Cell Division

Overview of the Cell Cycle

The cell cycle is the series of events that cells undergo as they grow and divide. It is fundamental to both growth and reproduction in all living organisms.

• Interphase: The phase where the cell grows, replicates its DNA, and prepares for division. It consists of G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis).

• M phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Example: Skin cells undergo the cell cycle to replace damaged or dead cells.

Mitosis

Role and Key Events

Mitosis is the process by which a cell divides to produce two genetically identical daughter cells. It is essential for growth, repair, and asexual reproduction.

• Prophase: Chromosomes condense, spindle fibers form, nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids are pulled apart to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Division of the cytoplasm, resulting in two cells.

End Product: Two diploid cells (2n), genetically identical to the original cell.

Example: Mitosis occurs in somatic (body) cells.

Meiosis

Role and Key Events

Meiosis is the process by which gametes (sperm and egg cells) are produced. It reduces the chromosome number by half and introduces genetic variation.

• Meiosis I: Homologous chromosomes separate.

  • Prophase I: Chromosomes condense, homologous chromosomes pair (synapsis), crossing over occurs.

  • Metaphase I: Homologous pairs align at the equator.

  • Anaphase I: Homologous chromosomes move to opposite poles.

  • Telophase I: Two cells form, each with half the chromosome number.

• Meiosis II: Sister chromatids separate (similar to mitosis).

  • Prophase II: Chromosomes condense in each cell.

  • Metaphase II: Chromosomes align at the equator.

  • Anaphase II: Sister chromatids move to opposite poles.

  • Telophase II: Four haploid cells form.

End Product: Four haploid cells (n), each genetically unique.

Example: Meiosis occurs in germ cells to produce gametes.

Comparison of Mitosis and Meiosis

The following table summarizes the main differences between mitosis and meiosis:

Key Genetic Terminology

Alleles

Alleles are different versions of a gene found at the same locus on homologous chromosomes. They can result in different traits.

• Dominant allele: Expressed trait when present.

• Recessive allele: Expressed only when both alleles are recessive.

Example: The gene for flower color in peas has a purple allele and a white allele.

Locus

The locus is the specific physical location of a gene on a chromosome.

• Each gene occupies a unique locus.

• Alleles of a gene are found at the same locus on homologous chromosomes.

Example: The locus for the gene controlling eye color is found on chromosome 15 in humans.

Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes (one from each parent) that have the same genes at the same loci, but may have different alleles.

• They pair during meiosis I.

• Homologous chromosomes are similar in size, shape, and gene content.

Example: Humans have 23 pairs of homologous chromosomes.

Additional info:

• During crossing over in meiosis, homologous chromosomes exchange genetic material, increasing genetic diversity.

• The formula for chromosome number after mitosis:

• The formula for chromosome number after meiosis: