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Unformatted text preview: Jessica Felder BIMS 320 Due: December 7 th BIMS 320 H Final Exam 1. A homeobox sequence is a DNA sequence that is found in genes that are most commonly a part of morphogenesis in organisms. Homeobox genes are genes that have a homeobox sequence and these genes collectively form the homeobox gene family. Typically a homeobox is approximately 129 bp long, and it has been found to encode for the protein domain, also know as the homeodomain, which can bind DNA. Homebox genes encode transcription factors, which are in charge of switching on and off cascades of other genes. Homeodomains bind DNA in a specific manner, but they are not usually enough to recognize only its desired target genes. For the most part, homeodomain proteins act in the promoter region of their target genes as complexes with other transcription factors, or even other homeodomain proteins. These complexes have much higher target specificity than a single homeodomain protein does. Homeobox genes are key components in the establishment of pattern formation and regional identity during development. Homeobox domains are regulatory signals that are a constant feature of genes with a very large role in embryonic development, because they control morphogenesis within early body segments. Genes with homeobox domains are found in all species of animals. Hox genes are a particular subgroup of homeobox genes. They are generally found in a special gene cluster, known as the Hox cluster or Hox complex. Hox genes are very important in morphogenesis and more specifically in the patterning of the body axis. Since they are important in providing the identity of the body regions, Hox genes also determine where limbs and other body segments will grow in a developing fetus. If there is a mutation in Hox genes in vertebrates, it usually results in miscarriages. Humans usually contain homeobox genes in four clusters: HOXA, HOXB, HOXC, HOXD. Also, there is a distal-less homeobox family: DLX1, DLX2, DLX3, DLX4, DLX, and DLX6. HESX1 is also a homeobox gene. A short stature homeobox gene also exists know as SHOX. 2. Okazaki fragments are a necessary part of DNA replication because they are involved in the synthesis of the lagging strand. Without Okazaki fragments, only one strand of the DNA could be replicated. Synthesis of the lagging strand involves synthesis away from the replication fork (5 3) therefore it can only be synthesized in short segments called Okazaki fragments, therefore no complete replication can occur without them. This is because DNA is replicated in the 5 3 direction, so only one strand, the leading strand, of double stranded DNA can be replicated continuously, where as the other is discontinuously synthesized. Lagging strand synthesis is discontinuous and each segment is initiated by RNA primers which are put on the strand by primases. After synthesis, the RNA primers are removed by RNAase and the spaces between all of the Okazaki fragments are sealed by DNA polymerase. The final phosphodiester bond that is left due to a nick is filled by DNA ligase to completely connect the Okazaki fragments, completing the lagging strand of the double stranded DNA; therefore, completing replication. If there Okazaki the double stranded DNA; therefore, completing replication....
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- Spring '08