6.6 Meiosis and Genetic Variationsimonscience.weebly.com/uploads/5/7/7/2/57722479/... · 6.6...
Transcript of 6.6 Meiosis and Genetic Variationsimonscience.weebly.com/uploads/5/7/7/2/57722479/... · 6.6...
6.6 Meiosis and Genetic Variation
KEY CONCEPT - Independent assortment and crossing over
during meiosis result in genetic diversity.
EQ – What is meiosis and how does it contribute to genetic
diversity (reproductive variability)?
SB1.a. Explain the role of cell organelles for both prokaryotic & eukaryotic cells,
including the cell membrane, in maintaining homeostasis and cell reproduction.
SB2.c. Using Mendel’s laws, explain the role of meiosis in reproductive variability.
6.6 Meiosis and Genetic Variation
6.6 Meiosis and Genetic Variation
• Meiosis I:
• occurs after DNA has been replicated.
• divides homologous chromosomes in four phases.
6.2 Meiosis
tetrad
Prophase IChromosomes
condense, homologous
chromosomes begin to
pair up, nuclear
envelope breaks down,
spindle fibers form
Metaphase ISpindle fibers align
homologous
chromosomes along the
cell equator
Anaphase IHomologous
chromosomes separate
to opposite sides of
cell, sister chromatids
remain attached
together
Telophase ISpindle fibers fall
apart, nuclear
membrane may
form again, cell
undergoes
cytokinesis
6.6 Meiosis and Genetic Variation
• Meiosis II:
• divides sister chromatids in four phases.• DNA is not replicated between meiosis I and meiosis II.
6.2 Meiosis
Prophase IINuclear envelope
breaks down if
necessary, spindle fibers
form
Metaphase IISpindle fibers align
chromosomes along the
cell equator
Anaphase IIChromatids separate
to opposite sides of
cell
Telophase IINuclear membranes
form around
chromosomes,
chromosomes begin to
uncoil, spindle fibers
fall apart, cytokinesis
occurs
6.6 Meiosis and Genetic Variation
• Meiosis differs from mitosis in significant ways.
– Meiosis has two cell divisions while mitosis has one.
– In mitosis, homologous chromosomes never pair up.
– Meiosis results in haploid cells; mitosis results in diploid
cells.
6.2 Meiosis
6.6 Meiosis and Genetic Variation
KEY CONCEPT - Independent assortment and crossing over
during meiosis result in genetic diversity.
EQ – What is meiosis and how does it contribute to genetic
diversity (reproductive variability)?
SB1.a. Explain the role of cell organelles for both prokaryotic & eukaryotic cells,
including the cell membrane, in maintaining homeostasis and cell reproduction.
SB2.c. Using Mendel’s laws, explain the role of meiosis in reproductive variability.
6.6 Meiosis and Genetic Variation
Sexual reproduction creates unique combinations of
genes.
• Sexual reproduction creates unique combination of genes.
– independent assortment of chromosomes in meiosis
– random fertilization of gametes
• Unique phenotypes (expressed traits) may give a
reproductive advantage to some organisms.
6.6 Meiosis and Genetic Variation
Fertilization
• Is random
• Increases unique combinations of genes
• In humans, the chance of getting one combination of
chromosomes from any one set of parents is 1 out of 223
x 223 (1 out of over 64 trillion) combinations
46
4646
23
23
23
23
1. 2. 3.
6.6 Meiosis and Genetic Variation
Meiosis
• Chromosomes independently assort in meiosis
• Increases unique combinations of genes (genetic variation)
• Homologous chromosomes pair randomly along the cell
equator
• In human cells, about 223 (8 million) different combinations of
chromosomes could result
6.6 Meiosis and Genetic Variation
Crossing over during meiosis increases genetic diversity.
• Crossing over is the exchange of chromosome segments
between homologous chromosomes.
– occurs during prophase I of meiosis I
– Creates new combinations of genes
– Recombined chromosomes are a combination of both
the mother and the father
6.6 Meiosis and Genetic Variation
• Chromosomes contain many genes.
– The farther apart two genes are located on a
chromosome, the more likely they are to be separated
by crossing over.
– Genes located close together on a chromosome tend to
be inherited together, which is called genetic linkage.
6.6 Meiosis and Genetic VariationConstruct a Crossing Over Model
1st: 1. Thread 2 colored marshmallows onto 2 toothpicks.
2. Tie the toothpicks together with a twist tie.
3. Thread 2 white marshmallows onto 2 toothpicks.
4. Tie the toothpicks together with a twist tie.
• The marshmallows represent the genetic information supplied by each parent.
• Each toothpick pair represents one duplicated chromosome (2 sister
chromatids) of a homologous pair – homologous chromosomes.
• The twist tie represents the centromere.
2nd: 1. Bring the pairs of homologous chromosomes together as in prophase I
(tetrad).
2. Model crossing over by exchanging one colored for one white marshmallow.
3. Line up the homologous chromosomes side-by-side as in metaphase I.
4. Separate the pairs as in anaphase I.
5. Then, separate the sister chromatids into individual chromosomes as in
anaphase II of Meiosis II.
Draw the process represented in your model. Label the phase of Meiosis
and the structures.
Answer this question: How is the genetic material that is exchanged during
crossing over alike and how is it different?