Download Mistakes in Meiosis

Meiosis Notes
Diploid Cells
Haploid Cells
• 2 chromosomes of each kind in a
• 1 chromosome of each kind in a
cell.
cell.
• 1 from mom, 1 from dad.
• 1n
• All sex cells are haploid.
• 2n
• All body cells are diploid.
• Made by meiosis.
• Made by mitosis.
• Gametes: Another name for sex cells: egg & sperm.
• Zygote: A fertilized cell.
• 2 gametes that are now together.
Meiosis
• The making of gametes (eggs & sperm).
• Cells contain 1/2 the normal amount of chromosomes. (haploid)
Why is Meiosis necessary?
To make sex cells for sexual reproduction
What is produced by Meiosis?
• 4 haploid cells (1n)
• Each cell is genetically different!
Meiosis Quick Review
• Makes 4 cells genetically different from parent cell & from each other
• Makes 1n cells (1n or 2n?)
• Makes gametes
• Used for reproduction
What is different about meiosis?
• Many different variations in the gametes
because of crossing over. Crossing over is
when tetrads bind tightly, parts break off
and are swapped between the chromatids
• Divides twice, but only copies DNA once.
Tetrad: when 2 homologous
chromosomes bind together during
prophase I of meiosis.
Meiosis I (2 new cells are formed)
Prophase I: *Tetrads condense
*__Crossing Over occurs
between chromosomes
* Centrosomes move to
opposite ends of cell to form
spindle fibers
Metaphase I: Tetrads line up in the middle of the
cell.
Anaphase I:
Tetrads _Split and each set of
sister chromatids moves to
Opposite __ ends of the cell
Telophase I:
Cytokinesis (division of the
cytoplasm) results in _2 new
cells._
Meiosis II (4 new cells are formed)
Prophase II:
Metaphase II:
Centrosomes move to opposite
ends of the cell to form
spindle fibers
Replicated Chromosomes line
up in the middle____
Anaphase II:
Replicated Chromosomes
seperte and move to opposite
ends of the cell
Telophase II:
*__nuclear membrane__
forms around each set of
chromosomes
*Cytokinesis
*___4 new daughter cells
Mistakes in Meiosis
Nondisjunction: when homologous chromosomes fail to separate from each other during
meiosis
 Result: One gamete may have too many chromosomes, while the other new gamete is missing
that chromosome
 Organisms with extra chromosomes can survive; organisms with too few usually do not
survive
 Polyploidy: when organisms have more than the normal number of chromosome sets
o Can be lethal for animals but not plants (plants just usually get bigger)

Examples of Nondisjunction
Genotype (X=Female sex chromosome,
Y= male sex chromosome)
X
Turner’s Syndrome
Trisomy X
XXX
Klinefelter’s syndrome
XXY
Jacob’s syndrome
XYY
Down syndrome
TRISOMY 21
Patau’s syndrome
TRISOMY 13
Edward’s syndrome
TRISOMY 18
Mitosis
DNA replicates in interphase
vs.
ONE division of chromosomes
 Separate sister chromatids
Meiosis
DNA replicates in interphase
Makes 2 identical DIPLOID cells
TWO divisions of chromosomes
 Separate homologous chromosomes
(meiosis I)
 Separate Replicated Chromosomes
(meiosis II)
Makes 4 HAPLOID cells
Chromosome # is identical to parent cell
Chromosome # is HALF of the parent cell
Used for replacing old cells, growth, and repair
Used to make gametes (for reproduction)