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  1. Chromatin
  2. Chromosomes
  3. After DNA replication
    • Each chromosomes consists of two identical DNA molecule
    • Held at the centromere
    • Each DNA molecule > sister chromatid (only referred to as this after undergone DNA replication)
  4. At the centromere
    • Specific proteins bind
    • Form kinetochore complex
    • Site of attachment of spindle fibres
  5. Chromosomes
    contain units of inheritence (gene) arranged along it
  6. A gene
    specific seq. of nucleotides along a DNA molecule that codes for a specific seq. of A.A in a polypeptide chain
  7. Gene
    • found at particular position of chromosome
    • location > locus (plural LOCI)
  8. Take note
    2 identical sister chromatid > 1 chromosome

    2 chromosome > 1 homologous chromosome
  9. Homologous chromosomes
    • Chromosome pairs that are
    • i) similiar in length, gene position, and centromere location
    • ii) derived from separate parents
    • iii) pair up during synapsis in prophase I form bivalents
  10. Sexual origin
    occurence of homologous pair where one chromosome is inherited from each parent
  11. Different forms of a gene is ?
  12. Number of sets of chromosomes is ?
  13. Haploid
    1 set of chromosomes
  14. Diploid
    2 sets of chromosomes
  15. Polyploid
    More than 2 sets of chromosomes
  16. Advantages of diploidy
    Increase genetic variation - mixture of characteristics from both parents

    Presence of backup - if a gene on 1 chromosome is faulty, the gene on 2nd chromosome can provide back up
  17. Cell Cycle
    • Interphase
    • M phase
  18. Interphase
    period of synthesis and growth

    • G1 - intensive cellular synthesis
    • S phase - semi conservative DNA replication
    • G2 - cellular synthesis
  19. M phase
    Karyokinesis PMAT - nuclear division

    Cytokinesis - cytoplasmic division with equal distribution of cytoplasm and organelles into each daugther cell
  20. Mitotic Cell Cycle
    • Interphase
    • Mitosis
    • Cytokinesis
  21. Mitosis
    process of nuclear division in eukaryotic cells

    • Prophase
    • Metaphase
    • Anaphase
    • Telophase

    • Conserves chromosome no.
    • By equally allocating replicated chromosomes to each of the daughter nuclei
  22. Interphase
    Intense synthesis and growth in cytolasm & nucleus

    • G1 & G2
    • growth and replication of organelles

    • S phase - DNA REPLICATION
    • loosely coiled threads of chromatin replicated via Semiconservative DNA replication
  23. Prophase
    Longest stage in M phase

    Condensation of chromatin into visible chromosomes > sister chromatids joined at centromere

    Centrioles move to 2 opp. poles of cells

    Aster (microtubules,radiate from centrioles)

    Spindle fibres formed

    Nuclear envelope and nucleolus disintegrate
  24. Metaphase
    Chromosomes aligh themselves singly along equatorial plane of cell, right angle to spindle axis

    Spindle fibres attached to kinetochore complex on both sides of centromere

    Pole to pole spindle fibres formed
  25. Anaphase
    Shortest stage where each centreomere divides into 2

    Spindle fibres pull separated chromosomes to opp. poles of cell

    Pole to pole spindle fibres lengthen push poles apart
  26. Telophase
    chromosomes reach poles of cell uncoil and lengthen to form chromatin again

    spindle fibres disintegrate

    nuclear envelope reforms arnd chromatin at each pole

    nucleoli reappear
  27. Cytokinesis in animals
    cell membrane invaginates at equatorial plane, form cleavage furrow

    cleavage furrow extends inwards until they meet, separate two cells
  28. Cytokinesis in plants
    spindle fibres disappear except at equatorial plane (phragmoplast region)

    organells attracted to region

    golgi produces vesicles containing cellulose, fuse to form cell plate

    cell plate extends outwards to parent wall at periphery, form new cell wall separate 2 daughter cells
  29. Significance of mitosis
    Maintaining genetic stability - same hereditary info (genetically identical)

    Growth in no. of cells - no. of cells increases by mitosis

    Replacement of cells - cells are constantly dying and replaced by mitosis

    Regeneration - by mitosis

    Asexual Reproduction - mitosis is the basis of AR

    Production of stem cells - allow for renewal and proliferation of stem cells while maintaing undifferentiated state

    *CANCER - uncontrolled mitosis responsible for formation of tumours
  30. Meiotic Cell Cycle
    • Interphase
    • Meiosis
    • Cytokinesis

