1. Cell Division, part 1 Prokaryotes – Binary Fission
Eukaryotes – Mitosis
Prophase
Metaphase
Anaphase
Telophase

2. Cellular Division Overview
Cell division requires
Duplication, organization and sorting of chromosomes
Single-celled organisms: asexual reproduction
Multi-celled organisms: growth, replacement

3. I. Prokaryotes – Binary Fission

4. Binary Fission: Prokaryotes, Yeast & Amoeba

5. II. Eukaryotes: Mitosis
Karyokinesis – nuclear division
Cytokinesis – cytoplasmic division

7. Interphase Comprises G1, S, G2 phases of cell cycle
Key event = replication of DNA during S, each chromosome will become two sister chromatids
Sister chromatids are not visible during this phase, & the nucleus remains in tact.

8. mitosis
interphase
Prophase, prometaphase
Metaphase
Anaphase
Telophase

9. Mitosis at the DNA level:

10. MITOSIS

11. A. Prophase, prometaphase
whitefish blastula cells
A. Prophase, prometaphase
First part of M phase
Chromatin condenses into visible chromosomes, sister chromatids are joined at the centromere (genetically identical to one another)
Nucleoli disappear, and nuclear membrane begins to break down
Centrosomes appear; Spindle fibers form

12. Prometaphase – chromosomes begin to move
Once the spindle apparatus is in place it attaches to the kinetochore on the centromere.
Homologues move toward the “equator”

13. B. Metaphase
Each chromosome lines up on the equatorial plane (metaphase plate), led by the centromere via the spindle apparatus

14. C. Anaphase
Sister chromatids of each chromosome separate from each other and migrate to opposite ends of the cell
Each centromeric region is split in two, once this occurs each chromatid is referred to as a daughter chromosome
Movement of each chromosome made possible by the spindle apparatus

16. D. Telophase Cleavage furrow forms in animal cells,
Cell plate in plants…
Final stage of mitosis
Cytokinesis occurs – cytoplasm is partitioned to each half, cleavage furrow forms in animal cells; cell plate forms in plant cells
Chromatids de-condense into chromatin
Nuclear membrane reforms around each nucleus; cell divides into two genetically identical daughter cells

17. SIGNIFICANCE: results in 2 new identical daughter cells, the exact distribution of the DNA (via chromosomes) to the daughter cells ensures the stability of cells and the inheritance of traits from one generation to the next.

18. 2n = 4 Know the number of chromosomes/chromatids during each phase
2n = 12, 24, 46… etc.

19. Cell Division, part 2 - Meiosis
Sexual reproduction requires gametes
Meisos v. mitosis
Prophase I
Letoneme stage
Zygoneme stage
Pachyneme stage
Diploneme stage
Diakinesis
Metaphase, Anaphase, Telophase I
The second meiotic division
Gamete development in animals
Spermatogenesis
Oogenesis

20. I. Sexual reproduction requires gametes
Meiosis = cell division that halves the genetic content for sexual reproduction
Meiosis is critical to the successful sexual reproduction of all diploid organisms-
Mechanism by which 2n is reduced to n
Leads to the formation of gametes
Basis for the production of extensive genetic variation among members of a population

21. Gametes: Haploid Gametes – isogamous vs. heterogamous
In many species, haploid (n) gametes are descended from germ cells that are originally diploid (2n) (via meiosis)
hapliod – they contain ½ the genetic content
Gametes then combine in fertilization to reconstitute the diploid complement found in parental cells

22. II. Meiosis, different from mitosis:
Two successive nuclear divisions, MI & MII, which produces haploid gametes that differ genetically
Homologous chromosomes pair up (synapse), forming tetrads
Non-sister chromatids exchange homologous sections of DNA (crossing over)

26. III. Prophase I Leptotene stage
Chromatin begins to condense, chromosomes become visible
Chromomeres develop along each chromosome
Localized condensations
Homology search underway – essential to the initial pairing of homologs

27. B. Zygotene stage Continued chromosomal condensation
Homologous chromosomes undergo initial alignment, wherein "pairing sites" on the chromosomes are matched
Synaptonemal complex begins to form between the homologs
Upon completion of this stage, paired homologs are referred to as bivalents

