Ribonucleic acid
From CreationWiki, the encyclopedia of creation science
Ribonucleic acid (RNA) is similar to DNA in that it is also a chain (or polymer) of nucleotides with the same 5' to 3' direction of its strands. However, the ribose sugar component of RNA is slightly different chemically than that of DNA. RNA has a 2' oxygen atom that is not present in DNA. Other fundamental structural differences exist. For example, uracil takes the place of the thymine nucleotide found in DNA, and RNA is, for the most part, a single-stranded molecule. DNA directs the synthesis of a variety of RNA molecules, each with a unique role in cellular function. For example, all genes that code for proteins are first made into an RNA strand in the nucleus called a messenger RNA (mRNA). The mRNA carries the information encoded in DNA out of the nucleus to the protein assembly machinery, called the ribosome, in the cytoplasm. The ribosome complex uses mRNA as a template to synthesize the exact protein coded for by the gene.[1]
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Composition
Ribonucleic Acid is made up of a pentose group, which is a five carbon sugar, a phosphoric acid and a organic base. The structure of RNA is very similar to DNA. RNA is a linear polymer. However it does not have a linear structure. It has many regions in its polynucleotide chain where it folds in on itself creating structures that are called hairpin loops. This results in the formation of a helical structure like DNA in certain portions of its chain. Nucleotides such as ATP, GTP, CTP or UTP attach to the molecule by phosphodiester bridges (bonds). These bonds attach to either that 3' carbon or the 5' carbon in the sugar group ribose. In its chemical structure each nucleotide that makes of RNA has only one subtle difference from its DNA counterpart. It has one extra OH group. As DNA changes into RNA an OH group is added and TTP switches out with UTP. DNA's double helix structure is broken into two as it becomes RNA. RNA has a single helical structure. [2]
RNA Functions
There are four types of RNA. There is the messenger RNA (mRNA), transfer-RNA (tRNA), ribosomal-RNA (rRNA), and catalytic-RNAs.
- Messenger RNA is used during the process of transcription. The mRNA travels from the DNA in the nucleus to the cytoplasm where it preforms protein synthesis.
- Transfer RNA plays a big role in the process in the creation of proteins. The tRNA decodes the mRNA and translates the information encoded from the DNA. This information is used to align the certain amino acids into their right sequences to make polypeptide chains. This process is called translation. Most protein is synthesized in the ribosome.
- Ribosomal RNA Over 50% of the ribosome is made up of rRNA and is required for the proteins to be properly made. (Purves 236-39)
Role in Gene Expression
- Main Article: Gene expression
Proteins in the body are made from the instructional code found in DNA. The DNA however stays in the nucleus and is not accessible to the ribosomes that catalyze the polymerization of amino acids. RNA is instead created as a "message" (called messenger RNA) to be sent to the ribosome with instructions for assembling a protein.
The Ribonucleic acid made during gene expression is a transcript or copy of DNA that is almost, but not 100% identical. In particular, DNA is a deoxy form of the molecule (one fewer OH group per nucleotide). Also, in the process of making RNA, one of the nucleotides is substituted - TTP (Thymidine Triphosphate) is replaced for UTP (Uridine Triphosphate).
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Transcription
- Main Article: Transcription
Transcription is the process whereby DNA is copied to a single-stranded RNA. This process is catalyzed by the enzyme RNA polymerase. This enzyme attaches itself to a portion of the DNA strand and replicates it sending out mRNA or rRNA. During transcription there needs to be a promoter. A promoter is a certain sequence of DNA which RNA polymerase can bind to. It creates a very tight bond with the promoter. There are at least one promoter for every gene in the genome. Promoters do three specific jobs. They tell RNA, 1. Where to start transcription, 2. Which strand of DNA to read, and 3. The direction to take from the start. After the specific promoter has been chosen by the RNA polymerase a process called elongation begins. This process is where the polymerase adds nucleotides (A, U, C, G) to a section of DNA of about 20 amino acids and replicates it. However it is created antiparallel to DNA. DNA is 5' to 3' and RNA transcribes as 3' to 5'. The elongation process will continue until it reaches a certain termination site in the DNA. There is also a specific initiation site the tells where the transcription is to start taking place. (Purves 236-39)
RNA Editing
RNA can be spliced to increase the number of proteins it can create. A pre-mRNA (in-mature mRNA) can go through a process of splicing. In the cases of the protein tropomyosin a pre-mRNA can be spliced to create five different end result mature mRNA's. This is the reason why the human body only has 21,000 genes rather than the 100,000 to 150,000 genes estimated by scientists. This type of editing greatly increases the number of mature mRNAs and therefore the number of proteins that can be created from a single gene.
Another way to further alter RNA is called RNA editing. This happens through two separate ways. The first way is called Intersection of Nucleotides. When this happens stretches of U's will be placed into the mRNA to make it longer. An example of this is:
(DNA) ACCTCC - becomes transcribed
(mRNA) UGGAGG - becomes edited
(Edited mRNA) UGGUUUAGG
The edit above will effectively alter the amino acid sequence from trp-arg, to trp-phe-arg.
The second way for editing to happen is called Alteration of Nucleotides. In this process an enzyme will catalyze and change the way the protein is formed. An example of this process is:
(DNA) GTA - becomes transcribed
(mRNA) CAU - becomes edited
(Edited mRNA) UAU
(Purves 296-97)
Gallery
 Electron micrograph of Ribosomal RNA transcription units of Chironomus thummi (Diptera). Magnification - 40.000x. |
References
- Molecular Biology more info
- Purves, William K. et al. Life: The Science of Biology. Gordensville, VA. 2004.
- What is a Genome? by the National Center for Biotechnology Information
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