It seems remarkable that the body can make millions of different proteins (called the proteome),

all from the same 20 amino acids and all encoded by genes made of just 4 nucleotides (A, T, C, G).

In the early years after the structure of DNA was discovered, an eminent scientist dismissed it as “a stupid molecule,”

too simple and repetitious in its sequence of just four bases, repeated over and over, to be the carrier of so much hereditary information.

Some thought heredity must be transmitted by the highly variable proteins of the nucleus, and DNA was just a boring, monotonous scaffold to support them.

But DNA, we now appreciate, is a striking illustration of how a great variety of complex structures can be made from a small variety of simpler components.

is a system that enables these 4 nucleotides to code for the amino acid sequences of all proteins.

It’s not unusual for simple codes to represent complex information. Computers store and transmit complex information, including pictures and sounds, in a binary code with only the symbols 1 and 0.

Thus, it shouldn’t be surprising that a mere 20 amino acids can be represented by a code of 4 nucleotides; all this requires is to combine these symbols in various ways

It requires more than 2 nucleotides to code for each amino acid, because the A, U, C, and G of mRNA can combine in only 16 different pairs (AA, AU, AC, AG, UA, UU, and so on).

The minimum code to symbolize 20 amino acids is 3 nucleotides per amino acid, and indeed, this is the case in DNA. A sequence of 3 DNA nucleotides that stands for 1 amino acid is called a base triplet.

When messenger RNA is produced, it carries a coded message based on these DNA triplets.

A 3-base sequence in mRNA is called a codon. The genetic code is expressed in terms of codons.

The reason for this is easy to explain mathematically. Four symbols ( N N) taken three at a time ( x x) can be combined in N x N x different ways;.

that is, there are 4 3 = 64 4 3 =64 possible codons available to represent the 20 amino acids

Only 61 of these, however, code for amino acids. The other 3—UAG, UGA, and UAA—are called stop codons; they signal “end of message,” like the period at the end of a sentence

A stop codon enables the cell’s protein-synthesizing machinery to sense that it has reached the end of the instruction for a particular protein.

The codon AUG plays two roles:

It serves as a code for methionine and as a start codon. This dual function is explained shortly.

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4.2b

Question	Answer
It seems remarkable that the body can make millions of different proteins (called the proteome),	all from the same 20 amino acids and all encoded by genes made of just 4 nucleotides (A, T, C, G).
In the early years after the structure of DNA was discovered, an eminent scientist dismissed it as “a stupid molecule,”	too simple and repetitious in its sequence of just four bases, repeated over and over, to be the carrier of so much hereditary information.
Some thought heredity must be transmitted by the highly variable proteins of the nucleus, and DNA was just a boring, monotonous scaffold to support them.	But DNA, we now appreciate, is a striking illustration of how a great variety of complex structures can be made from a small variety of simpler components.
The genetic code	is a system that enables these 4 nucleotides to code for the amino acid sequences of all proteins.
It’s not unusual for simple codes to represent complex information. Computers store and transmit complex information, including pictures and sounds, in a binary code with only the symbols 1 and 0.	Thus, it shouldn’t be surprising that a mere 20 amino acids can be represented by a code of 4 nucleotides; all this requires is to combine these symbols in various ways
It requires more than 2 nucleotides to code for each amino acid, because the A, U, C, and G of mRNA can combine in only 16 different pairs (AA, AU, AC, AG, UA, UU, and so on).	The minimum code to symbolize 20 amino acids is 3 nucleotides per amino acid, and indeed, this is the case in DNA. A sequence of 3 DNA nucleotides that stands for 1 amino acid is called a base triplet.
When messenger RNA is produced, it carries a coded message based on these DNA triplets.	A 3-base sequence in mRNA is called a codon. The genetic code is expressed in terms of codons.
The reason for this is easy to explain mathematically. Four symbols ( N N) taken three at a time ( x x) can be combined in N x N x different ways;.	that is, there are 4 3 = 64 4 3 =64 possible codons available to represent the 20 amino acids
Only 61 of these, however, code for amino acids. The other 3—UAG, UGA, and UAA—are called stop codons; they signal “end of message,” like the period at the end of a sentence	A stop codon enables the cell’s protein-synthesizing machinery to sense that it has reached the end of the instruction for a particular protein.
The codon AUG plays two roles:	It serves as a code for methionine and as a start codon. This dual function is explained shortly.

Created by: Russells3709

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