Bacteria. Protista. Fungi.

Plantae Animalia.

En la clasificación científica de los seres vivos, el reino Animalia (animales) o Metazoa (metazoos)constituye un amplio grupo de organismos eucariotas, heterótrofos, pluricelulares y tisulares. Se caracterizan por su capacidad para la locomoción, por la ausencia de clorofila y de pared en sus células, y por su desarrollo embrionario, que atraviesa una fase de blástula y determina un plan corporal fijo (aunque muchas especies puedensufrir posteriormente metamorfosis).

Figure 5-47. Depurination and deamination. These two reactions are the most frequent spontaneous chemical reactions known to create serious DNA damage in cells. Depurination can release guanine (shown here), as well as adenine, from DNA. The major type of deamination reaction (shown here) converts cytosine to an altered DNA base, uracil, but deamination occurs on other bases as well.

These reactions take place on double-helical DNA; for convenience, only one strand is shown.

Figure 12-23. Formation of a spontaneous point mutation by deamination of cytosine (C) to form uracil (U). If the resulting U·G base pair is not restored to the normal C·G base pair by repair mechanisms, it will be fixed in the DNA during replication. After one round of replication, one daughter DNA molecule will have the mutant U·A base pair and the other will have the wild-type C·G base pair. The uracil is removed and replaced by thymine, generating a mutant DNA in which a T·A pair replaces a C·G pair.

Figure 5-48. The thymine dimer. This type of damage is introduced into DNA in cells that are exposed to ultraviolet irradiation (as in sunlight). A similar dimer will form between any two neighboring pyrimidine bases (C or T residues) in DNA.

Figure 5-51. The recognition of an unusual nucleotide in DNA by base-flipping. The DNA glycosylase family of enzymes recognizes specific bases in the conformation shown. Each of these enzymes cleaves the glycosyl bond that connects a particular recognized base (yellow) to the backbone sugar, removing it from the DNA. (A) Stick model; (B) space-filling model.

Figure 5-53. Two different types of end-joining for repairing double-strand breaks. (A) Nonhomologous end-joining alters the original DNA sequence when repairing broken chromosomes. These alterations can be either deletions (as shown) or short insertions. (B) Homologous end-joining is more difficult to accomplish, but is much more precise.

El nucleolo

Figure 6-47. The function of the nucleolus in ribosome and other ribonucleoprotein synthesis. The 45S precursor rRNA is packaged in a large ribonucleoprotein particle containing many ribosomal proteins imported from the cytoplasm. While this particle remains in the nucleolus, selected pieces are added and others discarded as it is processed into immature large and small ribosomal subunits. The two ribosomal subunits are thought to attain their final functional form only as each is individually transported through the nuclear pores into the cytoplasm. Other ribonucleoprotein complexes, including telomerase shown here, are also assembled in the nucleolus.

Figure 6-45. Changes in the appearance of the nucleolus in a human cell during the cell cycle. Only the cell nucleus is represented in this diagram. In most eucaryotic cells the nuclear membrane breaks down during mitosis, as indicated by the dashed circles.

Figure 6-46. Nucleolar fusion. These light micrographs of human fibroblasts grown in culture show various stages of nucleolar fusion. After mitosis, each of the ten human chromosomes that carry a cluster of rRNA genes begins to form a tiny nucleolus, but these rapidly coalesce as they grow to form the single large nucleolus typical of many interphase cells. (Courtesy of E.G. Jordan and J. McGovern.)

Figure 11-50. Processing of pre-rRNA and assembly of ribosomes in eukaryotes. (a) Major intermediates and times required for various steps in pre-rRNA processing in higher eukaryotes. Ribosomal and nucleolar proteins associate with 45S pre-rRNA soon after its synthesis, forming an 80S pre-rRNP. Synthesis of 5S rRNA occurs outside of the nucleolus. The extensive secondary structure of rRNAs is not represented here. Note that RNA constitutes about two-thirds of the mass of the ribosomal subunits, and protein about one-third. (b) Pathway for processing of 6.6-kb (35S) pre-rRNA primary transcript in S. cerevisiae. The transcribed spacer regions (tan), which are discarded during processing, separate the regions corresponding to the mature 18S, 5.8S, and 25S rRNAs. All of the intermediates diagrammed have been identified; their sizes are indicated in red type. [Part (b) adapted from S. Chu et al., 1994, Proc. Nat'l. Acad. Sci. USA 91:659.]

DNA el material genetico

1944 Avery, MacLeod y McCarty.

• El principio activo transformante coincide con lo reportado para DNA.

• Las propiedades opticas,electroforéticas, difusivas y de ultracentrifugación fueron similares a las del DNA.

• No se pierde la actividad transformante con la extracción de proteínas o lípidos.

• La tripsina y quimotripsina no afectan la actividad del factor transformante.

• La ribonucleasa no afecta la actividad del factor transformante.

• La actividad del factor transformante se pierde con desoxiribonucleasas.

1967 Hershey y Chase.

1968 Gurdon.