Nucleic acid contains what kinds of atoms

This means Adenine pairs with Thymine, and Guanine pairs with Cytosine. This is known as the base complementary rule because the DNA strands are complementary to each other.

Antiparallel Strands : In a double stranded DNA molecule, the two strands run antiparallel to one another so one is upside down compared to the other. The phosphate backbone is located on the outside, and the bases are in the middle. Adenine forms hydrogen bonds or base pairs with thymine, and guanine base pairs with cytosine.

At this time it is possible a mutation may occur. A mutation is a change in the sequence of the nitrogen bases. Most of the time when this happens the DNA is able to fix itself and return the original base to the sequence. However, sometimes the repair is unsuccessful, resulting in different proteins being created.

DNA packaging is an important process in living cells. Without it, a cell is not able to accommodate the large amount of DNA that is stored inside. A eukaryote contains a well-defined nucleus, whereas in prokaryotes the chromosome lies in the cytoplasm in an area called the nucleoid. In prokaryotic cells, both processes occur together. What advantages might there be to separating the processes?

What advantages might there be to having them occur together? Eukaryotic and prokaryotic cells : A eukaryote contains a well-defined nucleus, whereas in prokaryotes, the chromosome lies in the cytoplasm in an area called the nucleoid.

The size of the genome in one of the most well-studied prokaryotes, E. So how does this fit inside a small bacterial cell? The DNA is twisted by what is known as supercoiling. Supercoiling means that DNA is either under-wound less than one turn of the helix per 10 base pairs or over-wound more than 1 turn per 10 base pairs from its normal relaxed state. Some proteins are known to be involved in the supercoiling; other proteins and enzymes such as DNA gyrase help in maintaining the supercoiled structure.

Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a different type of packing strategy to fit their DNA inside the nucleus. At the most basic level, DNA is wrapped around proteins known as histones to form structures called nucleosomes. The histones are evolutionarily conserved proteins that are rich in basic amino acids and form an octamer.

The DNA which is negatively charged because of the phosphate groups is wrapped tightly around the histone core.

This nucleosome is linked to the next one with the help of a linker DNA. This is further compacted into a 30 nm fiber, which is the diameter of the structure. At the metaphase stage the chromosomes are at their most compact, approximately nm in width, and are found in association with scaffold proteins.

Eukaryotic chromosomes : These figures illustrate the compaction of the eukaryotic chromosome. In interphase, eukaryotic chromosomes have two distinct regions that can be distinguished by staining.

The tightly packaged region is known as heterochromatin, and the less dense region is known as euchromatin. Heterochromatin usually contains genes that are not expressed, and is found in the regions of the centromere and telomeres. The high molecular weight nucleic acid, DNA, is found chiefly in the nuclei of complex cells, known as eucaryotic cells , or in the nucleoid regions of procaryotic cells , such as bacteria.

It is often associated with proteins that help to pack it in a usable fashion. In contrast, a lower molecular weight, but much more abundant nucleic acid, RNA , is distributed throughout the cell, most commonly in small numerous organelles called ribosomes.

Both have a more transient existence and are smaller than rRNA. As shown in the following diagram, the sugar component of RNA is ribose, and the pyrimidine base uracil replaces the thymine base of DNA. The RNA's play a vital role in the transfer of information transcription from the DNA library to the protein factories called ribosomes, and in the interpretation of that information translation for the synthesis of specific polypeptides.

These functions will be described later. A complete structural representation of a segment of the RNA polymer formed from 5'-nucleotides may be viewed by clicking on the above diagram.

In the early 's the primary structure of DNA was well established, but a firm understanding of its secondary structure was lacking. Indeed, the situation was similar to that occupied by the proteins a decade earlier, before the alpha helix and pleated sheet structures were proposed by Linus Pauling.

Rosalind Franklin , working at King's College, London, obtained X-ray diffraction evidence that suggested a long helical structure of uniform thickness. Francis Crick and James Watson, at Cambridge University, considered hydrogen bonded base pairing interactions, and arrived at a double stranded helical model that satisfied most of the known facts, and has been confirmed by subsequent findings.

