Meiosis CHAPTER 10

What is Meiosis? The mechanism that halves the number of chromosomes in cells is a form of cell division called meiosis Meiosis consist of 2 successful nuclear divisions 1. Meiosis I 2. Meiosis II Each round contains a prophase, metaphase, anaphase and telophase

Heredity is the transmission of traits from one generation to the next 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 is demonstrated by the differences in appearance that offspring show from parents and siblings Genetics is the scientific study of heredity and variation 3

What accounts for family resemblance? Figure 10.1 What accounts for family resemblance? 4

Inheritance of Genes Genes are the units of heredity and are made up of segments of DNA Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs) 5

Most DNA is packaged into chromosomes For example, humans have 46 chromosomes in their somatic cells, the cells of the body except for gametes and their precursors Each gene has a specific position, or locus, on a certain chromosome 6

The remaining 22 pairs of chromosomes are called autosomes The sex chromosomes, which determine the sex of the individual, are called X and Y Human females have a homologous pair of X chromosomes XX Human males have one X and one Y chromosome XY The remaining 22 pairs of chromosomes are called autosomes 7

A diploid cell (2n) has two sets of 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) 8

Each replicated chromosome consists of two identical sister chromatids In a cell in which DNA synthesis has occurred, each chromosome is replicated (copied) Each replicated chromosome consists of two identical sister chromatids 9

Each set of 23 consists of 22 autosomes and a single sex chromosome 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 10

Behavior of Chromosome Sets in the Human Life Cycle 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 11

Sexual Maturity in Organisms At sexual maturity, the ovaries and testes produce haploid gametes Gametes are the only types of human cells produced by meiosis rather than mitosis Meiosis results in one set of chromosomes in each gamete Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number 12

Multicellular diploid adults (2n  46) Key Haploid gametes (n  23) Haploid (n) Egg (n) Diploid (2n) Sperm (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n  46) Mitosis and development Multicellular diploid adults (2n  46) 13

Meiosis reduces the number of chromosome sets from diploid to haploid

Like mitosis, meiosis is preceded by the replication of chromosomes 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 15

Phases of Meiosis Meiosis takes place in two sets of cell divisions 1. Meiosis I 2. Meiosis II

duplicated chromosomes Meiosis II Figure 10.7 Interphase Pair of homologous chromosomes in diploid parent cell Chromosomes duplicate Duplicated pair of homologous chromosomes Sister chromatids Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes 17

Meiosis I Meiosis occurs in diploid reproductive cells Before meiosis begins, the DNA of the diploid reproductive cells are copied Meiosis I results in 2 haploid cells

MEIOSIS VIDEO SEGMENT

Prophase I DNA coils tightly into chromosomes (as in mitosis), spindle fibers appear Then the nucleus & nucleolus disassemble Every chromosome lines up next to its homologous pair The pairing of homologous chromosomes (which does not happen in mitosis) is called synapsis. In synapsis, homologous chromosomes pair up, aligned gene by gene

Each pair of homologous chromosomes is called a tetrad In each tetrad, chromatids of the homologous chromosomes are aligned lengthwise so that the genes on one chromosome are adjacent of the gene on the other chromosomes

Crossing Over During synapsis, the chromatids within a homologous pair twist around one another Portions of the chromatids may break off and attach to adjacent chromatids in a process called crossing over This permits the exchange of genetic material between maternal and paternal chromosomes Crossing over results in genetic recombination by producing a mixture of genetic material

Crossing over Crossing over occurs when chromosomes that make up a tetrad exchange portions of their chromatids. Crossing over results in an exchange of genes and in new combinations of genes

Prophase I video segment

Metaphase I The tetrads line up randomly along the midline of the dividing cell Spindle fibers from one pole attach to the Centromere of one homologous chromosome Spindle fibers from the opposite pole attach to the other homologous chromosome of the pair

Metaphase I video segment

Anaphase I Each homologous chromosome (consisting of 2 chromatids attached by a Centromere) moves to an opposite pole of the dividing cell The random separation is called independent assortment (results in a random separation of the maternal and paternal chromosome resulting in genetic variation)

Anaphase I video segment

Telophase I & Cytokinesis I The final phase of meiosis I The chromosomes reach the opposite ends of the cell and cytokinesis begins The new cells contain a haploid number of chromosomes

Telophase I video segment

Points to remember about meiosis I The original cell produces 2 new cells Each new cell contains 1 chromosome from each homologous pair New cells contain half the number of chromosomes of the original cell Each new cell contains 2 copies (as chromatids) because the original cell copied its DNA before meiosis I

