Cell Reproduction
Chapter 8
Chapter overview
 3 SECTIONS:
 SECTION 1
CHROMOSOMES
 SECTION 2
CELL DIVISION
 SECTION 3
MEIOSIS
meiosis
Section 3
p.161-164
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
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 look @ Meiosis
Development of gametes
 Occurs in testes
 Occurs in ovaries
 Called
 Called OOGENESIS
SPERMATOGENESIS
 Cytoplasm divides equally
 4 spermatids cells created
 Cytoplasm divides
 SPERM – when cells mature
unequally
 1 mature egg cell
created with 3 polar
bodies, which disintegrate
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