What is the difference between nucleic acid and dna

And as you know that's where the DNA, one of the types of nucleic acids that we've been talking about, is predominately found.

Lawrence C. Brody, Ph. Featured Content. Introduction to Genomics. As each nucleotide is added to the growing chain, the transcription bubble and the heteroduplex moves with respect to the DNA template. Once RNA polymerase has transcribed the gene, transcription will terminate. For some genes, transcription termination is signalled by a particular sequence within the DNA, a terminator sequence, which RNA polymerase recognises. In some cases, RNA polymerase requires the help of other protein factors to recognise the terminator sequence.

Finally, many eukaryotic genes do not contain a specific terminator sequence; instead, termination of transcription is linked to other events, for example cleavage of the RNA prior to addition of the polyA tail. In prokaryotes, mRNA does not need processing before it can be translated, in fact, as will be discussed below, mRNA is translated as it is being made.

However, the initial transcript in eukaryotes does need to be processed to produce a functional mRNA that can be exported to the cytoplasm for translation. At many genes in prokaryotes, RNA polymerase can bind to the gene and initiate transcription without other protein factors.

However, for most prokaryotic genes, the binding of RNA polymerase to the gene is controlled by transcription factors to ensure the correct genes are transcribed at the correct level within the cell. Each gene will have a different promoter sequence and can be controlled by different transcription factors.

A good example of this type of promoter is the promoter that controls the lac operon in E. Transcription factors that up-regulate transcription are called activators and those that down-regulate transcription are called repressors. In this example RNA polymerase on its own can bind the promoter and drive low levels of transcription. If the repressor binds it will stop all transcription and would override RNA polymerase and the activator.

In the absence of the repressor, if the activator is present then it can drive high levels of transcription. The lac operon codes for genes required to use lactose and needs to be controlled in response to glucose and lactose concentrations. A repressor protein is responsible for responding to lactose concentration and an activator is responsible for responding to glucose Figure In the absence of lactose, the lac operon is kept in an off state by the repressor protein binding to the promoter and stopping transcription.

If lactose is present in the cell, it will bind to the repressor and this stops the repressor binding the promoter, RNA polymerase can bind and drive low levels of transcription.

If the cell is starved of glucose, the activator is turned on and this binds the promoter and helps RNA polymerase to initiate transcription, resulting in high rates of transcription. In the examples above, RNA polymerase on its own drives low levels of transcription. This might not be the case for all promoters, at some promoters RNA polymerase may not be able to bind and drive transcription without an activator protein.

At other promoters RNA polymerase on its own will be able to drive high levels of transcription and a repressor protein would be needed to turn off transcription. Control of transcription in eukaryotes has to occur on a chromosome which is condensed into chromatin Figure In addition, transcription requires the assembly of a large multiprotein complex at the gene.

In addition, there will be further control sequences, enhancers, that can be just upstream or several base pairs away from the core promoter.

In the absence of activator proteins the chromatin structure will stop RNA polymerase and the GTFs binding to the core promoter. Here histone proteins act as generic repressors of transcription. In order for a transcription to be turned on activators will bind the enhancers and recruit co-activators which open up the chromatin structure and ensure the core promoter is not blocked by histone proteins.

The activators and co-activators will then assemble RNA polymerase and the GTF at the core promoter and drive transcription initiation. Transcription factors will also ensure the chromatin structure across the whole gene is in a conformation that is suitable for transcription. A When a gene is in a silent state the surrounding DNA will be in condensed chromatin and the histones will epigenetic modifications which facilitate gene repression red spheres.

B A gene that is being transcribed will have activators bound to enhancer sequences, the activators recruit co-activators that acetylate the histone and add other epigenetic modifications that facilitate gene transcription green spheres. Repressors are not normally required to block assembly of transcription complex at the core promoter, however, they are important in the regulatory patterns needed in complex multicellular organisms.

Eukaryotes have repressor proteins which can block the action of a specific activator and ensure the activator is only active when required. Repressors can work in a number of ways including binding to DNA and blocking the binding of the activator to the DNA, stopping the activator interacting with other proteins required for transcription or by binding to the activator and keeping the activator in the cytoplasm. As discussed above transcription initiation in eukaryotes requires the opening up of the chromatin structure.

