Episode 20 dives into all things transcription and translation with the Central Dogma! Don’t forget that you are what your proteins produce.
Episode 20 dives into all things transcription and translation with the Central Dogma! Don’t forget that you are what your proteins produce. The mRNA transcript is named after the Latin word for writing (1:50). RNA Polymerase will synthesize mRNA in the 5’-3’ direction, just like DNA polymerase. Do you know Chargaff’s rules? (3:22). The mRNA transcript will need to go through modification as it exits the nuclear pore (3:40). Translation involves mRNA, rRNA and tRNA to form the polypeptide (4:50). Almost all organisms use the same genetic code which supports common ancestry (6:00).
The Question of the Day asks (6:48) “Which enzyme copies the viral RNA genome into DNA for the assembly of new viral progeny?”
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap the Central Dogma
Lets Zoom out:
Unit 6 - Gene Expression and Regulation
Topic - 6.3 and 6.4
Big Idea: Information Storage and Transmission
You’ve likely heard the expression “You are what you eat.” This is partially true, (Google the Agouti Mouse study on epigenetics if you don’t believe me) but the expression would be better phrased as “You are what your proteins produce.” This episode will focus on the thematic mechanisms by which genetic information flows from the sequence of DNA nucleotides to RNA nucleotides and lastly, to a sequence of amino acids in a protein.
Lets Zoom in:
The central dogma consists of two stages: transcription and translation. I don’t know about you, but I could never remember which one happened first? A really simple trick is that the C of transcription comes before L of translation in the alphabet. Now that we’ve got that in order, let's focus on what occurs in each step.
DNA is found in the nucleus. It's pretty important - having all of the instructions for life and all. The cell cannot risk DNA damage by sending out it's only copy of instructions into the cytoplasm. Instead, an abbreviated version of the genetic message is produced. This message is called an mRNA transcript from the Latin scribere, meaning “to write.” The “m” in mRNA stands for messenger. Similarly, colleges request a high school transcript with any student application. This does not represent every grade you’ve ever received in school, but a shortened and abbreviated version of the most significant grades and courses. We also have transcripts of these episodes! Check out our website for details. And so, the mRNA transcript represents only the portion of DNA which codes for a specific gene.
Only one side of the DNA strand serves as the template for transcription at a time. This template strand is also known as the noncoding, minus, or antisense strand. Sorry about that. Focus less on its name and more on its directionality. The nitrogenous bases on the template strand are read in the 3’-5’ direction while the enzyme RNA Polymerase synthesizes the mRNA transcript in the 5’-3’ direction. Remember DNA polymerase in DNA replication? That enzyme also synthesizes new strands in the 5’-3’ direction.
The mRNA transcript is produced following Chargaff’s rules of complementary base pairs with one exception. Guanine is still complementary to Cytosine, but there is no Thymine in a molecule of RNA. Adenine will instead pair with a different pyrimidine, called Uracil. For example, if the DNA code was 3’-5’: ATCGAG, the mRNA transcript would be …..pause for you to think…...and now the result. 5’-3’: UAGCUC.
In Eukaryotic cells, the mRNA transcript undergoes a series of enzyme-regulated modifications before it leaves the nucleus through a pore. I consider this the step in which the message is cleaned up (like when you forget to cross your t’s and dot your i’s) and short “handles” are added to the physical message. A 3’ poly-A tail is added (just a bunch of adenines), a 5’ modified cap is added (made of guanine), and any short non-coding segments called introns are spliced out. The nucleotides which remain in the transcript are called exons which will eventually be expressed. And Tada! We’ve made an mRNA transcript - a short, single stranded message of nucleic acids.
But you aren’t made of nucleic acids - you are made of proteins! And all the products produced by proteins! We need to translate the nucleic acid message into amino acids, a different chemical language. Be careful and watch your words - mRNA doesn’t become or turn into protein. Instead, it codes for a sequence of amino acids.
Translation of the mRNA to generate a polypeptide chain occurs on ribosomes in both prokaryotic and eukaryotic cells. In short, rRNA (or ribosomal RNA) reads three nucleotides of mRNA at a time, called codons. These codons correlate to one specific amino acid, which is brought over by tRNA (or transfer RNA). For example the mRNA codon UCG encodes for the amino acid Serine. This amino acid, among others, is then transferred to the growing polypeptide chain and the process is continued until a stop codon is reached. The newly synthesized polypeptide is released for further processing and shipping.
Some quick math - how many different triplet combinations can you make with only four letters. There are 64 different codons. Of these, one is a start codon (AUG - which codes for the amino acid Methionine) and three are stop codons which do not translate into an amino acid. Since there are only twenty different amino acids, more than one codon correlates to each. Almost all organisms use the same genetic code. This evidence and overlapping codons further supports the common ancestry of all living organisms. Do not memorize any DNA sequences or the amino acid codon chart! Understand how to use it, and practice the process of transcription and translation.
To recap….
Genetic information flows from DNA to RNA to protein. The three types of RNA involved in the central dogma are mRNA, rRNA, and tRNA. The genetic code is first transcribed into mRNA which is translated into amino acids. These proteins bend, twist, and fold and make you who you are!
Today’s Question of the day is about retroviruses.
Question:Which enzyme copies the viral RNA genome into DNA for the assembly of new viral progeny?