Rucete ✏ AP Biology In a Nutshell
10. Meiosis and Genetic Diversity
This chapter explains how meiosis produces genetically unique gametes, maintains chromosome number across generations, and increases genetic diversity. It also compares meiosis with mitosis and introduces the concept of nondisjunction.
How Meiosis Works
• Meiosis produces haploid (n) gametes from diploid (2n) cells for sexual reproduction.
• Fertilization of two haploid cells (egg and sperm) results in a diploid zygote.
• Meiosis has two rounds of cell division: – Meiosis I (reduction division) – Meiosis II
• Final result: four genetically different haploid gametes.
• Count chromosomes by counting centromeres: – One chromatid = 1 chromosome (pre-replication) – Two chromatids = 1 chromosome (post-replication)
Meiosis I
• Four stages: Prophase I, Metaphase I, Anaphase I, Telophase I
– Prophase I: Chromosomes condense, nuclear membrane breaks down, homologous chromosomes pair up (synapsis), and crossing-over occurs → genetic diversity.
– Metaphase I: Homologous pairs align at the center of the cell (not single chromosomes as in mitosis).
– Anaphase I: Homologous chromosomes separate and move to opposite poles. Chromosome number stays the same.
– Telophase I: Nuclear membranes reform. Cytokinesis follows, producing two haploid cells.
Meiosis II
• Very similar to mitosis, but without DNA replication beforehand.
• Four stages: Prophase II, Metaphase II, Anaphase II, Telophase II
– Prophase II: Chromosomes condense again in each haploid cell.
– Metaphase II: Chromosomes line up at the center of each cell.
– Anaphase II: Sister chromatids separate, now considered individual chromosomes.
– Telophase II: Each cell divides, resulting in four genetically unique haploid gametes.
Meiosis vs. Mitosis
• Mitosis: one division, two genetically identical diploid cells → growth and repair.
• Meiosis: two divisions, four genetically different haploid cells → sexual reproduction.
• DNA replicates once before each process begins.
How Meiosis Creates Genetic Diversity
• Meiosis introduces variation through recombination and independent assortment.
• This variation ensures genetically unique gametes, increasing adaptability of populations.
Recombination (Crossing Over)
• Occurs during prophase I between homologous chromosomes.
• Exchange of genetic material between nonsister chromatids.
• Results in new allele combinations on a single chromosome.
• The closer genes are on a chromosome, the less likely they are to be separated → linked genes.
Independent Assortment
• During metaphase I, homologous chromosomes align randomly.
• Maternal and paternal chromosomes can be distributed in various combinations.
• 2ⁿ possible combinations of chromosomes, where n = haploid number.
• In humans, 2²³ = over 8 million possible combinations from independent assortment alone.
Random Fertilization
• Any sperm can fertilize any egg, further increasing variation.
• 8 million egg × 8 million sperm = over 64 trillion possible diploid zygotes.
Nondisjunction
• Error in meiosis where chromosomes fail to separate properly.
• Can occur in meiosis I (homologs) or meiosis II (sister chromatids).
• Leads to abnormal chromosome numbers (aneuploidy) in gametes.
• Example: – Trisomy 21 = Down syndrome (three copies of chromosome 21)
In a Nutshell
Meiosis produces genetically diverse haploid gametes through crossing over and independent assortment. These processes, along with random fertilization, ensure variation in sexually reproducing populations. Errors like nondisjunction can lead to genetic disorders but also provide insight into chromosomal behavior during division.