Lecture 6 - Finishing Genetics & DNA

Announcements

• Get exam from OH on T/H 7
• The final is everything from here on, no previous material.
  • I don't need to care about most of what we did the first 5 weeks

Last Time

We talked about the differences between DNA and RNA:

• DNA to hnRNA is the process of transcription (Tc'n).
• hnRNA to mRNA is RNA Processing
• mRNA moving to the outside of the nuclear envelope is called the process of Nuclear Export
• mRNA to polypeptide is called translation (Tl'n).
• Polypeptide to protein is called folding.

Recall that the DNA composes our genotype while the protein encodes our phenotype.

DNA's most important use is information storage in the nucleus. On the contrast, the many types of RNAs "mobilize" this information for translation in the cytoplasm.

Transcription

In transcription, the RNA polymerase synthesizes a copy of one gene from a DNA template as a heterogenous nuclear RNA (hnRNA).

Nucleotides are always made 5' to 3' so RNA is made 5' to 3' off of the 3' to 5' DNA template strand.

The image above also shows empty parts of the DNA, a place where the gene terminates.

The process occurs in 3 stages: initiation (starts), elongation (goes), & termination (stops)

1. Initiation: RNA polymerase binds DNA at the gene's promoter; the DNA helix unwinds; RNA synthesis begins

2. Elongation: DNA threaded through RNA polymerase at Transcription bubble; RNA strand growing 5' $\to$ 3'; A in DNA paired with U in RNA; 30-50 nucleotides per second.

3. Termination: RNA polymerase reaches "bumps" in terminator region and "falls off".

In bacterial cells, many genes are Transcribed at once, making a polycistronic RNA which codes for multiple proteins. Prokaryotic cells experience the same thing!

Genes and proteins are almost always colinear in bacteria, where earlier changes in the DNA affects the mRNA quickly, and thus the polypeptide chain as well. With colinearity, the number of nucleotides in the gene is proportional to the number of amino acids in the protein. This is the idea of how every 3 bases codes a unique amino acid, which chains to make a polypeptide chain, and was how it was discovered.

RNA Processing

In eukaryotic cells, we still have one more step after this for protein-coding messenger RNAs (mRNA): RNA Processing. Newly synthesized RNA called heterogeneous nuclear RNA must be modified before being fully functional.

Eukaryotic genes have comple internal organization. As the image above shows, the I's, or the introns (non-coding nucleotide sequences that are Tc'ed but not translated into the protein) are thrown out, while the exons are kept and read.

Many believed introns were composed of "junk-DNA". We now know introns are valuable via Evolutionary Conserved Regions or ECRs.

Newly syntehsized heterogeneous nuclear RNA must be modified before it is fully functional.

• Caps are added for initiation for translation and stability
• A poly-A tail is added for reduncancy for stability (in case of damage to the tail)
• This is an mRNA molecule (because now it can move without damaging the RNA internally)
• Introns are removed via a process called splicing

Remember that RNA processing is only possible in eukaryotic cells. The next step though is nuclear export into a polypeptide strand.

Difference RNA Molecules

In Translation (Tl'n), mRNA is translated into a stretch of amino acids by enzymes and other RNA molecules. Ribosomal RNA (rRNA) are a major component of the organelles (ribosomes) that construct proteins.

On the other hand, Transfer RNA (tRNA) are "interpreters" that read the mRNA code; insert amino acids to the growing protein.

• tRNA takes on a characteristic cloverleaf structure (see figure above)
• Amino acid attachment site (always "CCA") at 3' end
• Each has a unique anticodon, which pairs to a codon on an mRNA during Tl'n

The Genetic Code

We'll get to this next time.