There are two nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These two enable living organisms to reproduce their complex systems from generation to generation. DNA is the genetic material that organisms inherit from their parents. Each time a cell reproduces itself by dividing, the DNA is copied and passed along from one generation of cells to the next. Within the structure of DNA are instructions that program all the cell’s activities. These instructions are packaged in genes along the entire length of DNA molecule.
The other type of nucleic acid, RNA, functions in the synthesis of proteins specified by the DNA. Information flows from DNA to RNA and to proteins. The third nucleic acid is an energy carrier called adenosine triphosphate (ATP). ATP is responsible for transferring chemical energy from one molecule to another in cellular processes.
The DNA molecule is a polymer of nucleotides that are linked together by hydrogen bonds. Each nucleotide is itself composed of three parts: a nitrogenous base, which is joined to a five-carbon sugar (pentose) which in turn is bonded to a phosphate group.
There are two types of bases: purines (adenine, guanine), having a double ring and pyrimidines (thymine and cytosine) having a single ring. The nucleotides are called bases because they have basic properties that raise the pH a solution.
The bases are paired and bound to each other by hydrogen bonds. Adenine (A) is paired with thymine (T) and guanine (G) is paired with cytosine (C).
How nucleotides are linked in a DNA molecule
Another name for a nucleic acid is a polynucleotide. In a polynucleotide, the bases are linked by covalent bonds called phosphodiester linkages between the phosphate of one nucleotide and the sugar of the next nucleotide. This results in a backbone of repeating pattern of sugar-phosphate-sugar-phosphate with variable appendages of the four kinds of nitrogenous bases.
Watson and Crick model
Watson and Crick in 1953 proposed a double helix for the structure of DNA, somewhat like a twisted ladder in which the sugar-phosphate backbone of each polynucleotide strand makes up the sides of the ladder, with the nitrogenous bases making up the rungs or steps of the ladder.
DNA structure
Most DNA molecules are very large with thousands or even millions of base pairs. One long DNA molecule represents a large number of genes, occupying a particular segment along the double helix. The bases in the double helix occur in pairs: adenine always pairs with thymine, while guanine always pairs with cytosine. If we were to read the sequence of the bases along one strand along the length of the double helix, we would automatically know the sequence of bases along the other strand.
For example, if the sequence of bases along one strand is AGGTCCG, the sequence along the other strand should be TCCAGGC. This is because the two strands of DNA are complementary.
DNA replication
An important characteristic of a genetic material is its ability to be copied or duplicated and thus pass on the genetic material to the next generation. Watson and Crick proposed the following method for the duplication of the genetic material. DNA replication begins with an unzipping of the parent molecule, breaking the hydrogen bonds between the base pairs by the action of an enzyme called polymerase. The two pairs of the molecule unwind. Once exposed, the sequence of bases on each of the separated strands serves as a template or mould to guide the insertion of a complementary set of bases on the strand being assembled.
DNA replication
Thus, each C on the parent template guides the insertion of G on the new strand; each G on the old strand guides the insertion of C on the new strand. Similarly, each A on the parent strand guides the insertion of T on the new strand, etc. When the process is completed, two DNA molecules have been formed, identical to each other and to the parent molecule.
Protein Synthesis
Every cell in our bodies contains genes that specify every aspect of our heredity, such as the color of the eyes, the structure of the ears or teeth, height, etc. The genetic instructions stored in DNA allow the cells to produce particular proteins that will determine the hereditary characteristics of an individual.
In other words, proteins are the tools of heredity. In synthesizing proteins, cells use RNA.
The structure of RNA
Ribonucleic acid (RNA) is also a polynucleotide. The chain of nucleotides is formed in exactly the same way as in DNA, but the molecule has some very important differences:
1. It is a single stranded molecule.
2. The Thymine is replaced by Uracil.
3. It is much smaller than DNA.
4. It comes in three different forms, ribosomal (rRNA), transfer (tRNA) and messenger (mRNA).
Ribosomal RNA is 80% of the total RNA in a cell. It is involved with the formation of ribosomes and is therefore important as the site of protein synthesis in a cell.
