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I. Organization of Genetic Material
Figure 1: Genome Packaging
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Genome:
Figure 2: Chromosome Structure
The Centromere is a region of the chromosome that is constricted & attaches to spindle fibers during division.
Telomeres are the ends of the chromosomes & exist to protect genes from being eroded during DNA replication.
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Chromosome:
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Figure 2.1: Chromosome Changes During Division
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Double Stranded Chromosomes:
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Single Stranded Chromosomes:
II. Mitotic Cell Division
Figure 3: Cell Cycle: Interphase
The stages through which cells pass from one mitotic division to the next is the Cell Cycle. The cell cycle consists of 2
main stages: interphase & the M phase.
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Interphase:
a) G1 phase (1st Gap): longest stage of interphase defined as a period of intense growth & metabolic activity. Various
organelles replicated to ensure the resulting daughter cells are fully equipped to carry on all life processes.
b) S phase (Synthesis): period during which the cell’s DNA is replicated to ensure that each resultant daughter cell has
the same quantity & types of genes as the parent cell. Centrosomes & centrioles also replicate.
c) G2 phase (2nd Gap): shortest stage of interphase defined as a period during which final preparations are made for
division.
Figure 3.1: Cell Cycle: M Phase
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M Phase:
Figure 4: M Phase: Prophase - Prometaphase
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Prophase: chromosomes coil tightly to become visible as 46 Double Stranded Chromosomes, causing the nucleolus to
disappear. Centrosomes (& centrioles) migrate toward opposite poles of the cell as they begin to form spindle
(microtubule) fibers. When completed, the Mitotic Spindle serves as a structure on which chromosomes attach & are
evenly divided among daughter cells.
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Prometaphase (Late Prophase): nuclear envelope breaks down & formation of the spindle is complete. Each
chromosome interacts with the microtubules of the spindle to begin moving toward the middle of the spindle.
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Figure 4.1: M Phase: Metaphase – Telophase/Cytokinesis
Metaphase: interaction between chromosomes & spindle fibers causes all 46 double-stranded chromosomes to line up
on the Metaphase Plate, an imaginary plane equidistant from both poles.
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Anaphase: during a process called Disjunction, double-stranded chromosomes separate at their centromeres & begin
to move toward opposite poles of the cell as Single-Stranded Chromosomes. By the end of anaphase, each pole of the
spindle has a complete set of 46 single stranded chromosomes (equal to the amount of genetic material in the original
parent cell during G1 of interphase).
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Telophase: nuclear division concludes as the nuclear envelope reforms around each group of single stranded
chromosomes. Toward the end of telophase, Cytokinesis, the division of the cytoplasm begins.
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Cytokinesis: is the splitting of the cytoplasm to produce 2 daughter cells. Although each daughter cell may differ in
size & shape, they are genetically identical to each other & the original parent cell (2n to 2n). The means by which this
process occurs differs in animal & plant cells:
a) Animal Cells: a Contractile Ring consisting of actin fibers & myosin proteins forms on the inner side of the plasma
membrane. It contracts to form a Cleavage Furrow which deepens to pinch cell in two.
b) Plant Cells: involves the formation of a Cell Plate derived from the merging of vesicles from the Golgi apparatus
that contains materials to produce a new cell membrane & cell wall.
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Figure 4.2: Cytokinesis: Animal vs Plant Cells
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Figure 5: Prokaryotic Cell Division: Binary Fission
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Binary Fission:
III. Mitosis & Cancer
Figure 6: Cell Cycle Checkpoints
Cancers cells arise due to mutations in genes that control cell cycle “checkpoints”. These mutated Oncogenes,
increase the rate of the cell cycle, leading to other mutations in other genes.
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Ultimately, enough key mutations accumulate resulting in cancer cells that form masses called Tumors.
Benign tumors remain at the original site of formation whereas Malignant tumors metastasize, upon which cells
break away & travel to other parts of the body via the bloodstream or lymphatic system.
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Figure 6.1: Cancer Cell Spread: Metastasis
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Oncogene:
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Cancer:
IV. Sexual Reproduction
Figure 7: Human Sexual Lifecycle
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Sexual Reproduction:
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Figure 8: Diploid vs Haploid Cells
Cells other than the sex cells or Gametes are called Somatic Cells. These cells are considered to be Diploid (2n) in
their chromosome count because they possess 2 copies of each chromosome. In human somatic cells, the diploid
number is 46.
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In contrast to somatic cells, gametes are considered to be Haploid (n) in chromosome count because they possess
only 1 copy of each chromosome. In human gametes, the haploid number is 23. As seen in fig. 8, the union of haploid
gametes during fertilization gives rise to a diploid zygote  embryo  multi-celled individual.
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Figure 8.1: Diploid Cells: Homologous Chromosomes
Within the nucleus of diploid cells, each chromosome within the paternal set can be “matched” with a particular
chromosome within the maternal set with respect to similarities in size, shape, centromere position, & gene content.
Such chromosomes are said to be Homologous to each other.
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Based on the aforementioned physical characteristics, the 46 chromosomes within human diploid cells can be
organized into 23 Homologous Pairs.
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Figure 8.2: Homologous Chromosomes: Gene Content
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Homologous Chromosomes:
III. Mechanism of Meiosis
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Gametogenesis:
a) Testis: site of meiotic gametogenesis in males –produces gametes called Spermatozoa (Sperm).
b) Ovary: site of meiotic gametogenesis in females –produces a gamete called an Ovum.