Base Pairing Careful examination of the purine and pyrimidine base components of the nucleotides reveals that three of them could exist as hydroxy pyrimidine or purine tautomers, having an aromatic heterocyclic ring. Despite the added stabilization of an aromatic ring , these compounds prefer to adopt amide-like structures.

These options are shown in the following diagram, with the more stable tautomer drawn in blue. A simple model for this tautomerism is provided by 2-hydroxypyridine. As shown on the left below, a compound having this structure might be expected to have phenol-like characteristics, such as an acidic hydroxyl group.

These differences agree with the 2-pyridone tautomer, the stable form of the zwitterionic internal salt. Further evidence supporting this assignment will be displayed by clicking on the diagram. Note that this tautomerism reverses the hydrogen bonding behavior of the nitrogen and oxygen functions the N-H group of the pyridone becomes a hydrogen bond donor and the carbonyl oxygen an acceptor.

The additional evidence for the pyridone tautomer, that appears above by clicking on the diagram, consists of infrared and carbon nmr absorptions associated with and characteristic of the amide group.

The data for 2-pyridone is given on the left. Similar data for the N-methyl derivative, which cannot tautomerize to a pyridine derivative, is presented on the right. Once they had identified the favored base tautomers in the nucleosides, Watson and Crick were able to propose a complementary pairing, via hydrogen bonding, of guanosine G with cytidine C and adenosine A with thymidine T. This pairing, which is shown in the following diagram, explained Chargaff's findings beautifully, and led them to suggest a double helix structure for DNA.

Before viewing this double helix structure itself, it is instructive to examine the base pairing interactions in greater detail. The G C association involves three hydrogen bonds colored pink , and is therefore stronger than the two-hydrogen bond association of A T.

These base pairings might appear to be arbitrary, but other possibilities suffer destabilizing steric or electronic interactions. By clicking on the diagram two such alternative couplings will be shown.

The C T pairing on the left suffers from carbonyl dipole repulsion, as well as steric crowding of the oxygens. The G A pairing on the right is also destabilized by steric crowding circled hydrogens.

A simple mnemonic device for remembering which bases are paired comes from the line construction of the capital letters used to identify the bases. A and T are made up of intersecting straight lines. In contrast, C and G are largely composed of curved lines. After many trials and modifications, Watson and Crick conceived an ingenious double helix model for the secondary structure of DNA. Two strands of DNA were aligned anti-parallel to each other, i. Complementary primary nucleotide structures for each strand allowed intra-strand hydrogen bonding between each pair of bases.

These complementary strands are colored red and green in the diagram. Coiling these coupled strands then leads to a double helix structure, shown as cross-linked ribbons in part b of the diagram. The double helix is further stabilized by hydrophobic attractions and pi-stacking of the bases. A space-filling molecular model of a short segment is displayed in part c on the right.

The helix shown here has ten base pairs per turn, and rises 3. This right-handed helix is the favored conformation in aqueous systems, and has been termed the B-helix. As the DNA strands wind around each other, they leave gaps between each set of phosphate backbones. Two alternating grooves result, a wide and deep major groove ca. Other molecules, including polypeptides, may insert into these grooves, and in so doing perturb the chemistry of DNA. Other helical structures of DNA have also been observed, and are designated by letters e.

A and Z. A model of a short DNA segment may be examined by. Click Here. Frieda Reichsman, Univ. In their announcement of a double helix structure for DNA, Watson and Crick stated, "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material. The essence of this suggestion is that, if separated, each strand of the molecule might act as a template on which a new complementary strand might be assembled, leading finally to two identical DNA molecules.

Indeed, replication does take place in this fashion when cells divide, but the events leading up to the actual synthesis of complementary DNA strands are sufficiently complex that they will not be described in any detail. As depicted in the following drawing, the DNA of a cell is tightly packed into chromosomes. First, the DNA is wrapped around small proteins called histones colored pink below. These bead-like structures are then further organized and folded into chromatin aggregates that make up the chromosomes.

