Meiosis and Sexual Reproduction: Study Guide and Comparison with Mitosis

Study Guide - Smart Notes

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Haploid (n): A cell with a single set of chromosomes (e.g., gametes in animals).
Diploid (2n): A cell with two sets of chromosomes, one from each parent (e.g., somatic cells in animals).
Somatic Cell: Any cell in a multicellular organism except a sperm or egg or their precursors; typically diploid.
Gamete: A reproductive cell (sperm or egg) that is haploid and can unite with another gamete to form a diploid zygote.
Autosome: Any chromosome that is not a sex chromosome.
Sex Chromosome: Chromosomes involved in determining the sex of an organism (e.g., X and Y in humans).

Asexual Reproduction: Involves a single parent; offspring are genetically identical to the parent (clones). Common in prokaryotes (binary fission) and some eukaryotes (mitosis).
Sexual Reproduction: Involves two parents; offspring are genetically diverse due to the combination of gametes formed by meiosis.
Advantages of Asexual Reproduction: Rapid colonization, efficient in stable environments.
Disadvantages of Asexual Reproduction: Lack of genetic diversity, making populations vulnerable to disease.
Advantages of Sexual Reproduction: Genetic variation, which is the raw material for evolution.
Disadvantages of Sexual Reproduction: Slower, requires finding a mate, and involves complex processes.

Prokaryotic cells can evolve through:

Mutation: Random changes in DNA sequence.
Transformation: Uptake of foreign DNA from the environment.
Conjugation: Direct transfer of DNA between two cells via a pilus.
Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

Meiosis consists of two sequential divisions: Meiosis I and Meiosis II, each with distinct phases.

Phase	Key Events
Prophase I	Homologous chromosomes pair (synapsis) and crossing over occurs; spindle forms; nuclear envelope breaks down.
Metaphase I	Homologous pairs align at the metaphase plate.
Anaphase I	Homologous chromosomes separate and move to opposite poles.
Telophase I	Chromosomes arrive at poles; cell divides (cytokinesis); cells are haploid.
Prophase II	Spindle forms in each haploid cell; chromosomes condense.
Metaphase II	Chromosomes align at the metaphase plate (single file).
Anaphase II	Sister chromatids separate and move to opposite poles.
Telophase II	Nuclei form; cytokinesis occurs; four haploid cells result.
Interphase	DNA is replicated prior to meiosis I; no replication between meiosis I and II.

Meiosis I vs. Mitosis: Homologous chromosomes pair and separate in meiosis I, but not in mitosis.
Prophase I vs. Prophase II: Crossing over and synapsis occur only in prophase I.
Metaphase I vs. Metaphase II: Homologous pairs align in metaphase I; individual chromosomes align in metaphase II.
Anaphase I vs. Anaphase II: Homologous chromosomes separate in anaphase I; sister chromatids separate in anaphase II.

Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate properly during meiosis.
Consequences: Can result in aneuploidy (abnormal number of chromosomes), such as trisomy 21 (Down syndrome).
Lethality: Many cases are lethal, but some aneuploidies are viable and lead to genetic disorders.

Independent Assortment: Random orientation of homologous pairs during metaphase I leads to different combinations of maternal and paternal chromosomes in gametes.
Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I creates new allele combinations.
Random Fertilization: Any sperm can fertilize any egg, further increasing genetic diversity.

This table summarizes the main differences between mitosis and meiosis.

Comparison table of mitosis and meiosis

Characteristic	Mitosis	Meiosis
Homologous chromosome pairs or sister chromatids separated?	Sister chromatids	Homologous pairs (Meiosis I), then sister chromatids (Meiosis II)
Number of daughter cells produced	2	4
Cell type of daughter cell (diploid or haploid)	Diploid	Haploid
Number of cell divisions	1	2
Sexual or Asexual reproduction	Asexual (growth, repair)	Sexual (gamete production)
Involves duplication of chromosomes	Yes	Yes (before meiosis I)
Crossing over between homologous chromosomes occurs	No	Yes (prophase I)
Daughter cells are identical to parent cell	Yes	No
Due to crossing-over, daughter cells are NOT identical to parent cell	No	Yes
Produces gametes	No	Yes

Meiosis is essential for sexual reproduction, generating genetic diversity through independent assortment, crossing over, and random fertilization.
Mitosis produces genetically identical cells for growth and repair, while meiosis produces genetically unique gametes.