MOLECULAR BIOLOGY OF THE GENE
Chapter 12
THERE ARE SOME MAJOR CONTRIBUTORS TO THE STUDY OF DNA. THEY INCLUDE
* GRIFFITH
* AVERY
* FRANKLIN AND WILKINS
* CHARGAFF
* WATSON AND CRICK
YOU WILL NEED TO MAKE SURE YOU REVIEW THESE INDIVIDUALS, THEIR STUDY AND THEIR MAJOR CONTRIBUTION TO THE STUDY OF DNA....ON YOUR OWN TIME.
DNA Structure
DNA REPLICATION
Replication begins at many points and proceeds bidirectionally
Here is the overall picture of Replication
PROKARYOTIC REPLICATION
l Bacteria have a single circular loop
l Replication moves around the circular DNA molecule in both directions
l Produces two identical circles
l Cell divides between circles, as fast as every 20 minutes
REPAIR OF THE DNA MOLECULE BY NUCLEASES, DNA POLYMERASE, AND DNA LIGASE
PROTEIN SYNTHESIS
l Genes Specify Enzymes
l Beadle and Tatum:
l Experiments on fungus Neurospora crassa
l Proposed that each gene specifies the synthesis of one enzyme
l One-gene-one-enzyme hypothesis
l Genes Specify a Polypeptide
l A gene is a segment of DNA that specifies the sequence of amino acids in a polypeptide
l Suggests that genetic mutations cause changes in the primary structure of a protein
DNA serves as a template to make
RNA (transcription), which then
carries the code for making a protein.
The code is deciphered to make the
protein (translation).
Types of RNA
RNA is a polymer of RNA nucleotides
l RNA Nucleotides are of four types: Uracil, Adenine, Cytosine, and Guanine
l Uracil (U) replaces thymine (T) of DNA
l Types of RNA
l Messenger (mRNA) - Takes genetic message from DNA in nucleus to ribosomes in cytoplasm
l Ribosomal (rRNA) - Makes up ribosomes which read the message in mRNA
l Transfer (tRNA) - Transfers appropriate amino acid to ribosome when “instructed”
GENETIC CODE
TRANSCRIPTION
EUKARYOTIC RNA PROCESSING (EDITING)
PROKARYOTIC RNA PROCESSING (EDITING)
l In prokaryotes, introns are removed by “self-splicing”—that is, the intron itself has the capability of enzymatically splicing itself out of a pre-mRNA
BACKGROUND to TRANSLATION
tRNA Structure
TRANSLATION
During translation, a small ribosomal subunit attaches to a mRNA molecule. At the same time, an initiator tRNA molecule recognizes and binds to a specific codon sequence on the same mRNA molecule. A large ribosomal subunit then joins the newly formed complex. The initiator tRNA resides in one binding site of the ribosome called the P site, leaving the second binding site, the A site, open. When a new tRNA molecule recognizes the next codon sequence on the mRNA, it attaches to the open A binding site. A peptide bond forms connecting the amino acid attached to the tRNA in the P site to the amino acid attached to the tRNA in the A binding site. As the ribosome moves along the mRNA molecule, the tRNA in the P site is released and the tRNA in the A site is translocated to the P site. The A binding site becomes vacant again until another tRNA that recognizes the new mRNA codon takes the open position. This pattern continues as molecules of tRNA are released from the complex, new tRNA molecules attach, and the amino acid chain grows. The ribosome will translate the mRNA molecule until it reaches a termination codon on the mRNA. The E site is not shown, but after a tRNA loses its amino acid to the growing polypeptide, it briefly enters the E site before leaving to obtain another amino acid.