memorization

i have an essay quiz today in ap bio about transcription and translation. i'm not a fan of either of those and they just get all jumbled and mixed up in my head. or i just completely blank and nothing at all comes to mind. i have found that the more i write something down, or type it up for this case, the better chance i have to remember it so today i'm going to be running through the processes of transcription and translation.

transcription is the synthesis of RNA, specifically mRNA under the direction of DNA.

translation is the synthesis of a polypeptide chain and it occurs under the direction of mRNA.

((the base sequence of mRNA is translated into the amino acid sequence of polypeptides at the ribosomes))

some stuff to keep in mind...

• DNA splits into two separate strands, the coding and template strand. the template strand provides the sequence of bases while the coding strand just kinda hangs out and sits there. the coding strand does do one thing though, it has the TATA box which shows the prompter where to attach and then start scanning for the start codon, AUG.


• a codon is an mRNA triplet written 5'-->3', AUG is the start codon. it codes for methionine. redendancy is when multiple codons code for the same amino acid and wobble is when the third nucleotide in the sequence doesn't matter.
  

specifics of transcription:

step 1. initiation: RNA polymerase II binds to the prompter (the TATA box on the coding strand) and begins to unwind the DNA. RNA poly II then initiates RNA synthesis by finding the start codon AUG.

step 2. elongation: RNA poly II moves downstream, continually unwinding the DNA and elongating the RNA transcript. the RNA transcript is built 5'-->3', which means the nucleotides are added to the 3' end. confusing, yes i know.

step 3. termination: RNA transcript is released and RNA poly II detaches from the DNA. elongation continues for a bit after RNA poly II transcribes the termination sequence.

RNA processing because the mRNA just transcribed is not yet good enough to be used for translation

• a 5' cap consisting of a modified guanine nucleotide is added to the 5' end of the pre-mRNA transcript. a poly A tail consisting of multiple adenine nucleotides is added to the 3' end. these caps serve three purposes. 1- they facilitate the export of the mRNA from the nucleus. 2- the protect the mRNA transcript from enzymes. 3- they aid in the ribosomal attachment of the 5' end.

the RNA still isn't ready, it now has to go through splicing. RNA splicing is the removal of the noncoding segments (introns) so that the transcript only contains the coding segments (exons). EXONS-GOOD. INTRONS-BAD. these nifty little things called snRNPs (snerps) are needed to remove the introns and then a snRNP and protein combo form a spliceosome form and they reattach the exons back together. then there are these ribozymes and they're just RNA molecules that act as proteins and i'm not really sure what purpose they serve. TADA YOU HAVE JUST COMPLETED TRANSCRIPTION. FEEL FREE TO MOVE ON TO TRANSLATION NOW.

some more stuff to keep in mind...

• in translation the cell is interpreting a gentic message from mRNA and using it to build a polypeptide chain. it's tRNA [transfer RNA] that is used to interpret the codons on the mRNA.
• anticodons are found on tRNA and they're complementary to the codons found on mRNA. this is how the tRNA knows which amino acid comes next in the sequence. the amino acids attach to the 3' end of the tRNA when it pulls them to the ribosome from the nucleus.
• a nifty little guy called aminoacyl-tRNA synthetase is what attaches the amino acids and little tRNAs together.
• ribosomes are made up of rRNA [ribosomal RNA]. they have three binding sites, the P site, the A site, and the E site. the P site holds the tRNA carrying the growing polypeptide chain. the A site holds the tRNA carrying the next amino acid. the E site discharges the tRNAs.

specifis of translation:

step 1. initiation: mRNA, tRNA (with the first amino acid in tow)and ribosomal subunits are brought together. the mRNA and tRNA are bound to the small ribosomal subunit and then scans downstream until the start codon (AUG) is found. transcription starts here.

step 2. elongation: amino acids added to the polypeptide chain one by one as tRNA anticodons match up to the mRNA codons. there are three steps within elongation: 1- codon recognition 2- peptide bond formation 3- translocation. elongation continues until the stop codon reaches the A site.

step 3. termination: UAG, UAA, and UGA are the stop codons. release factors bind to the stop codon, adding a water molecule to the polypeptide chain. this causes the peptide chain to be released.

AN EXTREMELY SHORT VERSION OF WHAT HAPPENS ALL IN FIVE STEPS

1. RNA transcribed from DNA template
2. pre-mRNA spliced and modified into mRNA (eukaryotes only)
3. mRNA attaches to ribosome in preperation for translation
4. amino acids attach to appropriate tRNA anticodon
5. tRNAs and their amino acid meet the mRNA codons at the ribosome and polypeptide chain grows.