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Cell Division

Cell Division. Mitosis, meiosis and cytokinesis. Cell Division. Results in the formation of two identical “daughter” cells Each daughter cell will be roughly ½ the size of the parent cell Before the cell divides, the DNA of the cell must be copied and separated (mitosis)

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Cell Division

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  1. Cell Division Mitosis, meiosis and cytokinesis

  2. Cell Division • Results in the formation of two identical “daughter” cells • Each daughter cell will be roughly ½ the size of the parent cell • Before the cell divides, the DNA of the cell must be copied and separated (mitosis) • Actual cell division is called cytokinesis.

  3. Why do cells have to divide • Limits to size! • Surface area/volume ratio • Trouble exchanging materials with its outside environment • Too much demand placed on DNA • Larger the cell, more proteins need to be made! • Only one copy of each DNA molecule in a cell!

  4. Cell Division Mitosis occurs exclusively in eukaryotic cells. In multicellular organisms, the somatic (body cells) cells undergo mitosis, while germ cells (cells destined to become sperm in males or ova in females) divide by a related process called meiosis. Prokaryotic cells, which lack a nucleus, divide by a process called binary fission.

  5. Forms of DNA • DNA + Proteins = Chromosomes • Most of the time, DNA is “unspooled” into loose strands – called chromatin • can be used to provide instructions in this form. • Before a cell divides, the DNA winds around histone proteins and becomes visible as “chromosomes” • The chromosomes can be counted in this form.

  6. Structure of DNA Nucleosome Chromosome DNA double helix Coils Supercoils Histones

  7. Chromosome number • Eukaryotic organisms have a specific number of different types of chromosomes. • They have two of each type – so chromosomes come in pairs. • Cells with chromosomes present in pairs are said to be diploid (2N) • one of each is haploid

  8. Chromosome identity • Each chromosome type can be identified by shape and size. • If stained, characteristic patterns of bands can be seen as well. • A karyotype is a display of all of an individual’s chromosomes arranged by type. • Can be used to identify major genetic disorders.

  9. Chromosome number • Each organism has a different number of chromosomes: Camel 70 Chicken 78 Opossum 22 Housefly 22 Bat 44 Corn 24 Lentil 14 Rice 24 Goat 60 Barley 14 Apple 34 Lettuce 12 HUMANS HAVE 46!: 23 different pairs

  10. DNA  By the Numbers • In a diploid (2N) eukaryotic cell, one set of chromosomes comes from the mother and another from the father. • Each human gets 23 from each parent (46 total) • 44 are autosomes (general body characteristics) and two are sex chromosomes (determine sex and carry general characteristics)

  11. The two corresponding chromosomes are called homologouschromosomes. Homologous chromosomes need not be genetically identical. • For example, a gene for eye color at one locus (location) on the father chromosome may code for green eyes, while the same locus on the mother chromosome may code for brown.

  12. Regardless of the number, before a cell can divide, it must generate an exact copy of each chromosome. • When the cell divides, each daughter cell will get one copy of each of the total number of chromosomes.

  13. Before Cell Division, each chromosome must replicate! • Individual chromosomes replicate and form sister chromatids. • Each sister chromatid is destined for one of the two resulting daughter cells

  14. The copies are called sister chromatids, and together they are considered one chromosome. • After separation, however, each sister chromatid is considered a full-fledged chromosome by itself. The two copies of the original chromosome are then called sister chromosomes

  15. After Replication  Chromosomes

  16. The Cell Cycle • Sequence of events a cell goes through as it grows and divides • (G1 Phase) Cell grows and synthesizes proteins and new organelles • (S Phase) Chromosomes replicate • (G2 Phase) Organelles and molecule used in cell division are produced • (M Phase) mitosis and cytokinesis, the actual division of the cell into two daughter cells.

  17. Cell Division M-Phase • Consists of TWO steps (Mitosis and Cytokinesis) • Mitosis  process by which a cell separates its duplicated genome into two identical halves. Mitosis only separates the newly replicated chromosomes; DNA replication does not occur during mitosis. • broken down into five phases: (PMAT) Prophase, Metaphase, Anaphase, Telophase. • Cytokinesis which divides the cytoplasm and cell membrane.

  18. Mitosis  Prophase • Longest phase of mitosis • Chromosomes condense (become visible) • Centrioles (in cytoplasm) separate and move to opposite sides of cell • Nuclear membrane breaks-down • Microtubule structure called the spindle develops (attaches from centrioles to chromosomes

  19. Chromosome Structure • Prior to separation, the two sister chromatids are attached together in a specialized region of the chromosome known as the centromere.

  20. Mitosis  Metaphase • Chromosomes line-up along center of cell (metaphase plate) • Each chromosome is connected to its centromere by a spindle fiber

  21. Mitosis  Anaphase • Sister chromatids separate into separate chromosomes • Separated chromosomes pulled to opposite sides

  22. Mitosis  Telophase • Chromosomes move together at opposite ends of the cell and become less condensed • Spindle breaks apart • Two new nuclear membrane form • Result is one cell with 2 nuclei!

