Introduction to Genetics:

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• 9
• Chapter 9
• Introduction to Genetics
• One Cell Becomes Two
• Mitosis and Cytokinesis

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Why do cells divide?

• Growth
• Repair/regeneration
• Reproduction
• asexual

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Chromatin

• Invisible most of the time - Only visible during
  cell division (mitosis or meiosis)
• During S-phase of the cell cycle the DNA
  replicates (makes an exact copy of itself)
• This means the cell has twice as much DNA in it
  after replication
• Once a chromosome has replicated, it shortens and
  thickens and can now be seen in our microscopes.
• See animation

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• Each descendant of a cell, in addition to
  requiring nutrients, cell membrane, and
  organelles, must have this information of DNA to
  survive.
• 1. Simple division would mean the new cells had
  half of what the old cell had.
• 2. Duplication of both cytoplasmic and nuclear
  contents preceeds division so that new cells have
  a complete set of everything.
• 3. Replicationduplication of DNA Figure 9.5

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• When cells divide by mitosis, each daughter cell
  receives the same number of chromosomes as its
  mother cell has 2n.
• In order to do this, the chromosomes must be
  copied first, then one of each copy is placed in
  the new cells.

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46
46
46
46
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46
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• II. DNA Is Packaged in Chromosomes (Section 9.3)
• A. Organization of these long pieces of DNA
• 1. DNA is divided into long strands wrapped
  around protein (chromatin).
• 2. Each strand is packaged and condensed into a
  single chromosome Figure 9.6a. Why? Analogy of
  moving and packing boxes. You only need your
  stuff out when you are using it. When you move,
  you wont be using it and you want it to take up
  less space Figure 9.8

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One Chromosome

• Sister
• Chromatids
• Each strand is an identical
• copy of the other one
• Centromere
• Where the two chromatids
• Are attached to each other
• This is different for each
• chromosome

DNA
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• B. DNA is the key to reproduction, development,
  and maintenance.
• 1. Genome complete collection of an organisms
  genetic information as linked genes in a long
  strand of DNA.

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• 3. Information is found in letters A, C, G, and T
  in the double helix Figure 9.2
• 4. Path from DNA to protein
• 5. Humans have about 100,000 genes that have all
  the information to make all the proteins
  (especially enzymes) a cell needs.

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3. Replication takes one chromosome and makes two
identical copies, called sister chromatids
Figure 9.6b (Interactive Activity 3)
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Chromosome Number

• Each species has the same number of chromosomes
  in all their cells that are made by mitosis. This
  is the diploid number (2n). In humans this number
  is 46. So cells of your skin and muscle and liver
  each have 46 chromosomes in them. Look how many
  chromosomes are in the cells of these creatures

2n 42
2n 78
2n 38
2n 94
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Sex Chromosomes

• Homologous in females XX
• Not homologous in males XY

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• A male karyotype 22 pairs of homologous
  chromosomes one pair of sex chromosomes

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• The cell cycle keeps record of progress of a cell
  over time, like a clock See animation

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1. The cell cycle is made up of a repeating
pattern of growth, genetic duplication, and
division. 2. Typical animal cell cycle lasts
about 24 hours. 3. Two main phases interphase
and mitotic phase (about 30 minutes). 4.
Interphase G1 (gap 1 for growth, 12 hours) S
phase (synthesis, for replication of DNA, 6
hours) G2 (gap 2, 6 hours) -animation of
figure 9.9 available under the resources for this
chapter
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Mitosis

• Nuclear division resulting in nuclei identical to
  parent cell asexual reproduction for some
  organisms.
• Begins after interphase
• Ends before cytokinesis
• Four phases Prophase
• Metaphase
• Anaphase
• Telophase

P M A T
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• 1. Prophase (P for plain to see)Chromosomes
  condense, nuclear envelope breaks down, formation
  of spindle fibers (microtubules) from the
  centrosomes.
• 2. Metaphase (M for middle)Chromosomes are
  aligned on the equator by pushing along spindle
  with each sister chromatid facing opposite poles.
• 3. Anaphase (A for apart)Sister chromatids
  separate each new chromosome moves to the
  opposite pole.
• 4. Telophase (T for two nuclei)Chromosomes
  de-condense, spindle breaks down, nuclear
  envelope forms around the two separate
  complements of chromosomes.

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Prophase
Mother cell
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Prophase

• Chromosomes become visible
• Spindle forms from protein microtubules
• Nuclear envelope disintegrates
• Nucleolus disintegrates
• In animal cells, centrioles migrate to opposite
  ends of the cell (poles) and spindle fibers
  attach to them

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Metaphase

• Chromosomes line up single file at the equator of
  the cell

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Anaphase

• Sister chromatids are pulled apart toward
  opposite poles
• In animal cell, cleavage furrow begins to form

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Telophase

• Nuclear membrane forms around each group of
  chromosomes
• Nucleolus reappears in each nucleus
• Spindle fibers disappear
• Chromosomes become invisible again as chromatin
• Cytokinesis begins in plant cell by formation of
  cell plate cleavage furrow in animal cell
  completely separates the two nuclei into two
  different cells.

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Cytokinesis

• Occurs after nucleus has been duplicated
• Begins in anaphase in animal cells
• by the formation of a cleavage furrow

• Begins in telophase in plant cells
• by the formation of a
• cell plate.

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• V. Variations in Cell Division (Section 9.5)
• A. Plant cellsEverything is similar except for
  cytokinesis because plant cells have to break
  down and reform the cell wall Figure 9.12,
  animation available under the resources for this
  chapter
• 1. Vesicles fuse near the metaphase plate for
  form a cell plate that grows outward to form a
  cell wall.

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Find the different stages of mitosis in these
onion cells
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Meiosis

• Cell division producing cells that have half the
  number of chromosomes of the mother cell
• Produces gametes eggs and sperm
• Occurs so that fertilization doesnt increase the
  number of chromosomes in each generation.

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Gametes sex cells

• Eggs or sperm
• Have half the normal number of chromosomes
  haploid (n) 23 in humans
• Sexual reproduction needs these to combine DNA
  from two different parents, producing offspring
  that is different from each parent

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Meiosis Reduction Division

• Two complete cell divisions
• First cell division separates homologous
  chromosomes (reduction of chromosome number)
• Second cell division separates sister
  chromatids (like mitosis) - Division
• Produces 4 haploid cells

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• M
• E
• I
• O
• S
• I
• S
• II

M E I O S I S I
Crossing over can occur Homologous pairs are
separated
Sister chromatids are separated
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Gametogenesis

• Oogenesis
• Production of an egg
• One mother cell produces one egg cell and three
  polar bodies that die

• Spermatogenesis
• Production of sperm
• One mother cell produces 4 equally sized sperm
  cells

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Fertilization
The sperm unites with the egg forming a zygote
(fertilized egg). The zygote then divides by
mitosis to produce the trillions of cells that
make up a multicellular body like yours.
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B. Prokaryotes (no nucleus) binary fission
Figure 9.13, animation available under the
resources for this chapter
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• III. When Cell Division Runs Amok Cancer
  (Sidebar)
• A. Unrestrained cell divisioncancer
• 1. Mechanisms that induce cell division can
  become hyperactive. (carcinogens)
• 2. Mechanisms that suppress cell division can
  fail. (carcinogens)
• B. Genes
• 1. Oncogenes stuck accelerator
• 2. Tumor suppressor genes failed brakes
• 3. Cyclin-dependent kinases, act in linked chain
  of protein activity

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Skin cancer
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The End