Topic 10.1 Meiosis Essential idea: Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells.

10.1 Meiosis Nature of science: Making careful observations—careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies. (1.8) Understandings Chromosomes replicate in interphase before meiosis Crossing over is the exchange of DNA material between non-sister homologous chromatids Crossing over produces new combinations of alleles on the chromosomes of the haploid cells Chiasmata formation between non-sister chromatids can result in an exchange of alleles Homologous chromosomes separate in meiosis I Sister chromatids separate in meiosis II Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I

10.1 Meiosis Applications and skills Skill: Drawing diagrams to show chiasmata formed by crossing over

Review of Definitions Somatic cells: non sex cells Sex cells (gametes): egg (ova), sperm Autosome: chromosome that is not a sex chromosome (does not determine gender) Sex chromosomes: dissimilar chromosomes that determine an individual’s sex. In humans: X, Y Homologous chromosomes: pairs of chromosomes that have the same size, centromere position, staining pattern, location of genes

Review of Definitions Fertilization: the union of two gametes to form a zygote Zygote: a diploid cell that results from the union of 2 haploid gametes Meiosis: special type of cell division that produces haploid cells and compensates for the doubling of chromosome number that occurs at fertilization

Key differences between meiosis and mitosis Meiosis is a reduction division: gametes have ½ number of chromosomes as parent cell Meiosis creates genetic variation: 4 daughter cells genetically different from parent cell and from each other Meiosis is 2 successive nuclear divisions

Stages of Meiotic Cell Division Interphase I: precedes meiosis Chromosomes replicate Each duplicated chromosome consists of 2 identical sister chromatids attached at their centromere Centriole pairs in animal cells also replicate into two pairs

Meiosis I Prophase I Segregates the 2 chromosomes of each homologous pair and reduces the chromosome number by one-half Prophase I: 90% of the time required for meiosis. Longer and more complex than prophase of mitosis. Chromosomes condense- start to coil up and become shorter and thicker Synapsis occurs- homologous chromosomes come together as pairs. Uses the synaptenemal complex to accomplish this

Prophase I continued Each chromosome has 4 chromatids, so that each homologous pair in synapsis appears as a complex of 4 chromatids of a tetrad In each tetrad, sister chromatids of the same chromosome are attached at their centromeres Nonsister chromatids are linked by X-linked chiasmata, sites where homologous strand exchange or crossing over occurs Chromosomes thicken further and detach from the nuclear envelope. Centriole pairs move apart, toward poles, and spindle microtubules form between them Nuclear envelope and nucleoli disperse

Chromosome Tetrad

Metaphase I Metaphase I: tetrads (bivalents) are aligned on the metaphase plate, having moved there during this stage Chromosomes continue to shorten and thicken Spindle microtubules attach to the kinetochore region of the centromeres Bivalents line up on the equator so that centromeres of homologues point towards opposite poles Chiasmata slide towards the ends of the chromosomes causing the shapes of the bivalents to change At the end, chromosomes start to move

Metaphase 1: 2n = 4

Anaphase I Anaphase I: homologues separate and are moved towards the poles by the spindle apparatus. (Bivalents separate). This halves the chromosome number Each chromosome consists of 2 chromatids (sister chromatids remain attached at the centromeres) Because of crossing over, the 2 chromatids are not identical At the end, chromosomes reach the poles

Telophase I and cytokinesis Telophase I and cytokinesis: spindle apparatus continues to separate homologous chromosome pairs until the chromosomes reach the poles Each pole has a haploid set of chromosomes composed of 2 sister chromatids Nuclear membranes form around the groups of chromosomes at each pole Chromosomes uncoil partially Cytokinesis occurs simultaneously forming 2 daughter cells. Cleavage furrows form in animal cells and cell plates form in plant cells

Meiosis I: reduction division – separation of homologous chromosomes

Meiosis II: Separation of sister chromatids

Interphase II Interphase II: May not really exist per se Two cells either enter a brief period of interphase or immediately proceed to the second division of meiosis DNA is NOT replicated

