Nucleic Acids: DNA and RNA

Transcription

1 Nucleic Acids: DNA and RNA Living organisms are complex systems. Hundreds of thousands of proteins exist inside each one of us to help carry out our daily functions. These proteins are produced locally, assembled piece-by-piece to exact specifications. An enormous amount of information is required to manage this complex system correctly. This information, detailing the specific structure of the proteins inside of our bodies, is stored in a set of molecules called nucleic acids. The nucleic acids are very large molecules that have two main parts. The backbone of a nucleic acids made of alternating sugar and phosphate molecules bonded together in a long chain, represented below. Nucleic acids, which include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are made from monomers known as nucleotides. Each nucleotide consists of three components: a purine or pyrimidine nucleobase (sometimes termed nitrogenous base), a pentose sugar, and a phosphate group. The substructure consisting of a nitrogenous base plus sugar is termed a nucleoside. Also, the nucleobases found in the two nucleic acid types are different: adenine, cytosine, and guanine are found in both RNA and DNA and uracil occurs in RNA. The sugars and phosphates in nucleic acids are connected to each other in an alternating chain (sugar-phoshate backbone) through phosphodiester linkages. In conventional nomenclature, the carbons to which the phosphate groups attach are 3 -end and the 5 -end carbons of the sugar. This gives nucleic acids directionality, and the ends of nucleic acid molecules are rerred to as 5 -end and 3 -end. The nucleobases are joined to the sugars via an N-glycosidic linkage.

2 Though only four different nucleotide bases can occur in a nucleic acid, each nucleic acid contains millions of bases bonded to it. The order in which these nucleotide bases appear in the nucleic acid is the coding for the information carried in the molecule. In other words, the nucleotide bases serve as a sort of genetic alphabet on which the structure of each protein in our bodies is encoded. DNA In most living organisms (except for viruses), genetic information is stored in the molecule deoxyribonucleic acid, or DNA. DNA is made and resides in the nucleus of living cells. DNA gets its name from the sugar molecule contained in its backbone (deoxyribose). However, it gets its significance from its unique structure. Four different nucleotide bases occur in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T). Cytosine and thymine are pyrimedines, while adenine and guanine are purines. These nucleotides bind to the sugar backbone of the molecule. The versatility of DNA comes from the fact that the molecule is actually double-stranded. The nucleotide bases of the DNA molecule form complementary pairs: The nucleotides hydrogen bond to another nucleotide base in a strand of DNA opposite to the original.

3 This bonding is specific, and adenine always bonds to thymine (and vice versa) and guanine always bonds to cytosine (and vice versa). This bonding occurs across the molecule, leading to a double-stranded system as pictured below. Adenine always pairs with thymine by two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds. In the early 1950s, four scientists, James Watson and Francis Crick at Cambridge University and Maurice Wilkins and Rosalind Franklin at King's College, determined the true structure of DNA from data and X-ray pictures of the molecule that Franklin had taken. In 1953, Watson and Crick published a paper in the scientific journal Nature describing this research. Watson, Crick, Wilkins and Franklin had shown that not only is the DNA molecule double-stranded, but the two strands wrap around each other forming a coil, or helix. The true structure of the DNA molecule is a double helix, as shown at right.

4 The double-stranded DNA molecule has the unique ability that it can make exact copies of itself, or self-replicate. When more DNA is required by an organism (such as during reproduction or cell growth) the hydrogen bonds between the nucleotide bases break and the two single strands of DNA separate. New complementary bases are brought in by the cell and paired up with each of the two separate strands, thus forming two new, identical, double-stranded DNA molecules. This concept is illustrated in the animation below. RNA Ribonucleic acid, or RNA, gets its name from the sugar group in the molecule's backbone - ribose. Several important similarities and differences exist between RNA and DNA. Like DNA, RNA has a sugar-phosphate backbone with nucleotide bases attached to it. Like DNA, RNA contains the bases adenine (A), cytosine (C), and guanine (G); however, RNA does not contain thymine, instead, RNA's fourth nucleotide is the base uracil (U). Unlike the double-stranded DNA molecule, RNA is a single-stranded molecule. RNA is the main genetic material used in the organisms called viruses, and RNA is also important in the production of proteins in other living organisms. RNA can move around the cells of living organisms and thus serves as a sort of genetic messenger, relaying the information stored in the cell's DNA out from the nucleus to other parts of the cell where it is used to help make proteins. RNA are of three different types performing different functions during protein synthesis. Ribosomal and transfer RNA comprise about 98% of all RNA. All three forms of RNA are made on a DNA template. Transfer RNA and messenger RNA are synthesized on DNA templates of the chromosomes, while ribosomal RNA is derived from nucleolar DNA.

5 These are mrna (messenger RNA), trna (transfer RNA) and rrna (ribosomal RNA). Although all three types occur as single polyribonucleotide strands, they differ in their characteristic molecular weight, sedimentation coefficient (S), number of nucleotide residue and percent of total cell RNA.

6 Messenger RNA It is a single-stranded RNA molecule which carries the message or the information from the gene in the chromosomes to the ribosomes, the cytoplasmic sites of protein synthesis. Since mrna is the direct product of gene, the base sequence of mrna is complementary to the one strand of DNA. The mrna is synthesised on DNA by an enzyme called DNA dependent RNA polymerase. Transfer RNA (trna) Transfer RNA are those RNA that transports amino acids to the ribosome where the amino acids are assembled during the translation of genetic message (protein synthesis).

7 trnas function at specific sites in the ribosome during translation, which is a process that synthesizes a protein from an mrna molecule. Proteins are built from smaller units called amino acids, which are specified by three-nucleotide mrna sequences called codons. Each codon represents a particular amino acid, and each codon is recognized by a specific trna. The trna molecule has a distinctive folded structure with three hairpin loops that form the shape of a three-leafed clover. One of these hairpin loops contains a sequence called the anticodon, which can recognize and decode an mrna codon.

8 Each trna has its corresponding amino acid attached to its end. Only 20 amino acids (Lisomers) are used. When a trna recognizes and binds to its corresponding codon in the ribosome, the trna transfers the appropriate amino acid to the end of the growing amino acid chain. Then the trnas and ribosome continue to decode the mrna molecule until the entire sequence is translated into a protein. Ribosomal RNA (rrna) Ribosomal RNA or rrna is a type of RNA which acts as a structural component of ribosome. It builds up a ribosome in association with ribosomal protein or r-proteins. Ribosomal RNA (rrna) generally represents more than 80% of the RNA present in cells. The ribosome is a complex molecular machine, found within all living cells, that serves as the site of biological protein synthesis (translation). Ribosomes link amino acids together in the order specified by messenger RNA (mrna) molecules. Ribosomes consist of two major components: the small ribosomal subunit, which reads the RNA, and the large subunit, which joins amino acids to form a polypeptide chain. Each subunit is composed of one or more ribosomal RNA (rrna) molecule and a variety of proteins.

9 The ribosomes and associated molecules are also known as the translational apparatus. The sequence of DNA, which encodes the sequence of the amino acids in a protein, is copied into a messenger RNA chain. It may be copied many times into RNA chains. Ribosomes can bind to a messenger RNA chain and use its sequence for determining the correct sequence of amino acids.