what are the restrictions on the four different nucleotide bases in the double helix ?
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Purine Bases form hydrogen bonds with Pyrimidine Bases\n\nPurines are Adenine , Guanine\nPyrimidines are Cytosine , Thymine (and Uracil in RNA)\n\nAdenine only forms a double hydrogen bond with Thymine (or Uracil)\nGuanine only forms a triple hydrogen bond with Cytosine
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Although sometimes called "the molecule of heredity", DNA macromolecules as people typically think of them are not single molecules. Rather, they are pairs of molecules, which entwine like vines to form a double helix (see the illustration at the right).\n\nEach vine-like molecule is a strand of DNA: a chemically linked chain of nucleotides, each of which consists of a sugar (deoxyribose), a phosphate and one of five kinds of nucleobases ("bases"). Because DNA strands are composed of these nucleotide subunits, they are polymers.\n\nThe diversity of the bases means that there are five kinds of nucleotides, which are commonly referred to by the identity of their bases. These are adenine (A), thymine (T), uracil (U), cytosine (C), and guanine (G). U is rarely found in DNA except as a result of chemical degradation of C, but in some viruses, notably PBS1 phage DNA, U completely replaces the usual T in its DNA. Similarly, RNA usually contains U in place of T, but in certain RNAs such as transfer RNA, T is always found in some positions. Thus, the only true difference between DNA and RNA is the sugar, 2-deoxyribose in DNA and ribose in RNA.\n\nIn a DNA double helix, two polynucleotide strands can associate through the hydrophobic effect and pi stacking. Specificity of which strands stay associated is determined by complementary pairing. Each base forms hydrogen bonds readily to only one other -- A to T and C to G -- so that the identity of the base on one strand dictates the strength of the association; the more complementary bases exist, the stronger and longer-lasting the association.\n\nThe cell's machinery is capable of melting or disassociating a DNA double helix, and using each DNA strand as a template for synthesizing a new strand which is nearly identical to the previous strand. Errors that occur in the synthesis are known as mutations. The process known as PCR (polymerase chain reaction) mimics this process in vitro in a nonliving system.\n\nBecause pairing causes the nucleotide bases to face the helical axis, the sugar and phosphate groups of the nucleotides run along the outside; the two chains they form are sometimes called the "backbones" of the helix. In fact, it is chemical bonds between the phosphates and the sugars that link one nucleotide to the next in the DNA strand.
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DNA is composed of 4 bases: adenine (A), thymine (T), cytosine (C), and guanine (G). Uracil (U), is rarely found in DNA except as a result of chemical degradation of Cytosine, but the DNA of some viruses (notably PBS1 phage DNA) and RNA (Ribonucleic Acid), has Uracil instead of Thymine.\n\nEach base on one strand forms a bond with just one kind of base on another strand, called a "complementary" base: A bonds with T, and C bonds with G. Therefore, the whole double-strand sequence can be described by the sequence on one of the strands, chosen by convention. Two nucleotides paired together are called a base pair.\n\nIn a DNA double helix, two polynucleotide strands can associate through the hydrophobic effect and pi stacking. Which strands associate depends on complementary pairing. Each base forms hydrogen bonds readily to only one other base, A to T forming two hydrogen bonds, and C to G forming three hydrogen bonds. The GC content and length of each DNA molcule dictates the strength of the association; the more complementary bases exist, the stronger and longer-lasting the association, characterised by the temperature required to break the hydrogen bond, its melting temperature (also called Tm value))-.
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DNA has the form of a double helix. It is also anti parallel.
