Molecular biology ch4
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| What are exons? | Exons are the sequences retained i n the mature RNA product. |
| What’s a mature transcript? | Starts and ends with exons that correspond to the 5 ' and 3' ends of the RNA |
| What are introns ? | The intervening sequences that are removed when the primary RNA transcript is processed |
| Why is an interrupted gene longer than it’s mature RNA? | Because of the presence of introns |
| Interrupted genes requires an additional step that is not necessary in uninterrupted genes. What is it? | RNA splicing |
| What type of reaction do you consider is splicing? | An intramolecular reaction |
| What is trans splicing | Sequences from different mRNAs are ligated together into a single molecule for translation. |
| What are the effects of mutations in the introns? | They cannot directly affect the polypeptide but affect the processing of the mRNA production by inhibiting the splicing of exons. |
| What type of mutations In introns usually and how it affects the splicing? | Deleterious mutations The majority are single-base sub stitutions may cause an exon to be left out of the product, intron to be included, or make splicing occur at a different site. The most common outcome is a termination codon that shortens the polypeptide sequence. |
| What are the effects of intron mutations | Intron mutations may affect not only the production of a polypeptide, but also its sequence. |
| How does the genes in multicellular eukaryotes differ from prokaryotes? | In multicellular eukaryotes most genes are interrupted, and the introns are usu ally much longer than exons, so that genes are considerably larger than their coding regions. |
| What does the second parity rule say? | There are equal amounts of A and T, and equal amounts of C and G |
| Where does The second parity rule applies more closely? | More to introns than exons |
| What does the cluster rule say? | That purines tend to cluster on one DNA strand and pyrimidines tend to cluster on the other. |
| What is the CG rule? | The overall proportion of G +C in a genome (GCcontmt) tends to be a species-specific character |
| What is the segment loop structure? | The stems of which can display base parity and the loops of which can display some degree ofbase clustering. |
| What does chargaffs four rules relate to ? | Genomic characteristics, or "pressures," that constitute the genome phenotype. |
| How can you detect introns ? | When genes are compared with their RNA transcription products by either restriction mapping, electron microscopy, or sequencing. |
| What do you notice when comparing introns between homologous genes in different species? | The positions of introns are usually conserved |
| Why is the map appears different in mature mRNA and genes? | Because additional regions that are found in the gene are not represented in the mature mRNA. |
| Where is introns found beside multicellular eukaryotes? | Introns also are found in mitochondrial genes of plants, fungi, protists and in chloroplast genes |
| What do you notice in the pattern of restriction in the exons? | The pattern of restric tion sites in the exons is the same in both the cDNA and the gene. |
| How to identify introns when it comes to open reading frames? And how an open reading frame is made? | An intron usually has no open reading frame. An intact reading frame is created in an mRNA sequence by the removal of the introns from the primary transcript. |
| What is special about the globin genes introns? | Although intron lengths vary, the first intron is always fairly short and the second is usually longer. |
| How are variations of intron lengths made in globin genes? | Most of the variation in the lengths of different globin genes results from length variation in the second intron. |
| What happens to the introns when a gene have some exons related to those of another gene, with the remaining exons unrelated. | Usually, in such cases, the introns are not related at all. |
| How exon homologies between genes result? | Duplication and translocation of individual exons. |
| Why are exon sequence changes are conserved? | Changes in exon sequences are constrained by selec tion against mutations that alter or destroy the function of the polypeptide. the exon sequences are conserved by the negative selection of individuals in which the sequences have changed |
| Why Many of the preserved changes do not affect codon meanings ? | Because they change a codon into another for the same amino acid (they are synonymous substitutions). |
| What makes exons and introns different when exons are under negative selection pressure? | Introns evolve much more rapidly than exons when the exons are under negative selection pressure. |
| Explain why the mutation rate is different between exons and introns under negative selection pressure? | Mutations occur at the same rate in both exons and introns, but exon mutations are eliminated more effectively by selection. However, because of the low level of functional constraints, introns may more freely accumulate point substitutions and other changes. |
| What’s positive selection? | A mutation that confers a more advanta geous phenotype to an organism, relative to individuals in the same population without the mutation, |
| What’s the difference in the rates of evolution between introns and exons in positive selection ? | Exons evolve faster than introns. |
| What’s different about the fold potential in genes in positive selection? | That fold potential (measured in negative units) is high (more negative) in introns, and low (more positive) in exons |
| How is The overall length of a gene is determined? | By its introns |
| Is there a correlation between genome size and organism complexity? | The correlation is weak |
| Are very long genes the result of encoding Ionger products? | No, Very long genes are the result of very long introns |
| What is the correlation between total gene size and total exon size in multicellular eukaryotes? | There’s no correlation between total gene size and total exon size in multicellular eukaryotes |
| What is the result of alternative initiation and alternative termination codons? | Allows multiple variants of a polypeptide chain. |
| How is Different polypeptides can be produced fromthe same sequence of DNA | When the mRNA is read in different reading frames (as two overlapping genes). |
| How can alternative splicing produce identical polypeptides, differing by the presence or absence of certain regions? | When certain exons are included or excluded. This may take the form of including or excluding indi vidual exons, or of choosing between alternative exons. |
| How does an overlapping gene occur? | When the same sequence of DNA encodes two nonhomologous proteins because it uses more than one reading frame . |
| What does it mean when The exons of some genes appear homologous to the exons of others? | Suggesting a common exon ancestry. |
| How to know is homologous genes share common ancestry? | Share common features that preceded their evolutionary separation. |
| How do you know that globin genes share a single ancestral gene? | All globin genes have a common form of organization with three exons and two introns. |
| What does it suggest that Intron positions in the actin gene family are highly variable? | Suggests that introns do not separate functional domains. |
| What is a gene family? | Defined as a group of genes that encode related or identical products as a result of gene-duplication events. |
| What’s a superfamily? | Genes that are more distantly related but that still can be recognized a s having common ancestry. |
| What are orthologs? | Genes that are homologous (homologs) due to speciation (genes in different species) |