En la clasificación científica de losseres vivos, el reino Animalia(animales) o Metazoa (metazoos)constituye un amplio grupo deorganismos eucariotas, heterótrofos,pluricelulares y tisulares. Secaracterizan por su capacidad para lalocomoción, por la ausencia declorofila y de pared en sus células, ypor su desarrollo embrionario, queatraviesa una fase de blástula ydetermina un plan corporal fijo(aunque muchas especies puedensufrir posteriormente metamorfosis).
Figure 5-47. Depurination and deamination. These two reactions are the most frequent spontaneouschemical 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 aspontaneous point mutation bydeamination of cytosine (C) to formuracil (U). If the resulting U·G basepair is not restored to the normal C·Gbase pair by repair mechanisms, it willbe fixed in the DNA duringreplication. After one round ofreplication, one daughter DNAmolecule will have the mutant U·Abase pair and the other will have thewild-type C·G base pair. The uracil isremoved and replaced by thymine,generating a mutant DNA in which aT·A pair replaces a C·G pair.
Figure 5-48. The thymine dimer. This type of damage is introduced into DNA in cells that areexposed to ultraviolet irradiation (as in sunlight). A similar dimer will form between any twoneighboring pyrimidine bases (C or T residues) in DNA.
Figure 5-51. The recognition of an unusual nucleotide in DNA by base-flipping. The DNAglycosylase family of enzymes recognizes specific bases in the conformation shown. Each of theseenzymes cleaves the glycosyl bond that connects a particular recognized base (yellow) to thebackbone 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 brokenchromosomes. 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.
Figure 6-47. The function of the nucleolus inribosome and other ribonucleoprotein synthesis.The 45S precursor rRNA is packaged in a largeribonucleoprotein particle containing manyribosomal proteins imported from the cytoplasm.While this particle remains in the nucleolus,selected pieces are added and others discarded as itis processed into immature large and smallribosomal subunits. The two ribosomal subunitsare thought to attain their final functional formonly as each is individually transported throughthe nuclear pores into the cytoplasm. Otherribonucleoprotein complexes, including telomeraseshown here, are also assembled in the nucleolus.
Figure 6-45. Changes in the appearance of the nucleolusin a human cell during the cell cycle. Only the cellnucleus is represented in this diagram. In mosteucaryotic cells the nuclear membrane breaks downduring mitosis, as indicated by the dashed circles.
Figure 6-46. Nucleolar fusion. These light micrographs of human fibroblastsgrown in culture show various stages of nucleolar fusion. After mitosis, each ofthe ten human chromosomes that carry a cluster of rRNA genes begins to form atiny nucleolus, but these rapidly coalesce as they grow to form the single largenucleolus typical of many interphase cells. (Courtesy of E.G. Jordan and J.McGovern.)
Figure 11-50. Processing of pre-rRNA and assembly of ribosomes ineukaryotes. (a) Major intermediates and times required for varioussteps in pre-rRNA processing in higher eukaryotes. Ribosomal andnucleolar proteins associate with 45S pre-rRNA soon after itssynthesis, forming an 80S pre-rRNP. Synthesis of 5S rRNA occursoutside of the nucleolus. The extensive secondary structure of rRNAsis not represented here. Note that RNA constitutes about two-thirdsof the mass of the ribosomal subunits, and protein about one-third.(b) Pathway for processing of 6.6-kb (35S) pre-rRNA primarytranscript in S. cerevisiae. The transcribed spacer regions (tan),which are discarded during processing, separate the regionscorresponding to the mature 18S, 5.8S, and 25S rRNAs. All of theintermediates diagrammed have been identified; their sizes areindicated in red type. [Part (b) adapted from S. Chu et al., 1994, Proc.Natl. Acad. Sci. USA 91:659.]
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.