División celular

Introduction to Cell Division

Overview of Mitosis and Cell Structure

• Estela Roma introduces the topic of cell division, emphasizing that humans are composed of approximately 10 trillion cells, highlighting the complexity of multicellular organisms.

• The process of cell division is crucial not only for growth but also for wound repair, indicating its everyday relevance in biological functions.

The Cell Cycle Stages

• The life cycle of a cell includes several stages: G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis). In G1, cells grow and acquire organelles necessary for mature cellular functions.

• Different cell types exhibit varied morphologies during G1 due to differences in organelle quantity and quality, which reflects their specific cellular tasks such as secretion or nerve impulse transmission.

Cell Differentiation and G0 Phase

Specialized Cells in G1 Phase

• Some highly differentiated cells remain permanently in the G1 phase, referred to as being in the G0 state; examples include skeletal muscle cells, cardiac muscle cells, and neurons. These cells do not progress through the cell cycle from birth until death.

DNA Replication Process

Transition to S Phase

• During the S phase, DNA undergoes replication; this involves duplicating not just DNA but all chromatin within the nucleus with assistance from enzymatic complexes. This stage is critical for preparing genetic material for mitosis.

G2 Phase and Preparation for Mitosis

Final Preparations Before Division

• In the G2 phase, there is further preparation including duplication of centrioles which are essential for microtubule formation during mitosis. This sets up the cell for successful division into two daughter cells.

Understanding Chromosomes

Chromosome Structure

• Human somatic cells divide via mitosis while meiosis occurs only in gonads (ovaries/testes). Each human has 46 chromosomes (diploid) compared to gametes which have 23 chromosomes (haploid). Understanding chromosome structure is vital for grasping how they behave during division.

Components of Chromosomes

• A chromosome consists of a centromere that divides it into short arms and long arms; these arms contain sister chromatids that are identical copies joined at the centromere until separation during mitosis. Additionally, telomeres protect chromosome ends from degradation during replication cycles.

Cell Division and the Cell Cycle

Microtubules and Centrosomes

• Microtubules known as kinetochores are released, resembling animal whiskers. These structures play a crucial role in cell division.

• There are two pairs of centrioles or centrosomes that some authors suggest duplicate during the G2 phase, while others argue it occurs in the S phase.

• Centrioles release polar microtubules that extend from one pole of the cell to another, along with shorter microtubules called asters.

Phases of the Cell Cycle

• The cell cycle consists of phases: G1 (cell growth), S (DNA replication), and G2, collectively referred to as interphase before mitosis or meiosis begins.

• Adult cells divide through mitosis, producing two smaller daughter cells. During G1, they increase in size before potentially dividing again.

Mitosis Overview

• Mitosis is a type of cellular division occurring in somatic cells and comprises four classical phases: prophase, metaphase, anaphase, and telophase.

• An additional stage called prometaphase is introduced between prophase and metaphase due to its significant events.

Prophase Details

• In prophase, chromatin condenses into visible chromosomes within the nucleus; centriole pairs migrate to opposite poles while polymerizing microtubules.

• Summary: Chromosomes become visible as chromatin compacts; centrioles begin microtubule polymerization towards cell poles.

Prometaphase Events

• The nuclear envelope disintegrates along with the internal lamina that holds chromosomes against it. This allows chromosomes to move freely in the cytoplasm.

• Microtubules attach to chromosomes via kinetochores after the nuclear envelope's disappearance.

Metaphase Dynamics

• In metaphase, polar microtubules elongate while kinetochores anchor chromosomes at a structure called the spindle apparatus.

• Chromosomes reach their highest level of compactness during this phase and align at the equatorial plane due to balanced forces within the cell.

Anaphase Mechanism

• Anaphase begins when equilibrium breaks; polar microtubules lengthen while kinetochores shorten, splitting chromosomes into sister chromatids migrating toward opposite poles.

• A cleavage furrow forms at the center but still represents a single cell during this transition.

Telophase Conclusion

• In telophase, nuclear envelopes reform around separated chromatids which start decondensing back into chromatin form.

Cytokinesis Process

• Cytokinesis marks definitive separation where cytoplasm divides completely resulting in two distinct daughter cells.

Mitosis and Meiosis Overview

Mitosis Process

• Mitosis begins with a mother cell containing 46 chromosomes, classified as diploid (2n), which indicates the total number of chromosomes in humans.

• The mother cell has a specific amount of DNA referred to as 2c. It undergoes the G1 phase, followed by the S phase where DNA is duplicated, resulting in 4c.

• After the S phase, it progresses through G2 and enters mitosis, which includes prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis.

• The outcome of mitosis is two daughter cells that are genetically identical to each other and to the original mother cell; all three cells remain diploid (2n).

Meiosis Process

• Meiosis is a specialized type of division occurring only in ovaries (females) and testes (males), producing ovocytes and spermatozoa respectively.

• It consists of two divisions: meiosis I and meiosis II. Meiosis I is preceded by an S phase where DNA duplication occurs; meiosis II does not have this preceding S phase.

• The stages of meiosis include prophase I, prometaphase I, metaphase I, anaphase I, telophase I for the first division; then prophase II through telophase II for the second division.

• The result of meiosis from one diploid mother cell (2n 2c) is four daughter cells that each contain half the chromosome number and half the amount of DNA compared to the original cell.