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Ch.11.4 - Meiosis Formation of Gametes (egg & sperm) Our Chromosomes • 46 Chromosomes (23 pairs) • #1-22 Pairs are Autosomes (body cell chromosomes): • Determine all traits except gender • #23 pair are Sex chromosomes referred to as X & Y • male (XY) • female (XX) X - Chromosome Y-Chromosome Chromosome Numbers Vary in organisms A closer look at Chromosome Pairs Karyotype Picture of chromosomes arranged by size Pairs 1-22 = autosomes Pair 23 = sex chromo Detects abnormalities & XX female or XY male Homologous Chromosomes (Buddy-Buddy) • Matching pair = homologous chromosomes • Homologous chromosomes: 2 chromosomes (one from mom and one from dad) that are alike in: • size, • location of centromere, • dark/light banding pattern of genes Remember: only non matching pair are sex chromosomes Homologous Chromosome Pairs Mom’s Blue eye gene Dad’s Brown eye gene *Allele – different versions of the same gene (traits)* • If a cell has all 46 chromo (23 pairs), it’s called a diploid cell • Shorthand: 2N Homologous Pair Somatic Cell • Body Cell • Skin cell, etc. Gamete • Sex Cell • Egg/sperm • Mitosis • Meiosis • 2 sets of DNA • 46 total chromo • 1 set from each parent • 1 set of DNA • 23 total chromo • ½ set from each parent • DIPLOID (2N) • HAPLOID (N) Important Vocab • Somatic Cell vs. Gamete • Body cell vs. Sex cell • Diploid (2N) vs. Haploid (1N) • 2 sets of DNA vs. 1 set of DNA • Homologous Chromosomes • Matching pairs of chromo in 2N cell • Alleles • Different version of the same trait • Fertilization • When sperm meets egg and combines DNA • Zygote • Cell in growth state following fertilization • Tetrad • Structure containing 4 chromatids Meiosis Info… Similar but different from Mitosis: 1. Sex cell division only 2. Involves 2 cell divisions 3. Results in 4 cells with half the normal genetic info • Produces gametes (egg/sperm) • Male Testes (spermatogenesis) • Female Ovaries (oogenesis) Why Do we Need Meiosis? Basis of sexual reproduction • Accounts for individual genetic diversity • You are unique! You look a little bit like your mom and a little like your dad! Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote (fertilized egg) 1N Fertilization 1N 2N Here’s the key to your “uniqueness” Gene X Homologous Pair same genes, different alleles Sister Chromatids (same genes, same alleles) Homologous pairs separate in meiosis and therefore different alleles (versions of traits) separate. So many combos of traits are possible! Meiosis Forms Haploid Gametes Meiosis must reduce the chromosome # by half Fertilization then restores the 2n number 23 chromo from egg + 23 chromo from sperm = you 46! from mom from dad child too much! EGG SPERM YOU meiosis reduces genetic content The right number! EGG SPERM YOU Meiosis: 2-Part Cell Division Interphase 46 Homologous Pair separate Meiosis I Diploid (2x46 = 92) 1 replication of chromosomes is followed by 2 cell divisions (aka Interphase only happens once!) Sister chromatids separate Diploid (46) Meiosis II Haploid (23) Meiosis: Reduction Division • 2 part cell division • Meiosis I • Interphase I • PMAT I Interphase • Meiosis II • PMAT II Meiosis I • End result: 4 genetically different haploid cells • 4 sperm or 1 egg Meiosis II Meiosis I: Reduction Division Nucleus Early Prophase I (Chromosome number doubled) Chromo Spindle pair up fibers Late Prophase I Nuclear envelope Metaphase Anaphase Telophase I I I (diploid) Meiosis II: Reducing Chromo # Prophase II Metaphase Telophase II Anaphase 4 genetically II II different haploid cells Interphase I • SAME as MITOSIS • Chromosomes will double • G1, S, G2 Prophase I •Homologs pair up and form tetrad (a pair of homologous chromosomes Chromosomes condense. Spindle forms. Nuclear envelope disappears. Crossing over occurs Tetrads Form in Prophase I Homologous chromosomes Pair up (each with sister chromatids) Join to form a TETRAD Called Synapsis Crossing-Over occurs in Prophase I Tetrad Forms Definition: Pieces of chromosomes or genes are exchanged Advantage of sexual reproduction = genetic variation! Crossing-Over It’s hard to predict what traits you’ll get from mom and dad because there is so many possible combinations! **THIS IS ONE SOURCE OF GENETIC VARIABILITY!