1. Cell Increase and Decrease
Cell Increase and Decrease
Cell division increases the number of somatic (body) cells
Two parts of cell division:
Mitosis (division of nucleus)
Cytokinesis (division of cytoplasm)
Apoptosis (cell death) decreases the number of cells.

2. Cell division occurs when:
Body growth
Maintenance and repair
Fighting infection
Replacing worn/dead cells
Apoptosis occurs when:
Tail of tadpole disappears  frog
Skin between human fingers and toes dies during development
Both cell increase and apoptosis occur during normal development and growth – example of homeostasis.

3. The Cell Cycle
An orderly sequence of events that occurs from the time a cell is first formed until it divides into two new cells.

4. Most of the cell cycle is spent in interphase:
G1 stage – cell growth, cell doubles its organelles (cell structures), prepares for DNA replication
S stage – DNA replication occurs
G2 stage – cell makes proteins needed for cell division
Amount of time spent in interphase varies – average for adult mammals is 20 hours
Nerve cells and muscle cells exit the cell cycle  G0 phase

5. Following interphase is the M stage, including mitosis and cytokinesis (definitions slide #1).
During mitosis, two copies of DNA made during replication are separated, and become the nuclei of the two daughter cells – takes about 4 hours.
The cell cycle ends when cytokinesis, the splitting of the cytoplasm, is complete.

6. Control of the cell cycle
Three checkpoints:
During G1 prior to the S stage – if DNA is damaged, apoptosis occurs
During G2 prior to the M stage – will not proceed if DNA is damaged or not copied
During the M stage prior to the end of mitosis – if chromosomes are not properly aligned

8. Chromosome Structure
In a non-dividing cell – genetic material is in the form of chromatin (DNA & protein)
In a dividing cell, chromatin undergoes coiling to form chromosomes
Proteins called histones package the DNA so it can fit into the nucleus (2 meters of DNA fit into nucleus that is 5 micrometers)

11. After replication, there are 2 identical sister chromatids, held together by a centromere

12. Each species has a set number of chromosomes:
Humans – 46
Crayfish – 200
Corn – 20
Adder’s tongue fern – 1262
Chimpanzee - 48
Sand dollar – 52
Dog – 78
Cat - 32

13. Body cells contain the diploid (2n) number of chromosomes – 2 chromosomes of each kind (1 from each parent)
Sex cells (eggs and sperm) contain only 1 chromosome of each kind – haploid (n) number of chromosomes

14. Mitosis – occurs in body cells – diploid cells divide to produce diploid cells – daughter cells are genetically identical to parent cells
Animation of mitosis

15. Mitosis overview

16. Centriole
Chromatin
Nucleolus (in yellow)
Nuclear membrane
Spindle fibers
Chromosome (replicated)
Centromere
Sister Chromatids (each half of replicated chromosomes)
Daughter Chromosomes (once the replicated chromosome splits)
Cell membrane
Cleavage furrow
Asters
Centrosome (=aster + centriole)

24. Late Interphase
Centrosomes (which contain pair of centrioles and an aster – which are short microtubules) duplicate
Chromatin condenses into chromosomes

25. Early Prophase
Chromosomes become visible
Centrosomes move to opposite ends of the cell
Nuclear membrane and nucleolus disappear

26. Late Prophase
Spindle fibers form
Chromosomes become attached to spindle fibers – centromere attaches to spindle fibers

27. Metaphase
Chromosomes line up at metaphase plate – equidistant from poles

28. Anaphase
Centromeres holding sister chromatids divide
Sister chromatids separate, becoming daughter chromosomes, and move toward opposite ends of cell

29. Telophase
Spindle disappears
Nuclear membrane reappears
Chromosomes turn into chromatin
Nucleolus reappears

30. Mitosis in Plant Cells
Same phases as in animal cells
Have centrosome and spindle, but no centrioles or asters

32. Cytokinesis, or division of cytoplasm, accompanies mitosis.
Cleavage of the cytoplasm begins in anaphase, but is not completed until just before the next interphase.
Newly-formed cells receive a share of organelles made during interphase.
Division/Mitosis/Mitosis_Video_Quiz.asf

33. Cytokinesis in Animal Cells
A cleavage furrow (indentation of membrane where cell will divide) begins at the end of anaphase.
A band of actin and myosin filaments, called the contractile ring, slowly forms a constriction between the two daughter cells.
A narrow bridge between the two cells is apparent during telophase, then the contractile ring completes the division.

34. Cytokinesis in animal cells

35. Cytokinesis in Plant Cells
The rigid cell wall surrounding plant cells cannot form a cleavage furrow.
Instead, a cell plate forms from vesicles released by the Golgi apparatus (a part of the cell that processes proteins)
New plant cell walls form and are later strengthened by cellulose fibers.

