Chapter 11-4: Meiosis

Genes are lined up on chromosomes Chromosomes occur in pairs in body (somatic) cells 1 pair from mother 1 pair from father diploid cell (2n) cell with 2 of each kind of chromosome

Organisms produce gametes (sex cells) contain 1 of each kind of chromosome haploid cell (n) one of each kind of chromosome

Homologous Chromosomes Paired chromosomes 2 chromosomes of each pair in a diploid cell each have a corresponding chromosome from the opposite sex parent

Meiosis # of chromosomes per cell is cut in half through separation of homologous chromosomes cell division that produces gametes (sex cells) has ½ the number of chromosomes as the parent involves 2 cell divisions (I and II)

Haploid cells produced are sex cells or gametes Sperm: male gametes (n) 4 equal-sized Egg: female gametes (n) 1 large egg cell; 3 small polar bodies

Sperm + egg = fertilization (n + n = 2n) Fertilized egg called a zygote Zygote goes through mitosis to develop into a multicellular organism Meiosis produces genetic variation Genetic recombination Reassortment of chromosomes Basis for evolution (change through time)

Meiosis I: Interphase Begins with a diploid cell cell replicates its chromosomes Consists of 2 identical sister chromatids

Prophase I DNA coils up and spindle forms Homologous chromosomes line up with each other to form a tetrad Tetrad: consists of 2 homologous chromosomes, each made up of 2 sister chromatids Pair up tightly (very tightly)

Crossing-Over Non-sister chromatids from homologous chromosomes can break and exchange genetic material Can occur anywhere on a chromosome and at several locations at the same time Exchange of genetic material

Crossing-over provides genetic variation Increases variability of organisms Sometimes chromosomes fail to separate correctly Called nondisjunction Can lead to extra chromosomes or not enough chromosomes

Metaphase I Spindle fibers pull tetrads to middle of a cell and line up together Anaphase I Homologous chromosomes separate and move to opposite ends of the cell Telophase I Chromosomes uncoil and cytoplasm divides to make 2 cells

Phases of Meiosis II Second division Just like mitosis Prophase, Metaphase, Anaphase, Telophase No chromosomes are replicated before starting meiosis II Result of meiosis II: 4 genetically different haploid (n) cells formed Become gametes (sex cells)

Mitosis Meiosis 2 4 identical different Diploid Diploid 2n  2n Meiosis vs. Mitosis Mitosis Meiosis Number of cells produced 2 4 Are cells produced genetically identical or different? identical different Type of cells produced Diploid Diploid 2n  2n Somatic/body cells Diploid  Haploid 2n  n Sex cells Type of reproduction Asexual reproduction Sexual reproduction Number of divisions 1

Asexual Reproduction Involves one parent Binary fission Replicates DNA and divides in half  cell splits into 2 Budding Breaks off from parent and grows into new organism

Spore formation  fungi Vegetative propagation Making new plants from runners, stems, or roots Regeneration Making new organism from body parts

Sexual Reproduction Union of 2 cells

Bell Ringer: If a diploid cell containing 28 chromosomes undergoes meiosis, how many chromosomes will each daughter cell have? In what process do male and female reproductive cells join during sexual reproduction? a. segregation b. fertilization c. self-pollination d. cross-pollination 3. What type of cell has 2 sets of chromosomes? a. diploid b. haploid c. tetrad d. gene 4. There are four chromatids in a ___. a. polygenic trait b. tetrad c. gamete d. genotype