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 Gamete Body Cell Mitosis 2 sets of DNA 46 total chromo Skin cell, etc. Mitosis 2 sets of DNA 46 total chromo 1 set from each parent DIPLOID (2N) Sex Cell Egg/sperm Meiosis 1 set of DNA 23 total chromo ½ set from each parent HAPLOID (N)
Important Vocab Somatic Cell vs. Gamete Diploid (2N) vs. Haploid (1N) 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: Sex cell division only Involves 2 cell divisions 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 2N Fertilization
Here’s the key to your “uniqueness” Gene X Sister Chromatids (same genes, same alleles) Homologous Pair same genes, different 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 Sister chromatids separate Meiosis I Homologous Pair separate Meiosis II 46 Interphase Diploid (2x46 = 92) Diploid (46) Haploid (23) 1 replication of chromosomes is followed by 2 cell divisions (aka Interphase only happens once!)
Meiosis: Reduction Division 2 part cell division Meiosis I Interphase I PMAT I Meiosis II PMAT II End result: 4 genetically different haploid cells 4 sperm or 1 egg Interphase Meiosis I Meiosis II
Meiosis I: Reduction Division Nucleus Spindle fibers Nuclear envelope Chromo pair up Early Prophase I (Chromosome number doubled) Late Prophase I Metaphase I Anaphase I Telophase I (diploid)
Meiosis II: Reducing Chromo # Prophase II Metaphase II Telophase II Anaphase II 4 genetically 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… Crossing over Independent Assortment Random Segregation 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. Aka Mom’s chromosomes don’t necessarily all line up on one side Random Segregation Random chromatids separate into the newly forming eggs/sperm 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 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 Meiosis II produces gametes with one copy of each chromosome/gene. Only one homolog of each chromosome is present in the cell. Sister chromatids carry identical genetic information.
Meiosis II: Reducing Chromo # Prophase II Metaphase II Telophase II Anaphase II 4 genetically different haploid cells
Metaphase II Chromosomes align along the equator Prophase II Nuclear envelope disappears Spindle fibers form No Crossing Over
Anaphase II Sister chromatids separate and move to opposite poles. 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 37
THE PROCESS OF MAKING EGG AND SPERM 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 Polar bodies die Mitosis Meiosis I Oogonium (diploid) Mitosis Primary oocyte Meiosis I Secondary (haploid) Meiosis II 1st polar body may divide Polar bodies die Oocyte polar body (dies) a A X 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 # of divisions 1 2 Number of daughter cells 4 Genetically identical? Yes No Chromosome # Same as parent Half of parent Where Somatic cells Gamete cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction
What’s the difference between Mitosis & Meiosis ? 92 Single Chromosomes (46 Pairs) 46 Single Chromosomes (23 Pairs) 46 23 singles