A nucleic acid is a macromolecule composed of chains of monomeric nucleotides. In biochemistry these molecules carry genetic information or form structures within cells. The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are universal in living things, as they are found in all cells and viruses. Nucleic acids were first discovered by Friedrich Miescher in 1871.
FUNCTIONS OF DNA:
- Deoxyribonucleic acid (DNA) is the genetic code which ensures that daughter cells inherit the same characteristics as the parent cells
- DNA is the code from which all protein is synthesised
- All prokaryotes, eukaryote and some viruses have DNA
- All DNA code is composed from four nucleotide bases, Adenine, Cytosine, Guanine and Thymine.
DNA also contains all the genetic coding which is used to control functions, behaviour and development of an organism. DNA is also used as a long term storage device to store the genetic instructions. These instructions must be correct so the DNA can make an exact copy of itslef.
The function of DNA in a cell is to code the way proteins turn genes on or off.
FUNCTIONS OF RNA:
mRNA - messenger RNA - is a "copy" of the DNA base sequence of a gene after processing (capping, addition of 3' tail and splicing). It is used to transfer the genetic information from DNA, which is a storage molecule and quite inaccessible, to ribosomes, which perform translation to synthesise polypeptides.
tRNA - transfer RNA - is "charged" with an amino acid and used to recognize the code in the mRNA and "translate" it into the amino acid it is carrying. There are specific tRNA molecules for each amino acid.
rRNA - ribosomal RNA - makes up parts of the ribosome and has the catalytic transpeptidase action required to create polypeptide chains during translation.
snRNA - small nuclear RNA - regulates and provides the catalytic machinery for splicing or mRNA.
gRNA - guide RNA - directs editing of RNA to specific sites.
miRNA - micro RNA - inhibits translation by base pairing with complementary sequences of mRNA.
Signal Recognition Particle - RNA/protein molecule that binds to the "Signal Sequence" on polypeptides to be sent to the endoplasmic reticulum, causing translation to pause until polypeptide has been fed into translocon for entry into ER.
What components do DNA and RNA?
It is easier to say which componants are different as the two types of molecule are very similar. RNA uses ribose in the sugar-phosphate backbone rather than deoxyribose, as in DNA. And in RNA uracil (U) is used in place of (T) as a base. These are the two major differences. If you want a list of similarities then:
-both use a sugar phosphate backbone onto which bases are assembled
-Both use four bases to encode information (A,T,C,G - DNA) and(A,U,C,G- RNA)
-both use hydrogen bonding between bases to join sense and antisense strands (both sides of the ladder)
-all nucleotides (bases) used to make to both DNA and RNA have 3 phosphate groups attatched to them before they are added to the growing chain.
A chromosome is an organized building of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek χρῶμα (chroma, colour) and σῶμα (soma, body) due to their property of being very strongly stained by particular dyes.
A gene is a unit of heredity in a living organism. It is normally a stretch of DNA that codes for a type of protein or for an RNA chain that has a function in the organism. All living things depend on genes, as they specify all proteins and functional RNA chains. Genes hold the information to build and maintain an organism's cells and pass genetic traits to offspring.
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA.
Nucleosides are glycosylamines consisting of a nucleobase (often referred to as simply base) bound to a ribose or deoxyribose sugar via a beta-glycosidic linkage. Examples of nucleosides include cytidine, uridine, adenosine, guanosine, thymidine and inosine.
genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA.
histones are strongly alkaline proteins found in eukaryotic cell nuclei, which package and order the DNA into structural units called nucleosomes.[1][2] They are the chief protein components of chromatin, act as spools around which DNA winds, and play a role in gene regulation. Without histones, the unwound DNA in chromosomes would be very long (a length to width ratio of more than 10 million to one in human DNA). For example, each human cell has about 1.8 meters of DNA, but wound on the histones it has about 90 millimeters of chromatin, which, when duplicated and condensed during mitosis, result in about 120 micrometers of chromosomes.[3]
Nucleosomes are the basic unit of DNA packaging in eukaryotes (cells with a nucleus), consisting of a segment of DNA wound around a histone protein core.[1] This structure is often compared to thread wrapped around a spool.[2]
Chromatin is the combination of DNA and proteins that makes up chromosomes. It is found inside the nuclei of eukaryotic cells. It is divided between heterochromatin (condensed) and euchromatin (extended) forms.[1] [2] The major components of chromatin are DNA and histone proteins, although other proteins have prominent roles too. The functions of chromatin are to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis, and to serve as a mechanism to control expression and DNA replication. Chromatin contains genetic material-instructions to direct cell functions.
