ZOOHCC - 501: Molecular Biology (Theory)
Unit 1: Nucleic Acid
Salient Features of DNA Double-Helix
The bases on the opposite strands are connected through hydrogenbonds forming base pairs (bp). Adenine always forms two hydrogen bonds with thymine from the opposite strand and vice-versa. Guanine forms three hydrogen bonds with cytosine from the opposite strand and vice-versa.
DNA is made of chemical building blocks called nucleotides. These building blocks are made of three parts: a phosphate group, a sugar group and one of four types of nitrogen bases
Chemical composition of DNA
DNA is made of two linked strands that wind around each other to resemble a twisted ladder — a shape known as a double helix. Each strand has a backbone made of alternating ugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine(A), cytosine (C), guanine (G) or thymine (T) Unlike DNA, however, RNA is most often single-stranded. An RNA molecule has a backbone made of alternating phosphate groups and the sugar ribose, rather than the deoxyribose found in DNA. Attached to each sugar is one of four bases: adenine (A), uracil (U), cytosine (C) or guanine (G) RNA contain the same purines namely adenine (A) and anine (G). Further, the pyrimidine cytosine (C) is found in both DNA and RNA. However, the nucleic acids differ with respect to the second pyrimidine base. DNA contains thymine (T) whereas RNA contains uracil(U) DNA is negatively charged because of the presence of phosphate groups in nucleotides. The phosphate backbone of DNA is negatively charged, which is due to the presence of bonds created between the phosphorus and oxygen atoms
Watson and Crick model of DNA
The Watson Crick Structure of DNA James Watson and Francis Crick proposed that DNA must be formed like a double helix in 1953. This is called the Watson-Crick
Structure of DNA.
History
Watson-Crick Structure of DNA
Given below is the diagram for the Watson-Crick Structure Of DNA DNA is a helical, double-stranded molecule with two strands. On the surface, it has two backbones (with alternating glycosyl and phosphate groups) that are linked together on the inside by
hydrogen bonds between pairs of nitrogenous bases.
The bases are divided into four categories (A, C, G, and T), with A & T and C & G always matching.
James Watson and Francis Crick realised that these pairing principles indicated that either strand held all the information needed to build a new copy of the complete molecule and that the aperiodic sequence of bases might offer a “genetic code”.
History
Linus Pauling, in 1925, utilised X-ray crystallography to determine the secondary structure of proteins. X-raycrystallography was used for analysing molecular structure. One could see the patterns produced by the X-rays when they were shot through the crystal of a material. The patterns revealed important details about the structure of any molecule of interest. In 1953, Rosalind Franklin, a researcher at Wilkins Lab in the US, used X-ray crystallography to explore the structure of DNA. In the 1950s, while working together at Cambridge, England, Watson and Crick were able to piece together the jigsaw of the DNA molecule using Franklin’s discoveries.
Watson-Crick Structure of DNA
Crick presented a model for the DNA’s double-helix
structure.
A nucleotide polymer makes up the DNA molecule.
A nitrogenous base, a five-carbon sugar (deoxyribose), and a phosphate group are found in each nucleotide.
There are two purines (adenine and guanine) and two pyrimidines in the DNA (cytosine and thymine).Two strands of DNA make up a DNA molecule.Each strand is made up of nucleotides that are covalently linkedby their phosphate groups and deoxyribose sugars.The bases grow out of this backbone.Hydrogen bonds connect the bases of strands.Adenine is always linked with thymine, while cytosine is always linked with guanine.
The two strands wrap around each other in a shape termed the‘double helix’ because of the bonding.The second in the cells is ribonucleic acid(RNA). RNA is a nucleotide polymer with a single strand.It also contains sugar ribose instead of deoxyribose and nucleotide uracil instead of thymine. RNA molecules play a role in the formation of proteins using DNA’s genetic code.
The Watson-Crick model of DNA has the following important
Characteristics:
In a DNA molecule, a right-handed double-helix is madeup of a series or strands of two polynucleotides spirally wrapped around each other and twisted along a common path.
The two strands run in opposing directions, with the fifth ending of one chain facing the third ending of the other. In other words,they are antiparallel.
