Cell Division, Reproduction, and Nucleic Acids
Cell Division
Transmission gives information to do so. This occurs when a DNA cell multiplies during cell division. A cell divides and gives rise to two daughter cells with the same genetic information as the progenitor. This occurs during the process of sexual reproduction when gametes unite and give rise to a new zygote, inheriting characteristics from both progenitors.
Propagation of cell characteristics: A progenitor cell divides and distributes its content between two daughter cells. But before a cell divides, it must duplicate all the elements to ensure that the daughter cells receive the necessary components for their development.
Prokaryotic Cell Division
In prokaryotes, the chromosome consists of a circular DNA molecule dispersed in the cytoplasm. Division begins with the growth of a cell and the development of a transverse wall separating the cell into two daughters – binary fission. During the growth step, the identical chromosome forms two copies, which join at different points inside the plasma membrane. When the membrane thrives, the two chromosomes separate, and therefore, each daughter cell receives an exact copy of the genetic material.
Cell Division in Eukaryotes
Eukaryotic cell division occurs in two stages:
- Growth stage (Interphase): This is the longest stage. The daughter cell reaches adult size, and then DNA replication occurs, preparing for the division stage.
- Division stage: This stage involves the division of the nucleus (mitosis) and then the cytoplasm (cytokinesis).
Sexual Reproduction
Sexual reproduction consists of the union of two gametes, one from the maternal progenitor and the other from the paternal, to form a zygote. After successive divisions, the zygote originates an individual with characteristics from both progenitors. To maintain the chromosome number in sexually reproducing species, meiosis occurs at some point in the biological cycle to avoid duplication of chromosomes. Meiosis takes place during two successive cell divisions (meiosis I and meiosis II).
Cytokinesis
Cytokinesis is the division of the cytoplasm. This process is visible during telophase, and the cell divides into two smaller daughter cells. Cytokinesis occurs differently in animal and plant cells.
- Animals: Animal cells do not have a cell wall. Cytoplasmic division occurs by transverse division with the progressive strangulation of the plasma membrane around the cell’s equator, thanks to the presence of a contractile ring in the membrane.
- Plants: The rigid cell walls of plant cells prevent strangulation of the cytoplasm. Cell division is initiated by the formation of a partition: the phragmoplast, which progresses from the center of the cell to the margins and forms a cell wall between the daughter cells.
Life Cycle of Diploid Organisms
In diploid organisms (e.g., animals), life occurs in the diploid stage (2n), and the cells of their body divide by mitosis, originating new diploid cells. The haploid phase corresponds only to the training period of the gametes. These originate from diploid germ cells (2n) that undergo meiosis to obtain four haploid gametes (n). The fertilization of two gametes results in a diploid zygote (2n) that divides by mitosis to originate a new diploid organism.
Nucleic Acids
Nucleic acids are long chains consisting of carbon, oxygen, nitrogen, and phosphorus atoms. There are two types:
- DNA
- RNA
Differences between DNA and RNA
Structure: DNA is double-stranded (and therefore the information it contains is more secure), while RNA is single-stranded (single chain).
Composition: DNA and RNA have a different composition of pentose (a chain of five carbon atoms). DNA is composed of deoxyribose (derived from ribose, but carbon 2 lacks an alcohol group), and RNA is composed of ribose (which is a monosaccharide, a simple sugar, and a primary source of energy used by our cells). DNA is composed of adenosine, thymine, guanine, and cytosine. In contrast, RNA is composed of adenosine, uracil (instead of thymine), guanine, and cytosine.
Function: DNA stores, preserves, and transmits genetic information from generation to generation. RNA is an intermediary between the information that DNA carries in the form of genes and the final protein encoded by those genes. The translation occurs in the cytoplasm where the ribosomes are located. Ribosomes are made up of a small and a large subunit that surrounds the mRNA.
Genetic Code
The genetic code is the set of instructions used to make proteins from the order of nucleotides that make up DNA. This code states that each group of three nucleotides encodes an amino acid. This combination is formed by four nitrogenous bases.
DNA Duplication
DNA replication is the process of creating an exact copy of a DNA molecule. The double helix serves as a mold to copy a complementary chain through semi-conservative replication.
Interkinesis
Interkinesis is an interim period of rest, making way for the second meiosis.