2.3 Cell division and cell cycle

Cell division is the cornerstone of growth and reproduction. The cell cycle, a series of events leading to cell division, includes interphase and mitosis. Interphase prepares the cell for division, while mitosis divides the nucleus.

The cell cycle's phases—G1, S, G2, and M—each play a crucial role. G1 and G2 are growth phases, S phase replicates DNA, and M phase divides the cell. Understanding these processes is key to grasping cellular biology fundamentals.

Cell Cycle Phases

Interphase

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• Interphase is the longest phase of the cell cycle where the cell grows, develops, and prepares for cell division
• Consists of three sub-phases: G1 phase, S phase, and G2 phase
• During interphase, the cell is metabolically active and continuously grows but does not divide
• Interphase is crucial for the cell to grow, replicate its DNA, and prepare for mitosis

G1 Phase

• G1 phase is the first gap phase of interphase and the cell cycle
• Occurs immediately after the previous cell division and before the S phase
• During G1, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps
• Cells also monitor their environment to determine whether conditions are suitable for replication if not, cells delay progress through G1 and may enter a resting state called G0
• Once the cell reaches a certain size and has enough nutrients, it can advance to the next phase (S phase)

S Phase

• S phase is the synthesis phase of interphase where DNA replication occurs
• During S phase, the cell synthesizes a complete copy of the DNA in its nucleus by making an identical copy of each chromosome
• S phase ensures that each daughter cell will have a complete set of genetic instructions after cell division
• In most cells, S phase lasts between 8-10 hours and occupies about half of the cell cycle time
• By the end of S phase, the cell has a duplicated set of chromosomes and is ready to enter G2

G2 Phase

• G2 phase is the second gap phase of interphase that occurs after DNA replication (S phase) and before mitosis (M phase)
• During G2, the cell continues to grow and makes proteins necessary for cell division
• G2 phase serves as a checkpoint to ensure the cell is ready for mitosis if DNA damage is detected, the cell will pause in G2 to repair the damage before proceeding
• Once the cell has reached sufficient size and has undamaged DNA, it can enter M phase and begin cell division

M Phase

• M phase is the mitotic phase of the cell cycle where the duplicated chromosomes are separated into two new nuclei and the cell divides
• M phase consists of two major processes: mitosis (nuclear division) and cytokinesis (cytoplasmic division)
• During mitosis, the chromosomes condense, the nuclear envelope breaks down, and the chromosomes are pulled apart by the mitotic spindle
• Cytokinesis then divides the cytoplasm, organelles, and cell membrane into two genetically identical daughter cells
• M phase is the shortest part of the cell cycle, typically lasting only 1-2 hours in mammalian cells

Cell Division

Mitosis

• Mitosis is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei
• Mitosis is divided into five phases: prophase, prometaphase, metaphase, anaphase, and telophase
• During prophase, the chromatin condenses into visible chromosomes, and the nuclear envelope starts to break down
• In prometaphase, the nuclear envelope completely disappears, and the chromosomes attach to the mitotic spindle
• Metaphase aligns the chromosomes along the center of the cell, known as the metaphase plate
• Anaphase separates the sister chromatids to opposite poles of the cell
• Telophase marks the end of mitosis as the chromosomes decondense, the nuclear envelope re-forms, and the spindle fibers disappear

Meiosis

• Meiosis is a specialized type of cell division that produces reproductive cells, such as sperm and egg cells (gametes)
• Meiosis involves two rounds of cell division, meiosis I and meiosis II, resulting in four haploid daughter cells
• During meiosis I, homologous chromosomes pair up and exchange genetic material through crossing over, increasing genetic diversity
• Meiosis I separates the homologous chromosomes, while meiosis II separates the sister chromatids
• Meiosis reduces the chromosome number by half, from diploid (2n) to haploid (n), which is essential for sexual reproduction

Cytokinesis

• Cytokinesis is the division of the cytoplasm that occurs after the completion of mitosis or meiosis
• In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell membrane inward until the cytoplasm is divided into two
• Plant cells, which have a rigid cell wall, undergo cytokinesis by forming a cell plate at the center of the cell, which gradually grows outward to divide the cytoplasm
• Cytokinesis ensures that each daughter cell receives a complete set of organelles and sufficient cytoplasm to function independently

Chromosomal Components

Chromosomes

• Chromosomes are thread-like structures located inside the nucleus of cells, made up of DNA tightly coiled around proteins called histones
• Each chromosome consists of two identical sister chromatids joined together at a region called the centromere
• The number of chromosomes varies between species humans have 46 chromosomes (23 pairs) in their somatic cells
• Chromosomes carry genetic information in the form of genes, which are specific sequences of DNA that code for proteins
• During cell division, the replicated chromosomes condense and become visible under a microscope, allowing them to be separated accurately into daughter cells

Centromeres

• The centromere is a constricted region of a chromosome that joins the two sister chromatids
• Centromeres play a crucial role in the separation of sister chromatids during cell division
• During mitosis and meiosis, protein structures called kinetochores assemble at the centromeres
• Kinetochores serve as attachment points for the microtubules of the mitotic spindle
• The spindle microtubules pull on the kinetochores, allowing the sister chromatids to be separated and drawn towards opposite poles of the cell
• Centromere position can vary among chromosomes, and their location is used to classify chromosomes as metacentric, submetacentric, or acrocentric