This is facilitated by co-activator proteins that can move the relative position of the nucleosomes Figure 4 with respect to the DNA and hence make certain regions of the DNA more accessible. They can also add chemical tags to both the histone proteins and DNA Figure These epigenetic modifications can affect whether a gene or genomic region is available for transcription or is transcriptionally silenced.

Histones are acylated by enzymes which transfer an acetyl functional group to from acetyl-coenzyme A to lysine residues in the histone protein. This is linked to activation of transcription because it reduces the positive charge on histones and therefore reduces their affinity for the negatively charged DNA.

Acetylation can also act as a tag that is recognised by other proteins that drive gene transcription.

This modification of the DNA is described as epigenetic because it affects gene expression rather than the genetic code itself. Conversely some repressor proteins will recruit co-repressors that deacetylate histones, increasing their affinity for DNA causing the chromatin to be highly condensed and leading to transcriptional silencing.

Methylation of lysine residues is another epigenetic tag, a single lysine residue can have 1, 2 or 3 methyl groups added. Unlike acetylation, methylation of lysine residues does not change the positive charge. The consequences of histone methylation are more complex because depending on which lysine residue is methylated and the level of methylation, the tag may mark that region of the genome for transcription activation or repression.

DNA methylation is another important epigenetic modification which leads to transcriptional silencing of the genomic region that has been methylated. During differentiation in the developing embryo whole regions of the genome will be methylated and therefore transcriptionally silenced.

The DNA methylation patterns are maintained during cell division and future generations of that cell. In the last 15 years, many techniques have been developed to allow us to study transcriptional control of genes within a cell.

In the case of eukaryotes this will also show how they have been spliced. It is also possible to analyse the binding of transcription factors and study epigenetic changes within histone proteins across the genome using techniques such as ChIP-Seq.

So, combining techniques such as RNA-Seq and ChIP-Seq we can determine when and where a protein factor is bound to DNA and study epigenetic changes in a particular cell type and the consequences in terms of gene transcription.

In combination these techniques give a detailed picture of the factors that affect transcription; this has been used, for example to look at differences between cancer cells and normal cells from the same patient.

Transcription factors and promoters play major roles in health and disease, below are just a few examples to give an idea of their role in health and disease. The transcription factor p53 is a tumour suppressor protein, it guards against cancer and some human cancers have mutations that knock out p53 function.

The drug Tamoxifen used in the treatment of breast cancer binds the oestrogen receptor inhibiting its function. The oestrogen receptor is a transcription factor that turns on the transcription of genes in response to oestrogen. Rett Syndrome is a neurodevelopmental disorder that affects approximately 1 in female births. It is due to mutations in a transcription factor that would normally repress transcription of specific genes, the mutations lead to inappropriate transcription of these genes.

Cocaine use results in changes in expression of many genes, this can include epigenetic changes within genes involved in cognition and brain function. These epigenetic changes can be inherited and there is evidence that cocaine use by a father can result in epigenetic changes that result in male, but not female, offspring being cocaine resistant. The key player in protein synthesis is the ribosome, a complex structure composed of RNA and proteins.

The ribosome provides a framework that ensures that the mRNA and tRNA are correctly positioned enabling the deciphering of the genetic code. There are many other proteins that are important in protein synthesis; some of these are part of the ribosome and some are again correctly positioned by the framework of the ribosome.

As we will see, the small subunit ribosomal RNA is a ribozyme; an RNA molecule with catalytic properties similar to those of enzymes. Ribosomal RNA can form a peptide bond between two amino acids.

The other nucleic acid that you need for protein synthesis is the tRNA. The tRNA molecule is single stranded and folds up into a characteristic structure by base pairing Figure These act as adaptor molecules, each has an anticodon for a specific mRNA codon and each carries the amino acid specified by that codon.

The anticodon has a complementary sequence to the codon on the mRNA. B Clover leaf representation of the secondary structure of tRNA.

The reaction also requires ATP, it is carried out in two steps:. Aminoacyl tRNA synthetase enzymes are highly specific, they recognise specific amino acids and will only attach them to the correct tRNA. This ensures correct coupling of amino acids and tRNA molecules which is just as important in ensuring the fidelity of protein synthesis as the matching of the anticodon to the codon by the ribosome.

