DNA, Cell Division, and Reproduction: Core Biological Processes
Understanding DNA: The Molecule of Life
DNA is the biomolecule that contains the genetic information for making proteins in the cell. This fundamental molecule dictates the characteristics and functions of all living organisms.
Chromosomes and Chromatin Structure
The genetic material within a cell is organized into structures called chromosomes. These are formed from a complex material known as chromatin, which is an association of DNA and proteins.
- When the cell is at rest, the chromatin is dispersed throughout the nucleus, and individual chromosomes are not visible.
- Before cell division, the DNA is duplicated, and the chromatin condenses into shorter, thicker fibers, making the chromosomes visible.
Each duplicated chromosome consists of two identical arms called chromatids, which are joined by a structure called a centromere.
Genome and Genes
The complete set of chromosomes of an individual, ordered in pairs, is called a karyotype. The chromosomes of each pair are referred to as homologous chromosomes.
The genome is the entire set of genes of an organism. The human genome, for example, contains approximately 35,000 genes. A gene is a specific segment of DNA that contains the information needed to produce a particular protein.
Cell Division: The Basis of Life
Cell division is the fundamental process by which a cell divides into two or more identical daughter cells. In eukaryotic cells, the primary form of cell division is mitosis.
The Cell Cycle
The life of a cell is divided into two main stages: interphase and cell division.
Interphase: Growth and Preparation
Interphase takes up most of a cell’s life. It is a crucial growth stage where the cell:
- Increases its size and cytoplasm volume.
- Increases the size and number of its organelles.
- Performs its normal functions, such as absorbing nutrients, photosynthesizing, or producing substances and accumulating energy.
- Protein synthesis takes place during this phase.
Cells prepare for division by entering a stage called the S phase, during which DNA duplication occurs.
Mitosis: Nuclear Division for Growth and Repair
Cell division itself is a relatively short stage, consisting of the division of the nucleus (mitosis) and the division of the cytoplasm (cytokinesis).
Phases of Mitosis
Mitosis is a continuous process, but it is typically described in distinct phases:
- Interphase (Pre-Mitosis): Chromosomes have been duplicated but are still dispersed in the form of chromatin.
- Prophase: The chromatin condenses, and chromosomes start to become visible. The nuclear membrane begins to disappear (often referred to as late prophase or prometaphase).
- Metaphase: The nuclear membrane has completely disappeared, and chromosomes attach to the mitotic spindle microtubules, aligning at the cell’s equator.
- Anaphase: The two identical chromatids of each chromosome separate and move to opposite poles of the cell.
- Telophase: Chromosomes have reached the two poles of the cell and begin to decondense, forming new chromatin. New nuclear membranes begin to form around each set of chromosomes.
Mitosis specifically refers to the division of the cell nucleus and the precise distribution of chromosomes into two daughter cells. In animals, mitosis occurs in all tissues, while in plants, it is restricted to specific tissues, such as meristematic tissues.
Cytokinesis: Cytoplasmic Division
Cytokinesis is the division of the cytoplasm, which typically follows mitosis.
- In Animal Cells: A contractile ring of microtubules forms a furrow that pinches and strangles the cell until two identical daughter cells are formed.
- In Plant Cells: Cytokinesis is not caused by strangulation. Instead, a cell plate forms in the middle, separating the two daughter cells and giving rise to the new cell wall.
Mitosis is the type of cell division that occurs in all somatic cells for growth, development, and tissue repair.
Meiosis: Creating Genetic Diversity
In sexual reproduction, there is the union of two specialized cells called gametes, which results in the formation of a zygote. Meiosis is the process that forms these gametes.
Cells resulting from meiosis are not identical to each other, unlike those from mitosis. They contain different combinations of genes, formed by the exchange of chromosomal portions in a process called crossing over. The result of crossing over is genetic recombination, which contributes significantly to genetic diversity.
Meiosis is essential for sexual reproduction as it produces gametes with half the number of chromosomes (haploid) compared to the parent cell.
Gamete Formation
The formation of eggs (ova) and sperm cells is specifically called oogenesis and spermatogenesis, respectively. Both processes occur through meiosis.
- Sperm cells are formed in special cells of the testes called spermatogonia.
Reproduction Strategies
Reproduction is the ability of any living cell or organism to produce offspring similar to itself.
Asexual Reproduction
Asexual reproduction is characterized by a single progenitor giving rise to two or more genetically identical descendants. The cells that have the same genetic material as their parent are called clones.
Sexual Reproduction
In sexual reproduction, new individuals are formed from the fusion of gametes from two parents, leading to genetic variation.
Bacterial Reproduction
Bacteria are prokaryotes, meaning they lack a membrane-bound nucleus. Their genetic material is contained in a single, circular DNA strand, often called a circular chromosome or bacterial chromosome.
Bacteria reproduce asexually through a simple type of division called binary fission, where one cell divides into two identical daughter cells.