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Between Adenine and Thymine -- 2\n\n\nBetween Guanine and Cytosine -- 3
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Consider a regular climbing ladder.  If the ladder was sawed in half by sawing each rung crosscut thru the middle than each half of the ladder would represent one  DNA strand.  Each new organism receives a single strand from each of it's parents.  The two strands join to make the new organism's double stranded DNA.  The rail or sides of the ladder would represent the sugar and phosphate link backbone of the DNA.  The sawed rung would represent the nucleic acid bases.  The bases are classified as either purines or pyrimidines.  The two purines are Adenine (A) and Guanine (G) which are similiar to the compound Purine.  The two pyrimidine bases are Thymine (T) and Cytosine (C) which are structured similiar to the compound Pyrimidine.  In RNA Thymidine is replaced by Uracil.  The collective term for these structures is "nitrogenous base".  Each base contains either/or hydrogen (h), nitrogen (n) and oxygen (o) atoms located on the structure where the "cut" would exhist on the sawed ladder rungs.  When these bases move close together there is an attraction between these atoms and a bridge forms called a hydrogen bond  or h-bond.Thymine has an o and a h.  Adenine has a h which is attracted to T's o and an n which is attracted to T' h. A and T always h-bond or pair.  Cytosine has an h,n,and o.  Guanine has two h's and an o.  one h on guanine is attracted to the o on cytosine, the other to the n.  The o is attracted to the h on cytosine.  Therefore C always pairs with G and is held together by three h-bonds, where A and T are paired by two h-bonds.  In total the h-bonds strongly bind the two DNA single strands together but one at a time they can be broken to seperate the strands much like a zipper opening link by link.  Since C always pairs with G their proportion is is always the same.  The same is true for A and T.  Each Dna single strand has what is called a 3' (three prime hydroxyl) and a 5' (five prime phosphate) end.  When the two single strands pair to form thd Double strand they Form with the 3' end of one strand joining at the 5' end of the other strand.  This is known as antiparallel since they are running in opposite directions.
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DNA's most amazing characteristic is that it will bind to itself in a very specific manner, forming a helix. Figure 1 illustrates two DNA molecules binding. The variable groups of the polymer interact with eachother to mediate this binding. Adenine interacts with Thymine and Cytosine interacts with Guanine. No other bonds are energetically favorable, for example Cytosine cannot bond with itself. The bonds that these variable groups form are called are called base pairs and are illustrated in Figure 2. Thus, by knowing the sequence of bases of one strand of DNA we immediately know the sequence of the DNA strand which will bind to it, this strand is called the reverse complement or just the complementary strand. In Figure 1 the right hand strand is the reverse complement of the left hand side and vice-versa.\n\n\nFrom G. S. Sent, Molecular Biology of Bacterial Viruses. Copyright 1963 by W. H. Freeman and Company\n\nFigure Two:Illustration of DNA basepairing. Thermodynamically the interactions Adenine, Guanine, Cytosine, and Thymine are driven by both the hydrophobic stacking interactions hydrogen bond interactions. The specificity of the base pairing is driven mainly by hydrogen bonding and stearic interference.\nThis relationship to the complementary strand has many implications that make life as we know it possible, and allows all sorts of molecular biology techniques. In one of the greatest understatements of all time Watson and Crick said it best in their original paper on the structure of DNA:\n\n"It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.".
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The phosphate in the backbone is the same. The difference (other than uracil in RNA and thyamine in DNA) is the sugar group. RNA has ribose, while DNA has deoxyribose. Deoxyribose has one less hydroxyl group.
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It can be right because resonance can occur and cause that the hydrogen bonds link to another base ( that will cause a mutation). However there will be enzymes to correct that error. And some U residues are methylated so they cannot join to adenine.
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I assume you have some knowledge of transcription (DNA->RNA), basically they want to know how well you understand the process of translation (RNA-> protein).  Further I assume they give you a set of tRNAs taht you didn't include here.  You need the codon usage table and it various (slightly) depending on the organism.  \n\nThe first question starts with the result and asks you for the template.  Given an RNA, what was the complementary DNA that was used to encode it?  RNA transcription proceeds 5' to 3' (so does DNA synthesis - the only process that does obey this rule is the reverse translation of retroviruses, it proceeds 3' to 5').  Further, nucleic acids are always listed 5' to 3'.  So basically what you do is first find the complementary DNA base pair for the RNA nucleotide.  U pairs with A, A with T, C with G, and G with C.  I'll give the first six letters as an example.\n5' UACACC 3'\n3' ATGTGG 5'\nNow you have to flip the sequence over, to read it properly in the 5' to 3' orientation.  \nGGTGTA\n\nQ2)  Now that you know the template (GGTGTA), find it's complement.  Use the same process as above.\nGGTGTA\nCCACAT -> TACACC\n\nQ4)  The mRNA sequence is UACACCGGG...  The tRNA finds the first three bases (the first codon), and base pairs with it.  The tRNA has an amino acid coupled to it, and upon basepairing to mRNA this amino acid is released to form a growing polypeptide chain.  All you have to do is look at the codon UAC and figure out what amino acid it codes for.  \n\nUsing a codon usage table from humans, UAC codes for Tyr (tyrosine). ACC codes for Thr.  So the first two amino acids are Tyr-Thr (or YT in single letter abbreviation)
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No other information need be given.\n\nThe diagram represents a "untwisted" double helix.  The top row is a series of nitrogen bases, and the bottom row would be it's complimentary half.  The question is asking you which nitrogen base is paired with or across the ladder from Guanine in DNA. \n\nGuanine is a purine, and purines only bond with pyrimidines.  Since Adenine is also a purine, that is the incorrect answer.  Cytosine, thymine and uracil are pyrimidines.  Uracil is only found in RNA, so since the question specifies DNA, Uracil is incorrect.  That leaves us cytosine and thymine.  \n\nThe correct answer is (C) cytosine.  Guanine and cytosine are held together through three hydrogen bonds.  Adenine and thymine (or uracil in RNA) are joined by only two hydrogen bonds.  The molecules do not line up properly between guanine and thymine to form a bond.
