Cell Cycle and Interphase

Cell Cycle

• Cell cycle is the series of stages a cell goes through to grow, prepare for division, and divide into two daughter cells
• The cell cycle includes interphase (preparation phase) and mitotic phase (actual division phase)
• There are two types of the cell cycle: interphase and cell division

Interphase

• Interphase is the longest phase of the cell cycle
• During interphase the cell grows copies its DNA, and prepares for mitosis
• No visible chromosome separation occurs during interphase
• Interphase is divided into three phases: G1 phase, S phase, and G2 phase

G1 Phase (First Gap)

• G1 phase is the first stage of interphase
• Cells are metabolically active during G1 phase
• Cells accumulate the building blocks of chromosomal DNA and associated proteins during G1 phase
• Cells store energy reserves to replicate each chromosome in the nucleus during G1 phase
• G1 is the first growth phase
• During G1 phase a cell grows in size
• The synthesis of new proteins takes place during G1
• G1 starts immediately after cell division

S Phase (Synthesis of DNA)

• Replication of DNA takes place during the S phase
• Each chromosome becomes two sister chromatids during the S phase
• The S phase lasts for approximately 8-10 hours

G2 Phase (Second Gap)

• G2 phase is the stage where the cell replenishes its energy stores
• During G2 phase proteins are synthesized necessary for chromosome movement
• Some cell organelles are duplicated during the G2 phase
• Cells may continue growing during the G2 phase
• Cells make the final preparations before entering into the mitotic phase

Controlled Cell Cycle

• The cell cycle has an internal controlling system that enables it to follow regular patterns
• Regulatory system failure results in excessive cell division that, can lead to "cell madness"
• Cancer occurs as a result of regulatory system failure

Cell Division

• Cell division is a basic process where a parent cell divides into two daughter cells
• The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells
• Cells on the path to cell division proceed through timed and regulated stages of growth, DNA replication, and nuclear and cytoplasmic division
• Cell division ultimately produces two identical (clone) cells

Case of Cell Division

• As the cell grows, the volume of cytoplasm relative to the cell membrane decreases the surface area to volume ratio
• Material transport across the cell membrane by simple diffusion becomes inadequate
• As cell size increases, the nucleus's controlling power minimizes
• Cell division employs a mechanism by which one cell becomes two or more, solving these problems

Subdivisions of Cell Division

• Cell division consists of two subdivisions: nuclear division and cytokinesis
• Nuclear Division (Karyokinesis) results in the separation and distribution of duplicated genetic materials via mitosis or meiosis
• Cytokinesis (cytoplasmic division) is the separation of the cytoplasmic components into daughter cells

Mitosis

• Mitosis is a type of nuclear division where duplicated chromosomes of a mother cell are distributed between two identical daughter cells
• Daughter cells have the same number and kind of hereditary materials (chromosomes) as the parent nucleus
• A diploid (2n) mother cell gives rise to two diploid (2n) identical daughter cells
• Stages of mitosis in series: prophase, metaphase, anaphase, and telophase

Prophase

• Prophase is the first phase of cell division
• Chromatin condenses and becomes visible chromosomes
• Each chromosome consists of two sister chromatids joined at a centromere
• The nuclear envelope starts to break apart, allowing chromosomes to move freely
• Spindle fibers form from the centrioles (in animal cells) or other organizing centers (in plant cells)
• In animal cells, the centrioles move to opposite poles of the cell

Metaphase (arrangement)

• Chromosomes line up in the middle of the cell, called the metaphase plate
• Each chromosome (with two sister chromatids) attaches to spindle fibers at its centromere
• Spindle fibers from both poles pull on the chromosomes to align them in the center

Anaphase (migrating)

• Chromosomes are separated and moved toward opposite poles of the cell
• Sister chromatids (previously paired in metaphase) are pulled apart by spindle fibers attached to their centromeres
• Spindle fibers shorten, separating the chromatids into individual chromosomes
• One chromatid is dragged toward one pole, and the other to the opposite pole
• Daughter cells receive the same number and kind of chromosomes
• Separated chromosomes move toward opposite poles of the cell

Telophase

• Telophase is the last stage of cell cycle with the following activity:
• Spindle disappears
• Nucleolus reforms
• New nuclear membrane develops

Cytokinesis

• Cytokinesis involves the separation of cytoplasm