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What is Mitosis?

Mitosis is the process by which a single cell divides to produce two genetically identical daughter cells, each containing the full set of chromosomes (diploid). This process is essential for growth, repair, and asexual reproduction in organisms.

Key Steps of Mitosis:

Prophase

Chromatin condenses into visible chromosomes

Each chromosome consists of two sister chromatids joined at the centromere

Nuclear envelope begins to break down

Centrioles move to opposite poles of the cell

Metaphase

Chromosomes align at the cell's equator (metaphase plate)

Spindle fibers attach to kinetochores at centromeres

Cell checkpoint ensures all chromosomes are properly attached

Anaphase

Sister chromatids separate and move to opposite poles

Spindle fibers shorten, pulling chromatids apart

Cell begins to elongate

Telophase

Nuclear envelopes reform around each set of chromosomes

Chromosomes begin to decondense

Spindle apparatus disintegrates

Cytokinesis

Cytoplasm divides, creating two separate cells

In animal cells: cleavage furrow forms

In plant cells: cell plate forms

Key Characteristics:

Purpose: Growth, repair, asexual reproduction

Number of divisions: 1

Daughter cells produced: 2

Genetic makeup: Identical to parent cell (diploid)

Chromosome number: Same as parent (2n)

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What is Meiosis?

Meiosis is a specialized type of cell division that produces four genetically unique gametes (sex cells), each containing half the chromosome number of the parent cell (haploid). This process is crucial for sexual reproduction and genetic diversity.

Key Steps of Meiosis:

Meiosis I (Reduction Division):

1. Prophase I

Homologous chromosomes pair up (synapsis)

Crossing over occurs between non-sister chromatids

Genetic material is exchanged, creating genetic variation

Nuclear envelope breaks down

2. Metaphase I

Homologous chromosome pairs align at the metaphase plate

Independent assortment occurs (random orientation)

Spindle fibers attach to kinetochores

3. Anaphase I

Homologous chromosomes separate and move to opposite poles

Sister chromatids remain attached

Chromosome number is reduced by half

4. Telophase I

Nuclear envelopes may reform

Cytokinesis occurs, producing two haploid cells

Meiosis II (Similar to Mitosis):

5. Prophase II

Chromosomes condense again

New spindle apparatus forms

6. Metaphase II

Chromosomes align at the metaphase plate

Spindle fibers attach to kinetochores

7. Anaphase II

Sister chromatids finally separate

Move to opposite poles of the cell

8. Telophase II

Nuclear envelopes reform

Cytokinesis produces four haploid gametes

Key Characteristics:

Purpose: Sexual reproduction, gamete formation

Number of divisions: 2

Daughter cells produced: 4

Genetic makeup: Genetically unique (haploid)

Chromosome number: Half of parent (n)

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How meiosis generates variation& Mendel's Law 

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How meiosis generates variation& Mendel's Law 

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Sources of Genetic Variation in Meiosis:

1. Crossing Over (Recombination)

Occurs during Prophase I

Homologous chromosomes exchange genetic material

Creates new combinations of alleles on chromosomes

Results in chromosomes that differ from both parents

2. Independent Assortment

Occurs during Metaphase I

Homologous chromosome pairs orient randomly at the metaphase plate

Maternal and paternal chromosomes assort independently

Creates 2^n possible combinations (where n = number of chromosome pairs)

Mendel's Law of Independent Assortment Explained:

The behavior of chromosomes during meiosis directly explains Mendel's law. When homologous chromosomes align randomly during Metaphase I, genes located on different chromosomes are inherited independently of each other. This means the inheritance of one trait does not influence the inheritance of another trait (assuming they're on different chromosomes).

Example: In humans with 23 chromosome pairs, independent assortment alone can produce 2^23 = over 8 million different genetic combinations in gametes!

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Mitosis vs Meiosis - Key Differences:

Purpose:

Mitosis: Growth, repair, asexual reproduction

Meiosis: Sexual reproduction, genetic diversity

Number of Cell Divisions:

Mitosis: 1 division

Meiosis: 2 divisions (Meiosis I and II)

Daughter Cells Produced:

Mitosis: 2 identical diploid cells

Meiosis: 4 genetically unique haploid cells

Genetic Variation:

Mitosis: None (produces identical clones)

Meiosis: High variation (due to crossing over and independent assortment)

Chromosome Behavior:

Mitosis: No chromosome pairing occurs

Meiosis: Homologous chromosomes pair up (synapsis)

Crossing Over:

Mitosis: Does not occur

Meiosis: Occurs during Prophase I, creating genetic recombination

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Image 1: File:Mitosis stages.svg - Wikimedia Commons. (2016, June 26). https://commons.wikimedia.org/wiki/File:Mitosis_Stages.svg

Image 2: File:Meiosis diagram.jpg - Wikimedia Commons. (n.d.). https://commons.wikimedia.org/wiki/File:Meiosis_diagram.jpg

Image 3: File:Meiosis mx.png - Wikimedia Commons. (2012, July 7). https://commons.wikimedia.org/wiki/File:Meiosis_mx.png

Image 4: File:Mitosis vs Meiosis Daughter Cells.png - Wikimedia Commons. (2023, February 14). https://commons.wikimedia.org/wiki/File:Mitosis_vs_Meiosis_Daughter_Cells.png

Scitable by Nature Education. (2023). Mendel's law of independent assortment. Nature Education. https://www.nature.com/scitable/topicpage/mendel-s-experiments-and-heredity-12187/

National Human Genome Research Institute. (2023, April 12). Meiosis. Genome.gov. https://www.genome.gov/genetics-glossary/Meiosis

Nature Education. (2024). Cell division: Mitosis and meiosis. Nature Education. https://www.nature.com/scitable/topicpage/cell-division-mitosis-and-meiosis-205/

Fowler, S., Roush, R. & Wise, J. (2023, January 9). Concepts of biology. Open Stax College.

Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., & Walter, P. (2019). Molecular biology of the cell (6th ed.). W. W. Norton & Company.