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Introduction

The information content of DNA is in the form of specific sequences of nucleotides along the DNA strands.

The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins.

Proteins are the links between genotype and phenotype.

For example, Mendel’s dwarf pea plants lack a functioning copy of the gene that specifies the synthesis of a key protein, gibberellins.

Gibberellins stimulate the normal elongation of stems.

Transcription and translation are the two main processes linking gene to protein: an overview

Genes provide the instructions for making specific proteins.

The bridge between DNA and protein synthesis is RNA.

RNA is chemically similar to DNA, except that it contains ribose as its sugar and substitutes the nitrogenous base uracil for thymine.

An RNA molecules almost always consists of a single strand.

 

In DNA or RNA, the four nucleotide monomers act like the letters of the alphabet to communicate information.

The specific sequence of hundreds or thousands of nucleotides in each gene carries the information for the primary structure of a protein, the linear order of the 20 possible amino acids.

To get from DNA, written in one chemical language, to protein, written in another, requires two major stages, transcription and translation.

 

During transcription, a DNA strand provides a template for the synthesis of a complementary RNA strand.

This process is used to synthesize any type of RNA from a DNA template.

Transcription of a gene produces a messenger RNA (mRNA) molecule.

During translation, the information contained in the order of nucleotides in mRNA is used to determine the amino acid sequence of a polypeptide.

Translation occurs in ribosomes.

 

The basic mechanics of transcription and translation are similar in eukaryotes and prokaryotes.

Because bacteria lack nuclei, transcription and translation are coupled.

Ribosomes attach to the leading end of a mRNA molecule while transcription is still in progress.

 

In a eukaryotic cell, almost all transcription occurs in the nucleus and translation occurs mainly at ribosomes in the cytoplasm.

In addition, before the primary transcript can leave the nucleus
it is modified in various ways during RNA processing before the finished mRNA is exported to the cytoplasm.

To summarize, genes program protein synthesis via genetic messenger RNA.

The molecular chain of command in a cell is :
                 DNA -> RNA -> protein.

 

In the genetic code, nucleotide triplets specify amino acids

If the genetic code consisted of a single nucleotide or even pairs of nucleotides per amino acid, there would not be enough combinations (4 and 16 respectively) to code for all 20 amino acids.

Triplets of nucleotide bases are the smallest units of uniform length that can code for all the amino acids.

In the triplet code, three consecutive bases specify an amino acid, creating 43 (64) possible code words.

The genetic instructions for a polypeptide chain are written in DNA as a series of three-nucleotide words.

 

During transcription, one DNA strand, the template strand, provides a template for ordering the sequence of nucleotides in an RNA transcript.

The complementary RNA molecule is synthesized according to base-pairing rules, except that uracil is the complementary base to adenine.

During translation, blocks of three nucleotides, codons, are decoded into a sequence of amino acids. 

During translation, the codons are read in the 5’->3’ direction along the mRNA.

Each codon specifies which one of the 20 amino acids will be incorporated at the corresponding position along a polypeptide.

Because codons are base triplets, the number of nucleotides making up a genetic message must be three times the number of amino acids making up the protein product.

It would take at least 300 nucleotides to code for a polypeptide that is 100 amino acids long. 

 

The task of matching each codon to its amino acid counterpart began in the early 1960s.

Marshall Nirenberg determined the first match, that UUU coded for the amino acid phenylalanine.

He created an artificial mRNA molecule entirely of uracil and added it to a test tube mixture of amino acids, ribosomes, and other components for protein synthesis.

This “poly(U)” translated into a polypeptide containing a single amino acid, phenyalanine, in a long chain.

Other more elaborate techniques were required to decode mixed triplets such a AUA and CGA. 

 

By the mid-1960s the entire code was deciphered.

61 of 64 triplets code for amino acids.

The codon AUG not only codes for the amino acid methionine but also indicates the start of translation.

Three codons do not indicate amino acids but signal the termination
of translation.

The genetic code is redundant but not ambiguous.

There are typically several different codons that would indicate a specific amino acid.

However, any one codon indicates only one amino acid.

[If you have a specific codon, you can be sure of the corresponding amino acid, but if you know only the amino acid, there may be several possible codons.]

Both GAA and GAG specify glutamate, but no other amino acid.

Codons synonymous for the same amino acid often differ only in the third codon position.

To extract the message from the genetic code requires specifying the correct starting point.

This establishes the reading frame and subsequent codons are read in groups of three nucleotides.

The cell’s protein-synthesizing machinery reads the message as a series of nonoverlapping three-letter words.

In summary, genetic information is encoded as a sequence of non-overlapping base triplets, or codons, each of which is translated into a specific amino acid during protein synthesis.