenesis – One of the four nuclei receives the majority of the cytoplasm » Becomes the egg or ovum » Others wither away as polar bodies 34

The Cycle of Life • Human Life Cycle: § Sperm and egg are produced by meiosis § A sperm and egg fuse at fertilization § Results in a zygote • The one-celled stage of an individual of the next generation • Undergoes mitosis § Results in a multicellular embryo that gradually takes on features determined when the zygote was formed § All growth occurs as mitotic division § As a result of mitosis, each somatic cell in the body • Has same number of chromosomes as zygote • Has the same genetic makeup, which was determined when the zygote was formed 35

Spermatogenesis and Oogenesis in Mammals Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. SPERMATOGENESIS primary spermatocyte OOGENESIS primary oocyte 2 n 2 n Meiosis I secondary spermatocytes Meiosis II first polarbody n secondary oocyte n n spermatids n Meiosis II second polarbody Meiosis II is completed after entry of sperm (fertilization) n Metamorphosis and maturation egg Fertilization sperm n Sperm nucleus n n fusion of sperm nucleus and agg nucleus zygote 2 n