36. Cytokinesis in plant cells

37. Cell Division in Prokaryotes
The process of asexual reproduction in prokaryotes is called binary fission.
The two daughter cells are identical to the original parent cell, each with a single chromosome.
Following DNA replication, the two resulting chromosomes separate as the cell elongates.
Cell divides without cell structures seen in plants & animals

38. Animation of binary fission

39. Meiosis
Produces sex cells (gametes) – eggs & sperm
Reduces the chromosome number so that egg or sperm cells each have only one of each kind of chromosome (2n  1n).
The process ensures that the next generation will have:
the diploid number of chromosomes
a combination of traits that differs from that of either parent.

40. Overview of meiosis

45. Meiosis involves two cell divisions and produces four haploid cells.
Humans have 23 pairs of homologous chromosomes (chromosomes with the same genes), or 46 chromosomes total.
Prior to meiosis I, DNA replication occurs.

46. In many organisms, haploid daughter cells mature into gametes (sex cells – eggs and sperm)
Fertilization (fusion of egg and sperm) -restores the diploid number of chromosomes

47. Phases of Meiosis
The same four phases seen in mitosis – prophase, metaphase, anaphase, and telophase – occur during both meiosis I and meiosis II.
The period of time between meiosis I and meiosis II is called interkinesis.
No replication of DNA occurs during interkinesis
Animation
Meiosis square dance

48. Meiosis I
Prophase I
Nuclear memebrane & nucleolus disappear
Spindle forms
Homologous chromosomes pair during synapsis
Metaphase I
Homologous chromosomes line up at metaphase plate

50. Anaphase I
Homologous chromosomes separate & move to opposite poles
Telophase I
Nuclear membrane and nucleolus reappear
Cytokinesis occurs
Interkinesis
Period of time between meiosis I and II

52. Meiosis II
Prophase II
Spindle reappears, nucleolus and nuclear membrane disappear
Chromosomes attach to spindle
Metaphase II
Chromosomes line up at metaphase plate

53. Anaphase II
Sister chromatids separate, becoming daughter chromosomes
Telophase II
Spindle disappears, nuclear membrane and nucleolus reappear
Cytokinesis divides the cells

55. Genetic Recombination
Genetic variation occurs in several ways:
Crossing-over of nonsister chromatids – occurs during prophase I

56. 2) Independent assortment of homologous chromosomes – separate in a random manner – 223 or 8,388,608 possible combinations of the 23 pairs of chromosomes

57. Combining of chromosomes of genetically different gametes during fertilization
Between crossing over, combinations of gametes produced, and random combining of sperm and egg, variation is endless in the human population

58. Meiosis vs. Mitosis Mitosis
DNA replication occurs only once during interphase.
One cell division
Two diploid daughter cells – genetically identical to parent
Meiosis
DNA replication occurs only once during interphase.
Two cell divisions.
Four haploid daughter cells – genetically different from parent

59. Mitosis
Daughter cells are identical to each other
Occurs in all somatic cells for growth and repair
Meiosis
Daughter cells are different from each other
Occurs only in the reproductive organs for the production of gametes

62. Meiosis compared to mitosis

63. The Human Life Cycle
Requires both mitosis and meiosis.
In males, meiosis occurs as spermatogenesis and produces 4 haploid sperm.
In females, meiosis occurs as oogenesis and produces 1 egg cell.
Mitosis is involved in the growth of a child and repair of tissues during life.

64. Life cycle of humans

65. Spermatogenesis
Diploid primary spermatocytes undergo meiosis I to produce haploid secondary spermatocytes.
Secondary spermatocytes divide by meiosis II to produce 4 haploid spermatids.
Spermatids mature into sperm with 23 chromosomes.

66. Spermatogenesis

67. Oogenesis
Diploid primary oocyte undergoes meiosis I to produce one haploid secondary oocyte and one haploid polar body.
Secondary oocyte begins meiosis II, stops at metaphase II, and is released from the ovary.
Meiosis II will be completed only if sperm are present.
Following meiosis II, there is one haploid egg cell with 23 chromosomes and up to three polar bodies.
Polar bodies serve as a dumping ground for extra chromosomes – will disintegrate.

68. Oogenesis

69. In humans, both sperm cells and the egg cell have 23 chromosomes each (1n)
Following fertilization of the egg cell by a single sperm, the zygote has 46 chromosomes, the diploid number (2n) found in human somatic cells.
The 46 chromosomes represent 23 pairs of homologous chromosomes.
Cell differentiation occurs during development resulting in a variety of cell types