Meiosis Chapter 13: Meiosis and Sexual Life Cycles

Variation  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  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

Comparison of Reproductions  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  A life cycle is the generation-to- generation sequence of stages in the reproductive history of an organism  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  A life cycle is the generation-to- generation sequence of stages in the reproductive history of an organism

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)  Each gene has a specific location called a locus on a certain chromosome  Most DNA is packaged into chromosomes  One set of chromosomes is inherited from each parent  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)  Each gene has a specific location called a locus on a certain chromosome  Most DNA is packaged into chromosomes  One set of chromosomes is inherited from each parent

Chromosomes  Every organism has its own chromosome number  Somatic (body) cell chromosomes come in pairs  Called diploid (2n) number of chromosomes  The two chromosomes in each pair are called homologous chromosomes, or homologs  They are the same length and carry genes controlling the same inherited characters 1 from mom 1 from dad  In humans, somatic cells have 46 chromosomes  Every organism has its own chromosome number  Somatic (body) cell chromosomes come in pairs  Called diploid (2n) number of chromosomes  The two chromosomes in each pair are called homologous chromosomes, or homologs  They are the same length and carry genes controlling the same inherited characters 1 from mom 1 from dad  In humans, somatic cells have 46 chromosomes

Chromosomes  Gametes have only 1 of each chromosome  Called haploid (n) number of chromosomes  In humans, sex cells are haploid: n=23  A karyotype is an ordered display of the pairs of chromosomes from a cell  Gametes have only 1 of each chromosome  Called haploid (n) number of chromosomes  In humans, sex cells are haploid: n=23  A karyotype is an ordered display of the pairs of chromosomes from a cell

Homologous Chromosomes Exception  Sex chromosomes are called X and Y  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 (found in homologous pairs)  Sex chromosomes are called X and Y  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 (found in homologous pairs)

Fertilization  Each set of 23 chromosomes 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  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  Each set of 23 chromosomes 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  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

Meiosis  Meiosis is a type of cell division used to make gametes (sex cells)  2 nuclear divisions Meiosis I Meiosis II  Begins with 1 diploid (2n) cell  Ends with 4 haploid (n) cells  Meiosis is a type of cell division used to make gametes (sex cells)  2 nuclear divisions Meiosis I Meiosis II  Begins with 1 diploid (2n) cell  Ends with 4 haploid (n) cells

Meiosis  Gametes are the only types of human cells produced by meiosis  Meiosis results in one set of chromosomes in each gamete (23)  Gametes are the only haploid cells in animals  Gametes fuse ( ) to form a diploid zygote (46) that divides by mitosis to develop into a multicellular organism  Gametes are the only types of human cells produced by meiosis  Meiosis results in one set of chromosomes in each gamete (23)  Gametes are the only haploid cells in animals  Gametes fuse ( ) to form a diploid zygote (46) that divides by mitosis to develop into a multicellular organism

Cell Cycle Review  G1, S, G2, M  Mitosis has 4 main phases  Prophase  Metaphase  Anaphase  Telophase  Cells split by cytokinesis  Produce 2 identical cells  Growth and repair  G1, S, G2, M  Mitosis has 4 main phases  Prophase  Metaphase  Anaphase  Telophase  Cells split by cytokinesis  Produce 2 identical cells  Growth and repair

Meiosis I  Division in meiosis I occurs in four phases:  Prophase I  Metaphase I  Anaphase I  Telophase I and cytokinesis  Meiosis I results in two haploid daughter cells with replicated chromosomes  Focus is on splitting homologous chromosomes  Division in meiosis I occurs in four phases:  Prophase I  Metaphase I  Anaphase I  Telophase I and cytokinesis  Meiosis I results in two haploid daughter cells with replicated chromosomes  Focus is on splitting homologous chromosomes

 Meiosis I is preceded by interphase, in which chromosomes are replicated  Each replicated chromosome consists of two identical sister chromatids  Sister chromatids held together by centromere  The sister chromatids are genetically identical  Meiosis I is preceded by interphase, in which chromosomes are replicated  Each replicated chromosome consists of two identical sister chromatids  Sister chromatids held together by centromere  The sister chromatids are genetically identical Interphase

 Chromosomes begin to condense  Nuclear membrane & nucleoli dissolve  Homologous chromosomes pair up (align gene by gene); this is called synapsis  Each pair of chromosomes forms a tetrad, a group of four chromatids  Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurs  In crossing over, nonsister chromatids exchange DNA segments  Chromosomes begin to condense  Nuclear membrane & nucleoli dissolve  Homologous chromosomes pair up (align gene by gene); this is called synapsis  Each pair of chromosomes forms a tetrad, a group of four chromatids  Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurs  In crossing over, nonsister chromatids exchange DNA segments Prophase I

Prophase I: Tetrads Homologous chromosomes (each with sister chromatids) A Tetrad

 In metaphase I, tetrads independently line up across from each other “ sandwiching ” the equator (metaphase plate)  In mitosis the homologs made one single line on the equator  Microtubules from the poles are attached to the kinetochores of each chromosome of each tetrad  In metaphase I, tetrads independently line up across from each other “ sandwiching ” the equator (metaphase plate)  In mitosis the homologs made one single line on the equator  Microtubules from the poles are attached to the kinetochores of each chromosome of each tetrad Metaphase I

 Pairs of homologous chromosomes separate  One chromosome moves toward each pole, guided by the spindle fibers  Sister chromatids remain attached at the centromere and move as one unit toward the pole  Pairs of homologous chromosomes separate  One chromosome moves toward each pole, guided by the spindle fibers  Sister chromatids remain attached at the centromere and move as one unit toward the pole Anaphase I

