Molecular Structure Of Nucleic Acids Watson And Crick Pdf
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- Molecular structure of nucleic acids
- Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid
- Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid
- Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid
WE wish to suggest a structure for the salt of deoxyribose nucleic acid D.
Watson and F. Crick 1 April 25, 2 , Nature 3 , , We wish to suggest a structure for the salt of deoxyribose nucleic acid D. This structure has novel features which are of considerable biological interest. A structure for nucleic acid has already been proposed by Pauling 4 and Corey 1.
Molecular structure of nucleic acids
Steve Minchin, Julia Lodge; Understanding biochemistry: structure and function of nucleic acids. Essays Biochem 16 October ; 63 4 : — Nucleic acids, deoxyribonucleic acid DNA and ribonucleic acid RNA , carry genetic information which is read in cells to make the RNA and proteins by which living things function. The well-known structure of the DNA double helix allows this information to be copied and passed on to the next generation.
In this article we summarise the structure and function of nucleic acids. The article includes a historical perspective and summarises some of the early work which led to our understanding of this important molecule and how it functions; many of these pioneering scientists were awarded Nobel Prizes for their work.
We explain the structure of the DNA molecule, how it is packaged into chromosomes and how it is replicated prior to cell division. We look at how the concept of the gene has developed since the term was first coined and how DNA is copied into RNA transcription and translated into protein translation. Deoxyribonucleic acid DNA is one of the most important molecules in living cells.
It encodes the instruction manual for life. Genome is the complete set of DNA molecules within the organism, so in humans this would be the DNA present in the 23 pairs of chromosomes in the nucleus plus the relatively small mitochondrial genome. Humans have a diploid genome, inheriting one set of chromosomes from each parent.
A complete and functioning diploid genome is required for normal development and to maintain life. He wanted to determine the chemical composition of leucocytes white blood cells , his source of leucocytes was pus from fresh surgical bandages. Almost all molecular bioscience graduates would have repeated a form of this experiment in laboratory classes where DNA is isolated from cells. Kossel went on to show that nucleic acid contained purine and pyrimidine bases, a sugar and phosphate. Work in the s from many scientists further characterised nucleic acids including the identification of the four bases and the presence of deoxyribose, hence the name deoxyribonucleic acid DNA.
Erwin Chargaff had found that DNA molecules from a particular species always contained the same amount of the bases cytosine C and guanine G and the same amount of adenosine A and thymine T. DNA is a polymer made of monomeric units called nucleotides Figure 1 A , a nucleotide comprises a 5-carbon sugar, deoxyribose, a nitrogenous base and one or more phosphate groups. The building blocks for DNA synthesis contain three phosphate groups, two are lost during this process, so the DNA strand contains one phosphate group per nucleotide.
A A nucleotide guanosine triphosphate. A nucleoside is a base linked to a sugar. A nucleotide is a nucleoside with one or more phosphate groups. There are four different bases in DNA, the double-ring purine bases: adenine and guanine; and the single-ring pyrimidine bases: cytosine and thymine Figure 1 B. The phosphate group is acidic, hence the name nucleic acid. This linkage is called a phosphodiester bond. Although many scientists, including Miescher, had observed that prior to cell division the amount of nucleic acid increased, it was not believed to be the genetic material until the work of Fredrick Griffith, Oswald Avery, Colin MacLeod and Maclyn McCarty.
In , Griffith showed that living cells could be transformed by extracts from heat-killed cells and that this transformation had the potential to permanently change the genetic makeup of the recipient cell. Griffith was working with two strains of the bacterium Streptococcus pneumoniae. The encapsulated so-called S strain is virulent, whereas the non-capsulated R strain is nonvirulent.
If the S strain is injected subcutaneously into mice, the mice die, whereas, if either live R strain is injected or heat-killed S strain is injected, the mouse lives. However, if a mixture of live R strain and heat-killed S strain is injected into a mouse, the mouse will die, and live S strain can be isolated from the blood. So, in the Griffith experiment a component of the heat-killed S strain is transforming the R strain. They isolated a crude DNA extract from the S strain and destroyed any protein, lipid, carbohydrate and ribonucleic acid RNA component and showed that this purified DNA could still transform the R strain.
They used a virus that infects bacteria called a bacteriophage. The bacteriophage contains a protein capsid surrounding a DNA molecule. They showed that when bacteriophage T2 infects Escherichia coli , it is the phage DNA, not protein, that enters the bacterial cell.