    Forming 4 daughter cells with half the number of chromosomes of the parent
  31. Meiosis
    • Single replication of DNA in parent cell
    • Followed by 2 nuclear division and 2 cytokineses


    Reduction Division

    Formation of gametes
  32. Interphase I
    • G1 and G2 growth and replication
    • S phase DNA replication
  33. Meiosis I
    Separation of homologous chromosomes
  34. Prophase I - crossing over
    Chromatin condense to form visible chromosomes

    Each chromosome > 2 sister chromatids

    Homologous chromosomes pair up, form bivalents (synapsis)

    Joined at several pts along their length (chiasmata)

    Centrioles move to 2 opp. poles of cell

    Spindle fibres formed

    Nuclear envelope & nucleolus disintegrate
  35. Chiasma (singular) Chiasmata (plural) *note when labelling
    sites of exchange of genes btw non-sister chromatids of homologous chromosomes (crossing over)

    Breakage and reunion btw 2 non-sister chromatids at each site

    Allleles from 1 chromosome may swap with corresponding alleles from th other chromosome > lead to new combinations of alleles in resulting chromatids
  36. Metaphase I
    Bivalents arrange themselves at equator of spindle

    Spindle fibres attached to one side of kinetochore complex on centromere of each homologous chromosomes

    Pole to pole spindle fibres formed
  37. Anaphase I
    Centromeres do not divide (different from mitosis)

    Spindle fibres pull homologous chromosomes, towards opp poles of cell

    Homologous chromosomes separate into 2 haploid set (1 at each end of spindle)

    Pole to pole spindle lengthen to push pole apart
  38. Telophase I
    Arrival of homologous chromosomes at opp poles (each still composed of 2 sister chromatids)

    Crossing over causes sister chromatids to be not genetically identical

    Chromatids uncoil, nuclear envelope reforms

    Spindle fibres disintergrate

    Nucleoli reappear
  39. Cytokinesis I (end of Meiosis I)
    • Homologous chromosome separated
    • Cleavage occurs
  40. Interphase II
    No further DNA replication occurs

  41. Meiosis II
    Separation of sister chromatids (may not be identical anymore due to crossing over)
  42. Prophase II
    Chromatids shorten and thicken

    Centrioles move to opp. poles of cells

    Spindle fibres appear

    Arranged at right angles to first spindle axis of meiosis I

    Nucleoli and nuclear envelopes disintegrate
  43. Metaphase II
    Chromosomes align themselves singly along equatorial plane

    Spindle fibres attached to kinetochore complex on both sides of centromere

    Pole to pole spindle fibres formed
  44. Anaphase II
    Centromeres divide into 2

    Spindle fibres pull chromatids to opp poles of cell

    Pole to pole spindle fibres lengthen to push poles apart
  45. Telophase II
    Chromosomes uncoil lengthen decondense to form chromatin

    Spindle fibre disintegrate

    Nucleoli reappear

    Nuclear envelopes reform arnd each nucleus

    Half the no. of chromosomes of original parent cell
  46. Cytokinesis II
    Subsequent cleavage produce 4 daughter cells
  47. Significance of meiosis
    Reduction Division - no. of sets of chromosomes halved to haploid no.
  48. Allows for sexual reproduction
    Fertilization - fusion of 2 haploid gametes > diploid zygote

    Meiosis occurs to form haploid gametes

    Fusion of gametes will result in doubling of chromosomes if meiosis did not occur hence reduction division necessary prior to fertilization in sexual reproduction
  49. Leads to genetic variation
    • Gametes are formed - new combinations of alleles can occur via
    • i) Crossing Over (prophase I)
    • ii) Independent Assortment (metaphase I and II)
  50. Crossing Over
    Crossing over segments of non sister chromatids occurs at chiasmata - leads to formation of new combinations of alleles on chromosomes of gametes - recombinant chromosomes that contains genes from both parents
  51. Independent Assortment of chromosomes
    • Bivalents orientate themselves at the equator of spindle RANDOMLY
    • They line up independently of one another

    Hence undergo independent separation during anaphase I

    Different combinations of maternal and paternal chromosomes may result in different gametes being formed
  52. Calculate no. of variations
    • 2n = 46
    • n = 23

    Number of variations = 2n = 223
  53. Metaphase II
    Orientation of pairs of chromatids is random and determines which chromosomes migrate to opp poles during ANAPHASE II
  54. Sexual Reproduction
    • Fusion of gametes completely random
    • Random Fertilization - genetic variation
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