3S2+Bio+Essay+by+Ryan+Wong,+Joshua+Foo,+Lee+Yi+Liang

Bio Essay by Lee Yi Liang Joshua Foo **
 * Ryan Wong

Guiding General roles Introduction Nucleotides - phosphoric acid - pentose - nitrogenous base Bonding of nucleotides 3D structure of DNA

Task: **Write an essay describing DNA structure** 300-word essay describing the **components of DNA** and **how these associate** with each other to generate the **overall, 3-dimensional structure** of DNA

DNA stands for deoxyribonucleic acid, a nucleuic acid that contain genetic intstructions in a double helix structure. DNA make up genes and genes make up chromosomes in the nucleus of a cell. DNA carries information needed to construct and operate organisms.

DNA is made up of nucleotides consisting of a 5-carbon sugar, nitrogen base and phosphate group. The sugar and phosphate group form the backbone of the nucleotide. The nitrogenous base is located at the center of the 2 DNA strands. There are 4 types of nitrogen bases in DNA, Adenine, Guanine, Cytosine and Thymine. The nitrogen bases bond with the base on the opposite nucleotide. Adenine always bonds with thymine with 2 hydrogen bonds and guanine bonds with cytosine with 3 hydrogen bonds. In RNA, the four bases are Adenine, Guanine, Cytosine and Uracil, with Uracil replacing Thymine. The hydrogen bond bonding the bases are weak to allow the 2 strands to come apart. This allows for replication and transcription of DNA code by the mRNA. A base sequence of 3 bases is a codon. The bases in the codons can be arranged in any order, producing a total of 64, 4^3, codons. A similar group of codons produce the same amino acid. A sequence of amino acids can be coded from a sequence of bases. The amino acid is used protein synthesis.

Nucleotides different molecules that bond together to form DNA. What they consist of are stated above. The bases of nucleotides bond to another complementing base via weak hydrogen bonds. The 5-carbon sugar and phosphate group bond to form the "backbone" of the DNA. The nitrogen is bonded to the "backbone". This forms **one** strand of the DNA, and the other strand of the DNA is identically formed. The nucleotides are binded together by the weak hydrogen bonds between each bases. These bonds are the only bonds that hold the two strands of the DNA together.

For DNA, the purine bases adenine and guanine, pair with the pyrimidine bases thymine and cytosine, respectively. As shown in the picture above, a difference between the bonding of adenine-thymine and guanine-cytosine is that the bond between adenine and thymine consists of two hydrogen bonds while the bond between the guanine and cytosine consists of three hydrogen bonds (Hydrogen bond represented by dotted lines).

However, these bonds (hydrogen bonding) are extremely weak as compared to other intermolecular bonds, ionic and covalent bonding. This makes protein synthesis easier as less energy will be required to break the bonds between the two strands of the DNA, as the bond between the two strands is essentially the bond between the bases. This weak bond allows the two bonded strands to split into separate strands, allowing for transcription to take place easily.

An example of part of a DNA would be as follows: code GCA TAC GGC ATC CGA CGT ATG CCG TAG GCT code One strand will bond to the other (to its corresponding nucleotide) and this is also known as base pairing.

DNA consists of a double helix, with about 10 nucleotide pairs per helical turn. Each spiral strand consists of a sugar phosphate backbone, located on the outside of the helix. Attached bases, which are on the inside of the helix, are connected to a complementary strand by hydrogen bonding between paired bases, adenine with thymine, and guanine with cytosine. Components of DNA are summarised in the diagram below.



The sugar-phosphate backbone of DNA is formed of nucleotides linked by phosphodiester bonds. Nucleotides consists of 3 components, pentose sugar, nitrogen base and phosphate group. The pentose sugar is a ring sugar with 5 parts, 4 carbon and 1 oxygen. The pentose sugar in DNA is deoxyribose. The nitrogen base is a nitrogen containg basic ring compound. They are broadly classified into purines and pyrimidines. These are further divided into 5 nitrogen bases, Adenine,Guanine,Cytocine, Thymine, Uracil. DNA does not contain Uracil.

These DNA strands continues for a distance of up to 1.8 meters. A series of DNA strands coil up into single units called chromosome.The chromosomes are found in the nucleus of the cell only 6 micrometers in diameter.