Once it had been shown that DNA was the genetic material, there was a race to determine the three-dimensional structure of the DNA molecule. Their work was published in Nature in The Watson—Crick structure is shown in Figure 2 A. A The DNA double helix, with the sugar phosphate backbone on the outside and the nitrogenous bases in the middle.
In this figure, the atoms on the upper edge of the base pair face into the major groove and those facing lower edge face into the minor groove. The hydrogen bonds between the base pairs are indicated by the dotted line. DNA is a two-stranded helical structure, the two strands run in opposite directions. The helix is right-handed which means that if you are looking down the axis, the helix turns clockwise as it gets further away from you. The two chains interact via hydrogen bonds between pairs of bases with adenine always pairing with thymine, and guanine always pairing with cytosine.
The Watson—Crick structure therefore accounts for and explains the Chargaff data which showed that there was always an equal amount of C and G and of A and T. The diameter of the helix is 2 nm, adjacent bases are separated by 0.
DNA molecules are normally very long and the sequence of bases along the DNA chain is not restricted. For example, the genome of the bacterium E. The human genome is made up of 24 distinct chromosomes, chromosomes 1—22 and the X and Y chromosomes present in the nucleus plus mitochondrial DNA.
Within a single human diploid cell, which contains 23 chromosome pairs there is 2 m of DNA. Based on the assumption that humans contain 3 trillion cells with a nucleus, if all the DNA from a single human individual was put end to end, it would reach to the sun and back approximately 20 times.
The building blocks are nucleotides containing the 5-carbon sugar ribose, a phosphate and a nitrogenous base. RNA contains four bases adenine, guanine, cytosine and uracil. RNA is more labile easily broken down than DNA and most RNA molecules do not form stable secondary structures, some notable exceptions will be discussed below. An RNA strand containing the four nucleotides with the nitrogenous bases: adenine A , cytosine C , guanine G and uracil U respectively. DNA has to be highly condensed to fit into the bacterial cell or eukaryotic nucleus.
In eukaryotes, histone proteins are used to condense the DNA into chromatin. The basic structure of chromatin is the nucleosome, a nucleosome contains DNA wrapped almost two times around the histone octamer comprising two copies each of the histone proteins H2A, H2B, H3 and H4 Figure 4.
Further levels of compaction are required to fit the DNA into the nucleus Figure 4 , the nucleosomes are folded upon themselves to form the nm fibre, this is then folded again to form the nm fibre and during mitosis further compaction can occur forming the chromatid which is nm in diameter.
During mitosis there is further compaction not shown. Processes such as DNA replication and DNA transcription need to occur in the chromatin environment and because of the level of compaction, this acts as a barrier to proteins that need to interact with DNA.
Therefore, chromatin structure plays an important role in processes such as regulation of gene expression in eukaryotes. DNA and the histone proteins can be chemically modified, these are called epigenetic modifications as they do not change the DNA sequence, however, they can be passed on during cell division and to subsequent generations, a process known as epigenetic inheritance.
As these epigenetic modifications can alter the chromatin structure they regulate gene transcription and can affect the phenotype. Epigenetics plays key roles in many processes, including development, cancer and behaviour and addiction. This will be discussed further later in this article.
Nuclear organisation plays an important role in many biological processes including regulation of gene transcription. In recent years the development of several techniques, including microscopy, have allowed us to gain an understanding of the way the genome is organised in 3D. Individual chromosomes are not randomly spaced within the nucleus; each chromosome has a distinct territory.
Actively transcribed regions from different chromosomes are often close to each other and near the interior of the nucleus, whereas, inactive genes are on the periphery or near a special area called the nucleolus where ribosomal RNA is transcribed.
Whenever a cell divides there is a need to synthesise two copies of each chromosome present within the cell. For example in a human, prior to cell division, all 23 pairs of chromosomes need to be replicated to form 46 pairs, so that following cell division each daughter cell has a full complement 23 pairs of chromosomes. The evidence that DNA replication was semi-conservative came from an elegant experiment completed by Matthew Meselson and Franklin Stahl.
They then grew the bacteria, in a medium that contained 14 NH 4 Cl, in conditions such that any newly synthesised DNA would contain 14 N.
