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Unlock This CourseImagine your DNA as a recipe book locked away in a library vault. To make a meal (protein), you need to copy the recipe and take it to the kitchen. That's exactly what happens in your cells! mRNA copies the genetic instructions from DNA, whilst tRNA acts like a delivery service, bringing the right ingredients (amino acids) to build proteins. Codons are the three-letter words that make up this genetic language.
Key Definitions:
The genetic code is universal - almost all living things use the same three-letter codon system. There are 64 possible codons (4ยณ = 64) but only 20 amino acids, so most amino acids are coded by more than one codon. This redundancy helps protect against mutations!
mRNA is like a photocopy of a DNA recipe. During transcription, an enzyme called RNA polymerase reads the DNA template and creates a complementary mRNA strand. Unlike DNA, mRNA is single-stranded and contains uracil (U) instead of thymine (T).
Once mRNA is made in the nucleus, it travels through nuclear pores to reach ribosomes in the cytoplasm. The ribosome reads the mRNA sequence in groups of three nucleotides (codons), starting from a start codon (AUG) and ending at a stop codon (UAG, UAA, or UGA).
AUG always codes for methionine and signals where protein synthesis begins. Every protein starts with this amino acid!
The middle section contains codons that specify which amino acids to add to the growing protein chain.
UAG, UAA and UGA tell the ribosome to stop adding amino acids and release the completed protein.
tRNA molecules are shaped like a cloverleaf when drawn flat, but they actually fold into an L-shaped three-dimensional structure. Each tRNA carries a specific amino acid and has an anticodon that pairs with the corresponding codon on mRNA.
There are about 40 different types of tRNA in human cells, but only 20 amino acids. Some amino acids have multiple tRNA molecules that can carry them. The tRNA for methionine is special - it's the only one that can start protein synthesis at the AUG start codon!
Each tRNA goes through a charging process where specific enzymes (aminoacyl-tRNA synthetases) attach the correct amino acid. These enzymes are incredibly accurate - they make mistakes less than 1 in 10,000 times! The charged tRNA then travels to the ribosome where its anticodon pairs with the complementary codon on mRNA.
The genetic code is read in triplets because three nucleotides provide enough combinations (64) to code for all 20 amino acids plus start and stop signals. If we only used two nucleotides, we'd only have 16 combinations - not enough!
UUU and UUC both code for phenylalanine. GGU, GGC, GGA and GGG all code for glycine. This redundancy means that some mutations don't change the protein at all - they're called silent mutations.
The ribosome must read mRNA in the correct reading frame - starting from the right position. If it starts one nucleotide too early or late, it reads completely different codons and makes a totally different (usually useless) protein. This is like reading "THE CAT SAT" as "HEC ATS AT" - it makes no sense!
Protein synthesis happens in two main stages: transcription (making mRNA from DNA) and translation (making proteins from mRNA). During translation, the ribosome acts like a factory assembly line, with tRNA molecules bringing amino acids in the exact order specified by the mRNA codons.
This genetic condition is caused by a single nucleotide change in the ฮฒ-globin gene. The codon GAG (glutamic acid) changes to GUG (valine). This tiny change makes the haemoglobin protein stick together, causing red blood cells to become sickle-shaped and less efficient at carrying oxygen. It shows how important each codon is!
Not all mutations are harmful. Some have no effect (silent mutations), others might be beneficial. However, mutations in important codons can cause serious problems.
A single nucleotide change that might alter one amino acid in the protein. Can be silent, missense, or nonsense.
Insertion or deletion of nucleotides that shifts the reading frame, usually creating a completely non-functional protein.
Changes that create premature stop codons, resulting in shortened, usually non-functional proteins.
Understanding codons helps scientists develop treatments for genetic diseases. Gene therapy can potentially fix faulty codons, whilst understanding how mutations affect proteins helps doctors predict disease symptoms. Some bacteria have slightly different genetic codes, which is why certain antibiotics work - they interfere with bacterial protein synthesis without affecting human cells.
Scientists now use knowledge of the genetic code to engineer bacteria to produce human proteins like insulin. They insert human genes into bacterial DNA and the bacteria read our codons and make our proteins! This is called recombinant DNA technology.
Researchers are working on expanding the genetic code by creating artificial amino acids and new codons. Some have successfully added a 21st amino acid to the genetic code in laboratory conditions. This could lead to proteins with completely new properties for medicine and industry!