Cell Cycle and Cell Division

Overview of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It is essential for growth, development, tissue renewal, and reproduction in multicellular organisms.

• Cell Cycle: Begins as a single cell that divides repeatedly for growth or to replace damaged cells.

• Interphase: The cell grows and duplicates its chromosomes, ensuring the genetic material (DNA) is copied precisely.

• Mitosis: Duplicated chromosomes are separated and moved to opposite ends of the cell.

• Cytokinesis: The cell divides into two genetically identical daughter cells, which may continue to divide, repeating the cycle.

Key Concepts

• Genetically Identical Daughter Cells: During cell division, a parent cell divides to produce two genetically identical daughter cells. Each daughter cell receives an exact copy of the parent cell's DNA.

• Mitotic Phase and Interphase: The cell cycle consists of two main phases: the mitotic (M) phase and interphase. The mitotic phase includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm). Interphase is the period of cell growth and DNA replication, divided into three sub-phases: G1 (first gap), S (synthesis), and G2 (second gap).

• Molecular Control System: The cell cycle is regulated by a set of regulatory proteins and checkpoints. Key regulatory molecules include cyclins and cyclin-dependent kinases (Cdks). Checkpoints ensure that each phase of the cell cycle is completed accurately before the next phase begins.

Chromosomes and Genetic Material

Chromosome Structure and Function

Chromosomes are structures made of DNA and proteins that contain genetic information. They play a crucial role in the accurate distribution of genetic material during cell division.

• Chromosome: A structure made of DNA and proteins that contains genetic information.

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

• Chromatin: The material that makes up chromosomes, consisting of DNA and proteins.

Cellular Organization of Genetic Material

• A cell's genome contains its genetic information.

• Prokaryotic genomes typically consist of a single DNA molecule, while eukaryotic genomes consist of multiple DNA molecules.

• Before cell division, DNA must be replicated and separated so each daughter cell receives a complete genome.

• DNA is packaged into chromosomes, which facilitate the replication and distribution of genetic material.

• The DNA in chromosomes carries genes, which are units of information specifying inherited traits.

• Proteins associated with DNA help maintain chromosome structure and regulate gene activity.

• The combination of DNA and proteins in chromosomes is called chromatin, which varies in condensation during cell division.

• Eukaryotic species have a characteristic number of chromosomes in each cell's nucleus. For example, human somatic cells have 46 chromosomes, arranged in two sets of 23, one from each parent.

• Human gametes (sperm and eggs) have 23 chromosomes, half the number found in somatic cells.

Distribution of Chromosomes During Eukaryotic Cell Division

Chromosome Behavior During Cell Division

Chromosomes exist as long, thin chromatin fibers when a cell is not dividing and during DNA replication. After DNA replication, chromosomes condense, becoming visible under a light microscope.

• Each duplicated chromosome consists of two sister chromatids, joined copies of the original chromosome.

• Sister chromatids are attached along their lengths by protein complexes called cohesins.

• Each sister chromatid has a centromere, the region where chromatids are most closely attached.

• The portion of a chromatid on either side of the centromere is called an arm.

• During cell division, sister chromatids separate and move into two new nuclei, becoming individual chromosomes.

Phases of the Cell Cycle

Cell Cycle Phases

The cell cycle includes the mitotic phase (M) and interphase. The mitotic phase, which includes mitosis and cytokinesis, is the shortest part of the cycle. Interphase accounts for about 90% of the cycle and is divided into three phases:

• G1 Phase: Cell growth and normal functions.

• S Phase: DNA replication.

• G2 Phase: Preparation for mitosis.

M Phase: The cell divides its copied DNA and cytoplasm to form two new cells. It includes:

• Mitosis: Division of the nucleus.

• Cytokinesis: Division of the cytoplasm.

Events of Interphase and M Phase

• G1 of Interphase: The cell grows and replicates its DNA.

• G2 of Interphase: The nuclear envelope encloses the nucleus, which contains nucleoli. Centrosomes have duplicated, and chromosomes are not yet condensed.

