Meiosis and Its Wonders

1. Meiosis produces four sperm cells in males and one egg cell (oocyte) and three polar bodies (which don’t develop further) in females.

2. Meiosis I has four phases:

• Prophase I: Chromosomes condense and pair up with their homologs.
• Metaphase I: Paired chromosomes (tetrads) line up at the cell’s equator.
• Anaphase I: Homologous chromosomes separate and move towards opposite poles.
• Telophase I and Cytokinesis: Nuclear envelopes reform around separated chromosomes, and cytoplasm divides, forming two daughter cells.

3. Meiosis II also has four phases:

• Prophase II: Nuclear envelopes break down again, and chromosomes condense further.
• Metaphase II: Chromosomes line up at the equator of each daughter cell.
• Anaphase II: Sister chromatids separate and move to opposite poles.
• Telophase II and Cytokinesis: Nuclear envelopes reform, and cytoplasm divides, forming four daughter cells in total.

4. Prophase I starts with a diploid cell. This means it has two complete sets of chromosomes.

5. In a process called synapsis, each chromosome pairs up with and binds to its corresponding homologous chromosome, forming a bivalent.

6. A tetrad is a structure formed during synapsis where four chromatids (two from each homologous chromosome) are temporarily held together.

7. Genes are the basic units of heredity that code for specific traits. They are located on chromosomes.

8. Alleles are different versions of the same gene. For example, you might have an allele for brown eyes and another allele for blue eyes on your chromosome pair for eye color.

9. Crossing over (recombination) is the exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I. This reshuffles genes and creates genetic diversity.

10. Every gamete is genetically different because of two reasons:

• Independent assortment: Homologous chromosomes separate randomly during meiosis I, resulting in different combinations of chromosomes in daughter cells.
• Crossing over creates new combinations of alleles on the same chromosome.

11. Children are different from their parents and siblings because they inherit a unique combination of genes from each parent through meiosis. The random assortment of chromosomes and crossing over during meiosis creates genetic variation in the gametes, leading to unique combinations in offspring.

12. During prophase I, the nuclear membrane disappears, the centrioles move to opposite ends of the cell, and spindle fibers fan out from them to capture chromosomes.

13. During metaphase I, paired chromosomes (tetrads) line up along the equator of the cell, with each tetrad attached to spindle fibers at opposite poles.

14. During anaphase I, homologous chromosomes separate and are pulled towards opposite poles of the cell by the spindle fibers.

15. During telophase I, nuclear envelopes reform around the separated chromosomes at each pole, and cytokinesis divides the cytoplasm, forming two daughter cells.

16. Meiosis I ends with four genetically different haploid daughter cells. Each contains only one set of chromosomes consisting of paired sister chromatids.

17. Once the sister chromatids separate during anaphase II, they are called chromosomes.

18. Meiosis II ends with four genetically different haploid daughter cells. Each containing only one set of chromosomes (no longer sister chromatids).

19. Key points about meiosis:

• Reduces chromosome number from diploid to haploid.
• Creates genetic diversity through independent assortment and crossing over.
• Essential for sexual reproduction.

20. Differences between meiosis I and meiosis II:

• Homologous chromosomes separate in meiosis I, while sister chromatids separate in meiosis II.
• Crossing over can occur in meiosis I, not in meiosis II.
• Meiosis I results in two daughter cells, while meiosis II results in four.

21. Differences between mitosis and meiosis:

• Goal: Mitosis produces genetically identical daughter cells for growth and repair, while meiosis produces genetically diverse gametes for sexual reproduction.
• Number of cell divisions: Mitosis has one division, while meiosis has two.
• Chromosome number: Mitosis maintains the diploid chromosome number, while meiosis reduces it to haploid in gametes.
• Crossing over: Does not occur in mitosis, but can occur in meiosis I.