In this essay we will discuss about:- 1. Meaning of RNA 2. Features of RNA 3. Structure 4. Components 5. Differences between DNA and RNA 6. Types.
Contents:
- Essay on the Meaning of RNA
- Essay on the Features of RNA
- Essay on the Structure of RNA
- Essay on the Components of RNA
- Essay on the Differences between DNA and RNA
- Essay on the Types of RNA
Essay # 1. Meaning of RNA:
A nucleic acid that carries the genetic message from DNA to ribosomes and is involved in the process of protein synthesis is referred to as RNA [ribose nucleic acid]. Ribonucleic acid is one of the two types of nucleic acids found in all cells. The other is deoxyribonucleic acid (DNA).
RNA is similar to DNA but’ containing ribose in place of deoxyribose and uracil in place of thymine. Ribouncleic acid (RNA) is a nucleic acid polymer consisting of nucleotide monomers that acts as a messenger between DNA and ribosomes and is also responsible for making proteins out of amino acids. Some viruses use RNA instead of DNA as their genetic material.
Essay # 2. Features of RNA:
The main features of RNA are given below:
1. Location:
In eukaryotes, RNA is found both in nucleus and cytoplasm. In the nucleus it is a component of chromosome, whereas in cytoplasm it is found in ribosomes. In prokaryotes, it is found in the cytoplasm.
2. Synthesis:
The RNA synthesis takes place in the nucleus on DNA template. After synthesis it moves from nucleus to cytoplasm. Thus, RNA usually does not have self-duplication property. In certain viruses like TMV and plantango virus RNA can synthesize another RNA molecule.
3. Types:
There are different forms of RNA such as:
(i) Messenger RNA [mRNA],
(ii) Ribosomal RNA [rRNA], and
(iii) Transfer RNA [tRNA] or soluble RNA [sRNA].
Ribosomal and transfer RNAs constitute about 98% of the total RNA. All three forms of RNA are synthesized on DNA template.
4. Size:
RNA molecule is much smaller in size than DNA. It consists up to 12,000 nucleotides whereas DNA consists up to 4.3 million nucleotides.
5. Functions:
In most of the organisms, the usual function of RNA is transfer of genetic message from nucleus to the cytoplasm and synthesis of protein in ribosomes. In some viruses, RNA acts as the genetic material and regulates the gene action.
Essay # 3. Structure of RNA:
Mostly the RNA molecule is single stranded in both eukaryotes and prokaryotes. However, in some viruses RNA is double stranded. The double stranded RNA is found in Rio virus in animals and wound tumour virus in plants.
The single strand of RNA is folded upon itself cither entirely or in some regions. Most of the base in the folded region are complementary and are joined by hydrogen bonds. However, in the unfolded regions the bases do not have complements. As a result, the purine and pyrimidine ratio is not equal in RNA molecule.
The RNA is composed of ribose sugar, nitrogenous bases and phosphate group. The nitrogenous base in RNA are: adenine (A), guanine (G), cytosine (C), and uracil (U). In RNA uracil is present in place of thymine of DNA molecule. In RNA, the sugar is ribose, while in DNA it is deoxyribose.
Essay # 4. Components of RNA:
The RNA molecule is a polymer which is composed of several thousand pairs of nucleotide monomers. The polymer is known as a “polynucleotide.” Each nucleotide consists of a 5-carbon sugar (ribose), a nitrogen containing base attached to the sugar, and a phosphate group.
Thus there are three components of RNA, viz:
(1) Nitrogenous bases,
(2) Ribose sugar, and
(3) Phosphate group.
These are briefly discussed below:
1. Nitrogenous Bases:
Nucleotides are also known as nitrogenous bases or DNA bases. Nitrogenous base are of two types, viz. pyrimidines and purines. The features of pyrimidines and purines are given below.
Pyrimidines:
Main features of pyrimidines are given below:
(i) In RNA, pyrimidine base are of two types, viz. cytosine and uracil.
(ii) These are single ring structures.
(iii) They occupy less space in RNA structure.
(iv) Pyrimidine is linked with ribose sugar at position 3.
