Molecular lec 8

• Gene

  A length of DNA that directly corresponds to its polypeptide product
• Interrupted gene

  A gene in which the coding sequence is not continuous due to the presence of introns
• Exons

  The sequences retained in the mature RNA product
• Mature transcript

  Begins and ends with exons that correspond to the 5' and 3' ends of the RNA
• Introns

  The intervening sequences that are removed when the primary RNA transcript is processed to give the mature RNA product
• RNA splicing

  Excising introns from RNA and connecting the exons into a continuous mRNA
• Exons remain in the same order in mRNA as in DNA, but distances along the gene do not correspond to distances along the mRNA or polypeptide products
• Mutations in exons can affect polypeptide sequence
• Alternative splicing

  The process of selecting different combinations of splice sites within a messenger RNA precursor (pre-mRNA) to produce variably spliced mRNAs

  
• Mutations in introns cannot directly affect the polypeptide sequence, but they can affect the processing of the mRNA production by inhibiting the splicing of exons
• Mutations that affect splicing are usually deleterious, and the majority are single-base substitutions at the junctions between introns and exons
• Intron mutations can affect not only the production of a polypeptide but also its sequence, and about 15% of the point mutations that cause human diseases are because of disrupted splicing
• First parity rule

  A paired with T, G paired with C
• Second parity rule

  equal amounts of adenine (A) and thymine (T)

  and equal amounts of cytosine (C) and guanine (G)
• Cluster rule

  purines cluster on one DNA strand and pyrimidines tend to cluster on the other.
• GC rule

  the overall proportion of guanine (G) and cytosine (C) in a genome tends to be a species-specific character and the GC content tends to be greater in exons than in introns
• Introns can be detected when genes are compared with their RNA transcription products by sequencing, and the positions of introns are usually conserved when homologous genes are compared between different organisms, although the lengths of the corresponding introns can vary greatly
• Mammalian genes for DHFR have the same relative organization of rather short exons and very long introns, but vary extensively in the lengths of introns