2. Types of cell division Prokaryotes Binary fission Eukaryotes Mitosis:
Growth, development & repair
Asexual reproduction (yields genetically identical cells)
Occurs in somatic (body) cells
Meiosis:
Sexual reproduction (yields genetically different cells with half the # of chromosomes)
Occurs in specific reproductive cells
Yields gametes (e.g., eggs & sperm) or spores
So far, I’ve been talking about mitosis only
What? Somatic (body) cells
Why? Growth & development, repair lost or injured cells
Allows many organisms to reproduce asexually
But there is a 2nd type of cell division - meiosis that occurs only in select cells within certain tissue at particular phases of an organism’s lifetime.
meiosis is involved with organisms that undergo SEXUAL REPRODUCTION
--means of reducing number of chromosomes in sperm or egg so when combined through fertilization, the original number is restored
-- reduction of genetic state from diploidy to haploidy necessary
--occurs in reproductive organs in humans -- ovary & testis -- produces haploid cells called gametes (sperm, egg)
--completed with fertilization of male gamete & female gamete to produce diploid zygote

3. Mitotic cell division results in genetically identical daughter cells
Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA
A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells

4. Cellular Organization of the Genetic Material
A cell’s endowment of DNA (its genetic information) is called its genome
DNA molecules* in a cell are packaged into chromosomes
*Prokaryotes- circular DNA
Eukaryotes- linear DNA

5. Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus
Somatic (non-reproductive) cells (normally) have two sets of chromosomes
Gametes (reproductive cells: sperm and eggs) (and spores) have half as many chromosomes as somatic cells
Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division

6. DNA plus proteins is called chromatin.
Condensed, duplicated chromosome
chromatid
telomere
centromere
One half of a duplicated chromosome is a chromatid.
Sister chromatids are held together at the centromere.
Telomeres protect DNA and do not include genes.

7. DNA associates with special proteins to form more stable structure called chromosomes (different proteins in prokaryotes and eukaryotes, so chromosomes built different)
Chromosomes are found inside nucleus in eukaryotes
Human - 46 chromosomes, 23 pairs (1 set of 23 from egg, 1 set of 23 from sperm)
Each chromosome contains many genes
Gene is a segment of DNA that is responsible for controlling a trait (e.g., coding for a specific protein)

8. Fig. 19.1, p. 345: cartoon (left), EM (right)
Chromatin = helix wrapped around protein -- giving bead-like structure
These proteins are called ‘histones’
Nucleosome = DNA + histone complex
Chromatin = a string of these beads (thread-like)
Chromosome = looped and compacted chromatin

9. Human female karyotype

10. Human male karyotype

11. The cell cycle is a repeated pattern of growth and division that occurs in eukaryotic cells.
This cycle consists of the phases: G1, S, G2, M

12. The cell cycle has four main stages.
The cell cycle is a regular pattern of growth, DNA replication, and cell division.

13. The main stages of the cell cycle are gap 1, synthesis, gap 2, and mitosis.
Interphase
Gap 1 (G1): cell growth and normal functions
DNA synthesis (S): copies DNA
Gap 2 (G2): additional growth
Mitosis (M): includes division of the cell nucleus (mitosis) and division of the cell cytoplasm (cytokinesis)

14. Distribution of Chromosomes During Cell Division
In preparation for cell division, DNA is replicated and the chromosomes condense
Each duplicated chromosome has two sister chromatids, which separate during cell division
The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

15. 0.5 µm
Chromosome
duplication
(including DNA
synthesis)
Centromere
Sister
chromatids
Separation
of sister
chromatids
Centromeres
Sister chromatids

16. Interphase
Cells spend the majority of their cell cycle in interphase.
The purpose of interphase is for cell growth.
By the end of interphase a cell has two full sets of DNA (chromosomes) and is large enough to begin the division process.

17. Interphase is divided into three phases
Interphase is divided into three phases. Each phase is characterized by specific processes involving different structures.
During the G1 (gap 1) phase, the cell grows and synthesizes proteins.
During the S (synthesis) phase, chromosomes replicate and divide to form identical sister chromatids held together by a centromere.
During the G2 (gap 2) phase, cells continue to grow and produce the proteins necessary for cell division.

18. Mitosis
The purpose of mitosis is cell division: making two cells out of one.
Each cell has to have its own cytoplasm and DNA.

19. Mitosis is divided into four phases
Mitosis is divided into four phases. Each phase is characterized by specific processes involving different structures.
Prophase
Metaphase
Anaphase
Telophase

20. Chromosomes condense at the start of mitosis.
DNA wraps around proteins (histones) that condense it.
DNA double helix
DNA and histones
Chromatin
Supercoiled DNA

21. Prophase is characterized by four events:
Chromosomes condense and are more visible.
The nuclear membrane (envelope) disappears.
Centrioles have separated and taken positions on the opposite poles of the cell.
Spindle fibers form and radiate toward the center of the cell.

