Meiosis I and Meiosis II Notes

DNA Replication and Mitosis

DNA Replication Process

• Initiation: Begins at specific sites on DNA called origins of replication.

• Unwinding: The double helix unwinds and separates into two strands.

• Synthesis: DNA polymerase synthesizes new strands by adding nucleotides complementary to the template strand.

• Termination: Replication ends when the entire molecule has been copied, resulting in two identical DNA molecules.

Centrosomes, Centrioles, and Centromeres

• Centrosomes: Organize microtubules and provide structure for the cell, involved in the process of mitosis.

• Centrioles: Paired cylindrical structures within the centrosome, assist in cell division by helping to organize the mitotic spindle.

• Centromeres: Region of the chromosome where sister chromatids are joined and where spindle fibers attach during mitosis.

Chromosomes and Chromatids

• Chromosomes: Structure that carries genetic information, exists as single DNA molecules.

• Chromatids: Replicated forms of a chromosome joined at the centromere, exists as sister chromatids before cell division.

Phases of the Cell Cycle

1. Interphase: Preparation for division, consists of G1 (cell growth), S (DNA synthesis), and G2 (preparation for mitosis).

2. Mitosis (M phase): Actual process of cell division, consists of prophase, metaphase, anaphase, and telophase.

Phases of Mitosis

• Prophase: Chromosomes condense, the nuclear envelope breaks down, and the spindle apparatus forms.

• Metaphase: Chromosomes align at the metaphase plate.

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

• Telophase: Chromatids reach the poles, nuclear envelope re-forms.

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

Meiosis I and Meiosis II

Overview of Chromosomes in Humans

• Humans have 23 pairs of chromosomes (2n = 46).

• Most body cells are diploid (2n), containing two versions of each chromosome, one from each parent.

• Gametes (eggs/sperm) are haploid (n), containing one copy of each chromosome (23).

Meiosis Processes

• Meiosis consists of two rounds of division, meiosis I and meiosis II, with only one DNA replication.

• Functions of Meiosis:

  • Reduce chromosome number from diploid (2n) to haploid (n).

  • Increase genetic diversity through recombination and independent assortment.

Meiosis I

• Prophase I: Synapsis occurs, homologous chromosomes pair to form tetrads (4 chromatids). Crossing over can occur, exchanging genetic information, increasing diversity.

• Metaphase I: Tetrads align at the metaphase plate, with chromosomes pulled randomly.

• Anaphase I: Homologous chromosomes are separated into two cells, each containing duplicated chromosomes.

• Telophase I: May or may not reform nuclear envelope, no further DNA replication occurs before meiosis II.

Meiosis II

• Similar to mitosis, each haploid cell undergoes division without further DNA replication, leading to separate sister chromatids into four unique haploid daughter cells.

Comparison of Mitosis and Meiosis

• Mitosis vs. Meiosis:

  • Mitosis: One round of division, produces 2 diploid daughter cells identical to the parent.

  • Meiosis: Two rounds of division, produces 4 haploid daughter cells with genetic variation.

  • Meiosis involves synapsis and crossing over; mitosis does not.

Autophagy and Apoptosis

Autophagy

• Definition: Self-degradation process where cells dispose of nonfunctional organelles, forming autophagosomes that are degraded by lysosomes.

• Ubiquitin Tagging: Proteins marked for destruction by ubiquitin, which are recognized and degraded by proteasomes.

Apoptosis

• Definition: Programmed cell death that leads to neat self-destruction of cells, crucial for maintaining cellular health by removing damaged cells.

• Process: Initiated by mitochondrial membrane changes leading to caspase activation, which degrades cell components.

Theories of Aging

• Wear and Tear Theory: Cumulative damage from free radicals affects cell function over time.

• Mitochondrial Theory: Free radicals in mitochondria reduce energy production.

• Genetic Theory: Aging and cessation of cell division are programmed into genes through telomerase activity, with telomeres limiting replication.

Telomerase

• Definition: Telomerase is an enzyme that adds nucleotide sequences to the ends of chromosomes (telomeres), which shortens with each cell division.

• Function: Helps maintain the length of telomeres, thus allowing cells to divide more times than they would normally be able to without telomere shortening, which is particularly important in stem cells and cancer cells.

• Mechanism: Contains an RNA template that complements the DNA sequence of the telomere, enabling it to extend the telomeres by adding repetitive nucleotide sequences.

• Role in Aging: Shortening of telomeres during cell division is associated with aging; telomerase activity decreases as organisms age, which limits the ability of cells to divide.

• Cancer Connection: Many cancer cells reactivate telomerase, enabling them to divide indefinitely and contribute to tumor growth.