Purine:
Main features of purines are given below:
(i) They are same as in DNA i.e. adenine and guanine.
(ii) They are double ring compounds.
(iii) They occupy more space in RNA structure.
(iv) Ribose sugar is linked at position 9 of purine.
Thus, in RNA there are four different types of nitrogenous bases, viz. adenine (A), guanine (G), cytosine (C) and uracil (T). In DNA, the pyrimidine base uracil is replaced by thymine.
Base Pairing:
The purine and pyrimidine bases always pair in a definite fashion. Adenine will always pair with uracil and guanine with cytosine. Adenine and thymine are joined by double hydrogen bonds while guanine and cytosine are joined by triple hydrogen bonds.
2. Ribose Sugar:
RNA contains ribose sugar while DNA contains deoxyribose sugar. Both are pentose sugar having five carbon atoms. The four carbon atoms are inside the ring and the fifth one is with CH2 group. This has four OH groups on 1, 2, 3 and 5 carbon positions. Hydrogen atoms are attached to carbon atoms one to four. The ribose sugar has OH group on carbon atom 2, whereas DNA has H group.
3. Phosphate:
The phosphate molecule is arranged in an alternate manner to ribose sugar. Thus there is ribose sugar on both sides of phosphate. The phosphate is joined with carbon atom 3 of ribose at one side and with carbon atom 5 of ribose on the other side.
Nucleosides and Nucleotides:
In RNA, nucleoside is a combination of ribose sugar and nucleotide is a combination of nucleoside and 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 RNA are ribose adenosine (rA), ribose guanosine (rG), ribose cytosine (rC), and ribose urosine (rU).
RNA Backbone:
The RNA backbone is a polymer with an alternating sugar-phosphate sequence. The ribose sugar is joined at both the 3′-hydroxyl and 5′-hydroxyI groups to phosphate groups in ester links, also known as “phosphodiester” bonds.
RNA as Genetic Material:
Most of the plant viruses contain RNA and proteins and do not contain DNA. In such cases, RNA is the genetic material. The well-known example of plant virus which contains RNA as the genetic material is the tobacco mosaic virus [TMV].
Fraenkel Conract and Singer Experiment [1957]:
Organism Used:
They conducted their experiments with tobacco plant.
Principle Involved:
The experiment is based on the principle of transformation.
Procedure Adopted:
They isolated RNA and protein from the tobacco plants infected with tobacco mosaic virus [TMV] and used the isolated RNA, protein and their mixture to develop TMV symptoms in the healthy plants of tobacco and recorded the observations after a definite period.
Results:
Conclusion:
The TMV symptoms developed when either RNA or mixture of RNA and protein were applied to healthy plants. When only protein extract was applied, the symptoms did not develop. This clearly demonstrated that in TMV the genetic material is RNA.
Experiment with HRV:
Fraenkel Conract and Singer conducted another experiment with another virus known as Holmes Ribgrass Virus [HRV] to confirm their results. The details of the experiment are given below.
Organism Used:
They used Holmes Ribgrass Virus [HRV] which was isolated from Plantango lanceolata plant and used to develop symptoms in healthy tobacco plant.
Procedure Adopted:
They isolated RNA and proteins from TMV infected plants and also from HRV infected plantango plants. RNA from one virus and protein from other virus were mixed together and used to develop infection on healthy tobacco plants and observations were recorded. Symptoms of TMV and HRV differ. TMV causes mottling symptoms, while HRV develops ring pattern.
Conclusion:
The symptoms resembled the virus from which RNA was used. This provided conclusive evidence that in plant viruses RNA is the genetic material.
Essay # 5. Differences between DNA and RNA:
There are some similarities between DNA and RNA and some differences which are presented in the Table 19.1.
Essay # 6. Types of RNA:
On the basis of function RNA is of two types, viz:
(A) Genetic RNA, and
(B) Non-genetic RNA.
The non-genetic RNA is of three types, viz:
(1) Transfer RNA,
(2) Messenger RNA, and
(3) Ribosomal RNA.