Messenger RNA is 3-5% of the total RNA in a cell, depending on the protein synthesis activity at the time. It forms in the nucleus and it is a go-between for DNA in the nucleus and ribosomes in the cytoplasm.
Transfer RNA is about 15 % of the total RNA in the cell. It is involved in carrying the amino acids through the cytoplasm to their correct places in a growing polypeptide chain.
Table: DNA compared with RNA
DNA | RNA |
Found only in the nucleus | Found in the nucleus and in the cytoplasm |
Double stranded | Single stranded |
Thymine nucleotide | Uracil nucleotide |
The sugar is deoxyribose | The sugar is ribose |
While DNA is found only in the nucleus of a cell, RNA is present both in the nucleus and in the cytoplasm. mRNA takes coded messages from DNA to the cytoplasm where they are decoded and used to guide protein synthesis.
Transcription and Translation
Protein synthesis is a two-stage process. The first process is known as transcription, which involves the formation of mRNA and the copying of genetic information from DNA to mRNA. The process begins with the binding of an enzyme called RNA polymerase to special sites on the DNA molecule, followed by the separation of the two strands of DNA. The RNA polymerase then copies information from one of the two strands to mRNA.
Transcription takes place in the nucleus of the cell, after which the newly formed mRNA moves through pores in the nuclear membrane to the endoplasmic reticulum in the cytoplasm where translation or protein synthesis takes place.
Translation involves coded messages in which three nucleotides in a row on the mRNA form a codon, with each codon specifying an amino acid. On the other hand, each tRNA molecule carries at one end an anticodon of three nucleotides that are
complementary to the mRNA codon and an amino acid at the other end. There is at least one tRNA for each of the 20 amino acids needed to make protein. Some amino acids may have more than one tRNA molecules.
Protein synthesis begins when mRNA, with the genetic information copied from the DNA molecule, moves to the endoplasmic reticulum in the cytoplasm and binds to a ribosome. The binding occurs at a particular site on the mRNA called the initiation site.
Next, the tRNA with its anticodon, binds to the codon AUG at a site on the mRNA designated P that initiates every message. The AUG codon represents the amino acid methionine (met) and is called the initiation codon. This amino acid is always at the beginning of every protein chain. The tRNA- amino acid complex remains attached to the mRNA codon. The ribosome moves along the mRNA strand and exposes a new codon. Another tRNA, with an anticodon complementary to the next codon on the mRNA ‘assembly line’, moves in and occupies the next site on the mRNA called A. In this case, the codon on the mRNA is GCC, which codes for alanine. The first amino acid is then linked to the incoming amino acid by a peptide bond.
Protein synthesis
The ribosome moves one codon to the right. The tRNA at site P is released, and site P now becomes occupied by the second tRNA, leaving site A for the next incoming tRNA with an anticodon that can base pair with UGG codon. Each time the ribosome moves along the mRNA, an additional amino acid is added to the growing peptide. The process continues until a complete protein of a particular sequence is formed.
Termination of protein synthesis occurs at a specific nucleotide sequence on the mRNA where the last tRNA and completed polypeptide are detached from the ribosomal complex.
DNA Repair
As the two strands in a DNA molecule are complementary, the information carried by one strand is identical to the information carried by the other. Therefore, if a part of one of the strands breaks and gets lost, the cell will still have a full complement of information stored in the undamaged strand. The lost piece of the damaged strand can be regenerated with the undamaged strand acting as a template.
Study Questions
1. Describe the structure of DNA, including the double-helix model.
2. Describe how the processes of transcription and translation occur.
3. Write a few notes on each one of the following: DNA, mRNA, tRNA and rRNA
5. How does DNA replicate? Explain the significance of replication.
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