Transcription+and+Translation

The processes of transcription and translation are part of the central dogma of biology and genetics. The central dogma of moleculer genetics is: DNA → RNA (transcription) → Protein (translation). The process of transcription is the process of converting DNA to its complementary mRNA strand which can then be read as codons, three nucleotides, which encode for a specific amino acid which makes up a protein. The genetic code specifies which amino acid is encoded for by each codon. However, each amino acid is not always specified by one codon. Often, more than one closely related codons may specify the same amino acid. Also, specific codons signal the end of translation and to stop adding amino acids to the protein.



RNA is composed of a long chain of nucleotides. Each of these nucleotides are made up of a ribose sugar, a phosphate, and a nitrogenous base. One thing to keep in mind is that the base pairs in DNA and RNA differ. DNA is composed of Adenine, Guanine, Thymine and Cytosine. RNA on the other hand is composed of Adenine, Guanine, Uracil and Cytosine. Adenine pairs with Thymine in DNA and Uracil in RNA and Guanine and Cytosine pair with each other in both DNA and RNA. Other differences between DNA and RNA to note is that RNA is single-stranded and contains ribose whereas DNA is double-stranded and contains deoxyribose. Translation is a process where genetic information is translated from a `"nucleic acid language" to an "amino acid language". Translation is catalyzed by a large enzyme called a ribosome, which contains proteins and ribosomal RNA (rRNA). Translation also involves specific RNA molecules called transfer RNA (t-RNA) which can bind to three basepair codons on a messenger RNA (mRNA) and also carry the appropriate amino acid encoded by the codon. The ribosome assembles on the first AUG (start codon) in the mRNA. This codon encodes the amino acid methionine (Met).

There are different kinds of RNA that need to be considered:

Non-coding RNA (ncRNA) Non-coding RNA is simply an RNA molecule that does not code for protein and makes up 97% of RNA. Both transfer RNA and ribosomal RNA (listed below) are included in this group of RNA. Other, less know types of ncRNA include snoRNA, microRNA, siRNA, piRNA, Xist, HOTAIR, and thousands more. The various types of ncRNA all have very diverse biological functions.

Messenger RNA (mRNA) Messenger RNA contains genetic information. It is a copy of a portion of the DNA. It carries genetic information from the gene (DNA) out of the nucleus, into the cytoplasm of the cell where it is translated to produce protein. In mRNA, genetic information is encoded in the sequence of nucleotides arranged into codons which are made up of three bases each. Each codon indicates a specific amino acid, except for the stop codons, which terminate the sequence.

Ribosomal RNA (rRNA) This type of RNA is a structural component of the ribosomes. It does not contain a genetic message. The role of the rRNA is to supply a procedure used for decoding mRNA into amino acids and to work together with the tRNAs throughout the translation process by providing peptidyl transferase activity.

Transfer RNA (tRNA) Transfer RNA functions to transport amino acids to the ribosomes during protein synthesis.

Transcription occurs on DNA from 5 prime to 3 prime end. There is a template strand and there is a lagging strand. Transcription on the lagging strand has problems on the telomeres and after every transcription they get smaller. Current research shows this shortening of the telomeres may contribute to aging.

An important thing to remember, too, about transcription is that it occurs differently in different types of cells. In prokaryotes, one promoter region (the point on the strand of DNA that begins transcription) can contain multiple genes, meaning that in one strand of RNA, multiple proteins, affecting multiple different traits can be translated. Typically this does not occur in eukaryotic cells.

A couple of important terms to consider when looking at translation are: Initiation and Termination Codes

An initiation code signals the start of a genetic message. As the ribosome moves along a mRNA transcript, it will not begin synthesizing protein until it reaches an initiation code. Start codons are the following: AUG

Termination codes signal the end of the genetic message. Synthesis stops when the ribosome reaches a terminator codon. Stop codons are the following: UAA, UGA, UAG

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RNA synthesis, or transcription, is the process of transcribing DNA nucleotide sequence information into RNA sequence information. DNA is transferred to RNA (mRNA) molecule in a process called transcription. During transcription, the DNA of a gene helps with base-pairing, and an enzyme called RNA polymerase catalyzes the formation of the molecule mRNA.
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During translation, which is the second major step in gene expression, the mRNA is "read", which relates the DNA sequence to the amino acid sequence in proteins. Posted Oct 1, 2009 10:42 pm || One thing to add, the base pairs in DNA and RNA differ. DNA is composed of Adenine, Guanine, Thymine and Cytosine. RNA on the other hand is composed of Adenine, Guanine, Uracil and cytosine. Adenine pairs with Thymine in DNA and Uracil in RNA this pair is bonded with 2 hydrogen bonds between them. Guanine and Cytosine pair with each other in both DNA and RNA and are bonded together by 3 hydrogen bonds.
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Just to clarify, this process occurs during transcription. || There are three major steps of transcription: initiation, elongation and termination. During initiation the promoter region needs to be activated. During elongation the RNA polymerase moves along the DNA strand causing the new mRNA strand to grow longer. Finally, during termination the new mRNA strand is released.
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The above figure gives a breakdown of the processes of transcription and translation.

The sites of transcription and translation should be noted. In a eukaryotic cell, transcription occurs in the nucleus, however, translation occurs in the cytoplasm.

In a prokaryotic cell, transcription and translation are conjoined. This means that translation starts while the mRNA is still being synthesized.

||  ||   ||  || [|JH-1789] The key factors of translation are the ribosome which consists of two subunits, mRNA and tRNA as well as many protein factors. The three stages of translation (initiation, elongation, termination) previously discussed all describe the growth of the amino acid or polypeptide tha is the product of translation.

Also something to note in respect to transcription: Sometimes there is an alteration made in the central dogma of genetics. This is the case with retroviruses that use RNA instead of DNA. After transcription the RNA goes back to the DNA replication phase of the central dogma. This process is called reverse transcription. An example of a retrovirus would be HIV. The HIV virus copies itself and the genome is reversed transcribed to the DNA and is then inserted into the host genome. || J[|H-1789] It is also important to note that the DNA region where the RNA polymerase binds to initiate transcription is called the promoter. Transcription factors are proteins that help RNA polymerase bind to the promoter. Another thing that aids in the process of transcription is an enhancer. They bind with transcription factors and enhance transcription. They can be within a gene or upstream or downstream but most are located upsteam. In prokaryotes enhancers are close to the promoter but in eukaryotes they can be quite a distance away. || ||