71- Biología celular. Etapas de la mitosis

Introduction to the Cell Cycle

Overview of M Phase

• Alejandra introduces the topic of the cell cycle, focusing on the M phase, which involves cellular division.

• The discussion emphasizes that mitosis consists of various stages and encourages students to recall previous lessons about eukaryotic cells.

Prophase Details

• In prophase, DNA condenses into sister chromatids, and the nuclear envelope begins to disintegrate to make space for chromosome alignment.

• The process of chromatin condensation starts in prophase and reaches its peak during metaphase; this is crucial for proper chromosome segregation.

Cytoskeleton Disintegration

• The nuclear envelope's breakdown continues into prometaphase, while cytoskeletal elements must also be dismantled for effective cell shape change.

• As the cytoskeleton disintegrates, cells become more spherical and lose connections with neighboring cells or extracellular matrix.

Prometaphase and Metaphase Dynamics

Formation of Mitotic Spindle

• During prometaphase, microtubules form a mitotic spindle essential for chromosome movement; this structure is finalized in prometaphase.

• Different types of spindle fibers are identified: kinetochore fibers (attach to chromosomes), polar fibers (extend between poles), and astral fibers (anchor spindle).

Chromosome Alignment

• Kinetochore plates appear at centromeres during prophase; these structures are critical for connecting chromosomes to spindle fibers.

• Centrosomes move towards opposite poles as they organize microtubules necessary for forming the mitotic spindle.

Anaphase Mechanics

Chromosome Separation Process

• Anaphase is characterized by all chromosomes aligning at the cell's equator; this stage includes a checkpoint ensuring proper attachment to spindle fibers.

Mitosis Process Overview

Stages of Mitosis

• The migration phase in mitosis begins when chromosomes arrive at the poles, marking the conclusion of a specific phase and transitioning into the next.

• The nuclear envelope reorganizes to form daughter nuclei, while organelles begin orienting themselves for future daughter cells.

• Cytokinesis is crucial as it partitions the cytoplasm into two cells; without it, a binuclear cell would result, indicating that cytokinesis starts in one phase and ends in another.

Role of Mitotic Spindle

• The mitotic spindle consists of kinetochore fibers that pull sister chromatids toward the poles, along with polar fibers and astral fibers aiding in this process.

• Microtubules can depolymerize during this process, which helps in pulling daughter chromosomes towards their respective poles.

Cytokinesis Mechanism

• As kinetochore fibers depolymerize, polar fibers elongate; remnants of these polar fibers remain at the equator after spindle disassembly.