DR. SAGHIV EXPLAINS THE GENETIC CODE

OUR CELLS SPEAK IN CODE!

Gene expression is the end-result of multiple cellular (within the cell) and genetic processes that ultimately have take the genetic code and turns this into amino acids. Gene expression includes two major processes called transcription and translation. Simplified, during the transcription process, the genetic information becomes ready for translation into amino acids, involving DNA and RNA molecules.

The genetic code within DNA undergoes processes that allow the cell to "read" the genetic information and use it as instructions for the creation of amino acids. Thus, it is said that genes and the genetic information within them, act as the blueprints for the creation of proteins in the cells and body.

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At this point, let's explain as simply as possible the structure of the DNA molecule, as it is influential for the processes to come. DNA is structured as a double-helix molecule in the form of two strands that match each other in shape. Each strand is a long sequence of nucleotides arranged in a very specific and accurate order (sequence).

The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99% of those bases are the same in all people. In an RNA molecule, the thymine (T) is replaced by a base called uracil. Each nucleotide on one strands matches in its 3D conformation to a nucleotide on the other strand, causing them to create a connection.

Each such two pairs of nucleotides that connect are called a base pair. Adenine always pairs up with thymine, and guanine always pairs up with cytosine. Thus, in an RNA molecule, uracil pairs up with adenine. This structure is important and brilliant at the same time, since each strand (half of a DNA molecule) can function as the basis for replicating (doubling) a DNA molecule, and also as the basis for the transition from DNA to RNA.

DNA and RNA molecules are not in a form that allows the cell to "read" the genetic information within them, yet mRNA molecules are in a form that allows the cell to read the genetic information within them and create amino acids accordingly. Thus, the cell initiates the transcription process, where sequences of the DNA molecule are used to create mRNA molecules with a "mirror" sequence to that of the DNA's sequence it originated from.

In between, RNA is then used to create messenger RNA molecules or mRNA for short, that includes a very readable form of nucleotide sequences called "codons". Codons are short sequences of three nucleotides each, that code for the creation of an amino acid. One codon means the information needed to code for one amino acids.

Elongated chains of amino acids in a certain order/sequence make for a certain protein. Thus, proteins differ from each other in the number of amino acids they contain, and the specific order of their amino acids. Amino acids are the building blocks of proteins. In order to create the protein correctly, the codons within the mRNA molecules need to be in the right order and have no mistakes.

Now that the cell has a readable form of genetic information (mRNA) it can begin the second major process required to create a protein, called translation. As part of the translation process, the codons within the mRNA molecule are "read" and translated into amino acids. Each amino acid that exists is coded by one or more codons.

The following tables include the information for all amino acids, their three letter abbreviations, one letter abbreviations, and their possible codons:

Amino Acid	3 Letter Code	1 letter Code	Codon Option 1	Codon Option 2	Codon Option 3	Codon Option 4
Alanine	Ala	A	GCU	GCC	GCA	GCG
Arginine	Arg	R	CGU	GCG	CGA	CGG
Asparagine	Asn	N	AAU	AAC
Aspartic Acid	Asp	D	GAU	GAC
Cyteine	Cys	C	UGU	UGC
Glutamic Acid	Glu	E	GAA	GAG
Glutamine	Gln	Q	CAA	CAG
Glycine	Gly	G	GGU	GGC	GGA	GGG
Histidine	His	H	CAU	CAC
Isoleucine	Ila	I	AUU	AUC	AUA

Leucine	Leu	L	UUA; UUG	CUU; CUG	CUC	CUA
Lysine	Lys	K	AAA	AAG
Methionine	Met	M	AUG
Phenylalanine	Phe	F	UUU	UUC
Proline	Pro	P	CCU	CCC	CCA	CCG
Serine	Ser	S	AGU	AGC
Threonine	Thr	T	ACU	ACC	ACA	ACG
Tryptophan	Trp	W	UGG
Tyrosine	Tyr	Y	UAU	UAC
Valine	Val	V	GUU	GUC	GUA	GUG
STOP Codon			UAA	UAG	UGA

Molecules called transfer RNA or tRNA have an anti-codon, the matching nucleotide sequence to the mRNA codon on their one end, and the matching amino acid on its other end. mRNA enters the cell's protein creating apparatus called the ribosome. tRNA molecules connect to the mRNA in a base pairing method, causing the amino acids to be set in the order needed for the protein intended to be created. Then the amino acids are connected one to each other.

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The result is a polypeptide (protein) that mirrors the nucleotide sequence on the mRNA molecule, derived from the RNA molecule, derived from the DNA molecule. This shows how a gene's sequence within DNA leads to the creation of a certain polypeptide or protein. The genetic information is based on sequences of three nucleotide, and the ribosomes way of "reading" them is called the "reading frame".

The ribosome's function is based on the codons matching its "reading frame" of the codons in specific sequences of three nucleotides each. Any change to the correct sequence of nucleotides will cause a change in the codon that may result in the wrong codon and thus wrong amino acid, or something that is unreadable. Mistakes made to the codons are the basis for the phenomenon known as "genetic mutations" and also the basis for genetic diversity. To be continued in a future blog post.

In addition to protein coding genetic sequences (genes), DNA includes supervisory and regulatory mechanisms, indicators of where protein coding sequences begin (initiation codons), where protein coding sequencing end (stop codons), and non-coding sequences. About 90% of DNA has no known or understood functional role, while about 10% has a known and understood functional role. Human DNA includes about 25,000 genes.