The sugar-phosphate backbones remain on the exterior, while the purines and pyrimidines bases are located at the centre of the helix.
The two series are kept together by hydrogen bonds formed between the purines and pyrimidine bases of the opposing strands.
Adenine (A) will always couple with thymine (T) through twohydrogen interactions, while guanine (G) will always pair withcytosine (C) via three hydrogen bonds. This complementary nature is known as the rule of the base pair. As a result, the two chains are complementary to each other.
The base sequence varies throughout a polynucleotide chain. A certain sequence of bases carries genetic information.
The constitution of the base of the DNA follows Chargaff’s assumptions (E.E. Chargff, 1950), according to which A = T and G = C. As a corollary, ∑ purines (A+G) = ∑ pyrimidines (C + T). Also, (A + C) = (G + T). The rules also put forth the ratio of (A + T) and (G + C) as constants for a species (range 0.4 to 1.9).
The DNA has a diameter of 20 nm, or 20Å. The adjoining bases are 0.34 nm or 3.4Å along the axis apart. The length of a full helix turn is 3.4 nm or 34Å, implying that there are 10b/turning.
A small groove, called the minor groove (1.2 nm), and a deep groove, known as the major groove (2.2 nm), run across the DNA helix.
Conclusion
Watson and Crick postulated the “double helix” molecular structure of the DNA. The precise pairing of nucleotides is a critical aspect of the Watson and Crick model of DNA. Each DNA strand, in this case, is linear and long and is made up of smaller units known as nucleotides that link together to form a chain.
Structure and types of RNA
Structure of RNA
RNA is a ribonucleic acid that helps in the synthesis of proteins in our body. This nucleic acid is responsible for the production of new cells in the human body. It is usually obtained from the DNA molecule. RNA resembles the same as that of DNA, the only difference being that it has a single strand unlike the DNA which has two strands and it consists of an only single ribose sugar molecule in it. Hence is the name Ribonucleic acid. RNA is also referred to as an enzyme as it helps in the process of chemical reactions in the body.
RNA Types:
There are various types of RNA--
tRNA – Transfer RNA tRNA
The transfer RNA is held responsible for choosing the correct protein or the amino acids required by the body in-turn helping the ribosomes. It is located at the endpoints of each amino acid. This is also called as soluble RNA and it forms a link between the messenger RNA and the amino acid.
rRNA-Ribosomal RNA rRNA
The rRNA is the component of the ribosome and are located within the in the cytoplasm of a cell, where ribosomes are found. In all living cells, the ribosomal RNA plays a fundamental role in the synthesis and translation of mRNA into proteins. The rRNA is mainly composed of cellular RNA and are the most predominant RNA within the cells of all living beings.
mRNA – Messenger RNA.
This type of RNA functions by transferring the genetic material into the ribosomes and pass the instructions about the type of proteins, required by the body cells. Based on the functions, these types of RNA is called the messenger RNA. Therefore, the mRNA plays a vital role in the process of transcription or during the protein synthesis process.
Basic Structure of RNA
The ribonucleic acid has all the components same to that of the DNA with only 2 main differences within it. RNA has the same nitrogen bases called the adenine, Guanine, Cytosine as that of the DNA except for the Thymine which is replaced by the uracil. Adenine and uracil are considered as the major building blocks of RNA and both of them form base-pair with the help of 2 hydrogen bonds.
RNA resembles a hairpin structure and like the nucleotides in DNA, nucleotides are formed in this ribonucleic material(RNA). Nucleosides are nothing but the phosphate groups which sometimes also helps in the production of nucleotides in the DNA.
Functions of RNA:
The ribonucleic acid – RNA, which are mainly composed of nucleic acids, are involved in a variety of functions within the cell and are found in all living organisms including bacteria, viruses, plants, and animals. These nucleic acid functions as a structural molecule in cell organelles and are also involved in the catalysis of biochemical reactions. The different types of RNA are involved in various cellular process. The primary functions of RNA:
-Facilitate the translation of DNA into proteins
-Functions as an adapter molecule in protein synthesis
-Serves as a messenger between the DNA and the ribosomes.
-They are the carrier of genetic information in all living cells
-Promotes the ribosomes to choose the right amino acid which is required in building up of new proteins in the body.