** Genetic Variability is due to… 1. Crossing over 2. Independent Assortment 3. Random Segregation 4. Random Fertilization Meaning: You are unique for these 4 reasons! • Independent Assortment • Random Segregation • The way a pair of chromosomes lines up during metaphase is not dependent on other pairs. • Random chromatids separate into the newly forming eggs/sperm • Aka Mom’s chromosomes don’t necessarily all line up on one side • Aka if you inherit mom’s hair color, you might get her brown hair trait OR her blonde hair trait Independent Assortment Random Segregation Metaphase I and Anaphase I Metaphase I Homologous pairs of chromosomes align along the equator Anaphase I -Homologs separate and move to opposite poles. -Sister chromatids remain attached at their centromeres. Telophase I and Cytokinesis Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two new diploid cells. Meiosis II-occurs in 2 cells Gene X Only one homolog of each chromosome is present in the cell. Sister chromatids carry identical genetic information. Meiosis II produces gametes with one copy of each chromosome/gene. Meiosis II: Reducing Chromo # Prophase II Metaphase Telophase II Anaphase 4 genetically II II different haploid cells • Prophase II • Nuclear envelope disappears • Spindle fibers form • No Crossing Over Metaphase II Chromosomes align along the equator Anaphase II Equator Pole Sister chromatids separate and move to opposite poles. Telophase II Nuclear envelope reforms. Chromosomes loosen into chromatin. Spindle breakdown. Cytokinesis breaks the cells into 2 new daughter cells Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome Prophase II Prophase I Metaphase II Metaphase I Anaphase II Anaphase I Telophase II Telophase I Cytokinesis Cytokinesis Meiosis Animation Overview of Meiosis Oogenesis & Spermatogenesis THE PROCESS OF MAKING EGG AND SPERM Spermatogenesis “Creation of sperm” Testes 2 divisions produce 4 viable haploid spermatids Spermatids mature into sperm Men produce about 250,000,000 sperm per day Spermatogenesis in the Testes Spermatid Spermatogenesis Oogenesis “Creation of Eggs” Ovary 2 divisions produce 3 polar bodies that die + 1 viable egg Polar bodies die because of unequal division of cytoplasm Starting at puberty, if unfertilized, one immature oocyte matures into an ovum (egg) every 28 days MENSTRUAL CYCLE Oogenesis in the Ovaries **Egg cells are special…only one egg is made every time meiosis occurs; the other 3 cells (polar bodies) that are made are much smaller & are discarded (Remember: the egg cell is the largest cell you’ll come across) Oogenesis 1st polar body may divide (haploid) a Mitosis Oogonium (diploid) A X X a X a a Polar bodies die X Meiosis I Meiosis II X Primary oocyte (diploid) A X A X Secondary oocyte (haploid) Oocyte A X polar body (dies) Mature Egg (ovum) Meiosis is IMPORTANT… Genetic Variability 1. Independent assortment (late pro/early meta I&II) -chromosomes line up randomly 2. Law of Segregation (Late meta/ana I&II) -spindle fibers attach randomly to chromosomes and separate. 3. Crossing-over (Pro I) pieces of sister chromatids are switched 4. Random fertilization Random sperm + random egg are combined What are the possibilities? ***Total possible chromosome combinations due to independent assortment = 2n [for humans = 223 = 8,388,608] ***Total possible chromosomally different zygotes due to fertilization = (223)2 = 70,368,744,000,000 ***Possible genetically different zygotes per couple if crossing-over occurs only once = (423)2 = 4,951,760,200,000,000,000,000,000,000 --advantageous b/c variability needed for evolution Comparison of Divisions Mitosis Meiosis 2 # of divisions 1 Number of daughter cells 2 4 Yes No Same as parent Half of parent Where Somatic cells Gamete cells When Throughout life At sexual maturity Genetically identical? Chromosome # Role Growth and repair Sexual reproduction What’s the difference between Mitosis & Meiosis ? 46 Single 92 Chromosomes (23 Pairs) Single Chromosomes (46 Pairs) 46 Single Chromosomes (23 Pairs) 46 Single Chromosomes (23 Pairs) 46 Single Chromosomes (23 Pairs) 92 Single Chromosomes (46 Pairs) 46 Single Chromosome s (23 Pairs) 23 singles 23 singles 46 Single Chromosom es (23 Pairs) 23 singles 23 singles