MEIOSIS I: Separates homologous chromosomes Figure 10.8a MEIOSIS I: Separates homologous chromosomes Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Sister chromatids Centromere (with kinetochore) Sister chromatids remain attached Centrosome (with centriole pair) Cleavage furrow Chiasmata Metaphase plate Spindle Homologous chromosomes separate Fragments of nuclear envelope Microtubule attached to kinetochore Homologous chromosomes 32

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 34

Meiosis II Occurs in each cell formed during meiosis I and is not preceded by the copying of DNA In some species, Meiosis II begins after the nuclear membrane re-forms in the new cells In other species, Meiosis II begins immediately following meiosis I

Prophase II Nuclear membrane & nucleolus disappear Spindle fibers form and begin to move the chromosomes toward the midline of the dividing cell Chromosomes are visible

Prophase II video segment

Metaphase II The chromosomes move to the midline of the dividing cell, facing opposite poles of the dividing cells

Metaphase II video segment

Anaphase II The sister chromatids separate and move toward opposite poles of the cell

Anaphase II video segment

Telophase II & Cytokinesis II A nuclear membrane forms around the chromosomes in each of the 4 new cells Cytokinesis II happens resulting in 4 new cells (each containing half of the original cells number of chromosomes) Spindle disappears

Telophase II video segment

MEIOSIS II: Separates sister chromatids Figure 10.8b MEIOSIS II: Separates sister chromatids Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Sister chromatids separate Haploid daughter cells forming 44

At the end of meiosis, there are four daughter cells, each with a haploid set of unduplicated chromosomes Each daughter cell is genetically distinct from the others and from the parent cell 46

Mitosis Meiosis Reduces the number of chromosome sets from 2 (diploid) to 1 (haploid) Produce cells that differ from each other and the parent cell Includes 2 divisions after replication, each with specific stages Conserves the number of chromosomes sets 2 (diploid) Produce cells that are identical to the parent cell

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 Homologous pairs at the metaphase plate: Homologous pairs of chromosomes are positioned there in metaphase I Separation of homologs during anaphase I 48

Figure 10.9 A comparison of mitosis and meiosis in diploid cells Parent cell Chiasma MEIOSIS I Prophase Prophase I Chromosome duplication Chromosome duplication Homologous chromosome pair Duplicated chromosome 2n = 6 Individual chromosomes line up. Pairs of chromosomes line up. Metaphase Metaphase I Anaphase Sister chromatids separate. Homologs separate. Anaphase I Telophase Telophase I Sister chromatids separate. Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Figure 10.9 A comparison of mitosis and meiosis in diploid cells Daughter cells of meiosis II SUMMARY Property Mitosis Meiosis DNA replication Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins Number of divisions One, including prophase, prometaphase, metaphase, anaphase, and telophase Two, each including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Produces gametes; reduces number of chromosome sets by half and introduces genetic variability among the gametes 49

SUMMARY Property Mitosis Meiosis DNA replication Figure 10.9b SUMMARY Property Mitosis Meiosis DNA replication Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins Number of divisions One, including prophase, prometaphase, metaphase, anaphase, and telophase Two, each including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Produces gametes; reduces number of chromosome sets by half and introduces genetic variability among the gametes 50

Formation of Gametes In animals, meiosis produces haploid reproductive cells called gametes Because only those cells involved in the production of gametes divided by meiosis in animals, meiosis occurs only within their reproductive organs In humans, meiosis occurs in the testes and in the ovaries

Testes In testes, meiosis is involved in the production of male gametes known as sperm cells or spermatozoa In the development of sperm cells, a diploid reproductive cell divides meiotically to form 4 haploid cells called spermatids Each spermatids then develops into a mature sperm cell called spermatogenesis

Gametogenesis is the production of gametes It differs between females and males Eggs contribute DNA, cytoplasm, and organelles to an embryo. During meiosis, the egg gets most of the contents; the other cells from polar bodies. Sperm become streamlined and motile. Sperm primarily contribute DNA to an embryo.

Egg Cells Oogenesis is the production of mature egg cells or ova A diploid reproductive cell divides meiotically to produce 1 mature egg cell (ovum) During cytokinesis II of oogenesis, the cytoplasm of the original cell is divided unequally between new cells 1 cell which develops into a mature egg cell, receives most of the cytoplasm of the original cell As a result, 1 egg cell is produced by meiosis, the other 3 are called polar bodies & will eventually degenerate.