Meiosis (Super standard) Flashcards
Similarities Between Mitosis & Meiosis
Starting Cell: Both begin with a diploid starting cell.
Sequential Stages: Both undergo prophase, metaphase, anaphase, and telophase.
Chromosome Alignment: Chromosomes line up along the cell’s equator in metaphase.
Chromosome Separation: In anaphase, chromosomes are pulled apart to opposite poles.
Cytokinesis: Both processes end with cytokinesis, where the cell divides.
Differences Between Mitosis & Meiosis
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Rounds of Cell Division:
- Mitosis: One
- Meiosis: Two
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Number of Daughter Cells:
- Mitosis: Two
- Meiosis: Four
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Ploidy of Daughter Cells:
- Mitosis: Diploid
- Meiosis: Haploid
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Genetic Identity:
- Mitosis: Daughter cells are identical
- Meiosis: Daughter cells are different
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Occurrence:
- Mitosis: All organisms
- Meiosis: Animals, plants, fungi only
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Cells Created:
- Mitosis: Somatic
- Meiosis: Gametes
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Recombination:
- Mitosis: No
- Meiosis: Yes
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Chromosome Assortment:
- Mitosis: None
- Meiosis: Independent
Prophase I vs. Prophase II
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Prophase I:
- Chromosome Pairing: Homologous chromosomes pair up (bivalents).
- Genetic Recombination: Crossing over occurs between non-sister chromatids.
- Nuclear Changes: Nuclear envelope breaks down; spindle apparatus begins to form.
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Prophase II:
- Chromosome Configuration: No pairing; chromosomes exist as individuals.
- No Crossing Over: Genetic recombination is not present.
- Spindle Formation: New spindle apparatus forms again; nuclear envelope may reform briefly.
Metaphase I vs. Metaphase II
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Metaphase I:
- Chromosome Alignment: Homologous chromosome pairs (tetrads) align at the metaphase plate.
- Spindle Attachment: Spindle fibers attach to both sister chromatids of each homologous chromosome.
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Metaphase II:
- Chromosome Alignment: Individual chromosomes (each with two sister chromatids) align at the metaphase plate.
- Spindle Attachment: Spindle fibers attach to the centromeres of each sister chromatid.
Anaphase I vs. Anaphase II
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Anaphase I:
- Separation of Homologs: Homologous chromosomes are pulled apart toward opposite poles of the cell.
- Sister Chromatids: Sister chromatids remain attached at their centromeres.
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Anaphase II:
- Separation of Sister Chromatids: Sister chromatids are pulled apart toward opposite poles of the cell.
- Chromosome Count: Each pole receives individual chromosomes, halving the chromosomal content in each resulting cell.
Telophase I vs. Telophase II
- Telophase I: Homologous chromosomes move to opposite poles; cytokinesis may start. Results in 2 cells with half the chromosome number (two sister chromatids each). Nuclear envelope may reform, varying by organism.
- Telophase II: Individual chromosomes reach opposite poles; cytokinesis occurs, yielding 4 haploid cells (single chromatids). Nuclear envelope reforms, forming nuclei in the resulting cells.
How to understand chromosomes vs chromatids?
Count Chromosomes by Centromeres:
46 chromosomes = 46 centromeres, even after DNA duplication in interphase.
Chromatids:
After duplication, there are 92 chromatids (replicated chromosomes).
What is the importance of meiosis
Purpose of Meiosis: Reduces chromosome number by half, producing haploid gametes for sexual reproduction.
Genetic Diversity: Results in genetically diverse offspring through processes like crossing over and independent assortment.
Prevents Chromosome Doubling: Keeps chromosome number stable across generations during fertilization.
Crossing Over Def
Definition:
Exchange of genetic material between homologous chromosomes during Prophase I of Meiosis.
Where does Crossing over occur
Key Points:
Occurs in Meiosis I: Specifically during synapsis of homologous chromosomes.
Purpose of meiosis
Increases genetic diversity by creating new allele combinations.
What are the physical sites where crossing over occurs.
Chiasmata
Explain the process of crossing over
- Meiosis I: Homologous chromosomes pair closely.
- Nonsister Chromatids: Cross over at chiasmata.
- Process: DNA entanglement, breaks, rejoining.
- Outcome: Chromatid sections exchanged between chromosomes ➡️ recombinant chromatids
Explain the process of independent assortment
- Independent assortment leads to diverse allele combinations in daughter cells.
- Caused by random alignment of homologous pairs during metaphase I.
- In prophase I, homologous chromosomes pair up; in metaphase I, they align at the spindle equator.
- Each pair’s arrangement is random, allowing different orientations.
- The orientation of one homologous pair does not affect others.
- Homologous chromosomes are separated and pulled to opposite poles.
- The resulting allele combinations in daughter cells depend on the arrangement of homologous pairs.
Why is meiosis needed in a sexual life cycle?
- Organisms can have sexual or asexual life cycles.
- Asexual offspring are genetically identical to parents.
- Sexual offspring are genetically distinct due to differing chromosomes.
- Meiosis is essential in sexual reproduction to halve chromosome numbers.
- This allows gamete fusion to form a zygote.
- Halving prevents infinite chromosome doubling during generations.
Explain random fertilization
- Meiosis generates genetic variation via crossing over and independent assortment.
- Gametes carry different alleles.
- Random fusion of male and female gametes during fertilization forms unique zygotes.
- Each zygote has a distinct allele combination, enhancing genetic diversity in the species.
- Zygotes develop into genetically diverse adults.
Random fertilization combinations equation
- Different chromosome combinations occur post-fertilization.
- Random fertilization allows any two gametes to combine.
- The number of combinations is calculated as (2^n) where n is the haploid number.
- In humans, with a haploid number of 23, combinations total 70,368,744,177,664.
- Relatives can differ significantly genetically due to variations during meiosis, fertilization, mutations, and crossing over.