In addition this step is said to activate the aminoacyl tRNA as it not only produces the correct substrate for the ribosome but also provides much of the energy required for peptide bond formation during protein synthesis. All living things contain ribosomes. The ribosomes in bacteria are slightly smaller than those found in eukaryotic cells Table 2 but the overall structure and the way in which they work are essentially the same.

The Nobel Prize for Chemistry was awarded to three scientists, Ada Yonath, Thomas Steitz and Venkatraman Ramakrishnan, who used X-ray crystallography to solve the three-dimensional structure of the bacterial ribosome.

The ribosome is composed of two subunits, the small subunit which reads the messenger RNA and the large subunit which forms the bonds between amino acids, adding them to the growing polypeptide chain. There are three important binding sites for tRNAs in the ribosome which are at the interface between the two subunits and only formed when the two subunits come together. These sites are shown on the image in Figure 15 , they are referred to as the acceptor or aminoacyl A site, the peptidyl P site where the peptide bond between amino acids is formed and the exit E site from which spent tRNAs leave the ribosome.

In addition to the ribosome, the mRNA and tRNA, there are a number of small proteins that are not part of the structure of the ribosome, but are required for protein synthesis: initiation factors, elongation factors and termination factors. This serious neurodegenerative disease which results in lesions in the white matter in the brain is due to mutations in one of the initiation factors.

During protein synthesis the ribosome brings together the amino acid charged tRNA and the mRNA, the codon and anticodon are matched and the amino acids are joined together in the correct sequence. There are three phases to this process: initiation where the ribosome assembles on the mRNA, elongation where the triplet code is read and amino acids are added to the growing peptide chain and termination where protein synthesis stops.

The mRNA is then exported to the cytoplasm where it recruits initiation factors, tRNA charged with a methionine and the small 40S ribosomal subunit. This is recognised by the anticodon codon of the initiator tRNA, the large subunit then docks to give the translation complex. A During initiation, the mRNA recruits a tRNA charged with a methionine and the small ribosomal subunit, B the large subunit then docks to give the translation complex, C a tRNA with an amino acid attached enters the A site, D the peptide bond is formed between the amino acid in the P site and the one in the A site.

E Finally, everything moves along the mRNA by one codon in a process called translocation so the peptidyl tRNA with the growing peptide chain attached moves to the P site and the spent tRNA to the E site from where it leaves the ribosome. F When a stop codon is in the A site, a termination or release factor enters the A site, G the peptide is released from the ribosome and H the two subunits of the ribosome disassociate and are recycled.

With initiation complete, the mRNA is in the correct reading frame with the A site empty and the next codon exposed. In the elongation phase an aminoacyl tRNA, one charged with an amino acid, is brought to the ribosome in a complex with an elongation factor and enters the A site. If the anticodon it carries is complementary to the exposed codon it is correctly positioned in the acceptor site and GTP is hydrolysed on the elongation factor Figure 16 C.

A peptide bond Figure 17 is then formed between the C terminus of the amino acid in the P site and the N terminus of the amino acid in the A site, this reaction is catalysed in the peptidyl transfer centre of the large subunit of the ribosome.

Finally, the peptidyl tRNA with the growing peptide chain attached moves to the P site. The spent tRNA moves to the exit site from where it can leave the ribosome. During the elongation phase the ribosome cycles through this process, adding amino acids to the growing peptide chain until a stop codon is exposed in the A site. The new protein emerges from the ribosome through an exit tunnel in the large subunit. A Amino acids consist of a carbon atom with an amine group the N terminus , a carboxylic acid group the C terminus and a variable R group.

The simplest R group is a methyl group giving the amino acid alanine. B When two amino acids are joined together a peptide bond is formed between the N terminus of one amino acid and the C terminus of another.

This is a condensation reaction releasing one molecule of water. The stop codon is not decoded by being recognised by an anticodon on a tRNA. Instead it is detected by proteins called termination or release factors. In eukaryotes there is a single release factor RF1 that recognises all three stop codons enters the A site Figure 16 F. The ester bond linking the peptide chain to the tRNA in the P site is broken and the peptide is released from the ribosome Figure 16 G The two subunits of the ribosome disassociate and are recycled Figure 16 H.