An overall packing efficiency of 7, or more is thus achieved. Clearly a sequence of unfolding events must take place before the information encoded in the DNA can be used or replicated.

Once the double stranded DNA is exposed, a group of enzymes act to accomplish its replication. These are described briefly here:.

Topoisomerase : This enzyme initiates unwinding of the double helix by cutting one of the strands. Helicase : This enzyme assists the unwinding. Note that many hydrogen bonds must be broken if the strands are to be separated.. SSB : A single-strand binding-protein stabilizes the separated strands, and prevents them from recombining, so that the polymerization chemistry can function on the individual strands.

DNA Polymerase : This family of enzymes link together nucleotide triphosphate monomers as they hydrogen bond to complementary bases. These enzymes also check for errors roughly ten per billion , and make corrections. Ligase : Small unattached DNA segments on a strand are united by this enzyme. Polymerization of nucleotides takes place by the phosphorylation reaction described by the following equation.

Di- and triphosphate esters have anhydride-like structures and are consequently reactive phosphorylating reagents, just as carboxylic anhydrides are acylating reagents. Since the pyrophosphate anion is a better leaving group than phosphate, triphosphates are more powerful phosphorylating agents than are diphosphates. Formulas for the corresponding 5'-derivatives of adenosine will be displayed by Clicking Here , and similar derivatives exist for the other three common nucleosides.

The DNA polymerization process that builds the complementary strands in replication, could in principle take place in two ways.

Referring to the general equation above, R 1 could represent the next nucleotide unit to be attached to the growing DNA strand, with R 2 being this strand. Alternatively, these assignments could be reversed.

In practice, the former proves to be the best arrangement. Since triphosphates are very reactive, the lifetime of such derivatives in an aqueous environment is relatively short. However, such derivatives of the individual nucleosides are repeatedly synthesized by the cell for a variety of purposes, providing a steady supply of these reagents. In contrast, the growing DNA segment must maintain its functionality over the entire replication process, and can not afford to be changed by a spontaneous hydrolysis event.

As a result, these chemical properties are best accommodated by a polymerization process that proceeds at the 3'-end of the growing strand by 5'-phosphorylation involving a nucleotide triphosphate. This process is illustrated by the following animation, which may be activated by clicking on the diagram or reloading the page. The polymerization mechanism described here is constant. It always extends the developing DNA segment toward the 3'-end i. There is sometimes confusion on this point, because the original DNA strand that serves as a template is read from the 3'-end toward the 5'-end, and authors may not be completely clear as to which terminology is used.

Because of the directional demand of the polymerization, one of the DNA strands is easily replicated in a continuous fashion, whereas the other strand can only be replicated in short segmental pieces. This is illustrated in the following diagram. Separation of a portion of the double helix takes place at a site called the replication fork. As replication of the separate strands occurs, the replication fork moves away to the left in the diagram , unwinding additional lengths of DNA.

Since the fork in the diagram is moving toward the 5'-end of the red-colored strand, replication of this strand may take place in a continuous fashion building the new green strand in a 5' to 3' direction. This continuously formed new strand is called the leading strand. In contrast, the replication fork moves toward the 3'-end of the original green strand, preventing continuous polymerization of a complementary new red strand.

Short segments of complementary DNA, called Okazaki fragments, are produced, and these are linked together later by the enzyme ligase. This new DNA strand is called the lagging strand.

When you consider that a human cell has roughly 10 9 base pairs in its DNA, and may divide into identical daughter cells in 14 to 24 hours, the efficiency of DNA replication must be extraordinary. The procedure described above will replicate about 50 nucleotides per second, so there must be many thousand such replication sites in action during cell division.

A given length of double stranded DNA may undergo strand unwinding at numerous sites in response to promoter actions. The unraveled "bubble" of single stranded DNA has two replication forks, so assembly of new complementary strands may proceed in two directions. The polymerizations associated with several such bubbles fuse together to achieve full replication of the entire DNA double helix.

