Cell Cycle and Chromosome Structure

Overview of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis).

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

• S phase: DNA replication occurs, resulting in duplicated chromosomes.

• G2 phase: Cell prepares for division by producing necessary proteins and organelles.

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

Chromatin is the complex of DNA and proteins that forms chromosomes within the nucleus. Chromosomes are condensed chromatin visible during cell division. Homologous chromosomes are pairs of chromosomes with the same genes but possibly different alleles. Sister chromatids are identical copies of a chromosome connected by a centromere after DNA replication.

Stages of Mitosis

Mitosis is the process by which a cell divides its nucleus and contents. It consists of several stages:

• Prophase: Chromatin condenses into visible chromosomes; spindle apparatus begins to form.

• Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the cell's equatorial plate (metaphase plate).

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

• Telophase: Nuclear envelopes reform around the separated chromosomes; chromosomes decondense.

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

Spindle apparatus consists of microtubules and associated proteins that segregate chromosomes during mitosis. Kinetochore is a protein structure on chromatids where spindle fibers attach.

Functions and Importance of Mitosis

Mitosis is essential for growth, repair, and asexual reproduction in multicellular organisms.

• Growth: Increases cell number during development.

• Repair: Replaces damaged or dead cells.

• Asexual reproduction: Produces genetically identical offspring in some organisms.

DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division. It ensures that each daughter cell receives an identical set of chromosomes.

• Occurs during the S phase of interphase.

• Involves unwinding the DNA double helix and synthesizing new complementary strands.

Equation:

Animal vs. Plant Cell Cytokinesis

Cytokinesis differs between animal and plant cells due to structural differences.

• Animal cells: Cytokinesis occurs via formation of a cleavage furrow that pinches the cell in two.

• Plant cells: Cytokinesis occurs via formation of a cell plate that develops into a new cell wall separating the two daughter cells.

Comparison: Sister Chromatids vs. Homologous Chromosomes

Sister chromatids and homologous chromosomes are distinct structures involved in cell division.

Key Terms and Definitions

• Cell cycle: Sequence of growth and division in a cell.

• Chromosome: DNA molecule with associated proteins, carrying genetic information.

• Chromatin: DNA-protein complex forming chromosomes.

• Homologous chromosome: Pair of chromosomes with the same genes but possibly different alleles.

• Sister chromatid: Identical copy of a chromosome formed during DNA replication.

• Interphase: Period of cell growth and DNA replication (G1, S, G2).

• Spindle apparatus: Structure that separates chromosomes during mitosis.

• Kinetochore: Protein structure on chromatids for spindle attachment.

• Cytokinesis: Division of cytoplasm after mitosis.

• Cleavage furrow: Indentation that begins cytokinesis in animal cells.

• Cell plate: Structure that forms during cytokinesis in plant cells.

Example: Mitosis in Human Cells

During human cell division, a diploid cell (2n) undergoes mitosis to produce two genetically identical diploid daughter cells, each with the same number of chromosomes as the original cell.

Additional info: Prometaphase is sometimes considered a substage of prophase, and the spindle apparatus is composed of microtubules that originate from centrosomes.