The RNA template must then be “read” so that a protein can be synthesized
DNA mRNA tRNA protein <ul><li>DNA must first unzip so that ONE side may be read </li></ul><ul><li>Which side? The one that allows mRNA to be synthesized in the 5’ 3’ direction </li></ul><ul><li>mRNA must leave the nucleus and travel to the ribosome so protein synthesis can take place. </li></ul>
Leaving the nucleus <ul><li>The nucleus has pores formed by proteins </li></ul><ul><li>DNA is double stranded, too fat to fit through the pore! </li></ul><ul><li>mRNA is a single strand and once it is properly modified [more later…] it can exit through the pore so translation can occur at a ribosome </li></ul>
Complementary base pairs are added. Uracil replaces thymine in RNA
Ribonucleic Acid vs. Deoxyribonucleic Acid See the “missing” oxygen atom? What’s the difference here?
Before mRNA can leave the nucleus, it must be modified <ul><li>Not all of the DNA is expressed at once. </li></ul><ul><li>Usually only one gene or a few genes at a time </li></ul><ul><li>Exons are the part of the mRNA transcript that are EXPRESSED </li></ul><ul><li>Introns are the INERT part </li></ul>
An enzyme cuts out the intron and splices the exons together
This saves valuable ENERGY for the cell so it is not making proteins it doesn’t need.
Opening and closing the nuclear pore <ul><li>Next, the ends of the mRNA must be modified to signal to the pore to “open” and “close” </li></ul><ul><li>A cap is added 5’ end </li></ul><ul><li>A poly A tail is also added to the 3’ end </li></ul><ul><li>Remember synthesis is 5’ 3’ so the 3’ end is always the tail! </li></ul>
Overview <ul><li>First the mRNA leaves the nucleus </li></ul><ul><li>The 2 subunits of the ribosome attach to mRNA </li></ul><ul><li>tRNA carrying an amino acid matches its anticodon to the codon on mRNA </li></ul><ul><li>Peptide bonds form between the amino acids </li></ul><ul><li>tRNA is released from the ribosome </li></ul>
What does rRNA do? <ul><li>It is thought to have a catalytic role </li></ul>
The start codon on mRNA always codes for methionine Easy to remember… … we always start school in AUGust!
There are three stop codons—they do NOT code for any amino acid.
3 base pairs coding for an amino acid allows for minor mistakes in the DNA not becoming major mistakes in the protein!
These are the most forgiving—get the first 2 bases in the third doesn’t matter!
Notice tryptophan is not at all forgiving—it is only coded for by UGG
Here’s the whole picture: just memorize AUG as the start codon and remember all polypeptide chains begin with methionine. No need to memorize more.