Chromosomes & Inheritance MEIOSIS -two successful divisions of a diploid nucleus following only one DNA replication cycle - the original diploid cell contained a haploid set of chromosomes from mom and a haploid set from dad (except for self-fertilizing organisms - plants) - produces a haploid gamete

Chromosomes & Inheritance INTERPHASE - G 1, S, G 2 - chromosomes are duplicated - produces a temporary 4N - each “chromosome” consist of 2 sister chromatids attached at a centromere

Chromosomes & Inheritance MEIOSIS I -chromosome number is reduced - PROPHASE I -similar to Prophase of Mitosis except for crossing over of homologous pairs of chromosomes -4 stages -produces greater chromosomal individuality of offspring

Chromosomes & Inheritance MEIOSIS I - PROPHASE I -Leptonema – chromosomes begin to coil and become visible -Telomere – end of chromosomes that do not code for proteins; protect and secure the chromosome;

Chromosomes & Inheritance MEIOSIS I - PROPHASE I -during Meiosis I these are clustered on the nuclear envelope to produce a bouquet of the chromosomes -Zygonema – chromosomes continue to shorten (coil)

Chromosomes & Inheritance MEIOSIS I - PROPHASE I -homologous pairs align and undergo synapsis (formation of the synaptonemal complex) which aligns them base pair for base pair -made possible because of the telomeres and bouquet

Chromosomes & Inheritance MEIOSIS I - PROPHASE I -Pachynema – follows completion of synapsis -in this tetrad is where crossing-over now occurs -each synapsed set of homologous chromosomes has 4 chromatids, this is called a tetrad

Chromosomes & Inheritance MEIOSIS I - PROPHASE I -resulting genetic differences between the homologs can produce new gene combinations -there is usually no loss or gain of genetic material here, just exchange – each homolog gives and receives genes

Chromosomes & Inheritance MEIOSIS I - PROPHASE I -recombinant chromosome - chromosome that has a different combination of genes than what it began meiosis with -genetic recombinantion - process by which progeny receives genotypes different from the parents

Chromosomes & Inheritance MEIOSIS I - PROPHASE I -Diplonema - synaptonemal complex is disassembled and homologous chromosomes move apart -chiasma – cross-shaped structure resulting from the overlapping of chromatids

Chromosomes & Inheritance MEIOSIS I - PROPHASE I -at these locations the homologous chromosomes are very tightly connected -in most organisms the remainder of meiosis goes very quickly, except for egg development in animals -Humans: diplonema is completed by the 7 th month of fetal development, then they wait, wait, wait

Chromosomes & Inheritance MEIOSIS I - PROPHASE I -Humans: The onset of puberty begins a cycle of one oocyte per month completing meiosis I and is ovulated. -Humans: If fertilized, it quickly completes meiosis II as it passes down the fallopian tube and then a functional zygote is produced

Chromosomes & Inheritance MEIOSIS I - PROPHASE I -Diakinesis – nucleolus and nuclear envelope break down -spindle is assembled -chromosomes can be counted fairly easily * Crossing-over occurs between homologous chromosomes, mainly autosomes

Chromosomes & Inheritance

MEIOSIS I - METAPHASE I -nuclear envelope is completely broken down -tetrads become aligned at the equator of the cell -spindle is completely formed -microtubules are attached to the homologs

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Chromosomes & Inheritance MEIOSIS I - ANAPHASE I -chromosomes of the homologous pair (dyads) disjoin and move to opp. poles -centromeres segregate randomly -move to opp. poles is completed -sister chromatids remain joined - TELOPHASE I -new nuclear envelope forms -cytokinesis is happening concurrently

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Chromosomes & Inheritance MEIOSIS II - PROPHASE II -chromosomes condense -spindle is organized and attaches to centromere (still connecting sister chromatids) - METAPHASE II -centromeres line up on the equator of the cell

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Chromosomes & Inheritance MEIOSIS II - ANAPHASE II -centromeres split -chromatids are pulled to opp. sides - TELOPHASE II -nuclear envelope forms around each set of chromosomes -sister chromatids of each pair move to opp. poles -cytokinesis takes place -chromosomes become elongated

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Chromosomes & Inheritance MEIOSIS II - after TELOPHASE II -chromosomes become elongated and no longer easily visible -end product is 4 haploid cells (gametes) -each gamete has one chromosome (not exact copies) from each homologous pair