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1) D. A nucleotide is is just a molecule that makes up RNA or DNA. One single nucleotide won't make sense unless you have the rest of the sequence.\n2) I'm not sure about this one, but I think it's D. Google it.\n3) A. It's part of the central dogma of Biology. DNA --transcription--> RNA --Translation--> Protein\n4) C\n5) The closest answer that you have there is D. However, it's not really assembling new materials to form the new cell wall. It's pinching the cell in half so that the cells will bud off from each other. See below for source. \n\nHope this helps.
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nucleotides make up nucleic acids
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A strand of DNA contains genes, areas that regulate genes, and areas that either have no function, or a function which we do not (yet) know: also see last bullet point in this section for the difference between DNA and RNA; \nDNA is organized as two complementary strands, head-to-toe, with bonds between them that can be "unzipped" like a zipper, separating the strands; \nDNA is a chain of chemical "building blocks", called "bases", of which there are four types: these can be abbreviated A, T, C, and G. Each base can only "pair up" with one single predetermined other base: A+T, T+A, C+G and G+C are the only possible combinations; that is, an "A" on one strand of double-stranded DNA will "mate" properly only with a "T" on the other, complementary strand; \nU occasionally replaces T, notably in PBS1 phage DNA; you can thus substitute "U" for "T" throughout this section. \nBecause each strand of DNA has a directionality, the sequence order does matter: A+T is not the same as T+A, just as C+G is not the same as G+C; \nFor each given base, there is just one possible complementary base, so naming the bases on the conventionally chosen side of the strand is enough to describe the entire double-strand sequence; \nThe genetic
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Use the standard Watson-Crick base pairing rules, with the change that A in DNA base pairs with U in the RNA. The anticodons in the tRNA will then base pair with the mRNA strand by the same base pairing rules:  G with C and A with U.
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Each amino acid is coded for by a 3-nucleotide codon.  So the mRNA would need at least 450 nucleotides.\n\nBut real mRNA often has extra nucleotides at the beginning or the end that aren't translated into protein.  Sometimes, they are functional (for instance, Iron-Response elements, or IREs tie the translation of that mRNA to iron levels in the cell), sometimes, they aren't.  \n\nLikewise, with DNA you would need at least 450 DNA base pairs, but realistically, there are promotors, or other DNA sequences that don't code for protein, but control when the mRNA is or isn't made, depending on the situation.  Most eukaryotes also have introns, as sometimes, different splice variants are called for in different situations.
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One word contradicts your "facts". \n\nPlasmids.\n\nPlasmids are non-chromosomal loops of DNA found in bacteria.  They often contain antibacterial resistance genes, or give the ability to use different sugars.  These plasmids are commonly shared by bacteria.  This is a gain of genetic information.\n\nNow, it is true that we only have 4 types of nucleotides in DNA, and they make about 20 amino acids.  This is conserved in all life forms. But from this, a countless number of proteins can be made, and have a nearly infinite number of effects.  Your suggesting could be likened to saying that we can never invent a new word, since we only have 26 letters.  \n\nPlease provide a source for your claims, perhaps then we can understand how you arrived at such a conclusion.