  23. Cytokinesis • Remember, NOT part of mitosis • Animals • Cell membrane pinches off cytoplasm into two equal parts at a region called the cleavage furrow • Plants • Cell Plate develops between two new nuclei which grows into a separating membrane and ultimately a separating cell wall

  24. Mitosis Animation Cell Cycle Animation • http://www.cellsalive.com/mitosis.htm

  25. Limits to Division? • The big problem with eukaryotes is that they have to replicate linear chromosomes. The polymerase enzyme can’t work all the way to the end, so the chromosome gets shorter with each round of replication. • Solution: use special ends called telomeresthat get recreated with a special enzyme called telomerase. This may play a crucial role in human aging; if the chromosome ends fail to replicate properly, the chromosomes gradually lose parts of their end sequence.

  26. Regulation of Cell Cycle • Not all cells move through cell cycle at same rate • Bone marrow cells/skin cells  continuous division • Nerve and muscle cells  seldom or never

  27. Cycle Regulators • The cell cycle is regulated by special proteins called cyclins and cyclin-dependent kinases. • High concentrations of cyclin influences a cell to divide. • Internal Regulators proteins that respond to internal stimuli: cell cycle checkpoints! • Ex. Cell will not enter mitosis until all chromosomes are replicated • External Regulators proteins that respond to external stimuli • Ex. Cell will begin to divide rapidly after injury • Ex. When dividing cells come in contact with adjacent cells, division will slow

  28. WHEN CELLS GO BAD! • When a body’s cell lose the ability to control growth, cancer is the result. • Cancer cells do not respond to chemical signals that tell them to stop growing. • Form masses of cells called tumors that damage surrounding tissues.

  29. Meiosis • We know that regular somatic (body) cells contain TWO sets of chromosomes (diploid/ 2N) • When a sexually reproducing organism produces gametes (sex cells) they must somehow separate these pairs of chromosomes so gametes only get one set. • WHY?

  30. Ex. Humans • Normal Diploid (2N) somatic cell contains _____ chromosomes (_____ pairs) • Gametes (sperm and egg cells) need to contain _________ chromosomes. • We generate these HAPLOID (N) cells through the process of meiosis!

  31. Steps of Meiosis • Divided into two distinct stages • Meiosis I • Meiosis II • Starts with one diploid cell and ends with 4 haploid daughter cells • Before meiosis begins, DNA undergoes replication just like in mitosis!

  32. Meiosis I: Prophase I • Appearance of the chromosomes, the development of the spindle, and the breakdown of the nuclear membrane (envelope). • Each replicated chromosome pairs up with its corresponding homologous chromosome • Paired chromosomes (4 chromatids) form a tetrad

  33. Tetrads and crossing over • It is during this alignment that chromatid arms may overlap and temporarily fuse (chiasmata, or synapsis), resulting in crossovers • Segments of homologous chromosomes may switch places where overlap occurs.

  34. What is Crossing Over? • Paired-up homologous chromosomes, may exchange portions of their chromatids • Advantage?

  35. Meiosis I: Metaphase I • Here is where the critical difference occurs between Metaphase I in meiosis and metaphase in mitosis. In the latter, all the chromosomes line up on the metaphase plate in no particular order. In Metaphase I, the chromosome pairs are aligned on either side of the metaphase plate.

  36. Meiosis I: Anaphase I • During Anaphase I the homologous pairs separate from each other and move along the spindle fibers to each pole of the cell.

  37. End of Meiosis I • At the end, each daughter cell has a single set of chromosomes, half the total number in the original cell where the chromosomes were present in pairs. • While the original cell was diploid, the daughter cells are now haploid. This is why Meiosis I is often called reduction division.

  38. Meiosis II • Meiosis II is quite simple in that it is simply a mitotic division of each of the haploid cells produced in Meiosis I. • There is no Interphase between Meiosis I and Meiosis II

  39. Meiosis II: Prophase II • A new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell.

  40. Meiosis II:Metaphase II • All the chromosomes in the two cells align with the metaphase plate.

  41. Meiosis II: Anaphase II • Sister chromatids separate as they are pulled by spindle fibers

  42. Meiosis II: Telophase II • A cleavage furrow develops, followed by cytokinesis and the formation of the nuclear membrane (envelope). • When Meiosis II is complete, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell.

  43. Meiosis in Males and Females • In male animals, meiosis results in the formation of 4 ___________ cells • In female animals, meiosis results in the formation of one _______ cell and three small polar bodies which die.

  44. Advantages/Disadvantages of sexual reproduction? • Recombination of maternal and paternal chromosomes in the gamete results in genetic variation among the offspring. In an environment which changes, this allows the process of natural selection to occur.

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