Nucleic Acids
Nucleic Acids: DNA and RNA
Nucleic acids are the molecules of heredity. In most organisms, one type of nucleic acid, DNA, is used to store the hereditary information that codes for every protein in the body. Nucleic acids are divided into classes on the basis of the sugar used to form the nucleotides.
The two types of nucleic acids are:
The two types of nucleic acids are:
- deoxyribonucleic acid (DNA), the genetic material confined largely to the nucleus
- ribonucleic acid (RNA), found both in the nucleus and the cytoplasm
Formation of Nucleic Acids
Nucleic acids are made up of the monomer nucleotides. DNA and RNA have almost the same chemical structure, each consisting of a phosphate, a sugar, and a base. They contain four different bases. They share the same three bases in their nearly identical nucleotide structure: adenine, cytosine, and guanine; but DNA uses thymine while RNA uses uracil. Both have the same bases of adenine, cytosine, and guanine, but the fourth nucleotide in DNA is thymine, while RNA uses uracil to complete its quartet of nucleotides. These bases each go into one of two categories:
According to the rule of complementary base pairing, a purine on one strand can only bind with a pyrimidine on the other strand and vice versa. So, adenine always binds with thymine and guanine always binds with cytosine.
- purines, which have a double ring structure (e.g.- adenine and guanine)
- pyrimidines, which consist of a single ring (e.g.- thymine and cytosine)
According to the rule of complementary base pairing, a purine on one strand can only bind with a pyrimidine on the other strand and vice versa. So, adenine always binds with thymine and guanine always binds with cytosine.
DNA
Deoxyribonucleic Acid (DNA) is the substance of genes, the genetic instructions needed for an organism to develop, survive, and reproduce, the hereditary material in humans and almost all other organisms. To carry out these functions, DNA sequences must be converted into messages that can be used to produce proteins.
DNA is found inside the nucleus of a cell. Because the cell is very small, and because organisms have many DNA molecules per cell, each DNA molecule must be tightly packed.
DNA molecules consist of two long polymer strands of the nucleotides listed above. Scientist use the term "double helix" to describe the winding, two-stranded chemical structure. This shape- which looks much like a twisted ladder- gives DNA the power to pass along biological instructions with great precision. Each "rung" of the ladder is made up of two nitrogen bases, paired together by hydrogen bonds. Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. Information is stored or encoded in the DNA polymer by the pattern in which the four nucleotides are arranged. For example, the sequence ATCGTT might instruct for blue eyes, while ATCGCT might instruct for brown.
DNA's unique structure enables the molecule to copy itself during cell division. When a cell beings the process of dividing, the helix splits down the middle and becomes two single strands. These strands serve as templates for building two new, double-stranded DNA molecules, each a replica of the original DNA molecule.
DNA is found inside the nucleus of a cell. Because the cell is very small, and because organisms have many DNA molecules per cell, each DNA molecule must be tightly packed.
DNA molecules consist of two long polymer strands of the nucleotides listed above. Scientist use the term "double helix" to describe the winding, two-stranded chemical structure. This shape- which looks much like a twisted ladder- gives DNA the power to pass along biological instructions with great precision. Each "rung" of the ladder is made up of two nitrogen bases, paired together by hydrogen bonds. Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. Information is stored or encoded in the DNA polymer by the pattern in which the four nucleotides are arranged. For example, the sequence ATCGTT might instruct for blue eyes, while ATCGCT might instruct for brown.
DNA's unique structure enables the molecule to copy itself during cell division. When a cell beings the process of dividing, the helix splits down the middle and becomes two single strands. These strands serve as templates for building two new, double-stranded DNA molecules, each a replica of the original DNA molecule.
RNA
Ribonucleic Acid (RNA), like DNA, is assembled as a chain of nucleotides, but is usually single-stranded and has a much shorter chain. The lack of a paired strand allows RNA to fold into complex, three-dimensional structures (see video on fold link). The structure of RNA nucleotides is very similar to that of DNA nucleotides, with the main difference being that the ribose sugar backbone in RNA has a hydroxyl (-OH) group that DNA does not. This gives DNA its name; DNA stands for deoxyribonucleic acid.
RNA plays a central role in the pathway from DNA to proteins. During the process known as transcription- when a section of a DNA double helix is uncoiled and one of the DNA strands serves as a template to guide the synthesis- an RNA copy of a segment of DNA, or messenger RNA (mRNA), is created. This strand of RNA can then be read by a ribosome to form a protein. After the synthesis is complete, the RNA separates from the DNA and the DNA recoils into its helix.
Like many proteins, RNA can also act as a catalyst- something that increases the rate of a chemical reaction- for chemical reactions. This class of catalytic RNAs are known as ribozymes. The discovery of ribozymes supported a hypothesis, known as the RNA World Hypothesis. This idea proposed that earlier forms of life may have relied solely on RNA to store genetic information and to catalyze chemical reactions. According to the RNA World Hypothesis, life later evolved to use DNA and proteins due to RNA's relative instability and poorer catalytic properties. Perhaps the strongest evidence for the RNA World Hypothesis is the fact that the ribosome, a large molecular complex that assembles proteins, is a ribozyme. Although the ribosome is made up of both RNA and protein components, analysis revealed that the process of assembling a peptide chain based on a RNA sequence is catalyzed by RNA, not protein. This suggests that lifeforms used RNA to carry out chemical reactions before protein.
RNA plays a central role in the pathway from DNA to proteins. During the process known as transcription- when a section of a DNA double helix is uncoiled and one of the DNA strands serves as a template to guide the synthesis- an RNA copy of a segment of DNA, or messenger RNA (mRNA), is created. This strand of RNA can then be read by a ribosome to form a protein. After the synthesis is complete, the RNA separates from the DNA and the DNA recoils into its helix.
Like many proteins, RNA can also act as a catalyst- something that increases the rate of a chemical reaction- for chemical reactions. This class of catalytic RNAs are known as ribozymes. The discovery of ribozymes supported a hypothesis, known as the RNA World Hypothesis. This idea proposed that earlier forms of life may have relied solely on RNA to store genetic information and to catalyze chemical reactions. According to the RNA World Hypothesis, life later evolved to use DNA and proteins due to RNA's relative instability and poorer catalytic properties. Perhaps the strongest evidence for the RNA World Hypothesis is the fact that the ribosome, a large molecular complex that assembles proteins, is a ribozyme. Although the ribosome is made up of both RNA and protein components, analysis revealed that the process of assembling a peptide chain based on a RNA sequence is catalyzed by RNA, not protein. This suggests that lifeforms used RNA to carry out chemical reactions before protein.