Transcript Chapter 13: Meiosis & Sexual Life Cycles

CHAPTER 13: MEIOSIS & SEXUAL
LIFE CYCLES
You should know…
The difference between asexual and sexual
reproduction.
 The role of meiosis and fertilization in sexually
reproducing organisms.
 The importance of homologous chromosomes to
meiosis

You should know…
How the chromosome number is reduced from
diploid to haploid through the stages of
meiosis.
 Three important differences between mitosis
and meiosis.
 The importance of crossing over, independent
assortment, and random fertilization to
increasing genetic variability.
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Genes: segments of DNA that code for basic units
of heredity
Offspring acquire genes from parents by inheriting
chromosomes
Types of Reproduction
ASEXUAL
 Produces clones
(genetically identical)
 Single parent
 Little variation in
population - only through
mutations
 Fast and energy efficient
 Ex. budding, binary
fission
SEXUAL
 Meiosis produces
gametes (sex cells)
 2 parents: male/female
 Lots of
variation/diversity
 Slower and energy
consumptive
 Ex. humans, trees
Asexual vs. sexual reproduction
Chromosomes
•
Somatic (body) cell: diploid 2n = 46 chromosomes or 23
pairs
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Each pair of homologous chromosomes includes 1
chromosome from each parent
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Autosomes: 22 pairs of chromosomes that do not determine sex
Sex chromosomes: 1 pair X and Y determine sex
• Females: XX – can only give X to offspring
• Males: XY – can give X or Y, determine sex of offspring
Gametes haploid n=23: 22 autosomes + 1 sex
chromosome
• Egg: 22 + X
• Sperm: 22 + X
**or** 22 + Y
Duplicated Homologous Chromosomes in a Somatic Cell
Homologous pair
of nonsister
chromatids
Karyotype: a picture of an
organism’s complete set of
chromosomes
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Arranged from
largest 
smallest pair
Able to identify
chromosomal
abnormalities
Making a karyotype – unsorted chromosomes
22 pairs of autosomes + 1 pair of sex chromosomes
Male or female?
Male or female?
Karyotype - used to determine genetic abnormalities
Chromosomal abnormality
Chromosomal abnormality
Chromosomal abnormality
Cancer cells
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Some have abnormal #’s of chromosomes
Karyotype of
Metastatic
Melanoma
Cancer cells
Breast cancer cell
Life cycle: reproductive history of organism
from conception  production of own offspring
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Fertilization (2n) and meiosis (n) alternate in sexual
life cycles – alternation of generations
Distinct diploid and haploid stages for organism
 Meiosis:
cell division that reduces # of chromosomes
(2n  n), creates gametes
 Fertilization: combine gametes (sperm + egg)
 Fertilized
 Zygote
egg = zygote (2n)
divides by mitosis to make multicellular diploid
organism
Starfish zygote
Varieties of Sexual Life Cycles
Human Life Cycle
Meiosis = reduction division
Means by which gametes
are produced
 Cells divide twice
 Result: 4 haploid daughter
cells,
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 each
cell has half as many
chromosomes as parent cell
2.23.15

Compare your mitosis and meiosis chart with a
neighbor
 Discuss
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
the similarities and differences
We will highlight similarities and differences in
mitosis and meiosis
Ch. 13 test will be Wednesday
Meiosis I (1st division)
Interphase: chromosomes replicated
Prophase I:
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Synapsis: homologous nonsister chromosomes pair up
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Forming a tetrad = 4 sister chromatids or homologous pair of
chromosomes
Crossing over at the chiasmata – portions of the
chromosome switch spots
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Increases genetic variability
Metaphase I: Tetrads line up
Anaphase I:
 Pairs of homologous chromosomes separate
 Sister chromatids still attached by centromere
Telophase I & Cytokinesis:
 Haploid set of chromosomes in each cell
 2 sister chromatids
 Some species: chromatin & nucleus reforms
Meiosis II (2nd division) = create gametes
NO INTERPHASE – no replication of DNA
Prophase II:
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No crossing over
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Spindle forms
Metaphase II:
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Sister chromatids line up
Anaphase II:
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Sister chromatids separate
Telophase II:
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4 haploid cells
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Nuclei reappear
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Each daughter cell genetically unique
Events Unique to Meiosis I (not in mitosis)
1.
2.
3.
Prophase I: Synapsis and crossing
over
Metaphase I: pairs of homologous
chromosomes line up on metaphase
plate
Anaphase I: homologous pairs
separate  sister chromatids still
attached at centromere
3 Sources of Genetic Variation:
Crossing Over
1.
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
Exchange genetic
material
Recombinant
chromosomes
3 Sources of Genetic Variation:
2.
Independent
Assortment of
Chromosomes

Random orientation
of homologous
pairs in
Metaphase I
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Maternal &
paternal on same
sides
Or on opposite
sides
3 Sources of Genetic Variation:
Random Fertilization
3.
Any sperm + Any egg
223 x 223
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8 million X 8 million = about 70 trillion combinations!
Mitosis
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Somatic cells
1 division
2 diploid daughter cells
Clones
From zygote to death
Purpose: growth and
repair
No synapsis, crossing over
Meiosis
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Gametes
2 divisions
4 haploid daughter cells
Genetically different-less than
1 in 8 million alike
Females before birth follicles
are formed. Mature ova
released beginning puberty
Purpose: Reproduction
Evolution and Genetic Variation

Darwin
 Differential
 Best

success in populations because of variation
suited survive and reproduce, leave more offspring
Different combo of alleles make or break success- sexual
reproduction increases variation in population
 Heritable
variation makes evolution possible
Mitosis vs. Meiosis