Dna's Protein Instructions: Building Blocks Of Life

DNA, or deoxyribonucleic acid, is a molecule that contains the biological instructions that make each species unique. These instructions are passed from adult organisms to their offspring during reproduction. DNA's instructions are used to make proteins, which are large, complex molecules that perform essential functions in our bodies. This process involves two main steps: transcription and translation. During transcription, the DNA sequence is copied into an intermediary molecule called messenger ribonucleic acid, or mRNA. In the second step, translation, the information contained in the mRNA molecule is translated into the language of amino acids, which are the building blocks of proteins. This process tells the cell's protein-making machinery the precise order in which to link the amino acids to produce a specific protein.

DNA's instructions for protein building

Deoxyribonucleic acid, or DNA, is a molecule that contains the biological instructions that make each species unique. These instructions are passed from adult organisms to their offspring during reproduction. DNA is found inside a special area of the cell called the nucleus.

DNA's instructions are used to make proteins in a two-step process. First, enzymes read the information in a DNA molecule and transcribe it into an intermediary molecule called messenger ribonucleic acid, or mRNA. This is the transcription step. Next, the information contained in the mRNA molecule is translated into the "language" of amino acids, which are the building blocks of proteins. This is the translation step. This language tells the cell's protein-making machinery the precise order in which to link the amino acids to produce a specific protein.

Each DNA sequence that contains instructions to make a protein is known as a gene. The size of a gene may vary greatly, ranging from about 1,000 bases to 1 million bases in humans. Genes only make up about 1 percent of the DNA sequence. DNA sequences outside this 1 percent are involved in regulating when, how, and how much of a protein is made.

Transcription

DNA does contain the instructions for making proteins. These instructions are found in the form of genes, which are segments of DNA. Each gene provides the code for the assembly of a specific protein. The process of transcription involves converting DNA sequences into messages that can be used to produce proteins. This is a two-step process.

During transcription, the information stored in a gene's DNA is passed to a similar molecule called RNA (ribonucleic acid) in the cell nucleus. Both RNA and DNA are made up of a chain of building blocks called nucleotides, but they have slightly different chemical properties. The enzyme RNA polymerase moves along the DNA, unwinding the helix to expose a new region of the template strand for complementary base-pairing. The RNA chain produced by transcription is therefore elongated one nucleotide at a time, and it has a nucleotide sequence that is complementary to the DNA strand used as the template. The resulting mRNA is a single-stranded copy of the gene, which next must be translated into a protein molecule.

The type of RNA that contains the information for making a protein is called messenger RNA (mRNA) because it carries the information or message from the DNA out of the nucleus into the cytoplasm. The flow of information from DNA to RNA to proteins is one of the fundamental principles of molecular biology and is sometimes called the “central dogma”.

Messenger RNA (mRNA)

MRNA is made from a DNA template during the process of transcription. Enzymes read the information in a DNA molecule and transcribe it into mRNA, which is then translated into the "language" of amino acids. Each sequence of three nucleotides, called a codon, usually codes for one particular amino acid. This is a complex task, as there are 20 types of amino acids that can be placed in many different orders to form a wide variety of proteins.

The mRNA molecule is created during the process of transcription, where an enzyme (RNA polymerase) converts the gene into primary transcript mRNA (also known as pre-mRNA). This pre-mRNA usually still contains introns, regions that will not go on to code for the final amino acid sequence. These are removed in the process of RNA splicing, leaving only exons, regions that will encode the protein. This exon sequence constitutes mature mRNA, which is then read by the ribosome and used to create the protein.

The concept of mRNA was developed by Sydney Brenner and Francis Crick in 1960, and it was first identified and described in 1961 by two teams: one consisting of Brenner, Crick, and Matthew Meselson, and another led by James Watson.

Translation

The process of protein synthesis, also known as translation, occurs in the ribosomes. DNA contains the instructions to make proteins, which are found in the form of genes. Each gene provides the code for the assembly of a specific protein.

A type of RNA called transfer RNA (tRNA) assembles the protein, one amino acid at a time. Protein assembly continues until the ribosome encounters a “stop” codon (a sequence of three nucleotides that does not code for an amino acid).

The mRNA sequence is used as a template to assemble, in order, the chain of amino acids that form a protein. To go from 4 to 20 amino acids, the mRNA is read in groups of three. This is called translation.

During translation, the mRNA is "read" according to the genetic code, which relates the DNA sequence to the amino acid sequence in proteins. Each codon specifies a particular amino acid. The ribosome is composed of two subunits: the large (50S) subunit and the small (30S) subunit. The ribosomal subunits contain proteins and specialized RNA molecules—specifically, ribosomal RNA (rRNA) and transfer RNA (tRNA).

The translation of mRNA begins with the formation of a complex on the mRNA. First, three initiation factor proteins (known as IF1, IF2, and IF3) bind to the small subunit of the ribosome. This preinitiation complex and a methionine-carrying tRNA then bind to the mRNA, near the AUG start codon, forming the initiation complex.

The next phase in translation is known as the elongation phase. First, the ribosome moves along the mRNA in the 5'-to-3' direction, which requires the elongation factor G, in a process called translocation. The tRNA that corresponds to the second codon can then bind to the A site, a step that requires elongation factors and guanosine triphosphate (GTP) as an energy source.

This process is repeated until all the codons in the mRNA have been read by tRNA molecules, and the amino acids attached to the tRNAs have been linked together in the appropriate order. At this point, translation must be terminated, and the nascent protein must be released from the mRNA and ribosome.

Amino acids

There are 20 types of amino acids, which can be placed in various orders to form a wide variety of proteins. They can be classified into three groups: essential amino acids, nonessential amino acids, and conditionally essential amino acids. Essential amino acids cannot be made by the body and must be obtained from food. Nonessential amino acids are those that the body can produce, even if they are not obtained from food. Conditionally essential amino acids are usually not essential, except during illness or stress.

The first few amino acids were discovered in the early 1800s. The last of the 20 common amino acids to be discovered was threonine in 1935. However, there are over 500 amino acids that exist in nature.

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