• Prophase: Chromatin fibers condense into chromosomes, nucleoli disappear, and the mitotic spindle begins to form. Centrosomes move apart.

• Prometaphase: The nuclear envelope fragments, chromosomes condense further, and kinetochores form at centromeres. Microtubules attach to kinetochores and interact with opposite spindle poles.

• Metaphase: Chromosomes align at the metaphase plate, with kinetochores attached to microtubules from opposite poles.

• Anaphase: Cohesin proteins are cleaved, allowing sister chromatids to separate and move to opposite ends of the cell. The cell elongates.

• Telophase: Daughter nuclei form, nuclear envelopes reappear, chromosomes decondense, and spindle microtubules depolymerize. Mitosis is complete.

• Cytokinesis: The cytoplasm divides, forming two daughter cells. In animal cells, this involves the formation of a cleavage furrow that pinches the cell in two.

Cell Cycle Checkpoints

Checkpoints are control mechanisms that ensure the cell cycle progresses accurately.

• G1 Checkpoint: Ensures the cell is ready for DNA synthesis.

• G2 Checkpoint: Ensures the cell is ready to enter mitosis.

• M Checkpoint: Ensures all chromosomes are properly aligned before cell division proceeds.

The Mitotic Spindle and Chromosome Movement

Mitotic Spindle Structure and Function

The mitotic spindle forms during prophase and is crucial for mitosis. It consists of microtubules and associated proteins.

• Microtubules: Assemble by polymerizing tubulin subunits and shorten by depolymerizing them.

• In animal cells, spindle microtubules assemble at the centrosome, which contains a pair of centrioles.

• During interphase, the centrosome duplicates, and the two centrosomes move apart during prophase and prometaphase, forming the spindle poles.

• An aster is a radial array of short microtubules extending from each centrosome.

Chromosome Movement During Mitosis

• Kinetochores: Protein structures at centromeres where spindle microtubules attach.

• Kinetochores Microtubules: Attach to kinetochores and move chromosomes toward the spindle poles.

• Nonkinetochore Microtubules: Overlap and interact with those from the opposite pole, contributing to spindle stability.

• Anaphase: Begins when separase cleaves cohesins, allowing sister chromatids to separate and move to opposite poles.

• Chromosome movement involves motor proteins that "walk" chromosomes along depolymerizing microtubules or "reel in" chromosomes at the spindle poles.

Experiment: Chromosome Movement

• Researchers labeled microtubules with a fluorescent dye and used a laser to mark a region. Chromosomes moved toward the poles, and the microtubules shortened at the kinetochore side of the mark.

• Conclusion: Chromosome movement during anaphase is associated with the depolymerization of microtubules at the kinetochore end.

Summary Table: Phases of the Cell Cycle

Key Terms and Definitions

• Chromosome: Structure made of DNA and proteins containing genetic information.

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

• Chromatin: Material that makes up chromosomes, consisting of DNA and proteins.

• Centromere: Region where sister chromatids are most closely attached.

• Mitotic Spindle: Structure made of microtubules that separates chromosomes during mitosis.

• Kinetochores: Protein structures at centromeres where spindle microtubules attach.

• Cohesins: Protein complexes that hold sister chromatids together.

• Checkpoints: Control mechanisms that ensure proper cell cycle progression.

Important Equations

• DNA Replication: Each daughter cell receives an exact copy of the parent cell's DNA.

• Cell Cycle Regulation:

Examples and Applications

• Human Somatic Cells: Have 46 chromosomes (23 pairs), one set from each parent.

• Human Gametes: Have 23 chromosomes, produced by meiosis.

• Fertilization: Fuses two gametes, restoring the chromosome number to 46.

• Cell Division in Chickens: Somatic cells have 78 chromosomes; each gamete contains 39 chromosomes.

Additional info:

• Cell division is essential for growth, development, and tissue renewal in multicellular organisms.

• Meiosis is a special type of cell division that produces gametes (sperm and eggs) and reduces the chromosome number by half.

• Chromosome movement during mitosis is powered by motor proteins and microtubule dynamics.