 Reverse of prophase I  Spindle fibers breaks down  Chromosomes uncoil  Nuclear envelope reforms  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  Reverse of prophase I  Spindle fibers breaks down  Chromosomes uncoil  Nuclear envelope reforms  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 Telophase I

 Cytokinesis forms two haploid daughter cells  In animal cells, a cleavage furrow forms  In plant cells, a cell plate forms  Each new cell has ½ the genetic information as the original  1 chromosome from each pair  Need a second division for sister chromatids to split  Each chromosome from the pair is still doubled  No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated  Cytokinesis forms two haploid daughter cells  In animal cells, a cleavage furrow forms  In plant cells, a cell plate forms  Each new cell has ½ the genetic information as the original  1 chromosome from each pair  Need a second division for sister chromatids to split  Each chromosome from the pair is still doubled  No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated Cytokinesis & Interkinesis

Meiosis I

Meiosis II  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  Focus is on splitting sister chromatids  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  Focus is on splitting sister chromatids

Prophase II  Spindle apparatus forms; nuclear membrane and nuclelous dissolve  Chromosomes condense (each still composed of two chromatids) and move toward the metaphase plate Metaphase II  Sister chromatids are arranged at the metaphase plate  The two sister chromatids of each chromosome are no longer genetically identical  The kinetochores of sister chromatids attach to microtubules extending from opposite poles Prophase II  Spindle apparatus forms; nuclear membrane and nuclelous dissolve  Chromosomes condense (each still composed of two chromatids) and move toward the metaphase plate Metaphase II  Sister chromatids are arranged at the metaphase plate  The two sister chromatids of each chromosome are no longer genetically identical  The kinetochores of sister chromatids attach to microtubules extending from opposite poles Prophase II & Metaphase II

Anaphase II  Sister chromatids separate and move to opposite poles  Each chromatid is now its own chromosome Telophase II  Nuclei reform  Spindles break down  Chromosomes uncoil  Cytokinesis again separates the cytoplasm Anaphase II  Sister chromatids separate and move to opposite poles  Each chromatid is now its own chromosome Telophase II  Nuclei reform  Spindles break down  Chromosomes uncoil  Cytokinesis again separates the cytoplasm Anaphase II & Telophase II

 At the end of meiosis, there are four haploid daughter cells  Each daughter cell is genetically distinct from the others and from the parent cell  Each has 1 chromosome from each homologous pair  Each will mature into eggs or sperm (gametogenesis)  At the end of meiosis, there are four haploid daughter cells  Each daughter cell is genetically distinct from the others and from the parent cell  Each has 1 chromosome from each homologous pair  Each will mature into eggs or sperm (gametogenesis) Final Products

Meiosis II

 Three events are unique to meiosis, and all three occur in meiosis I:  Synapsis and crossing over in prophase I  In metaphase I, paired homologous chromosomes (tetrads) independently arrange on either side of the equator  In anaphase I, homologous chromosomes, instead of sister chromatids, separate  Three events are unique to meiosis, and all three occur in meiosis I:  Synapsis and crossing over in prophase I  In metaphase I, paired homologous chromosomes (tetrads) independently arrange on either side of the equator  In anaphase I, homologous chromosomes, instead of sister chromatids, separate Meiosis Distinctions

Meiosis & Variation  Three mechanisms contribute to genetic variation:  Independent assortment of chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis  Crossing over Nonsister chromatids of a tetrad exchange genetic information  Random fertilization Over 8 million different gametes possible (2^23) 70 trillion chromosome combinations possible for zygotes! (2^23)x(2^23)  Three mechanisms contribute to genetic variation:  Independent assortment of chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis  Crossing over Nonsister chromatids of a tetrad exchange genetic information  Random fertilization Over 8 million different gametes possible (2^23) 70 trillion chromosome combinations possible for zygotes! (2^23)x(2^23)

Mitosis vs. Meiosis

 Mitosis  1 division  2 daughter cells  Exact copies of parent cells  Diploid to diploid  Purpose Growth Repair Asexual reproduction  Mitosis  1 division  2 daughter cells  Exact copies of parent cells  Diploid to diploid  Purpose Growth Repair Asexual reproduction  Meiosis  2 divisions 1st separates pairs  4 daughter cells  Each unique  Diploid to haploid  Purpose Make gametes/ sex cells Leads to genetic variation  Meiosis  2 divisions 1st separates pairs  4 daughter cells  Each unique  Diploid to haploid  Purpose Make gametes/ sex cells Leads to genetic variation

Review Questions 1.Define genetics and differentiate between heredity and variation. 2.Differentiate between asexual and sexual forms of reproduction in regards to the life cycles of various organisms. 3.Define the following vocabulary associated with meiosis: gene, gamete, locus, somatic cell, karyotype, homologous chromosomes, & zygote. 4. Differentiate between autosomes and sex chromosomes. 5.Describe the process of fertilization. 6.Define meiosis and explain why there must be 2 divisions. 7.Define the 4 major phases of meiosis I, along with the important events that occur during those phases and how they are unique from those phases of mitosis. 8.Explain the relationship between synapsis, tetrads, chiasmata, and crossing over. 9.Define the 4 major phases of meiosis II, along with the important events that occur during those phases and how they are unique from those phases of mitosis. 10.Describe the 3 events that are unique to meiosis. 11.Name and describe 3 mechanisms that contribute to genetic variation. 12.Describe 5 major differences between mitosis and meiosis as processes.