Development Stages of the Human Life Cycle: From Conception to Adulthood

1. Chapter 3Development

2. Stages of the Human Life Cycle • Genes orchestrate our physiology after conception through adulthood • Development is the process of forming an adult from a single-celled embryo • In humans, new individuals form from the union of sex cells or gametes • Sperm from the male and oocyte from the female form a zygote

3. vas deferens seminal vesicle bladder urethra prostate bulbourethral gland epididymis testis Male Reproductive Tract Figure 3.1

4. uterine tube ovary uterus cervix vagina Female Reproductive Tract Figure 3.2

5. Gametes • Form from cell division of germline cells • Meiosis is cell division to produce gametes • Meiosis has two divisions of the nucleus (Meiosis I and Meiosis II) and produces cells with half the number of chromosomes (haploid)

6. from mother from father child too much! Meiosis • Reduces the genetic material by half • Why is this necessary? meiosis reduces genetic content

7. Homologous Chromosomes • Carry the same genes • Pair during Meiosis I • Separate in the formation of gametes • One copy of each pair is from the mother and one is from the father. Figure 1.2

8. Sexual Reproduction • Meiosis and sexual reproduction increases genetic diversity in a population • Variation is important in a changing environment • Evolution is the genetic change in a population over time

9. Comparison of Mitosis and Meiosis Table 3.1

10. Meiosis Interphase precedes meiosis I Meiosis I Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II Figure 2.13

11. Spindle fibers Nucleus Nuclear envelope Prophase I (early) (diploid) Prophase I (late) (diploid) Metaphase I (diploid) Anaphase I (diploid) Telophase I (diploid) Meiosis I : the reduction division Figure 3.4

12. Prophase I • Late prophase • Chromosomes condense • Spindle forms • Nuclear envelope fragments • Early prophase • Homologs pair • Crossing over occurs Figure 3.4

13. Metaphase I • Homolog pairs align • along the equator of the cell Figure 3.4

14. Anaphase I • Homologs separate and move to opposite poles • Sister chromatids remain attached at their centromeres Figure 3.4

15. Telophase I • Nuclear membrane reforms • Spindle disappears • Cytokinesis divides cell Figure 3.4

16. Prophase II (haploid) Metaphase II (haploid) Anaphase II (haploid) Telophase II (haploid) Four nonidentical haploid daughter cells Meiosis II : like mitosis; sister chromatids separate Figure 3.4

17. Prophase II • Nuclear envelope fragments • Spindle forms Figure 3.4

18. Metaphase II • Chromosomes align • along equator of cell Figure 3.4

19. Anaphase II • Centromeres divide • Sister chromatids separate Figure 3.4

20. Telophase II • Nuclear envelopes reform • Chromosomes decondense • Spindle disappears • Cytokinesis divides cells Figure 3.4

21. Results of Meiosis • Gametes • Four haploid cells • Contain one copy of each chromosome and one allele of each gene • Each cell is unique Figure 3.4

22. Meiosis I (reduction division) Meiosis II (equational division) Diploid Haploid Haploid Meiosis: Cell Division in Two Parts Figure 3.3 Result: one copy of each chromosome in a gamete.

23. Table 3.1

24. A A a a B B b b C C c c D D d d E E e e F F f f Recombination (crossing over) • Occurs in prophase of meiosis I • Homologous chromosomes exchange genes • Generates diversity Figure 3.5

25. Recombination (crossing over) A a a A B • Exchange between homologs • Occurs in prophase I b b B c C C c D D d d E E e e F F f f Figure 3.5 Letters denote genes and case denotes alleles

26. Recombination (crossing over) a A a A B b B b c c C C • Creates chromosomes with new combinations of alleles for genes A to F D D d d E E e e F F f f Figure 3.5

27. Chiasmata In prophase I, crossing over or recombination events create chiasmata. Figure 3.5

28. Independent Assortment The homolog of one chromosome can be inherited with either homolog of a second chromosome. Figure 3.6

29. Spermatogenesis: sperm formation Figure 3.7

30. 3.3 Gamete Maturation • The cells of the maturing male and female proceed through similar stages but with sex specific terminology and different time tables • Males begin manufacturing sperm at puberty and continue throughout life. • Females begin meiosis as a fetus and complete meiosis only if a sperm fertilizes and oocyte

31. Spermatogenesis- formation of sperm cells • Begins in the diploid cell-spermatogonium. • The spermatogonium divides by mitosis to yield two daughter cells. • One daughter cell will specialize into mature sperm. • One daughter cell will remain a stem cell.

