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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Codons and Anticodons
DNA contains the instructions for making proteins, which are essential for almost everything your body does. But how exactly does the information in DNA get turned into proteins? This is where codons and anticodons come in - they're like the alphabet and translator of the genetic language!
Key Definitions:
DNA (deoxyribonucleic acid) is like the instruction manual for your body. It's made up of two strands twisted into a double helix. Each strand contains a sequence of nucleotides with four different bases: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). These bases pair up specifically - A with T and G with C. The sequence of these bases forms the genetic code that determines your traits.
Making proteins from DNA involves two main steps: transcription and translation. During transcription, the DNA code is copied into messenger RNA (mRNA). During translation, this mRNA code is read by transfer RNA (tRNA) and converted into a chain of amino acids, which fold to form a protein. This whole process is called protein synthesis.
Codons are the key to understanding how the genetic code works. They're like the words in the language of life!
A codon is a sequence of three nucleotide bases in mRNA. Each codon specifies a particular amino acid (or a stop signal) during protein synthesis. With four possible bases (A, U, G, C) in RNA, there are 4³ = 64 possible codons. Since there are only 20 amino acids, most amino acids are coded for by more than one codon.
AUG is the start codon. It signals the beginning of protein synthesis and codes for the amino acid methionine. Every protein begins with this codon!
UAA, UAG and UGA are stop codons. They don't code for any amino acid but signal the end of protein synthesis. They're like the full stops in genetic sentences.
The genetic code is redundant, meaning multiple codons can code for the same amino acid. For example, GGU, GGC, GGA and GGG all code for glycine.
In the early 1960s, scientists Marshall Nirenberg and Heinrich Matthaei conducted a groundbreaking experiment. They created an artificial mRNA molecule made entirely of uracil (U) and added it to a cell-free protein synthesis system. The result was a polypeptide made entirely of phenylalanine. This revealed that the codon UUU codes for phenylalanine, marking the first step in cracking the genetic code. By 1966, all 64 codons had been deciphered, revolutionising our understanding of how genes work.
If codons are the words in mRNA, then anticodons are the translators that help turn those words into proteins.
Anticodons are sequences of three nucleotide bases found on transfer RNA (tRNA) molecules. They're complementary to codons in mRNA. During translation, the anticodon on a tRNA molecule pairs with a matching codon on the mRNA strand. Each tRNA carries a specific amino acid, so when the anticodon pairs with a codon, it brings the correct amino acid to the growing protein chain.
Transfer RNA (tRNA) has a cloverleaf shape with an anticodon loop at one end and an attachment site for a specific amino acid at the other. There are different tRNA molecules for different amino acids. When a tRNA's anticodon matches a codon on the mRNA, the ribosome adds that tRNA's amino acid to the growing protein chain. It's like a delivery service bringing the right building blocks to the construction site!
Now let's put it all together to see how DNA, codons and anticodons work together to make proteins.
Transcription is the first step in protein synthesis. During this process:
Translation is the second step in protein synthesis. During this process:
Codon-anticodon pairing follows complementary base pairing rules: A pairs with U and G pairs with C. For example, if the codon on mRNA is AUG, the matching anticodon on tRNA would be UAC.
The "wobble hypothesis" explains why some tRNAs can recognize more than one codon. The first two bases of a codon form strict base pairs with the anticodon, but the third position can sometimes "wobble" and allow non-standard pairing.
The genetic code is nearly universal across all living organisms. This means that a codon codes for the same amino acid whether it's in a human, a plant, or a bacterium. This universality is strong evidence for the common ancestry of all life on Earth.
Sickle cell anaemia is a genetic disorder caused by a single base change in the DNA coding for haemoglobin. The mutation changes the codon GAG to GTG, which results in the amino acid valine being incorporated instead of glutamic acid. This small change alters the structure of haemoglobin, causing red blood cells to become sickle-shaped when oxygen levels are low. This demonstrates how even a tiny change in the genetic code can have significant effects on protein structure and function, leading to serious health conditions.
In your iGCSE exam, you might be asked to explain how the genetic code works or to describe the roles of codons and anticodons in protein synthesis. Remember to use specific examples and correct terminology. Be prepared to explain the processes of transcription and translation and how changes in DNA sequence can affect protein structure and function.
To deepen your understanding, try modelling protein synthesis with paper cut-outs or online simulations. Practice translating DNA sequences into mRNA and then into amino acid sequences using a genetic code chart. This will help you understand how changes in DNA can affect the final protein product.
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