Structure And Function Of Dna Essay

In this article we will discuss about:- 1. Meaning of DNA 2. Features of DNA 3. Molecular Structure 4. Components 5. Forms.

Contents:

  1. Essay on the Meaning of DNA
  2. Essay on the Features of DNA
  3. Essay on the Molecular Structure of DNA
  4. Essay on the Components of DNA
  5. Essay on the Forms of DNA


Essay # 1. Meaning of DNA:

A nucleic acid that carries the genetic information in the cell and is capable of self-replication and RNA synthesis is referred to as DNA. In other words, DNA refers to the molecules inside cells that carry genetic information and pass it from one generation to the next. The scientific name for DNA is deoxyribonucleic acid.

Essay # 2. Features of DNA:

The main features of DNA are given below:

i. Location:

In eukaryotes, DNA is found both in nucleus and cytoplasm. In the nucleus it is a major component of chromosome, whereas in cytoplasm it is found in mitochondria and chloroplasts. In prokaryotes, it is found in the cytoplasm.

ii. Structure:

Mostly the DNA structure is double stranded in both eukaryotes and prokaryotes. However, in some viruses DNA is single stranded. DNA is a double stranded molecule held together by weak bonds between base pairs of nucleotides. The four nucleotides in DNA contain the bases: adenine (A), guanine (G), cytosine (C), and thymine (T).

iii. Shape:

In eukaryotes, the DNA is of linear shape. In prokaryotes and mitochondria; the DNA is circular.

iv. Replication:

The DNA is capable of self-replication. This is the only chemical which has self-replicating capacity. The DNA replicates in semi-conservative manner. In eukaryotes, DNA replicates during the S-Phase of the cell cycle.

v. Types:

There are different forms of DNA such as A, B, C, D and Z DNA. The DNA may be linear or circular. It may be double stranded or single stranded. It may be right handed or left handed. The DNA may be repetitive or unique. It may be nuclear or cytoplasmic.

vi. Functions:

DNA plays important role in various ways. It is used in transcription i.e. synthesis of mRNA which in turn is used in protein synthesis. It carries genetic information from one generation to the next generation. DNA stores and transmits the genetic information in cells.

It forms the basis for genetic code. The genes are made of DNA and are responsible for passing on traits from generation to generation. DNA contains the genetic instructions for the development and functioning of living organisms. Thus it is the substance of heredity.

Essay # 3. Molecular Structure of DNA:

The double helical structure of DNA was identified by Watson and Crick in 1953. This brilliant research work resulted in significant breakthrough in understanding the gene function. This structure has been verified in many different ways and is universally accepted. James Watson and Francis Crick were awarded Nobel prize in 1958 for this significant contribution in the field of molecular biology.

The important features of their model of DNA are as follows:

1. Two helical polynucleotide chains are coiled around a common axis, the chains run in opposite directions.

2. The purine and pyrimidine bases are on the inside of the helix whereas the phosphate and deoxyribose units are situated on the outside. Also the planes of the base residues are perpendicular to the helix axis. While the planes of the sugar residues are almost at right angles to those of the bases.

3. The diameter of the helix is 2 nm. Adjacent bases are separated by 0.34 nm along the helix axis. Hence the helix repeats itself every 10 residues on each chain at intervals of 3.4 nm.

4. The two chains are held together by hydrogen bonds formed between pairs of bases. Pairing is highly specific. Adenine pairs with thymine, guanine always pairs with cytosine. A = T, G = C.

5. The sequence of bases along the polynucleotide chain is not restricted. The precise sequence of bases carries the genetic information.

6. The sugar-phosphate backbones of the two DNA strands wind around the helix axis like the railing of a spiral staircase.

7. The bases of the individual nucleotides are on the inside of the helix, stacked on top of each other like the steps of a spiral staircase.

Essay # 4. Components of DNA:

DNA molecule is a polymer which is composed of several thousand pairs of nucleotide monomers. Union of several nucleotides together leads to the formation of polynucleotide chain. The monomer units of DNA are nucleotides, and the polymer is known as a “polynucleotide.” Each nucleotide consists of a 5-carbon sugar (deoxyribose), a nitrogen containing base attached to the sugar, and a phosphate group.

There are three components of DNA, viz:

(1) Nitrogenous bases,

(2) Deoxyribose sugar, and

(3) Phosphate group.

These are briefly discussed below.

i. Nitrogenous Bases:

Nucleotides are also known as nitrogenous bases or DNA bases. Nitrogenous base are of two types, viz. pyrimidines and purines.

Pyrimidines:

Main features of pyrimidines are given below:

(i) These are single ring structures,

(ii) These are of two types namely cytosine and thymine,

(iii) They occupy less space in DNA structure,

(iv) Pyrimidine is linked with deoxyribose sugar at position 3.

Purine:

Main features or purines are given below:

(i) They are double ring compounds.

