Cell Cycle and Chromosomes Overview

Cell Cycle Overview

• Cells replicate through cell division from pre-existing cells.
• Embryos in plants and animals start as single cells.
• Growth occurs through cell divisions.
• Meiosis produces gametes (reproductive cells).
• Mitosis produces somatic (body) cells.
• Mitosis and meiosis are usually accompanied by cytokinesis (division of cytoplasm).
• During mitosis, genetic material is copied and divided equally between two cells.
• Daughter cells are genetically identical to the parent cell and to each other during mitosis.
• Meiosis produces cells with half the amount of hereditary material as the parent cell.
• Daughter cells are genetically different during meiosis.
• Cell replication involves copying DNA, separating copies, and dividing the cytoplasm.
• Humans have 46 chromosomes.

Chromosomes

• A chromosome is a single, long DNA double helix wrapped around proteins called histones.
• DNA encodes the cell's genetic information.
• A gene is a section of DNA coding for a specific RNA, which in turn codes for a specific protein.
• Before mitosis, each chromosome is replicated.
• Each double-stranded DNA copy is called a chromatid.
• Chromatids are initially attached along their entire length by proteins called cohesions.
• Once mitosis begins, they are attached only at the centromere.
• Chromatids attached at the centromere are called sister chromatids.
• Two attached sister chromatids are still considered a single chromosome.
• Unreplicated chromosomes consist of a single DNA double helix wrapped around proteins.
• Replicated chromosomes consist of two copies of the same DNA double helix.
• Condensed replicated chromosomes are 10,000 times shorter than their original length due to the condensation around proteins.

Cell Cycle Phases

• Growing cells cycle between two phases: mitosis (M phase) and interphase.
• M phase is the dividing phase.
• Interphase is the nondividing phase.
• Chromosomes condense into compact structures during mitosis.
• Chromosomes are uncoiled during interphase.
• Cells grow and prepare or fulfill specialized functions during interphase.
• Cells mainly spend time in interphase.
• Chromosome replication occurs only during interphase, specifically during the S phase (synthesis).
• Interphase includes two gap phases (G1 and G2) where no DNA synthesis occurs.

Mitosis

• Mitosis is a continuous process with 5 subphases (prophase, prometaphase, metaphase, anaphase, and telophase).
• During prophase, chromosomes condense, becoming visible under a light microscope, and the spindle apparatus forms.
• The spindle apparatus is composed of microtubules, forming from microtubule-organizing centers (MTOCs).
• Polar microtubules extend from opposing spindle poles and overlap.
• In animal cells, MTOCs are centrosomes containing centrioles.
• During prometaphase, the nuclear envelope breaks down, and microtubules attach to chromosomes at kinetochores, structures at the chromosomes' centromere.
• Kinetochore microtubules pull chromosomes to the middle of the spindle.
• During metaphase, the mitotic spindle is complete.
• Chromosomes align on the metaphase plate, an imaginary plane between the spindle poles.
• Astral microtubules hold spindle poles in place.
• During anaphase, cohesions holding sister chromatids together split.
• Sister chromatids are pulled to opposite poles of the cell.
• Two identical sets of daughter chromosomes are created.
• Kinetochore microtubules shrink.
• Motor proteins of polar microtubules push the poles apart.
• During telophase, a new nuclear envelope forms around each set of chromosomes.
• Chromosomes decondense.
• Mitosis is complete.
• Cytokinesis typically follows mitosis.
• The cytoplasm divides to form two daughter cells.

Bacterial Cell Replication

• Bacteria divide through binary fission.
• A process similar to eukaryotic mitosis.
• Bacterial chromosomes replicate.
• Proteins attach to and separate the replicated chromosomes.
• Proteins divide the cytoplasm.

Cell Cycle Control

• Cell-cycle length varies among cell types, primarily due to variations in the G1 phase length.
• Rapidly dividing cells essentially eliminate G1 phase.
• Nondividing cells are permanently stuck in G1 phase, known as the G0 state.
• Division rate can change based on various conditions.
• Cell-cycle length variations suggest regulation.
• M-phase checkpoints occur: between metaphase and anaphase, and between anaphase and telophase.
• Cell cycle checkpoints ensure replicated chromosomes are correctly segregated into daughter cells.

Cancer

• Cancer is a family of diseases caused by cells that grow uncontrollably, invading nearby tissues and spreading to other sites.
• Cancer is caused by defects in checkpoints and regulation of the cell cycle.
• Cancerous cells have two types of defects: activating proteins that should not be active, and preventing the suppression of tumor suppressor genes.
• Malignant tumors are cancerous and invasive, spreading throughout the body.
• Benign tumors are noncancerous and noninvasive.
• Many cancers arise from cells with defects in the checkpoint.

Social Control

• Cells respond to signals from other cells.
• Cells only divide when the organism benefits.
• Social control is based on growth factors, small proteins that stimulate cell division.
• Cells in culture need growth factors (e.g., serum) to pass the G1 checkpoint.
• Cancer cells divide without growth factors.

How Checkpoints Work

• G1 checkpoint: Determines if cells continue through the cycle or enter the G0 resting state. Factors include: size, nutrient availability, social signals, and DNA damage.
• G2 checkpoint: Checks if chromosome replication was successful and if DNA is undamaged. If so it triggers MPF activation. If not, it prevents MPF activation to stop mitosis.
• M-phase checkpoints occur: between metaphase and anaphase, and between anaphase and telophase.
• G1, G2, and M phase checkpoints influence cell cycle progression based on various factors, including intracellular events and external signals.

MPF (M-phase Promoting Factor)

• MPF is present in the cytoplasm of M-phase cells and induces mitosis in eukaryotes.
• Composed of two subunits: a protein kinase (Cdk) and a cyclin.
• Cyclin concentration increases during interphase, peaking in M phase before decreasing again.
• Cdk is a cyclin-dependent kinase (Cdk).
• MPF is active only when bound to cyclin.
• Target proteins are phosphorylated, initiating mitosis.
• MPF deactivation illustrates negative feedback in cells.
• An enzyme complex activated during anaphase marks cyclin for destruction, deactivating MPF.

Explore the fascinating processes of the cell cycle, including mitosis and meiosis, which are crucial for growth and reproduction. Understand the structure and function of chromosomes, and how genetic information is transferred during cell division. This quiz will test your knowledge on the fundamentals of cellular biology.