Genetic code

The genetic code is a set of important rules used by all living cells to turn information stored in DNA or RNA into proteins, the building blocks of life. This process is like a language that cells understand. Special sequences of three tiny parts called nucleotides form codes known as codons. These codons tell the cell which proteinogenic amino acids to link together to make proteins.

The job of reading these codes is done by tiny structures in the cell called ribosomes. They use another kind of RNA called messenger RNA (mRNA) as a guide. They match each group of three nucleotides with the right amino acid carried by transfer RNA (tRNA) molecules. This linking of amino acids in the right order is how the cell builds proteins.

The genetic code is almost the same in every living thing, from tiny bacteria to humans. It can be shown in a simple chart with 64 different codes. Most genes follow one main pattern called the standard genetic code, but there are a few variant codes, like the one used in cell powerhouses called mitochondria. Understanding the genetic code helps scientists learn how living things grow, stay healthy, and pass traits from one generation to the next during protein biosynthesis.

History

Further information: Adaptor hypothesis

People have been trying to understand how proteins are made since we learned about DNA in 1953. Two scientists, Francis Crick and James Watson, thought that information moves from DNA to make proteins. Another scientist, George Gamow, suggested that groups of three DNA building blocks could tell cells which protein pieces to use.

Later, scientists found out that special molecules called codons, made of three DNA building blocks, tell cells which protein pieces to use. In 1961, Marshall Nirenberg and J. Heinrich Matthaei discovered that a certain sequence of RNA building blocks would only make one kind of protein piece. This helped scientists figure out many of the codes used in proteins. More work helped complete the whole set of these codes.

Codons

"Codon" redirects here. For other uses, see Codon (disambiguation).

See also: DNA and RNA codon tables § Translation table 1

Experiments showed that codons are made of three DNA building blocks. Scientists discovered which protein pieces each codon makes. This work helped us understand how cells build proteins.

Expanded genetic codes (synthetic biology)

Main article: Expanded genetic code

See also: Nucleic acid analogues

Scientists have added new building blocks to the genetic code to make new proteins. They have created special codes and molecules to do this. Some experiments have even made bacteria with extra building blocks in their genetic code.

Features

Main article: Reading frame

A reading frame is set by the first three parts in a DNA or RNA sequence. It decides how the code is read in groups of three. This grouping decides the order of building blocks for proteins.

Translation starts with a special signal, most commonly AUG. This stands for the amino acid methionine. There are also stopping signals that tell the cell when to finish building a protein. These stopping signals include UAG, UGA, and UAA.

Errors can happen when DNA is copied. These errors are called mutations. Some changes might cause health problems, while others might help an organism survive better.

Alternative genetic codes

See also: DNA and RNA codon tables § Alternative codons

Some organisms use slightly different rules to build proteins than the standard genetic code. For example, certain signals in the messenger RNA can make a stop signal code for a special amino acid instead. This lets some organisms use an extra amino acid, making their proteins unique.

Scientists have found many small differences in how the genetic code works in different living things. For example, some bacteria read a usual stop signal as an instruction to add a different amino acid. Even humans have a few places where stop signals are used in unusual ways. Despite these differences, all organisms use the same basic system of three-letter codes to build proteins.

Origin

The genetic code is an important part of life's history. There are different ideas about how it began. One idea is that the code was randomly created and then stayed the same because big changes would have been harmful. Another idea is that each amino acid naturally sticks to its matching codon or anti-codon. A third idea is that the current code is the best one because it helps reduce mistakes when copying DNA.

Scientists study how the code might have grown over time by adding new amino acids. They look at how the code protects against mistakes and balances the need for many different amino acids with the need to be accurate and use resources wisely. These studies help us understand how life may have developed its way of building proteins.

Main articles: RNA world hypothesis, aminoacyl-tRNA synthetases, ribozymes, robustness, synonymous, substitution of a biochemically similar amino acid, peptides, fitness, aptamers, metabolism, Last Universal Common Ancestor, Miller–Urey experiment, S-adenosyl methionine, mutations, ribosome, Information-theoretic, "rate-distortion", topology, map coloring problem, signaling games, frame shift