1. College Prep Biology Mr. Martino
Chapter Meiosis
College Prep Biology
Mr. Martino

2. I. Introduction Germ Cells: develop into sex cells (sperm and egg)
Gametes: mature sex cells – sperm and egg

3. III. Sexual and Asexual Reproduction
Sexual Reproduction: reproductive process that includes the union of sperm and egg
Two parents – each donate a complete set of genes
Asexual Reproduction: production of offspring without the participation of sperm and egg
Single parent
Allele: unique form of the same gene (a gene)

4. IV. Meiosis Halves Chromosome Number
Meiosis: nuclear division that divides chromosomes twice
Diploid number: (2n) pair of each type of chromosome
Homologous chromosomes: both chromosomes of a pair
Same length and shape
Genes are for same traits
Haploid number: (n) results from the second division of meiosis (sex cells)

5. Sister chromatids: two attached chromosomes
Once they pull apart, they are considered chromosomes
Meiosis consists of 2 divisions:
Meiosis I: DNA is replicated and divides
Meiosis II: another cell division without more chromosome doubling

6. V. Stages of Meiosis
Meiosis: the process that produces haploid gametes (sex cells)
2 consecutive divisions: meiosis I and meiosis II
Meiosis II is essentially the same as mitosis
4 daughter cells produced
Only 1 duplication of chromosomes, second division halves chromosome number

7. Interphase I Like interphase in mitosis
Each pair replicates forming a 4 chromatid structure

8. Prophase I Most complex phase of meiosis Lasts 90% of meiosis
homologous chromosomes form tetrads (4 chromatids)

9. Structure of a Tetrad
Tetrad = structure consisting of 2 pairs of chromosomes
2 chromosomal pairs = 4 sister chromatids
Visible during Prophase I only

10. Metaphase I
Tetrads align on metaphase plate

11. Anaphase I
Begins as homologous chromosomes (still attached at centromere) separate and move towards opposite poles
Only tetrads are separated

12. Telophase I and Cytokinesis
Chromosomes arrive at opposite poles
Cytokinesis results in 2 diploid cells
No DNA replication occurs between meiosis I and meiosis II

13. Prophase II
Chromosomes must condense again since there was a pause prior to meiosis II

14. Metaphase II
Chromosomes are aligned at metaphase plate

15. Anaphase II
Begins when centromeres of sisters chromatids separate

16. Telophase II and Cytokinesis
Nuclei form at opposite poles
Cytokinesis occurs
Forms 4 haploid daughter cells

17. VI. From Gametes to Offspring
Spermatogenesis: sperm formation
results in 4 viable sperm
Oogenesis: egg formation
First meiotic division results in a polar body and a secondary oocyte
Second division results in 3 polar bodies and 1 egg
Polar bodies are reabsorbed – egg is viable

18. VII. Meiosis and Mitosis Compared
Chromosomes of both replicate only once
Mitosis consists of one cell division and two diploid cells
Meiosis entails two cell divisions and four haploid cells
Mitosis and meiosis II are virtually identical

19. 9.6 Meiosis Increases Genetic variation among offspring
Genetic variation – differences among the gene combinations in the offspring due to sexual reproduction
Assortment of Chromosomes – the aligning of chromosomes during metaphase I is random; which results in many possible chromosome combinations

21. Crossing Over – the exchange of genetic material in between homologous chromosomes; during prophase I when tetrads are present