Cell reproduction

Chapter 8Cell ReproductionChapter overview3 SECTIONS:

SECTION 1CHROMOSOMES

SECTION 2CELL DIVISION

SECTION 3MEIOSISmeiosisSection 3p.161-164meiosisPurpose:To create haploid cells from a diploid cellResult:4 cells with half the number of chromosomes as in the original cellReason:When sexual reproduction occurs and gametes (sex cells) join, they have the correct number of chromosomesmeiosis

Formation of haploid cellsCell cycle begins and includes all the phases of interphaseDNA IS COPIED DURING S PHASE!Cell division occurs TWICEMEIOSIS I: prophase I, metaphase I, anaphase I and telophase IMEIOSIS II: prophase II, metaphase II, anaphase II and telophase IIThe chromosome number is reduced to half during the first divisionFormation of haploid cells

MEIOSIS I Separate the HomologuesPROPHASE IDNA coils into chromosomesSpindle fibers appearHomologous chromosomes pair in SYNAPSIS forming TETRADSCrossing over may occurIncreases genetic varietyEach organism receives one set of chromosomes from each parent. During this stage, the chromosome from the mother pairs with the SAME chromosome from the fatherSAME= has same GENES in same order, but not necessarily the same TRAITS (characteristics)

Meiosis I: prophase I

Meiosis IMetaphase ITetrads line up in middle of cellSpindle fibers attach to centromere of homologous chromosomeSpindle fibers from opposite pole attach to other centromere of homologous pairFYI:Due to random orientation of chromosomes, you will never look exactly like your parents..There are 8, 388, 608 possible combinations for our chromosomes!Meiosis I: metaphase I

Meiosis IAnaphase IHomologous pairs are separated randomlyKnown as INDEPENDENT ASSORTMENT

MEIOSIS ITelophase I and Cytokinesis IChromosomes reach opposite polesSister chromatids are still attached at centromere2 new cells result Each cell has half the original number of chromosomes, but still have copies attachedIn other words, are still SISTER CHROMATIDS

MEIOSIS II Separate the Chromatids******Not preceded by copying of DNA******Can occur immediately after meiosis ICan be delayed for considerable time

Meiosis II (similar to mitosis)Prophase IISpindle fibers reformMetaphase IISister chromatids moved to center of cellAnaphase IISister chromatids separated at centromere and move to opposite poles of cellTelophase IINuclear membrane reformsCytokinesis II4 new HAPLOID cells createdA look @ Meiosis

Development of gametesMalesOccurs in testesCalled SPERMATOGENESISCytoplasm divides equally4 spermatids cells createdSPERM when cells matureFemalesOccurs in ovariesCalled OOGENESISCytoplasm divides unequally1 mature egg cell created with 3 polar bodies, which disintegrate

Sexual reproductionProduction of offspringGametes fuse into new organismCreates genetic varietyRandom fertilization which sperm?Independent assortment (meiosis I)Separation of homologous pairs randomlySegregation of traits during meiosis IIEach gamete only gets one chromosome of each pair