22. Metaphase (the shortest phase of mitosis) is characterized by two events:
Chromosomes line up across the middle of the cell at the metaphase plate
Spindle fibers connect the centromere of each sister chromatid to the poles of the cell.

23. Anaphase is characterized by three events:
Centromeres that join the sister chromatids split.
Sister chromatids separate becoming individual chromosomes.
Separated chromatids move to opposite poles of the cell.

24. Telophase (the last phase of mitosis) consists of four events:
Chromosomes (each consisting of a single chromatid) uncoil.
A nuclear envelope forms around the chromosomes at each pole of the cell.
Spindle fibers break down and dissolve.
Cytokinesis begins.

25. Cytokinesis
Cytokinesis is the actual division of the cytoplasm into two individual cells.
The process of cytokinesis differs somewhat in plant and animal cells.
In animal cells the cell membrane forms a cleavage furrow that eventually pinches the cell into two nearly equal parts, each part containing its own nucleus and cytoplasmic organelles.

26. Telophase/Cytokinesis
Cleavage furrow
100 µm
Contractile ring of
microfilaments
Cleavage of an animal cell (SEM)
Animal Cell
Telophase/Cytokinesis
Daughter cells

27. Mitosis

28. In plant cells a structure known as a cell plate forms midway between the divided nuclei, which gradually develops into a separating membrane.
The cell wall forms in the cell plate.

29. Plant Cell Telophase/ Cytokinesis Vesicles forming cell plate Wall of
parent cell
1 µm
Cell plate
New cell wall
Daughter cells
Cell plate formation in a plant cell (TEM)

30. Cells divide at different rates.
The rate of cell division varies with the need for those types of cells.
Some cells are unlikely to divide (G0).

31. Cell size is limited.
Volume increases faster than surface area.

32. Surface area must allow for adequate exchange of materials.
Cell growth is coordinated with division.
Cells that must be large have unique shapes.

33. The cell cycle is regulated by a molecular control system
The frequency of cell division varies with the type of cell
These cell cycle differences result from regulation at the molecular level
The cell cycle appears to be driven by specific chemical signals present in the cytoplasm
The levels of these chemical signals are influenced by biotic & abiotic factors

34. The Cell Cycle Control System
The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock
The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received
For many cells, the G1 checkpoint seems to be the most important one

35. G1 checkpoint
Control
system S G1 G2 M
M checkpoint
G2 checkpoint

36. G0
G1 checkpoint G1 G1
If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues on in the cell cycle.
If a cell does not receive a go-ahead signal at the G1 checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.

37. G1
Cyclin S
Cdk
Degraded
cyclin M G2
accumulation G2
checkpoint
Cdk
Cyclin is
degraded
Cyclin
MPF
Molecular mechanisms that help regulate the cell cycle

38. An external signal is density-dependent inhibition, in which crowded cells stop dividing
Most animal cells also exhibit anchorage dependence, in which they must be attached to a substrate in order to divide

39. Cells anchor to dish surface and
divide (anchorage dependence).
When cells have formed a complete
single layer, they stop dividing
(density-dependent inhibition).
If some cells are scraped away, the
remaining cells divide to fill the gap and
then stop (density-dependent inhibition).
25 µm
Normal mammalian cells

40. Cancer cells do not exhibit
anchorage dependence
or density-dependent inhibition.
25 µm
Cancer cells

41. Loss of Cell Cycle Controls in Cancer Cells
Cancer cells do not respond normally to the body’s control mechanisms
Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue
If abnormal cells remain at the original site, the lump is called a benign tumor
Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors

42. Reduction-Division Genetic Recombination
Meiosis
Reduction-Division
Genetic Recombination

43. DIPLOID (2n)  HAPLOID (n) Meiosis is SEXUAL reproduction
The form of cell division by which GAMETES, with HALF the number of CHROMOSOMES, are produced
DIPLOID (2n)  HAPLOID (n)
Meiosis is SEXUAL reproduction
TWO divisions (MEIOSIS I and MEIOSIS II)

44. Sex cells divide to produce GAMETES (sperm or egg)
Meiosis
Sex cells divide to produce GAMETES (sperm or egg)
Gametes have HALF the # of chromosomes
Occurs only in GONADS (testes or ovaries)
Male: SPERMATOGENESIS -sperm
Female: OOGENESIS - egg or ova

45. Spermatogenesis n=23 n=23 2n=46 Meiosis sperm haploid (n) Meiosis II
human
sex cell
diploid (2n)
n=23
Meiosis I

46. Oogenesis n=23 n=23 2n=46 Meiosis Haploid (1n) human egg sex cell
Meiosis II
2n=46
human
sex cell
diploid (2n)
n=23
Meiosis I
Polar Bodies (die)

47. Interphase I Similar to mitosis interphase. Meiosis
CHROMOSOMES (DNA) replicate in the S phase
Each duplicated chromosome consist of two identical SISTER CHROMATIDS attached at their CENTROMERES.
CENTRIOLE pairs also replicate.