(ii) They are of two types, viz. adenine and guanine.

(iii) They occupy more space in DNA structure.

(iv) Deoxyribose sugar is linked at position 9 of purine.

Thus, in DNA there are four different types of nitrogenous bases, viz. adenine (A), guanine (G), cytosine (C) and thymine (T). In RNA, the pyrimidine base thymine is replace by uracil.

Base Pairing:

The purine and pyrimidine bases always pair in a definite fashion. Adenine will always pair with thymine and guanine with cytosine. Adenine and thymine are joined by double hydrogen bonds while guanine and cytosine are joined by triple hydrogen bonds. However, these bonds are weak which help in separation of DNA strands during replication.

ii. Deoxyribose Sugar:

This is a pentose sugar having five carbon atoms. The four carbon atoms are inside the ring and the fifth one is with CH2 group. This has three OH groups on 1, 3 and 5 carbon positions. Hydrogen atoms are attached to carbon atoms one to four. In RNA, the sugar ribose is similar to deoxyribose except that it has OH group on carbon atom 2 instead of H group.

iii. Phosphate:

The phosphate molecule is arranged in an alternate manner to deoxyribose molecule. Thus there is deoxyribose on both sides of phosphate. The phosphate is joined with carbon atom 3 of deoxyribose at one side and with carbon atom 5 of deoxyribose on the other side.

Nucleosides and Nucleotides:

A combination of deoxyribose sugar and nitrogenous base is known as nucleoside and a combination of nucleoside and phosphate is called nucleotide.

Nucleoside = Deoxyribose Sugar + Nitrogenous base

Nucleotide = Deoxyribose + Nitrogenous base + Phosphate

Thus, a nucleotide is a nucleoside with one or more phosphate groups covalently attached to it. Nucleosides differ from nucleotides in that they lack phosphate groups. The four different nucleosides of DNA are deoxyadenosine (dA), deoxyguanosine (dG), deoxycytosine (dC), and deoxythymidine (dT).

DNA Backbone:

The DNA backbone is a polymer with an alternating sugar-phosphate sequence. The deoxyribose sugars are joined at both the 3′-hydroxyl and 5′-hydroxyl groups to phosphate groups in ester links, also known as “phosphodiester” bonds.

Essay # 5. Forms of DNA:

Depending upon the nucleotide base per turn of the helix, pitch of the helix, tilt of the base pair and humidity of the sample, the DNA can be observed in four different forms namely, A, B, C and D. The comparison of A, B and Z forms of DNA is presented in Table 15.1.

i. B-form:

This is the same form of DNA proposed by Watson and Crick.

Main features of B form of DNA are given below:

1. This is the most common form of DNA.

2. It is observed when humidity is 92% and salt concentration is high.

3. The coiling is in the right direction.

4. The number of base is 10 per turn of helix.

5. The pitch is 3.4 nm.

6. The sugar phosphate linkage is normal.

7. The helix is narrower and more elongated than A form.

8. The major groove is wide which is easily accessible to proteins.

9. The minor groove is narrow.

10. The conformation is favored at high water concentrations.

11. The base pairing is nearly perpendicular to helix axis.

12. The sugar puckering is C2′-endo.

ii. A-form:

1. This form is observed when the humidity of the sample is 75%.

2. The coiling is in the right direction.

3. The number of bases is 10.7 per turn of helix.

4. The pitch is 2.8 nm.

5. The sugar phosphate linkage is normal.

6. The major groove is deep and narrow which is not easily accessible to proteins.

7. The minor groove is wide and shallow which is accessible to proteins, but information content is lower than major groove.

8. The helix is shorter and wider than B form.

9. The conformation is favored at low water concentrations.

10. The base pairs are tilted to helix axis.

11. The sugar puckering is C3′-endo.

iii. Z-form:

1. The helix has left-handed coiling pattern.

2. The number of bases is 12 per turn of helix.

3. The pitch is 4.5 nm.

4. The sugar phosphate linkage is zigzag.

5. The major “groove” is not really a groove.

6. The minor groove is narrow.

7. The helix is narrower and more elongated than A or B form.

8. The base pairing is nearly perpendicular to helix axis.

9. The conformation is favored by high salt concentrations.

10. The sugar puckering is C: C2 endo, G : C2 exo.


Biologists in the 1940s had difficulty in accepting DNA as the genetic material because of the apparent simplicity of its chemistry. DNA was known to be a long polymer composed of only four types of subunits, which resemble one another chemically. Early in the 1950s, DNA was first examined by x-ray diffraction analysis, a technique for determining the three-dimensional atomic structure of a molecule (discussed in Chapter 8). The early x-ray diffraction results indicated that DNA was composed of two strands of the polymer wound into a helix. The observation that DNA was double-stranded was of crucial significance and provided one of the major clues that led to the Watson-Crick structure of DNA. Only when this model was proposed did DNA's potential for replication and information encoding become apparent. In this section we examine the structure of the DNA molecule and explain in general terms how it is able to store hereditary information.

