뒤로Cell Cycle and Mitosis: An Overview
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Cell Cycle and Mitosis
Introduction to the Cell Cycle
The cell cycle is the series of events that cells go through as they grow and divide. All cells arise through cell division of pre-existing cells. This process is fundamental for growth, development, and tissue repair in multicellular organisms.
Observation: Plants and animals start life as single-celled embryos and grow through a series of cell divisions.
There are two main types of cell division:
Mitosis: Produces most cells, called somatic cells.
Meiosis: Produces the gametes (egg and sperm).
Types of Cell Division
Mitosis: Produces cells that are genetically identical to the original cell and to each other.
Genetic material is copied and divided equally between two cells.
Produces the somatic cells that make up most of the organism (including cells replaced throughout adulthood).
Meiosis: Produces cells that are genetically different from each other and from the parent cell.
Genetic material is divided in half into two cells.
Occurs only in specialized reproductive cells.
Produces gametes: egg and sperm.
12.1 How Do Cells Replicate?
Phases of the Cell Cycle
Cells alternate between phases of the "cell cycle," which consists of interphase and the M phase.
Interphase (G1, S, G2): Non-dividing phase.
Cell grows and fulfills its specialized function.
Genes on chromosomes are transcribed to make proteins the cell needs.
Chromosomes are replicated only before proceeding to M phase.
M phase (mitotic or meiotic): Dividing phase.
Cell separates chromosomes and divides to create two cells.
Chromosomes and Genetic Material
Chromosome: Contains a single long double helix of DNA wrapped around proteins called histones.
Each species has a characteristic number of chromosomes in each somatic cell (e.g., humans have 46).
Gene: A specific segment of DNA on a chromosome that codes for a specific mRNA, which is used to make a specific protein.
Chromosome Replication
Before a cell divides, each chromosome is replicated during the S phase of interphase.
The two identical copies are called sister chromatids, attached at a region called the centromere.
Chromosomes condense only when cells begin mitosis.
Summary Table: Cell Cycle Phases
Phase | Main Events |
|---|---|
G1 | Cell growth, normal function |
S | DNA replication (chromosome duplication) |
G2 | Preparation for mitosis |
M | Mitosis (chromosome separation) and cytokinesis (cell division) |
12.2 What Happens During M Phase?
Overview of Mitosis
Mitosis is a continuous process divided into five sub-phases, followed by cytokinesis. It ensures equal distribution of genetic material to daughter cells.
Prophase: Chromosomes condense, and the spindle apparatus begins to form.
Prometaphase: Nuclear envelope breaks down; microtubules contact chromosomes at the kinetochore.
Metaphase: Chromosomes align at the middle of the cell (metaphase plate).
Anaphase: Sister chromatids separate and are pulled to opposite poles of the spindle apparatus.
Telophase: Nuclear envelope reforms around each set of chromosomes; chromosomes de-condense.
Cytokinesis follows, dividing the cytoplasm and completing cell division.
Key Structures in Mitosis
Centrosome: Organelle that organizes microtubules and contains a pair of centrioles (in animal cells).
Spindle apparatus: Structure made of microtubules that separates chromosomes during mitosis.
Kinetochore: Protein complex at the centromere where spindle fibers attach.
Summary Table: Mitosis Phases and Key Events
Phase | Main Events |
|---|---|
Prophase | Chromosomes condense, spindle forms |
Prometaphase | Nuclear envelope breaks down, spindle fibers attach to kinetochores |
Metaphase | Chromosomes align at metaphase plate |
Anaphase | Sister chromatids separate to opposite poles |
Telophase | Nuclear envelope reforms, chromosomes de-condense |
Cytokinesis | Cytoplasm divides, two daughter cells form |
Cytokinesis
In animal cells, a contractile ring of actin and myosin filaments pinches the cell membrane to form a cleavage furrow, dividing the cell into two.
In plant cells, vesicles form a cell plate that develops into a new cell wall between the daughter cells.
Mechanism of Chromosome Movement
During anaphase, microtubules attached to kinetochores shorten at the kinetochore end, pulling sister chromatids toward opposite poles.
Motor proteins "walk" the chromatids along the microtubules.
Practice and Application
Be able to identify the phase of the cell cycle based on chromosome state (replicated/unreplicated, condensed/uncondensed), spindle attachment, and nuclear envelope status.
Draw and label the stages of mitosis and the cell cycle for practice.
Key Terms
Somatic cells: All body cells except gametes.
Gametes: Reproductive cells (egg and sperm).
Sister chromatids: Identical copies of a chromosome connected at the centromere.
Centromere: Region where sister chromatids are joined and spindle fibers attach.
Kinetochore: Protein structure on the centromere for spindle attachment.
Example
In a human cell (2n = 46), after S phase, there are 46 chromosomes, each consisting of two sister chromatids (92 chromatids total).
After mitosis and cytokinesis, each daughter cell has 46 chromosomes (identical to the parent cell).
Additional info: For further study, watch time-lapse videos of mitosis in animal and plant cells to visualize the dynamic changes during cell division.
