Título del gráfico
Módulo 2. Implementación de las tic
2. Ventajas y desventajas del uso de las tics
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A
basic principle of modern evolutionary theory is that organ-isms attain their diversity through hereditary modifi cations of populations. All known lineages of plants and animals are related by descent from common ancestral populations.Heredity establishes the continuity of living forms. Although off-spring and parents in a particular generation may look different, there is nonetheless a genetic continuity that runs from generation to genera-tion for any species of plant or animal. An offspring inherits from its parents a set of coded information (genes), which a fertilized egg uses, together with environmental factors, to guide its development into an adult bearing unique physical characteristics. Each generation passes to the next the instructions required for maintaining continuity of life.
The gene is the unit entity of inheritance, the germinal basis for every characteristic that appears in an organism. The study of what genes are, how they are transmitted, and how they work is the sci-ence of genetics. It is a scisci-ence that reveals the underlying causes of resemblance, as seen in the remarkable fi delity of reproduction, and of variation, the working material for organic evolution. All liv-ing forms use the same information storage, transfer, and translation system, which explains the stability of all life and reveals its descent from a common ancestral form. This is one of the most important unifying concepts of biology.
a pair of gametes unites in fertilization, each gamete contributes its set of chromosomes to the newly formed cell, called a zygote, which has two complete sets of chromosomes. The number of chromosomes in two complete sets is called the diploid (2 n ) number. In humans the zygotes and all body cells normally have the diploid number (2 n ), or 46 chromosomes; the gametes have the haploid number ( n ), or 23, and meiosis reduces the number of chromosomes per cell from diploid to haploid.
Thus each cell normally has two copies of each gene coding for a given trait, one on each of the homologous chromosomes.
Alternative forms of genes for the same trait are allelic forms, or alleles. Sometimes only one of the alleles has a visible effect on the organism, although both are present in each cell, and either
may be passed to progeny as a result of meiosis and subsequent fertilization.
Alleles are alternative forms of the same gene that have arisen by mutation of the DNA sequence. Like a baseball team with several pitchers, only one of whom can occupy the pitcher’s mound at a time, only one allele can occupy a chromosomal locus (position).
Alternative alleles for the locus may be on homologous chromo-somes of a single individual, making that individual heterozygous for the gene in question. Numerous allelic forms of a gene may be found among different individuals in a population, a condition called “multiple alleles” (p. 85).
Round vs. wrinkled seeds F1 = all round
F2 = 5474 round 1850 wrinkled Ratio: 2.96:1
Purple vs. white flowers F1 = all purple F2 = 705 purple 224 white Ratio: 3.15:1 Green vs. yellow pods
F1 = all green F2 = 428 green 152 yellow Ratio: 2.82:1
Long vs. short stems F1 = all long F2 = 787 long 277 short Ratio: 2.84:1
Terminal vs. axial flowers F1 = all axial F2 = 651 axial 207 terminal Ratio: 3.14:1 Inflated vs. constricted pods
F1 = all inflated F2 = 882 inflated 299 constricted Ratio: 2.95:1
Yellow vs. green seeds F1 = all yellow F2 = 6022 yellow 2001 green Ratio: 3.01:1
Figure 5.1
Seven experiments on which Gregor Mendel based his postulates. These are the results of monohybrid crosses for fi rst and second generations.
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w w w . m h h e . c o m / h i c k m a n i p z 1 4 e CHAPTER 5 Genetics: A Review 79
Prophase I Late prophase I
Metaphase I
Anaphase I
Prophase II
Metaphase II
Anaphase II MEIOSIS I
MEIOSIS II
Telophase II
Homolog Homolog Sister chromatids
Centromere Region of close association, where
crossing over occurs SYNAPSIS B
A
Figure 5.2
A, Meiosis in a sex cell with two pairs of chromosomes. Prophase I, homologous chromosomes come to lie with side-to-side contact, or synapsis, forming bivalents. A bivalent comprises a pair of homologous chromosomes, with each of the chromosomes containing a pair of identical chromatids joined by a centromere. Metaphase I, bivalents align at the spindle equator. Anaphase I, chromosomes of former bivalents are pulled toward opposite poles. Prophase II, daughter cells contain one of each homologous chromosome (haploid) but each chromosome is in replicated form (two chromatids attached at a centromere). Metaphase II, chromosomes align at the spindle equator. Anaphase II, chromatids of each chromosome separate. Telophase II, four haploid cells (gametes) formed, each with unreplicated chromosomes (one chromatid per chromosome). B, Synapsis occurs in prophase I, in which homologous chromosomes can break and exchange corresponding portions. The labelled sister chromatids and region of close association extend the full length of the bivalent.
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During an individual’s growth, all dividing cells contain the double set of chromosomes (mitosis is described on p. 52). In the reproductive organs, gametes (germ cells) are formed after meiosis, which separates the chromosomes of each homologous pair. Without this reductional division, the union of ovum (egg) and sperm would produce an individual with twice as many chromosomes as the parents. Continuation of this process in just a few generations could yield astronomical numbers of chromo-somes per cell.
Most unique features of meiosis occur during prophase of the fi rst meiotic division ( Figure 5.2 ). Prior to meiosis, each chro-mosome has already replicated to form two chromatids joined at one point, the centromere. The two members of each pair of homologous chromosomes make side-by-side contact (synap-sis) to form a bivalent, which permits genetic recombination between the paired homologous chromosomes (p. 89). Each bivalent is composed of two pairs of chromatids (each pair is a dyad, sister chromatids held together at their centromere), or four future chromosomes, and is thus called a tetrad. The position or location of any gene on a chromosome is the gene locus (pl., loci ), and in synapsis all gene loci on a chromatid normally lie exactly opposite the corresponding loci on the sis-ter chromatid and both chromatids of the homologous chromo-some. Toward the end of prophase, the chromosomes shorten and thicken and then enter the fi rst meiotic division.
In contrast to mitosis, the centromeres holding the chro-matids together do not divide at anaphase. As a result, each of the dyads is pulled toward one of the opposite poles of the cell by microtubules of the division spindle. At telophase of the fi rst meiotic division, each pole of the cell has one dyad from each tetrad formed at prophase. Therefore at the end of the fi rst meiotic division, the daughter cells contain one chromosome of each homologous pair from the parent cell, so that the total chromosome number is reduced to haploid. However, because each chromosome contains two chromatids joined at a centro-mere, each cell contains twice the amount of DNA present in a gamete.
The second meiotic division more closely resembles events in mitosis. The dyads are split at the beginning of anaphase by division of their centromeres, and single-stranded chromosomes move toward each pole. Thus by the end of the second meiotic division, the cells have the haploid number of chromosomes, and each chromatid of the original tetrad exists in a separate nucleus. Four products are formed, each containing one com-plete haploid set of chromosomes and only one copy of each gene. Only one of the four products in female gametogenesis becomes a functional gamete (p. 146).