A DNA Molecule Consists of Two Complementary Chains of Nucleotides

A DNAmolecule consists of two long polynucleotide chains composed of four types of nucleotide subunits. Each of these chains is known as a DNA chain, or a DNA strand. Hydrogen bonds between the base portions of the nucleotides hold the two chains together (Figure 4-3). As we saw in Chapter 2 (Panel 2-6, pp. 120-121), nucleotides are composed of a five-carbon sugar to which are attached one or more phosphate groups and a nitrogen-containing base. In the case of the nucleotides in DNA, the sugar is deoxyribose attached to a single phosphate group (hence the name deoxyribonucleic acid), and the base may be either adenine (A), cytosine (C), guanine (G), or thymine (T). The nucleotides are covalently linked together in a chain through the sugars and phosphates, which thus form a “backbone” of alternating sugar-phosphate-sugar-phosphate (see Figure 4-3). Because only the base differs in each of the four types of subunits, each polynucleotide chain in DNA is analogous to a necklace (the backbone) strung with four types of beads (the four bases A, C, G, and T). These same symbols (A, C, G, and T) are also commonly used to denote the four different nucleotides—that is, the bases with their attached sugar and phosphate groups.

Figure 4-3

DNA and its building blocks. DNA is made of four types of nucleotides, which are linked covalently into a polynucleotide chain (a DNA strand) with a sugar-phosphate backbone from which the bases (A, C, G, and T) extend. A DNA molecule is composed of two (more...)

The way in which the nucleotide subunits are lined together gives a DNA strand a chemical polarity. If we think of each sugar as a block with a protruding knob (the 5′ phosphate) on one side and a hole (the 3′ hydroxyl) on the other (see Figure 4-3), each completed chain, formed by interlocking knobs with holes, will have all of its subunits lined up in the same orientation. Moreover, the two ends of the chain will be easily distinguishable, as one has a hole (the 3′ hydroxyl) and the other a knob (the 5′ phosphate) at its terminus. This polarity in a DNA chain is indicated by referring to one end as the 3end and the other as the 5end.

The three-dimensional structure of DNA—the double helix—arises from the chemical and structural features of its two polynucleotide chains. Because these two chains are held together by hydrogen bonding between the bases on the different strands, all the bases are on the inside of the double helix, and the sugar-phosphate backbones are on the outside (see Figure 4-3). In each case, a bulkier two-ring base (a purine; see Panel 2-6, pp. 120–121) is paired with a single-ring base (a pyrimidine); A always pairs with T, and G with C (Figure 4-4). This complementary base-pairing enables the base pairs to be packed in the energetically most favorable arrangement in the interior of the double helix. In this arrangement, each base pair is of similar width, thus holding the sugar-phosphate backbones an equal distance apart along the DNA molecule. To maximize the efficiency of base-pair packing, the two sugar-phosphate backbones wind around each other to form a double helix, with one complete turn every ten base pairs (Figure 4-5).

Figure 4-4

Complementary base pairs in the DNA double helix. The shapes and chemical structure of the bases allow hydrogen bonds to form efficiently only between A and T and between G and C, where atoms that are able to form hydrogen bonds (see Panel 2-3, pp. 114–115) (more...)

Figure 4-5

The DNA double helix. (A) A space-filling model of 1.5 turns of the DNA double helix. Each turn of DNA is made up of 10.4 nucleotide pairs and the center-to-center distance between adjacent nucleotide pairs is 3.4 nm. The coiling of the two strands around (more...)

The members of each base pair can fit together within the double helix only if the two strands of the helix are antiparallel—that is, only if the polarity of one strand is oriented opposite to that of the other strand (see Figures 4-3 and 4-4). A consequence of these base-pairing requirements is that each strand of a DNAmolecule contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand.

The Structure of DNA Provides a Mechanism for Heredity

Genes carry biological information that must be copied accurately for transmission to the next generation each time a cell divides to form two daughter cells. Two central biological questions arise from these requirements: how can the information for specifying an organism be carried in chemical form, and how is it accurately copied? The discovery of the structure of the DNAdouble helix was a landmark in twentieth-century biology because it immediately suggested answers to both questions, thereby resolving at the molecular level the problem of heredity. We discuss briefly the answers to these questions in this section, and we shall examine them in more detail in subsequent chapters.

DNA encodes information through the order, or sequence, of the nucleotides along each strand. Each base—A, C, T, or G—can be considered as a letter in a four-letter alphabet that spells out biological messages in the chemical structure of the DNA. As we saw in Chapter 1, organisms differ from one another because their respective DNA molecules have different nucleotide sequences and, consequently, carry different biological messages. But how is the nucleotide alphabet used to make messages, and what do they spell out?

