4. 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 pueden
sufrir posteriormente metamorfosis).
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20. 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.
21. 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.
22. 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.
23.
24. 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.
25. 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.
35. 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.
36. 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.
37. 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.)
38. 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.]
42. 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.
