Cell Reproduction

Overview

Cell reproduction is a fundamental process in all living organisms, allowing for growth, development, and maintenance of tissues. In eukaryotes, cell division occurs through two main processes: mitosis and meiosis. Mitosis produces genetically identical cells, while meiosis generates gametes with half the chromosome number, introducing genetic diversity.

Chromosome Structure

Chromosomes and Chromatin

Chromosomes are highly organized structures of DNA and proteins found in the nucleus. The DNA is wrapped around histone proteins, forming a complex called chromatin. During cell division, chromatin condenses to form visible chromosomes.

• Chromosome: A single, long DNA molecule with associated proteins.

• Chromatin: The less condensed form of DNA-protein complex present during interphase.

• Chromatid: Each of the two identical halves of a duplicated chromosome.

• Centromere: The region where sister chromatids are joined and where spindle fibers attach during division.

• Centrosome: Organelle that organizes microtubules and is crucial for spindle formation.

Chromosome Number and Types

Each species has a characteristic number of chromosomes. In humans, somatic cells are diploid (2n = 46), while gametes are haploid (n = 23). Chromosomes are classified as autosomes or sex chromosomes.

• Diploid (2n): Cells with two sets of chromosomes (e.g., somatic cells).

• Haploid (n): Cells with one set of chromosomes (e.g., gametes).

• Autosomes: Non-sex chromosomes.

• Sex chromosomes: Chromosomes that determine biological sex (X and Y in humans).

Cell Cycle

Phases of the Cell Cycle

The cell cycle is the sequence of events that a cell undergoes from one division to the next. It consists of interphase (G1, S, G2) and the mitotic (M) phase.

• G1 phase: Cell grows and carries out normal functions.

• S phase: DNA is replicated.

• G2 phase: Cell prepares for division.

• M phase: Includes mitosis and cytokinesis.

Cell Cycle Checkpoints

Checkpoints are control mechanisms that ensure the cell cycle progresses only when certain conditions are met. Major checkpoints include G1, G2, and M checkpoints.

• G1 checkpoint: Checks for cell size, nutrients, and DNA integrity.

• G2 checkpoint: Ensures DNA replication is complete and undamaged.

• M checkpoint: Ensures all chromosomes are properly attached to the spindle before separation.

Mitosis

Phases of Mitosis (PMAT)

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical nuclei. It consists of four main phases: Prophase, Metaphase, Anaphase, and Telophase.

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

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

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Nuclear envelopes reform around the two sets of chromosomes, which decondense.

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two daughter cells. In animal cells, this occurs via cleavage furrow formation, while in plant cells, a cell plate forms.

• Cleavage (animal cells): Contractile ring of microfilaments pinches the cell in two.

• Cell plate formation (plant cells): Vesicles fuse at the center to form a new cell wall.

Meiosis

Overview and Purpose

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing four genetically unique haploid gametes. It is essential for sexual reproduction and genetic diversity.

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

Stages of Meiosis

Each meiotic division has four phases: Prophase, Metaphase, Anaphase, and Telophase. Meiosis I separates homologous chromosomes; Meiosis II separates sister chromatids.

• Prophase I: Homologous chromosomes pair and exchange segments (crossing over).

• Metaphase I: Homologous pairs align at the metaphase plate.

• Anaphase I: Homologs separate to opposite poles.

• Telophase I: Two haploid cells form; chromosomes are still duplicated.

• Prophase II: New spindle forms in each haploid cell.

• Metaphase II: Chromosomes align at the metaphase plate.

• Anaphase II: Sister chromatids separate.

• Telophase II: Four haploid cells result.

Origins of Genetic Variation

Mechanisms of Variation

Genetic variation in sexually reproducing organisms arises from three main mechanisms during meiosis and fertilization:

• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I.

• Independent Assortment: Random orientation of homologous pairs during Metaphase I leads to different combinations of maternal and paternal chromosomes in gametes.

• Fertilization: Random fusion of gametes from two parents increases genetic diversity.

Possible chromosome combinations: The number of possible combinations due to independent assortment is , where is the haploid number of chromosomes. For humans, , so possible combinations.

Comparison: Mitosis vs. Meiosis

Key Terms to Know

• Chromosome

• Chromatin

• Chromatid

• Centromere

• Centrosome

• Cytokinesis