CHAPTER 8 Cellular Reproduction: Cells from Cells – Part Two

Mitosis Review Why does mitosis take place? A: grow B: heal C: make babies D: a and b

How many chromosomes are in a human skin cell? A: 2 B: 23 C: 46 D: 92

Mitosis Review What stage of the cell cycle does the DNA double? A: Prophase B: S-phase C: Cytokinesis D: G 1

How do animals reproduce? A: Asexually B: Sexually C: Mitosis D: Binary Fission

Sexual reproduction depends on –Meiosis. –Fertilization. Meiosis, The Basis of Sexual Reproduction Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings

Humans have 46 chromosomes –One set from each parent –Two different sex chromosomes, X and Y. –Twenty-two pairs of matching chromosomes, called autosomes.

A karyotype is an orderly arrangement of chromosomes. Homologous chromosomes are matching pairs of chromosomes.

Gametes and the Life Cycle of a Sexual Organism The life cycle of a multicellular organism is the sequence of stages leading from the adults of one generation to the adults of the next.

Humans are diploid organisms. –Their cells contain two sets of chromosomes One from mom, one from dad –Their gametes are haploid, having only one set of chromosomes.

Fertilization –Is the fusion of sperm and egg. –Creates a zygote, or fertilized egg. Sexual life cycles involve an alternation of diploid and haploid stages.

Figure 8.15

The Process of Meiosis In meiosis, –Haploid daughter cells are produced in diploid organisms. –Two consecutive divisions occur, meiosis I and meiosis II, preceded by interphase. –Crossing over occurs. – the five stages of prophase one. Meiosis Overview Interphase Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis

Figure

Figure

Figure

PROPHASE 1 : DNA  chromosomes Spindle appears Homologous Chromosomes find each other Crossing over occurs

METAPHASE 1: Tetrads line up Spindle fibers attach

ANAPHASE 1: Homologous Chromosomes separate and move toward opposite ends of the cell

TELEPHASE 1: chromosomes reach ends of the cell New nuclear membranes form Cytokinesis 1 occurs

PROPHASE 2: Spindle fibers form Nucleus dissolves (again) Chromosomes start moving to middle

METAPHASE 2: chromosomes (without pairs) line up in middle of cell Spindle fibers attach

ANAPHASE 2: Chromatids separate and move to opposite ends of the cell Cleavage furrow is seen

TELOPHASE 2: Chromatids make it to end of cell New nuclear membrane forms Cytokinesis 2 occurs

Review: Comparing Mitosis and Meiosis All the events unique to meiosis occur during meiosis I.

Figure 8.17

The Origins of Genetic Variation Offspring of sexual reproduction are genetically different from their parents and from one another.

Independent Assortment of Chromosomes In independent assortment, every chromosome pair orients independently of the others during meiosis. Genetic Variation

Crossing Over In crossing over, –Homologous chromosomes exchange genetic information. –Genetic recombination occurs. Crossing Over

How Accidents During Meiosis Can Alter Chromosome Number In nondisjunction, –The members of a chromosome pair fail to separate during anaphase. –Gametes with an incorrect number of chromosomes are produced. Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings

Figure 8.20

The result of nondisjunction

Down Syndrome: An Extra Chromosome 21 Down Syndrome –Is a condition where an individual has an extra chromosome 21. –Is also called trisomy 21.

Figure 8.22

The incidence of Down Syndrome increases with the age of the mother.

Abnormal Numbers of Sex Chromosomes Nondisjunction –Also affects the sex chromosomes.