As discussed above, it was known well before the structure of DNA was determined that genes contain the instructions for producing proteins. The DNA messages must therefore somehow encode proteins (Figure 4-6). This relationship immediately makes the problem easier to understand, because of the chemical character of proteins. As discussed in Chapter 3, the properties of a protein, which are responsible for its biological function, are determined by its three-dimensional structure, and its structure is determined in turn by the linear sequence of the amino acids of which it is composed. The linear sequence of nucleotides in a gene must therefore somehow spell out the linear sequence of amino acids in a protein. The exact correspondence between the four-letter nucleotide alphabet of DNA and the twenty-letter amino acid alphabet of proteins—the genetic code—is not obvious from the DNA structure, and it took over a decade after the discovery of the double helix before it was worked out. In Chapter 6 we describe this code in detail in the course of elaborating the process, known as gene expression, through which a cell translates the nucleotide sequence of a gene into the amino acid sequence of a protein.

Figure 4-6

The relationship between genetic information carried in DNA and proteins.

The complete set of information in an organism's DNA is called its genome, and it carries the information for all the proteins the organism will ever synthesize. (The term genome is also used to describe the DNA that carries this information.) The amount of information contained in genomes is staggering: for example, a typical human cell contains 2 meters of DNA. Written out in the four-letter nucleotide alphabet, the nucleotide sequence of a very small human gene occupies a quarter of a page of text (Figure 4-7), while the complete sequence of nucleotides in the human genome would fill more than a thousand books the size of this one. In addition to other critical information, it carries the instructions for about 30,000 distinct proteins.

Figure 4-7

The nucleotide sequence of the human β-globin gene. This gene carries the information for the amino acid sequence of one of the two types of subunits of the hemoglobin molecule, which carries oxygen in the blood. A different gene, the α-globin (more...)

At each cell division, the cell must copy its genome to pass it to both daughter cells. The discovery of the structure of DNA also revealed the principle that makes this copying possible: because each strand of DNA contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand, each strand can act as a template, or mold, for the synthesis of a new complementary strand. In other words, if we designate the two DNA strands as S and S′, strand S can serve as a template for making a new strand S′, while strand S′ can serve as a template for making a new strand S (Figure 4-8). Thus, the genetic information in DNA can be accurately copied by the beautifully simple process in which strand S separates from strand S′, and each separated strand then serves as a template for the production of a new complementary partner strand that is identical to its former partner.

Figure 4-8

DNA as a template for its own duplication. As the nucleotide A successfully pairs only with T, and G with C, each strand of DNA can specify the sequence of nucleotides in its complementary strand. In this way, double-helical DNA can be copied precisely. (more...)

The ability of each strand of a DNAmolecule to act as a template for producing a complementary strand enables a cell to copy, or replicate, its genes before passing them on to its descendants. In the next chapter we describe the elegant machinery the cell uses to perform this enormous task.

In Eucaryotes, DNA Is Enclosed in a Cell Nucleus

Nearly all the DNA in a eucaryotic cell is sequestered in a nucleus, which occupies about 10% of the total cell volume. This compartment is delimited by a nuclear envelope formed by two concentric lipid bilayer membranes that are punctured at intervals by large nuclear pores, which transport molecules between the nucleus and the cytosol. The nuclear envelope is directly connected to the extensive membranes of the endoplasmic reticulum. It is mechanically supported by two networks of intermediate filaments: one, called the nuclear lamina, forms a thin sheetlike meshwork inside the nucleus, just beneath the inner nuclear membrane; the other surrounds the outer nuclear membrane and is less regularly organized (Figure 4-9).

Figure 4-9

A cross-sectional view of a typical cell nucleus. The nuclear envelope consists of two membranes, the outer one being continuous with the endoplasmic reticulum membrane (see also Figure 12-9). The space inside the endoplasmic reticulum (the ER lumen) (more...)

The nuclear envelope allows the many proteins that act on DNA to be concentrated where they are needed in the cell, and, as we see in subsequent chapters, it also keeps nuclear and cytosolic enzymes separate, a feature that is crucial for the proper functioning of eucaryotic cells. Compartmentalization, of which the nucleus is an example, is an important principle of biology; it serves to establish an environment in which biochemical reactions are facilitated by the high concentration of both substrates and the enzymes that act on them.

Summary

Genetic information is carried in the linear sequence of nucleotides in DNA. Each molecule of DNA is a double helix formed from two complementary strands of nucleotides held together by hydrogen bonds between G-C and A-T base pairs. Duplication of the genetic information occurs by the use of one DNA strand as a template for formation of a complementary strand. The genetic information stored in an organism's DNA contains the instructions for all the proteins the organism will ever synthesize. In eucaryotes, DNA is contained in the cell nucleus.

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