A cartoon illustrating these concerted replications will appear by clicking on the above diagram. Note that the events shown proceed from top to bottom in the diagram. One of the benefits of the double stranded DNA structure is that it lends itself to repair, when structural damage or replication errors occur. Several kinds of chemical change may cause damage to DNA:.

All these transformations disrupt base pairing at the site of the change, and this produces a structural deformation in the double helix.. Inspection-repair enzymes detect such deformations, and use the undamaged nucleotide at that site as a template for replacing the damaged unit.

These repairs reduce errors in DNA structure from about one in ten million to one per trillion. The genetic information stored in DNA molecules is used as a blueprint for making proteins. Why proteins? Because these macromolecules have diverse primary, secondary and tertiary structures that equip them to carry out the numerous functions necessary to maintain a living organism.

As noted in the protein chapter , these functions include:. The critical importance of proteins in life processes is demonstrated by numerous genetic diseases, in which small modifications in primary structure produce debilitating and often disastrous consequences. Such genetic diseases include Tay-Sachs, phenylketonuria PKU , sickel cell anemia, achondroplasia, and Parkinson disease.

The unavoidable conclusion is that proteins are of central importance in living cells, and that proteins must therefore be continuously prepared with high structural fidelity by appropriate cellular chemistry. Please check your Internet connection and reload this page.

If the problem continues, please let us know and we'll try to help. An unexpected error occurred. Previous Video 2. Monomers comprised of a pentose sugar, a phosphate group, and a nitrogen base.

Within the sugar molecule, the five carbons are numbered one through five prime, with each notation revealing what group attaches to it. For example, the second carbon can either be attached to a hydrogen atom, creating a deoxyribose sugar, as in deoxyribonucleic acid or DNA.

Alternatively, a hydroxyl group can occur in that space, forming a ribose sugar, as in ribonucleic acid, or RNA. To form the alternating phosphate sugar backbone, the five prime carbon is linked to the phosphate group. This connects to the three prime carbon of the next nucleotide, creating a chain via phosphodiester linkage.

This arrangement dictates directionality, the sequence in which nucleic acids are read. Five prime to three prime on a single strand. The last important carbon, one prime, is bound to a nitrogen base with one of two ring structures.

Pyrimidines, such as cytosine, thymine, and uracil, have a single organic ring, where as purines, adenine and guanine, contain double rings. Hydrogen bonds between nitrogen bases of two separate strands create the double-helix of DNA.

Guanine always links with cytosine, and adenine links with thymine. RNA contains uracil rather than thymine and remains a single strand. Nucleic acids are long chains of nucleotides linked together by phosphodiester bonds. DNA and RNA differ very slightly in their chemical composition, yet play entirely different biological roles.

Chemically, nucleic acids are polynucleotides—chains of nucleotides. A nucleotide is composed of three components: a pentose sugar, a nitrogen base, and a phosphate group. The sugar and the base together form a nucleoside.

Hence, a nucleotide is sometimes referred to as a nucleoside monophosphate. Each of the three components of a nucleotide plays a key role in the overall assembly of nucleic acids. As the name suggests, a pentose sugar has five carbon atoms, which are labeled 1 o , 2 o , 3 o , 4 o , and 5 o. The pentose sugar in RNA is ribose, meaning the 2 o carbon carries a hydroxyl group. The sugar in DNA is deoxyribose, meaning the 2 o carbon is attached to a hydrogen atom.

The sugar is attached to the nitrogen base at the 1 o carbon and the phosphate molecule at the 5 o carbon. The phosphate molecule attached to the 5 o carbon of one nucleotide can form a covalent bond with the 3 o hydroxyl group of another nucleotide, linking the two nucleotides together.

This covalent bond is called a phosphodiester bond. The phosphodiester bond between nucleotides creates an alternating sugar and phosphate backbone in a polynucleotide chain. Linking the 5 o end of one nucleotide to the 3 o end of another imparts directionality to the polynucleotide chain, which plays a key role in DNA replication and RNA synthesis.

At one end of the polynucleotide chain, called the 3 o end, the sugar has a free 3 o hydroxyl group.