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Meiosis:
Figure 9: Meiosis I: Early Prophase I – Late Prophase I
Meiosis requires two stages (meiosis I & II) in order to reduce the chromosome count of a diploid stem cell within the
ovaries or testis to the haploid number. Meiosis I is preceded by an interphase similar to that which precedes mitosis.
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Chromosomes condense into 46 double stranded chromosomes & the nucleolus disappears. Centrosomes (centrioles)
migrate toward opposite poles of the cell to form the beginning of Meiotic Spindle.
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Chromosomes organize into 23 homologous pairs during a process called Synapsis, forming groupings of 4 chromatids
called Tetrads. At numerous places along their length, chromatids of a tetrad are crisscrossed at points called
Chiasmata (helps hold homologous chromosomes together).
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In a process called Crossing Over, nonsister chromatids of each tetrad break & exchange genetic material at a
chiasma. This serves to separate linked genes & “shuffle” the genetic makeup of the chromosomes, resulting in genetic
combinations different from that of the original stem cell.
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Figure 9.1: Crossing Over & Genetic Recombination
*Because crossing over results in producing chromatids with new & unique gene combinations, it is considered to be a form of
Genetic Recombination. Another major form of genetic recombination, Independent Assortment, is described below.
The nuclear envelope breakdown by late prophase I. In addition, formation of the meiotic spindle is complete.
Tetrads attach to the microtubules of the spindle & start migrating toward its center.
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Figure 9.2: Meiosis I: Metaphase I – Telophase I/Cytokinesis
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Metaphase I: Due to the interaction between the tetrads & mitotic spindle, the tetrads line up on the Metaphase
Plate, an imaginary plane equidistant from both poles. In the process of aligning along the metaphase plate, tetrads
exhibit a second form of genetic recombination, Independent Assortment:
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a) When aligning along the metaphase plate, maternal & paternal homologs of each tetrad may line up on the right
OR left side. The side they occupy is determined at RANDOM. As a result, each side of the metaphase plate will
exhibit a random assortment of maternal & paternal chromosomes …
Figure 9.3: Independent Assortment
Figure 9.4: Independent Assortment & Genetic Recombination
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Anaphase I: during Disjunction, the homologs of each tetrad separate at their centromeres & begin to move toward
opposite poles of the spindle. At the end of anaphase, each spindle pole possesses 23 genetically recombined doublestranded chromosomes.
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Telophase I: meiosis I ends as the cell continues to elongate, & the nuclear membrane reforms around each set of
dyads. Cytokinesis begins as a cleavage furrow, formed by a contractile ring consisting of actin & myosin proteins,
serves to pinch the cell in two. The result is two daughter cells containing 23 genetically recombined double-stranded
chromosomes.
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*Despite the haploid number (23) being reached at the end of meiosis I (“reduction division”) the chromosomes as still in the double
stranded state. This requires a second division (meiosis II) whereby each dyad will be separated into single stranded chromosomes.
Figure 9.5: Meiosis II
Meiosis II is similar to mitosis. The end result is 4 haploid (n) gametes that are genetically different from the original
primary diploid stem cell in terms of chromosome number & gene combinations.
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IV. Specific Forms of Meiosis: Spermatogenesis & Oogenesis
Figure 10: Spermatogenesis vs Oogenesis
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Spermatogenesis:
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Oogenesis:
Figure 11: Mitosis vs Meiosis
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V. Chromosomal Mutations
Figure 12: Human Karyotype
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Karyotype:
a) The first 22 pairs of homologs are called Autosomes, which contain genes coding mainly for body traits. Pair 23
represents the Sex Chromosomes, for they contain genes that determine an individual’s gender.
Figure 13: Nondisjunction & Aneuploidy
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Nondisjunction:
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Figure 13.1: Effects of Aneuploidy
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Aneuploidy:
a) 2n+1 =
b) 2n-1 =
Aneuploidy & Disease
● Downs Syndrome: 1 in 700 children born & results from trisomy of chromosome 21. Nondisjunction of chromosome 21
occurs more often during oogenesis. Results in unique facial features, short stature, heart defects, susceptibility to
respiratory infection, & mental retardation. Affected individuals have shorter life spans & are usually sexually
underdeveloped & sterile. Nondisjunction of chromosome 21 occurs more frequently in women 45 years or older.
Klinefelter Syndrome: results from an extra X chromosome in males, producing XXY individuals. Affected individuals
have severely underdeveloped testis & are always sterile. In addition, this disorder also leads to breast enlargement,
paucity of body hair, high-pitched voice & the formation of other feminine contours. Are usually capable of normal
sexual function, but many, if not most, are unable to produce sufficient amounts of sperm for conception. Klinefelter
syndrome males with more than two X chromosomes usually have extreme symptoms & are often mentally retarded.
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Turner Syndrome: results from monosomy of the X chromosome in females (XO). Are short of stature, averaging 4’7”
in adults. Exhibit abnormally small, widely spaced breasts; have broad, shield-shaped breasts & turned out elbows.
Ovaries do not develop & individuals are sterile, but usually of normal intelligence.
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Chromosomal Structural Mutations
Figure 14: Deletions
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Deletions:
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Figure 14.1: Duplication
Duplications:
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Figure 14.2: Chromosome Inversion
Inversions:
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Figure 14.3: Chromosome Translocation
Translocations:
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