29. C. Pachytene stage
Further development of synaptonemal complex occurs between the two members of each bivalent…leading to Synapsis
Chromomeres align in the bivalents, producing a distinctive pattern for each pair (completes the “homology search”)
Physical exchange between non-sister chromatids occurs (but not visible yet)
An identical pattern of twinned loops occurs on both pairs of sister chromatids

30. D. Diplotene stage
Tetrads highly visible, each consisting of two pairs of sister chromatids
Within each teterad, each pair of sister chromatids begins to separate
One or more areas remain in contact where chromatids are intertwined = Chiasma (crossing over visible)

32. Crossing over at the DNA level –
Holliday Structure

33. Significance of CROSSING OVER:
Crossing over results in recombinant chromosomes that now have paternal alleles at some loci and maternal alleles at other loci.

34. Additional Crossing over info:
The exchange is reciprocal, such that each chromosome gets the same region of the chromosome segment from the other parent that it donated to the other chromosome.
Crossovers are frequent and there is usually at least one on each chromosome and 3-4 on the larger chromosomes.

35. E. Diakinesis Final stage of prophase I
Chromosomes pull farther apart, but nonsister chromatids remain loosely associated via the chaismata
Terminalization occurs = Chiasmata move toward the ends of the tetrad
Nucleolus and nuclear envelope break down & the two centromeres of each tetrad attach to spindle fibers

38. IV. Metaphase, Anaphase & Telophase I
Metaphase I – tetrads move to the metaphase plate, alignment is random
Anaphase I – ½ of each tetrad (dyad) is pulled to one or the other pole at random, and the the other ½ moves to the opposite pole [disjunction]
Telophase I – nuclear membrane forms around the dyads, cleavage furrow forms, nucleus enters short interphase

39. tetrad
dyad

40. V. Second meitotic division
Prophase II
Metaphase II
Anaphase II
Telophase II
monad

41. nondisjunction

42. Review questions
A cell containing 64 chromatids at the start of mitosis would, at its completion, produce cells containing how many chromosomes?
What if the same cell undergoes meiosis – how many chromatids would there be in the daughter cells after MI, and how many chromosomes after MII?
When ½ of the gametes produced are trisomic, what has occurred during meiosis? (be specific)
Which of the following is FALSE in comparing prophase I of meiosis and prophase of mitosis?
The chromosomes condense
Homologs pair up side by side
The nuclear envelope disassembles
A spindle apparatus forms from centrosomes
Each chromosome has two chromatids 32
32, 16
First division nondisjunction

43. The significance of meiosis is that it produces genetic variation by reducing the genetic material by ½ that is recombined, to novel produce gametes for fertilization
Meiosis produces new Combinations of genes in 3 ways:
Random assortment of maternal and paternal chromosomes, and the alleles of genes they contain
each monad is a combination of maternal & paternal genetic information
Recombination due to crossing over and exchange of chromosome parts between non-sister chromatids
Random fertilization

44. Random assortment

45. The number of possible combinations of maternal and paternal homologues is 2n, where n = the haploid number of chromosomes. In this diagram, the haploid number is 3, and 8 (23) different combinations are produced.

47. VI. Gamete development in animals
Spermatogenesis
takes place in testes
Germ cell = spermatogonium, enlarges to become a primary spermatocyte
Primary spermatocyte undergoes MI, producing 2 secondary spermatocytes – producing haploid spermatids
Spermatids undergo modifications – becoming spermatozoa

48. B. Oogenesis Occurs in ovary
Germ cell = oogonium, enlarges to form primary oocyte
Primary oocyte undergoes MI, producing 1 large secondary oocyte & 1 small polar body
Unequal division of cytoplasm results in polar body
Secondary oocyte undergoes MII, producing 1 large haploid ootid & 1 small polar body
Ootid differentiates into mature ovum

50. 2n = 8, what phase of mitosis, MI or MII is this cell in?
This cell is in metaphase II, if it were in mitosis there would be 8 pairs of sister chromatids!

51. 2n = 4, what phase of cell division is each cell in?
Metaphase
of mitosis
Interphase
Metaphase I
Metaphase II