32. As the mature spermatogonium accumulate cytoplasm, replicate DNA-become primary spermatocytes. • Meiosis I- each primary spermatocyte divides forming two equal sized haploid cells called secondary spermatocytes. • Meiosis II- each secondary spermatocyte divides to yield two spermatids.

33. Each spermatid then develops the characteristic tail-flagellum. • The base of the tail has many mitochondria that release ATP propelling the sperm in the female tract. • After spermatidid differentiation some of the cytoplasm connecting the cells falls away leaving mature tadpole shaped speramtozoa or sperm.

34. Spermatogenesis • Stem cells in testes divide mitotically to produce spermatocytes • Spermatocytes divide bymeiosis to produce four equal sized haploid spermatids that mature into four sperm . Figure 3.9

35. Each sperm has a tail, body or midpiece, and a head region. • The membrane covered front end-acrosome- enzymes to penetrate the oocyte. • In the head DNA is wrapped around proteins- inactive. • Built in protections- • Spermatogonia exposed to toxins do not mature into sperm or cannot swim.

36. Oogenesis Figure 3.11

37. Oogenesis: Ovum Formation • Cells of the ovary divide to form oocytes • Oocytes divide by meiosis • Unequal cytoplasmic division • A discontinuous process • At birth, oocytes are arrested in prophase I • At ovulation, an oocyte continues to metaphase II • The four meiotic products produce a functional ovum and three polar bodies.

38. Fertilization The ovum completes meiosis II after fertilization Figure 3.13 • Fertilization is the union of sperm and ovum.

39. Fertilization • Hundreds of millions of sperm are deposited in the vagina during sexual intercourse. A sperm cell can survive for up to 3 days but the oocyte can only be fertilized 12-24 hrs. after ovulation. • Woman’s body helps sperm reach the oocyte. • Capacitation chemically activates sperm. • Oocyte release an attractant chemical. • Female muscle contractions assist • Moving sperm tails • Only 200 sperm come near the oocyte.

40. The encounter of sperm and oocyte is dramatic. • Wave of electricity • Physical and chemical changes occur over the entire oocyte surface. • These chemical reactions prevent additional sperm from entering the ovum. • Additional sperm can enter but there is too much genetic material for development to follow.

41. Usually only the sperm’s head enter the oocyte. • The ovum’s nuclear membrane disappears and the two sets of chromosomes called pronuclei approach. • Within each pronucleus, DNA replicates. • Fertilization completes when the two genetic packages merge . • The fertilized ovum is called a zygote.

42. Multiple Births • Dizygotic twins • Form from two differ zygotes • Two ova are fertilized • Same genetic relationship as any siblings • Monozygotic twins • One ova is fertilized • Developing embryo splits during early development • Genetically identical

43. Figure 3.16

44. Abnormal Chromosome Number • Atypical chromosomes account for at least 50 percent of spontaneous abortions, yet only 0.65 percent of newborns have them. • Therefore, most embryos and fetuses with atypical chromosomes stop developing before birth. • See Table 13.2

46. Polyploidy -most extreme - an entire extra set . An individual whose cells have three copies of each chromosome is a triploid (designated 3N, for three sets of chromosomes). Two-thirds of all triploids result from fertilization of an oocyte by two sperm. The other cases arise from formation of a diploid gamete, such as when a normal haploid sperm fertilizes a diploid oocyte. Triploids account for 17 percent of spontaneous abortions (figure 13.11). Very rarely, an infant survives as long as a few days, with defects in nearly all organs. However, certain human cells may be polyploid. The liver, for example, has some tetraploid (4N) and even octaploid (8N) cells.

48. Aneuploidy • Cells missing a single chromosome or having an extra one. • A normal chromosome number is euploid, which means “good set.” • Most autosomal aneuploids (with a missing or extra non-sex chromosome) are spontaneously aborted • Intellectual disability is common in aneuploidy because development of the brain is so complex • Sex chromosome aneuploidy usually produces milder symptoms. • Most children born with the wrong number of chromosomes have an extra chromosome (a trisomy) rather than a missing one (a monosomy).

49. Nondisjunction • The meiotic error that causes aneuploidy is called nondisjunction. • In normal meiosis, homologs separate and each of the resulting gametes receives only one member of each chromosome pair. • In nondisjunction, a chromosome pair fails to separate at anaphase of either the first or second meiotic division. This produces a sperm or oocyte that has two copies of a particular chromosome, or none, rather than the normal one copy. • When such a gamete fuses with its partner at fertilization, the zygote has either 45 or 47 chromosomes, instead of the normal 46.