The structure and function of ribosomes are highly conserved with a large core of structurally conserved proteins and rRNAs found in both eukaryotic and prokaryotic ribosomes. However, there are some differences both in the rRNAs and in some of the additional proteins involved in translation Table 2. The elongation phase is highly conserved but there are important differences in how protein synthesis is initiated. Bacterial mRNAs have a specific sequence called the ribosome binding site or Shine—Dalgarno sequence.

In order to ensure that the mRNA is correctly positioned in the ribosome the Shine—Dalgarno sequence binds to a complementary sequence of the 16S rRNA in the small subunit. Differences between the structure of bacterial and eukaryotic ribosomes can be exploited by antibiotics which are selective in that they affect protein synthesis in bacteria but not in mammalian cells. Macrolide antibiotics like erythromycin, block the exit tunnel in the large subunit of bacterial ribosomes and halt protein synthesis.

The exit tunnel in eukaryotic ribosomes is slightly narrower which means that eukaryotic ribosomes are not affected. Streptomycin, an important antibiotic in the treatment of tuberculosis binds to the 16S of bacterial ribosomes. This distorts the structure of the decoding site and results in misreading of the mRNA.

Protein synthesis can proceed very quickly, particularly in rapidly growing cells or those that are differentiating. In bacteria between 15 and 20, new peptide bonds can be formed per second. In eukaryotes it is slower, more like five peptide bonds per second. A small human protein like insulin would take only 10 seconds to make whereas the largest human protein titin, which is found in human muscle cells, takes about an hour and a half per molecule. One of the mechanisms that ensures that protein synthesis is carried out efficiently is the polyribosome.

As soon as one ribosome has started translation another ribosome binds to initiate synthesis of another protein copy. This gives rise to polyribosomes or polysomes which can be seen by electron microscopy. Sometimes these polyribosomes can form circular structures so that, as soon as the ribosome has finished synthesis of one polypeptide it can rebind the same mRNA molecule and start synthesis of another copy of the protein.

Cyro-electron micrograph reconstruction of eukaryotic polyribosome. Reprinted from Myasnikov by permission.

The study of nucleic acids, from their first identification as the genetic material is littered with landmarks in molecular biosciences, many of them marked with Nobel Prizes. The topics introduced in this article are important topics covered in all bioscience programmes; understanding them is key to all areas of biosciences from evolution and animal diversity to health and disease.

Recent developments in the techniques that we can use to study DNA, often in living cells means that new and exciting developments in our understanding of the way nucleic acids work are occurring all the time. Given the scope of this article we have barely scratched the surface of the topic, however, the reader can find more detail from the articles in the bibliography below and even more detail from a few minutes searching on the internet.

The authors declare that there are no competing interests associated with the manuscript. National Center for Biotechnology Information , U. Essays in Biochemistry. Essays Biochem.

Published online Oct Steve Minchin and Julia Lodge. Author information Article notes Copyright and License information Disclaimer. Steve Minchin: ku. Correspondence: Steve Minchin ku. This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution License 4.

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 euchromatin usually contains genes that are transcribed, with DNA packaged around nucleosomes but not further compacted.

RNA is the nucleic acid that makes proteins from the code provided by DNA through the processes of transcription and translation.

DNA is the genetic material found in all living organisms and is found in the nucleus of eukaryotes and in the chloroplasts and mitochondria. In prokaryotes, the DNA is not enclosed in a membranous envelope. Each nucleotide is made up of three components: a nitrogenous base, a pentose five-carbon sugar called ribose, and a phosphate group.

RNA Structure : A nucleotide is made up of three components: a nitrogenous base, a pentose sugar, and one or more phosphate groups. Adenine A , guanine G , and cytosine C are present, but instead of thymine T , a pyrimidine called uracil U pairs with adenine. The DNA molecules never leave the nucleus but instead use an intermediary to communicate with the rest of the cell.

This is called transcription. The mRNA then carries the code out of the nucleus to organelles called ribosomes for the assembly of proteins. Once the mRNA has reached the ribosomes, they do not read the instructions directly. It then reads the sequence in sets of three bases called codons.

Each possible three letter arrangement of A,C,U,G e. The ribosome acts like a giant clamp, holding all of the players in position, and facilitating both the pairing of bases between the messenger and transfer RNAs, and the chemical bonding between the amino acids. These subunits do not carry instructions for making a specific proteins i. Privacy Policy. Skip to main content.