Chapter 8 – Section 3

Bell Work What would children be like if humans reproduced using the process of mitosis? List some reasons why it is good for a species overall for every individual to be a little bit different.

Chromosomes are matched in homologous pairs Human cells have 46 chromosomes, making up 23 pairs of homologous chromosomes MEIOSIS Chromosomes Centromere Sister chromatids Figure 8.12

Gametes have a single set of chromosomes Cells with two sets of chromosomes are said to be diploid (2n = 46 for humans) Gametes are haploid, with only one set of chromosomes (n = 23 for humans)

Sexual Life Cycle At fertilization, a sperm fuses with an egg, forming a diploid zygote Repeated mitotic divisions lead to the development of a mature adult The adult makes haploid gametes by meiosis All of these processes make up the sexual life cycle of organisms

The human life cycle Figure 8.13 MEIOSISFERTILIZATION Haploid gametes (n = 23) Egg cell Sperm cell Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development

Meiosis reduces the chromosome number from diploid to haploid Meiosis, like mitosis, is preceded by chromosome duplication (during interphase) However, in meiosis the cell divides twice to form four daughter cells, each of which is haploid (n = 23).

Meiosis I In the first division, meiosis I, homologous chromosomes are paired While they are paired, they cross over and exchange genetic information The homologous pairs are then separated, and two daughter cells are produced, which at this point are haploid (n = 23). But because each chromosome has double the genetic info (2 sister chromatids), another division is necessary.

Figure 8.14, part 1 MEIOSIS I : Homologous chromosomes separate INTERPHASEPROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Nuclear envelope Chromatin Sites of crossing over Spindle Sister chromatids Tetrad Microtubules attached to kinetochore Metaphase plate Centromere (with kinetochore) Sister chromatids remain attached Homologous chromosomes separate

Meiosis II Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is four haploid daughter cells, each of which are haploid (n = 23).

Figure 8.14, part 2 MEIOSIS II : Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow Sister chromatids separate TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming

Review: A comparison of mitosis and meiosis For both processes, chromosomes replicate only once, during interphase

Figure 8.15 MITOSISMEIOSIS PARENT CELL (before chromosome replication) Site of crossing over MEIOSIS I PROPHASE I Tetrad formed by synapsis of homologous chromosomes PROPHASE Duplicated chromosome (two sister chromatids) METAPHASE Chromosome replication 2n = 4 ANAPHASE TELOPHASE Chromosomes align at the metaphase plate Tetrads align at the Metaphase plate METAPHASE I ANAPHASE I TELOPHASE I Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I ; sister chromatids remain together No further chromosomal replication; sister chromatids separate during anaphase II 2n = 4 Daughter cells of mitosis Daughter cells of meiosis II MEIOSIS II Daughter cells of meiosis I Haploid n = 2 nnnn Haploid n = 2

Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Each chromosome of a homologous pair comes from a different parent Each chromosome thus differs at many points from the other member of the pair

The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes (Independent Assortment) Random fertilization also increases variation in offspring (Which sperm will fertilize the egg?)

Figure 8.16 POSSIBILITY 1POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1Combination 2Combination 3Combination 4 Independent Assortment

Crossing over further increases genetic variability Crossing over is the exchange of corresponding segments between two homologous chromosomes Genetic recombination results from crossing over during prophase I of meiosis This increases variation further

What leads to variability/diversity? Why are we not all identical? Independent assortment Crossing over Random fertilization