CSS Menus Tutorial Css3Menu.com


Samford University -- Department of Biological and Environmental Sciences
Genetics -- Biol 333

Gene Expression part I: Transcription

DNA's role as the genetic material includes 1) carrying information (in its base sequence), 2) copying that information (replication), and 3) giving meaning to that information (determining traits). DNA does this last job by determining what proteins (including enzymes) are made in the cell. Protein synthesis involves RNA synthesis (transcription) and polypeptide synthesis (translation). Crick proposed that his information transfer was one way (The central dogma of genetics).
  • Genes and Polypeptides: Proteins are made up of one or more polypeptides, which are chains of amino acids joined by peptide bonds. A gene can be defined as the DNA coding for one polypeptide ("One gene, one polypeptide"). Yanofsky demonstrated colinearity of the gene and polypeptide.

  • Transcription: Transcription is DNA-directed RNA synthesis. The sequence of a segment of a DNA molecule determines the sequence of an RNA molecule. RNA is very similar to DNA in structure, but is usually shorter, is usually single stranded, has ribose in place of 2'-deoxyribose, and has uracil in place of thymine (uracil base pairs with adenine just like thymine does). Transcription is the process of making mRNA, ribosomal RNA (rRNA), transfer RNA (tRNA), as well as other small RNAs.
 
    • RNA Polymerase: The enzyme that transcribes DNA is called RNA polymerase. RNA polymerase uses the nucleoside triphosphates (ATP, GTP, CTP, UTP) and RNA polymerization occurs just like DNA polymerization does. That is, it begins with the 5' end and the new RNA molecule grows in the 5' to 3' direction (a nucleotide is added to the 3'-OH). RNA polymerase uses one strand of the DNA molecule as the template strand with the 4 bases of DNA (A, G, C, T) specifying which RNA nucleotides will be added (U is added when the template DNA nitrogen base is A). As with DNA replication, the newly-made RNA molecule is antiparallel to the template DNA. However, unlike DNA polymerase, RNA polymerase does not need a primer but can add start a new RNA molecule with a single NTP (therefore, the first nucleotide of an RNA molecule has 3 phosphates on its 5' end). Only one strand of the DNA double helix is used as a template. That template strand is used is called the template strand and the strand not used is the non-template strand. An RNA molecule made by transcription is called a transcript.

    • Promoters and Termination Sites: The site where RNA polymerase binds and begins transcription is called the promoter. Transcription stops at a termination site.
  • RNA Processing: In eukaryotes, pre-mRNA molecules undergo considerable processing before leaving the nucleus and directing translation. This includes the addition of a cap at the 5' end, a poly-A tail, and splicing: the removal of introns leaving in exons.
Things I Learned at the Movies:
Should you decide to defuse a bomb, don't worry which wire to cut. You will always choose the right one.