48. Meiosis I (four phases)
Cell division that reduces the chromosome number by one-half.
Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I

50. Prophase I Longest and most complex phase (90%). Chromosomes condense.
Meiosis
Prophase I
Longest and most complex phase (90%).
Chromosomes condense.
Synapsis occurs - Homologous chromosomes come together to form a tetrad.
Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids)

51. Non-Sister Chromatids-HOMOLOGS
Meiosis
Non-Sister Chromatids-HOMOLOGS
Homologs contain DNA that codes for the same genes , but different versions of those genes
Genes occur at the same loci

52. Homologous chromosomes
Meiosis
Meiosis
Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

53. Homologous Chromosomes
Meiosis
Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs (tetrads) carry GENES controlling the SAME inherited traits.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes:
a. First 22 pairs of autosomes
b. Last pair of sex chromosomes
LOCI

54. Homologous Chromosomes
Meiosis
Homologous Chromosomes
eye color
locus
hair color
Paternal
Maternal

55. Meiosis
Crossing Over
Crossing over may occur between non-sister chromatids at sites called chiasmata.
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.
Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.)
Causes Genetic Recombination

56. Genetic Recombination/Crossing Over
Meiosis
Genetic Recombination/Crossing Over
Tetrad
nonsister chromatids
chiasmata: site of crossing over
variation

57. Meiosis I
MEIOSIS I
Homologs separate

58. Prophase I Meiosis Nucleus & Nucleolus disappear Spindle forms
Chromosomes coil & Synapsis (pairing) occurs
Tetrads form & Crossing over Occurs
centrioles
spindle fiber
aster
fibers
TETRAD

59. Metaphase I Meiosis Shortest phase Tetrads align on the equator.
Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION

60. Formula: 2n Example: 2n = 4 then 1n = 2 thus 22 = 4 combinations
Meiosis
Formula: 2n
Example: 2n = 4
then 1n = 2
thus 22 = combinations

61. Meiosis
Question:
In terms of Independent Assortment -how many different combinations of sperm could a human male produce?

62. Answer Formula: 2n Human chromosomes: 2n = 46 n = 23
Meiosis
Answer
Formula: 2n
Human chromosomes: 2n = 46
n = 23
223 = ~8 million combinations

63. Anaphase I Homologous chromosomes separate and move towards the poles.
Meiosis
Anaphase I
Homologous chromosomes separate and move towards the poles.
Sister chromatids remain attached at their centromeres.

64. Telophase I Each pole now has haploid (1n) set of chromosomes.
Meiosis
Telophase I
Each pole now has haploid (1n) set of chromosomes.
Cytokinesis occurs and two haploid daughter cells are formed.

65. Meiosis
Telophase I
cytokinesis

66. Sister Chromatids Separate
Meiosis
MEIOSIS II
Sister Chromatids Separate

67. Meiosis II No Interphase II No DNA Replication
Meiosis II is similar to mitosis

68. Prophase II Same as Prophase in mitosis Meiosis
Nucleus & nucleolus disappear
Chromosomes condense
Spindle forms

69. Metaphase II Same as Metaphase in mitosis Meiosis
Chromosomes (not homologs) line up at metaphase plase

70. Anaphase II Same as Anaphase in mitosis SISTER CHROMATIDS separate
Meiosis
Anaphase II
Same as Anaphase in mitosis
SISTER CHROMATIDS separate

71. 1n Sperm cell fertilizes 1n egg to form 2n zygote
Meiosis
Telophase II
Same as Telophase in mitosis.
Nuclei and Nucleoli reform, spindle disappears
CYTOKINESIS occurs.
Remember: FOUR HAPLOID DAUGHTER cells are produced.
Called GAMETES (eggs and sperm)
1n Sperm cell fertilizes 1n egg to form 2n zygote

72. Meiosis
Telophase II

73. Variation Meiosis Also known as GENETIC RECOMBINATION
Important to population as the raw material for NATURAL SELECTION.
All organisms are NOT alike
Strongest “most fit” survive to reproduce & pass on traits

74. Genetic Variation CROSSING OVER (prophase I)
Meiosis
Genetic Variation
CROSSING OVER (prophase I)
INDEPENDENT ASSORTMENT (metaphase I)
RANDOM FERTILIZATION