Chapter 8 Cell Reproduction

Chapter overview 3 SECTIONS: SECTION 1CHROMOSOMES SECTION 2CELL DIVISION SECTION 3MEIOSIS

meiosis Section 3 p

meiosis Purpose: To create haploid cells from a diploid cell Result: 4 cells with half the number of chromosomes as in the original cell Reason: When sexual reproduction occurs and gametes (sex cells) join, they have the correct number of chromosomes

meiosis

Formation of haploid cells Cell cycle begins and includes all the phases of interphase DNA IS COPIED DURING S PHASE! Cell division occurs TWICE MEIOSIS I: prophase I, metaphase I, anaphase I and telophase I MEIOSIS II: prophase II, metaphase II, anaphase II and telophase II The chromosome number is reduced to half during the first division

Formation of haploid cells

MEIOSIS I – “Separate the Homologues” PROPHASE I DNA coils into chromosomes Spindle fibers appear Homologous chromosomes pair in SYNAPSIS forming TETRADS Crossing over may occur Increases genetic variety Each organism receives one set of chromosomes from each parent. During this stage, the chromosome from the mother pairs with the SAME chromosome from the father SAME= has same GENES in same order, but not necessarily the same TRAITS (characteristics)

Meiosis I: prophase I

Meiosis I Metaphase I Tetrads line up in middle of cell Spindle fibers attach to centromere of homologous chromosome Spindle fibers from opposite pole attach to other centromere of homologous pair FYI : 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 I Anaphase I Homologous pairs are separated randomly Known as INDEPENDENT ASSORTMENT Known as INDEPENDENT ASSORTMENT

MEIOSIS I Telophase I and Cytokinesis I Chromosomes reach opposite poles Sister chromatids are still attached at centromere 2 new cells result Each cell has half the original number of chromosomes, but still have copies attached In other words, are still SISTER CHROMATIDS

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

Meiosis II (similar to mitosis) Prophase II Spindle fibers reform Metaphase II Sister chromatids moved to center of cell Anaphase II Sister chromatids separated at centromere and move to opposite poles of cell Telophase II Nuclear membrane reforms Cytokinesis II 4 new HAPLOID cells created

A Meiosis

Development of gametes

Sexual reproduction Production of offspring Gametes fuse into new organism Creates genetic variety Random fertilization which sperm? Independent assortment (meiosis I) Separation of homologous pairs randomly Segregation of traits during meiosis II Each gamete only gets one chromosome of each pair