The Cell Cycle and Mitosis: Structure, Function, and Regulation

The Cell Cycle

Introduction to the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It is fundamental to the growth, development, and maintenance of all living organisms. The cell cycle ensures that genetic material is accurately replicated and distributed to daughter cells.

• Cell division is necessary for:

  • Growth

  • Healing

  • Cell replacement

  • Reproduction

• Cell division occurs in somatic (body) cells and germ cells (for reproduction).

Cell Theory and Historical Context

Hooke & Cell Theory

The cell theory is a fundamental concept in biology, first articulated by scientists such as Robert Hooke. It states that all living things are composed of cells, and that all cells arise from pre-existing cells through cell division.

• Robert Hooke first observed cells in cork tissue.

• Cell theory highlights the importance of cell division for life processes.

Chromosome Structure and DNA

Chromosome Structure

Chromosomes are thread-like structures located within the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA).

• Chromosomes are visible during cell division.

• Each chromosome consists of two identical halves called chromatids.

• Chromatids are joined together by a centromere.

• Chromosomes are made of DNA tightly coiled around proteins called histones.

• Histones help maintain chromosome shape and aid in the tight packing of DNA.

Homologous chromosomes are pairs of chromosomes (one from each parent) that are similar in shape, size, and genetic content.

DNA Structure

DNA (deoxyribonucleic acid) is the hereditary material in almost all living organisms. It consists of two antiparallel strands forming a double helix.

• Each DNA strand has a 5' (five prime) end and a 3' (three prime) end.

• Nucleotides are the building blocks of DNA, each containing a phosphate group, a deoxyribose sugar, and a nitrogenous base (adenine, thymine, cytosine, or guanine).

• The two strands are held together by hydrogen bonds between complementary bases: Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G).

Example: The complementary DNA strand for 5'-CGTATG-3' is 3'-GCATAC-5'.

DNA Replication

Overview of DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division, ensuring that each new cell receives an identical set of genetic instructions.

• Occurs during the S phase of interphase in the cell cycle.

• Each original DNA strand serves as a template for a new strand.

• Replication is semiconservative: each new DNA molecule consists of one old strand and one new strand.

Steps in DNA Replication

1. Unwinding: The DNA double helix is unwound by the enzyme helicase.

2. Priming: Primase synthesizes short RNA primers to provide a starting point for DNA synthesis.

3. Elongation: DNA polymerase adds new nucleotides to the 3' end of the primer, synthesizing the new DNA strand.

4. Leading and Lagging Strands:

   • The leading strand is synthesized continuously in the 5' to 3' direction.

   • The lagging strand is synthesized discontinuously, forming short segments called Okazaki fragments.

5. Primer Removal and Ligation: Exonuclease removes RNA primers, and DNA ligase joins Okazaki fragments to form a continuous strand.

DNA Damage and Repair

DNA can be damaged by chemicals and ultraviolet radiation. Cells have repair mechanisms to maintain genetic integrity.

• Excision repair: Over 50 repair enzymes can remove damaged DNA segments.

• DNA polymerase and DNA ligase replace and bond new nucleotides together.

The Cell Cycle Phases

Interphase

Interphase is the longest phase of the cell cycle, accounting for about 75% of a cell's life. It consists of three subphases:

• G1 phase (Gap 1): Rapid cell growth and normal metabolic activities.

• S phase (Synthesis): DNA replication occurs; centrioles also replicate.

• G2 phase (Gap 2): Cell prepares for division; microtubular structures form.

Mitotic Phase (M Phase)

The mitotic phase includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

• Mitosis: Division of nuclear components (chromosomes).

• Cytokinesis: Division of cytoplasmic components, resulting in two daughter cells.

Stages of Mitosis

Overview of Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets, resulting in two genetically identical daughter cells. It consists of four main stages:

1. Prophase: Chromatin condenses into visible chromosomes; nuclear envelope breaks down; spindle fibers form; centrioles move to opposite poles.

2. Metaphase: Chromosomes align at the metaphase plate (center of the cell); spindle fibers attach to centromeres.

3. Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell.

4. Telophase: Chromatids reach the poles; nuclear envelope reforms; chromosomes decondense; nucleolus reappears.

Cytokinesis usually overlaps with telophase, dividing the cytoplasm and completing cell division.

Summary Table: Stages of Mitosis

Stage	Main Events
Prophase	Chromosomes condense, spindle forms, nuclear envelope dissolves
Metaphase	Chromosomes align at metaphase plate, spindle fibers attach
Anaphase	Sister chromatids separate and move to opposite poles
Telophase	Nuclear envelope reforms, chromosomes decondense
Cytokinesis	Cytoplasm divides, two daughter cells form

Key Terms and Definitions

• Chromatid: One of two identical halves of a duplicated chromosome.

• Centromere: Region where two sister chromatids are joined.

• Spindle fibers: Microtubules that separate chromosomes during mitosis.

• Kinetochore: Protein structure on the centromere where spindle fibers attach.

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

Additional info:

• Human somatic cells have 46 chromosomes (23 pairs).

• Errors in DNA replication or cell division can lead to mutations or diseases such as cancer.