UGA and listen to Episode 67! Translation - it's kind of like riding a bike. Once you learn how, you never forget. This episode is all about codons.
UGA and listen to Episode 67! Translation - it's kind of like riding a bike. Once you learn how, you never forget. This episode is all about codons - what they are (1:13), where they come from (2:08), how to read them, and what happens if there's a mistake (5:18).
The Question of the Day asks (8:08) What is the greatest number of codons that give the same amino acid?
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Codons
Let’s Zoom out:
Unit 6 - Gene Expression and Regulation
Topic - 6.3 and 6.4
Big Idea: Information Storage and Transmission
Codon - what a great term for the secret of life. The genetic DNA code, transcribed into mRNA and translated three letters at a time into amino acids. Put a few (more likely hundreds or thousands) of amino acids together at a ribosome, bend, fold and tada! Polypeptide.
Let’s Zoom in:
This episode is all about codons - what they are, where they come from, how to read them, and what happens if there's a mistake. Translation - it's kind of like riding a bike. Once you learn how, you never forget.
Codons - what are they? A codon is a three nucleotide sequence of an mRNA transcript. Each codon will indicate a particular amino acid during the process of translation. How many different triplet combinations can you make with only four letters? (pause) The answer? - 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 (UAA, UAG, and UGA). 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.
Where do they come from? Remember, mRNA is assembled during the process of transcription. This is the first step of the central dogma. The template strand of DNA is read 3’ to 5’ with RNA polymerase synthesizing the mRNA strand 5’ to 3’ according to complementary base pairs. This is transcription, so we are still in the language of nucleic acids with Uracil substituted for Thymine. Next for eukaryotes, an mRNA transcript is processed - introns spliced out (the non coding sections), mG cap added to the 5’ end and the 3’ end gets a poly - tail (essentially a few hundred adenines). Transcription and translation happen simultaneously in prokaryotes, so there is no mRNA modification.
How to read them? Translation of the mRNA to generate a polypeptide chain occurs on ribosomes in both prokaryotic and eukaryotic cells. Remember, ribosomes are found free floating in the cytoplasm as well as on the rough endoplasmic reticulum. In short, rRNA (or ribosomal RNA) reads mRNA one codon at a time. 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. The tRNA which delivers serine contains the anticodon AGC and temporarily binds at its complement within the ribosome. 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. 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.
What if there's a mistake!? Dun dun duh! DNA mutations can be positive, negative or neutral. This is determined by the effect the mutation has on the nucleic acid sequence, resulting protein, and phenotype. First - the mutation in question has to be within the coding region of a gene to have effect, and within an exon of mRNA Second, the location of the change within a codon matters. If the mutation changes the amino acid sequence of the polypeptide and alters the structure and function of the protein, then it affects the phenotype of the organism.
Point mutations substitute one nucleotide base pair for another. They can be silent, having no effect on the amino acid sequence, missense - causing an amino acid change, or nonsense - coding for a stop. If the point mutation is in the third base of a codon, the chances of a silent mutation increase. For example, any codon that begins with CU will give you leucine. The third letter can be U,C,G or A and you still get leucine. This allows for mistakes in transcription and translation to occur, while still getting the intended protein product. A frameshift mutation is caused by the insertion or deletion of a base. This causes a shift in the ribosomes reading frame and every subsequent codon downstream of the mutation may be different. Frameshift mutations can also cause a stop of translation.
Time for unit connections. Unit 1: Chemistry of life with nucleic acids and protein structure, Unit 3: Cell structure and function with ribosomes. Plus Unit 7: Natural selection. The codon chart consistency is further evidence of the continuity of living things.
Alright - what about the exam? It is unlikely that you will be asked a question that is a direct forward translation. More likely a disruption to the process or predict the effect of the mutation on protein function. Don’t memorize the codon chart! It will be provided for you within a question if you need it. Just remember, it is the mRNA sequence you read for codons, not DNA or tRNA.
To recap……
Codons are three nucleotide sequences within an mRNA molecule. They are produced during transcription and decoded into amino acids during translation. There are three stop codons and 1 start codon and 20 amino acids. Mutations have varied effects on phenotype depending on their position within a codon.
Coming up next on the Apsolute RecAP Biology Edition: Transcription vs. Replication
Today’s question of the day is about translation
Question of the day: What is the greatest number of codons that give the same amino acid?