Ch 11 4 Meiosis Formation of Gametes egg

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 üPicture of chromosomes arranged by size üPairs 1 -22 = autosomes üPair 23 = sex chromo üDetects abnormalities & üXX 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: 2 N 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 (2 N) • HAPLOID (N)

Important Vocab • Somatic Cell vs. Gamete • Body cell vs. Sex cell • Diploid (2 N) vs. Haploid (1 N) • 2 sets of DNA vs. 1 set of DNA • Homologous Chromosomes • Matching pairs of chromo in 2 N 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? üBasis 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! üTwo haploid (1 n) gametes are brought together through fertilization to form a diploid (2 n) zygote (fertilized egg) 1 N Fertilization 1 N 2 N

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 üMeiosis must reduce the chromosome # by half üFertilization then restores the 2 n number ü 23 chromo from egg + 23 chromo from sperm = you 46! from mom from dad child too much! EGG SPERM meiosis reduces genetic content YOU The right number! EGG SPERM YOU

Meiosis: 2 -Part Cell Division Interphase 46 Homologous Pair separate Meiosis I Diploid (2 x 46 = 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 Chromo Spindle pair up fibers Early Late Prophase I (Chromosome Prophase I number doubled) Nuclear envelope Metaphase Anaphase Telophase I I I (diploid)

Meiosis II: Reducing Chromo # Prophase Metaphase II Telophase II Anaphase 4 genetically II II different haploid cells

Interphase I • SAME as MITOSIS • Chromosomes will double • G 1, S, G 2

Prophase I • Homologs pair up and form tetrad (a pair of homologous chromosomes üChromosomes condense. üSpindle forms. üNuclear envelope disappears. üCrossing 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 üTetrad Forms üDefinition: Pieces of chromosomes or genes are exchanged üAdvantage 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 Metaphase II Telophase II Anaphase 4 genetically II II different haploid cells

• Prophase II • Nuclear envelope disappears • Spindle fibers form • No Crossing Over ¡ Metaphase II l 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 I Metaphase I Anaphase I Telophase I Cytokinesis Prophase II Metaphase II Anaphase II Telophase II Cytokinesis

Meiosis Animation

Overview of Meiosis

Oogenesis & Spermatogenesis THE PROCESS OF MAKING EGG AND SPERM

Spermatogenesis ü“Creation of sperm” üTestes ü 2 divisions produce 4 viable haploid spermatids üSpermatids mature into sperm üMen produce about 250, 000 sperm per day

Spermatogenesis in the Testes Spermatid

Spermatogenesis

Oogenesis ü“Creation of Eggs” üOvary ü 2 divisions produce 3 polar bodies that die + 1 viable egg üPolar bodies die because of unequal division of cytoplasm üStarting 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 1 st polar body may divide (haploid) a Mitosis Oogonium (diploid) A X Polar bodies die X Meiosis I X Primary oocyte (diploid) X a a Meiosis II 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 = 2 n [for humans = 223 = 8, 388, 608] ***Total possible chromosomally different zygotes due to fertilization = (223)2 = 70, 368, 744, 000 ***Possible genetically different zygotes per couple if crossing-over occurs only once = (423)2 = 4, 951, 760, 200, 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) 92 Single Chromosomes (46 Pairs) 46 Single Chromosome s (23 Pairs) 23 singles 46 Single Chromosom es (23 Pairs) 23 singles