Compare mitosis and meiosis: (a) create a table comparing number of divisions, number of daughter cells, chromosome number, and genetic variation, (b) explain how crossing over and independent assortment create genetic diversity, and (c) calculate possible gamete combinations for an organism with n=3.
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Mitosis vs Meiosis:
(a) Comparison Table:
Feature
Mitosis
Meiosis
Explain using:
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ā± 3 hoursš 66 questionsš 3 sections
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š Real-World Applications: Meiosis and Sexual Reproduction
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Crossing over: exchange of DNA between homologs
Occurs at chiasmata
Creates genetic variation
Nuclear envelope breaks down
Spindle forms
Metaphase I
Tetrads align at metaphase plate
Random orientation of homologs
Independent assortment occurs
Anaphase I
Homologous chromosomes separate
Sister chromatids stay together
Different from mitosis!
Telophase I and Cytokinesis
Two haploid cells form
Each has one chromosome from each homologous pair
Sister chromatids still joined
Meiosis II (Equational Division)
Similar to mitosis but with haploid cells
Prophase II
Chromosomes condense
Spindle forms
No crossing over
Metaphase II
Chromosomes align at metaphase plate
Single file (not pairs)
Anaphase II
Sister chromatids separate
Move to opposite poles
Telophase II and Cytokinesis
Nuclear envelopes reform
4 haploid cells produced
Each genetically unique
Sources of Genetic Variation
1. Crossing Over (Prophase I)
Exchange of DNA between homologous chromosomes
Creates new allele combinations
Recombinant chromosomes
2. Independent Assortment (Metaphase I)
Random orientation of homologous pairs
2āæ possible combinations (n = # of chromosome pairs)
Humans: 2²³ = ~8 million combinations
3. Random Fertilization
Any sperm can fertilize any egg
(2²³)² = ~70 trillion combinations
Mitosis vs. Meiosis
Feature
Mitosis
Meiosis
Purpose
Growth, repair
Gamete production
Divisions
One
Two
Daughter cells
2 diploid
4 haploid
Genetic identity
Identical to parent
Different from parent
Crossing over
No
Yes (Prophase I)
Homolog pairing
No
Yes (synapsis)
Errors in Meiosis
Nondisjunction: Homologs or sister chromatids fail to separate
Results:
Aneuploidy: abnormal chromosome number
Monosomy: 2n - 1 (one chromosome missing)
Trisomy: 2n + 1 (one extra chromosome)
Examples:
Down syndrome: Trisomy 21 (3 copies of chromosome 21)
Turner syndrome: Monosomy X (45, X)
Klinefelter syndrome: XXY (47, XXY)
Key Concepts
Meiosis has two divisions producing 4 haploid cells
Meiosis I: homologous chromosomes separate
Meiosis II: sister chromatids separate
Crossing over (Prophase I) exchanges DNA
Independent assortment randomizes chromosome distribution
Three sources of variation: crossing over, independent assortment, random fertilization
Nondisjunction causes aneuploidy
Purpose
Growth, repair, asexual reproduction
Sexual reproduction (gamete formation)
Number of divisions
1
2 (Meiosis I + II)
Daughter cells produced
2
4
Chromosome number
Diploid (2n) ā 2 diploid (2n)
Diploid (2n) ā 4 haploid (n)
Genetic variation
Identical to parent (clones)
Genetically unique
Homologous pairing
No synapsis
Yes (Prophase I)
Crossing over
No
Yes (Prophase I)
Where occurs
Somatic cells (body)
Germ cells (gonads)
In humans
2n=46 ā 2 cells with 46
2n=46 ā 4 cells with 23
(b) Sources of Genetic Diversity:
1. Crossing Over (Recombination):
When: Prophase I of meiosis I
Process:
Homologous chromosomes pair up (synapsis)
Form tetrads (4 chromatids total, 2 per chromosome)
Chiasmata form - points where chromatids cross
Non-sister chromatids exchange DNA segments
Chromatids break and rejoin at same position
Result:
Recombinant chromosomes with new allele combinations
Mix of maternal and paternal genes on same chromosome
Occurs multiple times per chromosome pair (1-3 crossovers typical)
Example:
Before crossing over:
Maternal: A---B---C
Paternal: a---b---c
After crossing over:
Recombinant 1: A---b---c (maternal-paternal mix)
Recombinant 2: a---B---C (paternal-maternal mix)
Frequency:
Genes far apart ā more likely to cross over
Genes close together ā less likely (linked genes)
Used to create genetic maps
2. Independent Assortment:
When: Metaphase I of meiosis I
Process:
Homologous pairs line up randomly at metaphase plate
Maternal and paternal chromosomes orient independently
Each pair's orientation independent of other pairs
Random which homolog goes to which pole
Number of combinations:
PossibleĀ combinations=2n
where n = haploid number
Example (n=3):
3 homologous pairs
Each pair can orient 2 ways
Total: 2³ = 8 different gamete combinations
Visualization:
Pair 1: Mā or Pā
Pair 2: Mā or Pā
Pair 3: Mā or Pā
Possible gametes (8 total):
1. MāMāMā (all maternal)
2. MāMāPā
3. MāPāMā
4. MāPāPā
5. PāMāMā
6. PāMāPā
7. PāPāMā
8. PāPāPā (all paternal)
3. Random Fertilization:
Additional diversity:
Any male gamete can fuse with any female gamete
For humans (n=23):
Male: 2²³ = 8.4 million possible sperm
Female: 2²³ = 8.4 million possible eggs
Combinations: (2²³)² = 70 trillion possible zygotes!
Without crossing over! With crossing over ā essentially infinite variation
(c) Calculation for n=3:
From independent assortment alone:
GameteĀ types=2n=23=8
8Ā differentĀ gameteĀ typesĀ possibleā
From one parent: 8 types
From both parents (after fertilization):
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