Meiosis and Sexual Reproduction

2. Meiosis and Sexual Reproduction

3. Sexual Reproduction • New individuals arise from fusion of 2 nuclei from 2 parents • Common in multicellular organisms • Few offspring • Involves meiosis as well as mitosis

4. Advantages • Offspring is a genetic mixture of both parents (variation) • Better adapted for survival of species • More likely to survive environmental changes

5. Before we get started, some definitions • Gametes: cells that provide nuclear material for sexual reproductions; also known as sex cells • Egg cells (female) • Sperm cells (male)

6. Fertilization: fusion of nuclei of 2 gametes • Zygote: single cell formed by fusion of gametes; fertilized egg cell • Primary Sex Cell: make up gonads • Oocyte (female) • Spermatocyte (male)

7. Gametogenesis: production of gametes • Oogenesis (female) • Spermatogenesis (male)

8. Chromosome Number • Somatic cells: all body cells except gametes • Contain 46 chromosomes • Present as homologous pairs • Each homologous pair are similar in size and shape- control same hereditary characteristics

11. Homologous Chromosomes • 22 pairs of homologous chromosomes • 1 pair of sex chromosomes

12. Female Karyotype

13. Cells that contain the full number of chromosomes are called diploid (2n) • Gametes do not contain homologous pairs • They only contain one of the chromosomes of each homologous pair • Monoploid or haploid (n)

14. Male (46) Female (46) meiosis meiosis Gametes Gametes (sperm) (23) (ova) (23) fertilization zygote (46)

16. What is meiosis? • Type of cell division that occurs only in sex cells (gametes) • Daughter cells receive ½ the number of chromosomes that are present in body cells

17. Stages of Meiosis (Reduction Division) • Starting out, cells have diploid number of chromosomes • Each cell replicates once but divides twice • Replication occurs before 1st meiotic division • No replication at 2nd meiotic division

18. 1st Meiotic Division • Chromosomes replicate (2 chromatids) • Each chromosome lines up with a homologous chromosome and attach at centromeres • Line up must be exact • Pairing process known as synapsis

19. Each chromosome pair now consists of 4 chromatids (tetrads) • Strands of the tetrad twist around each other and may exchange segments • Crossing Over

20. During these chromosomal changes, the nuclear membrane disappears and the spindle fibers form • Homologous pairs move toward equator • Then homologous chromosomes separate and move to poles= disjunction • At the end of the first meiotic division, cytoplasm divides resulting in 2 daughter cells

23. 2nd Meiotic Division • No further replication of chromosomes • Remaining steps… • Each daughter cells forms a spindle and double stranded chromosomes move to equator • Chromosomes become attached to spindle at centromere • Centromeres divide and sister chromatids separate

24. Chromatids move to poles • Both daughter cells divide forming 4 monoploid cells • Nuclear membrane reappears

26. Primary sex cell 46 replication 1st meiotic division 92 46 46 2nd meiotic division 23 23 23 23 gametes

29. Development of Gametes • Gemaetogenesis: process of development of gametes in gonads

30. Oogenesis • Development of egg • Occurs in ovaries • Production of oocytes occurs in early development and stops at birth • Limited number (~400)

31. Oogenesis • At this stage the cell is diploid • After 1st meiotic division, 2 unequal cells are formed • 2nd meiotic division results in 1 egg and 3 polar bodies • Each is haploid

33. Spermatogenesis • Development of sperm • Occurs in testes • Production of spermatocytes is continuous throughout life • Unlimited number

34. At this stage the cell is diploid • After 1st meiotic division 2 equal cells are formed • 2nd meiotic division results in 4 functional gametes • Each is haploid

36. Comparison of Egg and Sperm

37. Fertilization and Zygote Development • Union of egg and sperm cells • Called fertilization • Results in zygote • There must be fluid for sperm to swim to egg

38. 2 forms of fertilization • External fertilization • Eggs are fertilized outside female body • Limited to aquatic animals • Large production of eggs and sperm • Process is known as spawning

39. Internal Fertilization • Occurs within female body • Characteristic of land animals • Requires specialized sex organ of male • After fertilization, either enclosed in shell or remains in body • Few eggs produced