A brief discussion of all these types of RNA is presented as follows:
A. Genetic RNA:
RNA which acts as genetic material like DNA is called genetic RNA. Such RNA is found in most of the plant viruses [TMV, HRV, etc.], some animal viruses and certain bacteriophages. Genetic RNA may be single stranded or double stranded. Genetic RNA has self-replication property.
B. Non-Genetic RNA:
RNA which does not act as genetic material is known as non-genetic RNA. This is found in higher organisms where DNA is the genetic material. Such RNA is usually single stranded. This type of RNA does not have self-replication property. Such RNA is synthesized from DNA template in the presence of DNA dependent RNA polymerase enzyme. Thus genetic RNA differs from non-genetic RNA in several aspects.
1. Transfer RNA:
RNA which carries amino acids and attaches them with, ribosome mRNA complex for use in protein synthesis is termed as transfer RNA. It is also known as soluble RNA. It constitutes about 10 to 15% of the total RNA of the cell. Main features of transfer RNA are given below.
i. Synthesis:
Transfer RNA is synthesized on a DNA template using small section of the DNA molecule. It is synthesized at the end of cleavage.
ii. Types:
There are more than a hundred types of tRNA per bacterial cell. There are 20 amino acids, hence there should be at-least 20 types of tRNA. However, tRNA is always more than 20 and in some cases, there are two types of tRNA which specify for one amino acid.
iii. Molecular Weight:
It has molecular weight of about 25,000 to 30,000 with sedimentation coefficient of 3.8 S. It contains 73 to 93 nucleotides.
iv. Function:
The main function of tRNA is to carry various types of amino acids and attach them to mRNA template for protein synthesis.
2. Messenger RNA:
The RNA which carries information from nuclear DNA to cytoplasm for protein synthesis is referred to as messenger RNA. It is also known as template RNA. It constitutes about 5 to 10% of the total cellular RNA. The main features of messenger RNA are presented below.
i. Synthesis:
The new mRNA is synthesized during early cleavage on a DNA strand in the presence of RNA polymerase enzyme. In eukaryotes, heterogeneous nuclear RNA [hnRNA] is synthesized from DNA. This is considered as precursor of mRNA.
The hnRNA may be 5,000 to 50,000 thousand nucleotides long. It consists of both coding and non-coding nucleotide sequences. The non-coding sequences are removed during processing. The mature mRNA may be a friction of original hnRNA.
Synthesis of mRNA differs from DNA replication in following three main aspects:
a. Ribose nucleotides are used instead of deoxyribose nucleotides.
b. Adenine pairs with uracil instead of thymine.
c. Only one strand of DNA takes part in the synthesis of mRNA.
ii. Types:
Usually each gene transcribes its own RNA. There are as many types of mRNA molecules as there are genes. There may be 1,000-10,000 types of mRNA in a cell. These types of mRNA differ only in the sequence of their bases and in their length.
iii. Molecular Weight:
The molecular weight of an average molecule of mRNA is 5,00,000 and its sedimentation coefficient is 8S.
iv. Life:
In bacteria, it is short lived. For example, in E. coli the average half-life of some mRNA is about 2 minutes. However, in mammals it may live for many hours and even days.
v. Function:
The main function of mRNA is protein synthesis in ribosomes in the cytoplasm.
3. Ribosomal RNA:
RNA which is found in ribosomes in the cytoplasm is called ribosomal RNA. It constitutes about 80% of the total cellular RNA. Its main features are given below.
i. Synthesis:
In eukaryotes, ribosomal RNA is synthesized from nucleolar DNA. In prokaryotes it is synthesized from a part of DNA. Synthesis of ribosomal RNA begins during gastrulation and increases as embryo develops.
ii. Types:
On the basis of molecular weight and sedimentation rate, ribosomal RNA is of following three types:
(a) Molecular weight over a million [21S-29S RNA],
(b) Molecular weight below one million [12 S-18S],
(c) Low molecular weight [5S RNA].
iii. Life:
Ribosomal RNA is more stable than mRNA.
iv. Function:
The function of ribosomal RNA is binding of mRNA and tRNA to ribosomes in the cytoplasm.