v動(dòng)物身體圖式的模式建成ii-在線瀏覽
2025-06-24 12:13本頁(yè)面
【正文】 ter gastrulation. They forms sequentially in pairs on either side of the notochord, starting at the anterior end of the embryo or head end. The somites give rise to the vertebrae, to the muscles of the trunk and limbs, and to the dermis of the skin. ? Somites differentiate into particular axial structures depending on their position along the AP axis. The anteriormost somites skull Those posterior to them cervical vertebrae More posterior ones thoracic vertebrae with ribs ? The presomatic mesoderm is patterned along its AP axis before somite formation begins during gastrulation. ? The positional identity of the somites is specified by the binatorial expression of genes of the Hox plexs along the AP axis, from the hindbrain to the posterior end, with the order of expression of these genes along the axis corresponding to their order in the cluster along the chromosome ? Mutations or overexpression of a Hox gene results, in general, in localized defects in the region in which the gene is expressed, and cause homeotic transformations（ 同源異型轉(zhuǎn)化 ） . Somites are formed in a welldefined order along the anteroposterior axis Specification of the presomitic mesoderm by position along the anteroposterior axis has occurred before somite formation begins during gastrulation Identity of somites along the anteroposterior axis is specified by Hox gene expression (I) ? The Hox (Homeobox) genes of vertebrates encode a large group of gene regulatory proteins that all contain a similar DNAbinding region of around 60 amino acids known as the homeodomain. The homeodomain is encoded by a DNA motif of around 180 base pairs termed the homeobox, a name that came originally from the fact that this gene family was discovered through mutations that produce a homeotic transformation— a mutation in which one structure replaces another. For example, the fourwinged fly. ? Hox genes that specify positional identity along the AP axis were originally identified in Drosophila and it turned out that related genes are involved in patterning the vertebrate axis Identity of somites along the anteroposterior axis is specified by Hox gene expression (II) ? All the Hox genes whose functions are known encode transcriptional factors. Most vertebrates have four separate clusters of Hox genes. ? A particular feature of the Hox gene expression in both insects and vertebrates is that the genes in each cluster are expressed in a temporal and spatial order that reflects their order on the chromosome. That isa spatial pattern of genes on a chromosome corresponds to a spatial expression pattern in the embryo (The order of the genes in each cluster from 3， to 5， in the DNA is the order in which they are expressed along the AP axis). ? The overall pattern suggests that the bination of Hox genes provides positional identity for each somite. In the cervical region, for example, each somite, and thus each vertebra, could be specified by a unique pattern of Hox gene expression Specification of the identity (characteristic strucutre) of each segment is acplished by the homeotic selector (同源異型選擇者 ) genes lab and Dfdthe head segments Scr and Antp the thoracic segments Ubx the third thoracic segment AbdA and AbdBthe abdominal segments Homeotic gene expression in Drosophila There are 2 clusters of the homeotic genes encoding the Antennapedia and bithorax plexes Lossoffunction mutations in the Ultrabithorax gene can transform the 3rd thoracic segment into another 2nd thoracic segment, producing a fourwinged fly Almost every region in the mesoderm along the anteroposterior axis is characterized by a particular set of expressed Hox genes Patterning the body plan in animals 1 Development of the Drosophila body plan Specification
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