Prophase II Prophase II: chromosomes become shorter and thicken again by coiling Centrioles move to poles (animal cells) Nuclear membrane breaks down Spindle apparatus forms and chromosomes move towards the metaphase II plate

Metaphase II Metaphase II: chromosomes align singly on the metaphase plate. Spindle microtubules attach to the centromeres Chromosomes line up on equator Centromeres divide Kinetochores of sister chromatids point towards opposite poles

Meiosis II continued Anaphase II: sister chromatids separate and move toward opposite poles of the cell Telophase II and Cytokinesis: nuclei form at opposite poles of the cell Nuclear membranes reform around the groups of chromatids at each pole Cytokinesis produces 4 cells total Chromosomes uncoil, nucleoli appear In most organisms, develop into gametes

Meiosis and Fertilization Primary sources of genetic variation in sexually reproducing organisms Sexual reproduction provides genetic variation by: Independent Assortment Crossing over during Prophase I Random fertilization by gametes

Independent Assortment of chromosomes Orientation of the homologous pair of chromosomes (one maternal and one paternal) is RANDOM. 50-50 chance that each gamete receives maternal or paternal derived chromosome Each homologous pair of chromosomes orients independently of the other pairs at metaphase I. First meiotic division results in independent assortment of maternal and paternal chromosomes

Alternative arrangements of 2 homologous chromosome pairs

Independent Assortment Definitions Independent Assortment: The random distribution of maternal and paternal homologues to the gametes OR Independent Assortment: The presence of an allele of one of two genes in a gamete has no influence over which allele of another gene is present in the same gamete

Independent Assortment leads to Genetic Variation Process produces 2n possible combinations of maternal and paternal chromosomes in gametes If n = 2 then 22 = 4 If n = 23 (human) then 223 or 8,000,000 different possibilities before crossing over

Crossover Definition: the exchange of genetic material between homologues (homologous portion of 2 non-sister chromatids trade places) X-shaped chiasmata are the visible evidence of this process Produces chromosomes that contain genes from both parents In humans, an average of 3 crossovers/chromosome pair

Crossover Most of the time, crossing over can occur without loss of genetic material because of the precise base to base pairing of homologues involving the formation of the synaptonemal complex, a protein structure that brings the chromosomes into close association

Results of crossing over during meiosis

Benefits of Crossing over Chiasmata hold together homologues during prophase and metaphase I Allows recombination of linked genes. Breaks up linkage groups/parental combinations

Crossing over results in an exchange of alleles Parental combinations of linked genes cannot be broken up without crossing over Crossing over occurs between non-sister chromatids of a homologous pair in prophase 1 between the loci of the 2 linked genes Parentals: abc, ABC Crossing over occurs between B and C Position of chiasma formed by crossing over The 4 chromatids separate into 4 nuclei produced by meiosis Recombinants produced = abC, ABc

Linkage Groups Definition: All the genes that have their loci on the same chromosome type form a linkage group

Example of Gene Linkage

Random Fertilization: Result of sexual reproduction In humans an egg cell with 1 of 8,000,000 different possibilities will be fertilized by a sperm cell that is also 1 of 8,000,000 possibilities, resulting in a zygote that can have 1/64,000,000,000,000 possible diploid combinations

S 10.1.1 Drawing diagrams to show chiasmata formed by crossing over A chiasma is an X-shaped knot-link structure that forms where crossover has occurred Draw two homologous chromosomes using two different colors- one from mom and one from dad- close to each other

S 10.1.1 Drawing diagrams to show chiasmata formed by crossing over Since the position of crossover is random you can draw it anywhere (and you can draw more than one) The chromosomes will break where they will cross over Draw an X-shaped chiasma

S 10.1.1 Drawing diagrams to show chiasmata formed by crossing over Separate and draw the newly crossed over homologous chromosomes