This was shown by analysing the density of the DNA using density-gradient centrifugation. As predicted, they observed that the new daughter DNA molecule had a density consistent with the fact that it contained both 15 N and 14 N and that this daughter DNA contained one strand with 15 N and another strand with 14 N.
It cannot synthesise DNA in the absence of a template. DNA polymerase catalyses the formation of a phosphodiester bond. B The chemical reaction during the formation of a phosphodiester bond, showing the addition of a nucleotide containing guanine and the release of pyrophosphate.
Pyrophosphate is the two phosphate residues within the deoxynucleoside triphosphate building block that are not incorporated into the DNA chain. This is analogous to using the delete key to remove a letter that you have typed incorrectly before adding the correct one and continuing typing.
DNA polymerase will then extend from the primer copying the template and synthesising the daughter DNA strand. A large multiprotein complex, called the replisome, is responsible for DNA replication. In prokaryotes, two replisomes form at a specific point on the chromosome called the Origin of Replication ori. The two replisomes then travel in opposite directions around the circular prokaryotic chromosome, each replisome forming a replication fork, a schematic representation of one replication fork is shown in Figure 6.
A single replication fork showing the leading and lagging strands. The lagging strand is synthesised discontinuously, in short Okazaki fragments bases in prokaryotes and bases in eukaryotes. Each Okazaki fragment will be started with an RNA primer and is synthesised in the opposite direction to the movement of the replication fork. In prokaryotes, Okazaki fragments are — bases in length. In Figure 6 you will see that the DNA polymerase synthesising the Okazaki fragment will eventually reach the primer for the previous Okazaki fragment.
When all the primer has been removed, there will be two DNA strands adjacent to each other but not joined by a phosphodiester bond, these two strands are joined together by the enzyme DNA ligase.
The replisome contains a number of other important proteins required for DNA replication. Single-stranded binding proteins SSBs bind the lagging strand template to stabilise and protect the single-stranded DNA.
The two replication forks that form at the ori will move in opposite directions around the circular prokaryotic genome until they reach the terminator sequence, ter , which is on the opposite side of the genome compared with the ori , i.
This results in the complete replication of the genome. Once DNA replication has been completed a post-replication DNA repair process will correct errors that were not corrected by the proofreading activity of DNA polymerase.
The fidelity of DNA replication is extremely high, resulting in an error rate of 1 mistake per 10 9 —10 10 nucleotides added.
Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid
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MOLECULAR STRUCTURE OF. NUCLEIC A structure for nucleic acid has already been proposed by the salt of deoxyribose nucleic acid. This structure has two helical chains each coiled round the same J. D. 'WATSON. F. H. C. CRICK.
Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid
From the DNA double helix model, it was clear that there must be some correspondence between the linear sequences of nucleotides in DNA molecules to the linear sequences of amino acids in proteins. Other advances in molecular biology stemming from the discovery of the DNA double helix eventually led to ways to sequence genes. Next, Watson and Crick describe is the specific identity of each base in a base pair. In particular, in late , Franklin had submitted a progress report to the Medical Research Council, which was reviewed by Max Perutz, then at the Cavendish Laboratory of the University of Cambridge. Later in , Watson and Crick wrote a second paper, "Genetical Implications of the Structure of Deoxyribonucleic Acid," that addressed how DNA might self-replicate to pass on the genetic information encoded within it.
Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid
Steve Minchin, Julia Lodge; Understanding biochemistry: structure and function of nucleic acids. Essays Biochem 16 October ; 63 4 : — Nucleic acids, deoxyribonucleic acid DNA and ribonucleic acid RNA , carry genetic information which is read in cells to make the RNA and proteins by which living things function. The well-known structure of the DNA double helix allows this information to be copied and passed on to the next generation. In this article we summarise the structure and function of nucleic acids. The article includes a historical perspective and summarises some of the early work which led to our understanding of this important molecule and how it functions; many of these pioneering scientists were awarded Nobel Prizes for their work.
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Steve Minchin, Julia Lodge; Understanding biochemistry: structure and function of nucleic acids. Essays Biochem 16 October ; 63 4 : — Nucleic acids, deoxyribonucleic acid DNA and ribonucleic acid RNA , carry genetic information which is read in cells to make the RNA and proteins by which living things function. The well-known structure of the DNA double helix allows this information to be copied and passed on to the next generation. In this article we summarise the structure and function of nucleic acids.
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