BASES OF DNA AND RNA:
The DNA bases are adenine, guanine, thymine, and cytosine . In RNA, Thymine is replaced by Uracil. Adenine and guanine are known as purines and cytosine and thymine are known as pyrimidines.
Although scientists have made some minor changes to the Watson and Crick model, or have elaborated upon it, since its inception in 1953, the model's four major features remain the same yet today. These features are as follows:
- DNA is a double-stranded helix, with the two strands connected by hydrogen bonds. A bases are always paired with Ts, and Cs are always paired with Gs, which is consistent with and accounts for Chargaff's rule.
- Most DNA double helices are right-handed; that is, if you were to hold your right hand out, with your thumb pointed up and your fingers curled around your thumb, your thumb would represent the axis of the helix and your fingers would represent the sugar-phosphate backbone. Only one type of DNA, called Z-DNA, is left-handed.
- The DNA double helix is anti-parallel, which means that the 5' end of one strand is paired with the 3' end of its complementary strand (and vice versa). As shown in Figure 4, nucleotides are linked to each other by their phosphate groups, which bind the 3' end of one sugar to the 5' end of the next sugar.
- Not only are the DNA base pairs connected via hydrogen bonding, but the outer edges of the nitrogen-containing bases are exposed and available for potential hydrogen bonding as well. These hydrogen bonds provide easy access to the DNA for other molecules, including the proteins that play vital roles in the replication and expression of DNA (Figure 4).
Summary
Watson and Crick were not the discoverers of DNA, but rather the first scientists to formulate an accurate description of this molecule's complex, double-helical structure. Moreover, Watson and Crick's work was directly dependent on the research of numerous scientists before them, including Friedrich Miescher, Phoebus Levene, and Erwin Chargaff. Thanks to researchers such as these, we now know a great deal about genetic structure, and we continue to make great strides in understanding the human genome and the importance of DNA to life and health.
Putting the Evidence Together: Watson and Crick Propose the Double Helix
Chargaff's realization that A = T and C = G, combined with some crucially important X-ray crystallography work by English researchers Rosalind Franklin and Maurice Wilkins, contributed to Watson and Crick's derivation of the three-dimensional, double-helical model for the structure of DNA. Watson and Crick's discovery was also made possible by recent advances in model building, or the assembly of possible three-dimensional structures based upon known molecular distances and bond angles, a technique advanced by American biochemist Linus Pauling. In fact, Watson and Crick were worried that they would be "scooped" by Pauling, who proposed a different model for the three-dimensional structure of DNA just months before they did. In the end, however, Pauling's prediction was incorrect.
Using cardboard cutouts representing the individual chemical components of the four bases and other nucleotide subunits, Watson and Crick shifted molecules around on their desktops, as though putting together a puzzle. They were misled for a while by an erroneous understanding of how the different elements in thymine and guanine (specifically, the carbon, nitrogen, hydrogen, and oxygen rings) were configured. Only upon the suggestion of American scientist Jerry Donohue did Watson decide to make new cardboard cutouts of the two bases, to see if perhaps a different atomic configuration would make a difference. It did. Not only did the complementary bases now fit together perfectly (i.e., A with T and C with G), with each pair held together by hydrogen bonds, but the structure also reflected Chargaff's rule (Figure 3).
complementary base pairs in the DNA:
The mRNA transcribed from the antisense DNA strand is not identical to that DNA strand; it is complementary. -the mRNA has the 'partners' of the bases on the DNA template(remembering that RNA uses U instead of T)
-it IS identical to the sense strand; therefore, it carries the code for the protein.
-if the DNA says ACC, the mRNA says UGG.
Complementary base pairs are: adenine and thymine (A - T )
guanine and cytosine (G - C)
Base pairs in DNA
DNA (Deoxyribonucleic acid), is a chemical found primarily in the nucleus of cells. DNA carries the instructions for making all the structures and materials the body needs to function.
DNA is organized as two complementary strands, head-to-toe, with bonds between them that can be "unzipped" like a zipper, separating the strands.
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