Fig. 9-CO, p.215
Nucleic Acids: How Structure Conveys Information 1. What Is the Structure of DNA? 2. What Are the Levels of Structure in Nucleic Acids? 3. What Is the Covalent Structure of Polynucleotides? 4. Knowing the types & general features of B- DNA. 5. How Does the Denaturation of DNA Take Place?
DNA structure DNA is the largest macromolecule in the cell. In eukaryotic cells, 99% of the cell DNA is present in linear form folded on itself several times to occupy small space within the chromosomes of the nucleus. Each chromosome contains single DNA molecule. Small amounts of DNA (about 1%) are circular shape present inside the mitochondria.
The DNA can be described as a polymer of nucleotides (Polynucleotide).That is a long chain of repeating nucleotide units connected together strongly (by covalent bonds). Therefore, nucleotide is the unit of DNA structure that has complex structure made of 3 different components: 1.Nitrogen base 2. Pentose sugar 3. Phosphate groups.
Without phosphate groups, the combination of nitrogen base and sugar is called a nucleoside. The nitrogen bases are heterocyclic (combination of carbon and nitrogen atoms) present in the cell with 5 different types. Two purines : adenine and guanine (with 9 atoms of 4 nitrogens and 5 carbons arranged in two rings) and Three pyrimidine: Cytosine, Thymine and Uracil having 6 atoms ring including 2 nitrogens and 4 carbons
In the nucleotide structure, the pentose sugar connects to nitrogen base at carbon one from one direction and to phosphate group at carbon 5 of the other side
There are 4 different types of nitrogen bases present in each nucleic acid. Adenine, Guanine and cytosine are present in both DNA and RNA.However only RNA contains Uracil and only DNA contains Thymine nitrogen bases. Also RNA nucleotides have ribose while DNA has deoxyribose as pentose sugars.
Fig. 9-1, p.216
The difference is - 2'-OH in ribo and 2'-H in deoxyribose The difference is - 2'-OH in ribose and 2'-H in deoxyribose
In single polynucleotide chain of DNA(and similarly for RNA structure) the chain is made from covalent linkage of sugar phosphate backbone.in this arrangement the nitrogen bases are exposed freely to the inside of the backbone structure.
In DNA structure two polynucleotides are twisted around each others in double helical arrangement so that bases of opposite polyncleotides are specifically connected by weak hydrogen bonds.
Opposite direction of DNA polynucleotide chains The two strands have their 3 and 5 terminals at opposite ends ( antiparallel ) 5 terminal: at one end of each DNA strand is a phosphate group linked to carbon atom 5 of deoxyribose 3 terminal: at one end of each DNA is a hydroxyl group attached to carbon atom 3 of deoxyribose.
Fig. 9-5, p.219
Has polarity Has a Hydroplilic side Has a Hydrophobic side Fig. 9-6, p.219
James Watson and Francis Crick, 1959 won the 1962 Nobel Prize in Medicine for their discovery of the structure of DNA. This was one of the most significant scientific discoveries of the 20th century
ROSALIND FRANKLIN 1920-58 Gave an idea that the structure of DNA is a helical structure. Physical Chemistry & X-ray crystallography expert Died of ovarian cancer
The 3-dimensional double helix structure of DNA, correctly elucidated by James Watson and Francis Crick. Complementary bases are held together as a pair by hydrogen bonds Fig. 9-7, p.221
Fig. 9-8a, p.222
Fig. 9-8b, p.222
Less Common Nucleobases Fig. 9-2a, p.216
Hypoxanthine + Sugar = Inosine Fig. 9-2c, p.216
5 Fig. 9-2b, p.216
Fig. 9-3, p.217
Commonly Occurring Nucleotides Fig. 9-4a, p.218
Fig. 9-4b, p.218
commonly occurring nucleotides Fig. 9-4c, p.218
Fig. 9-4d, p.218
Three different conformations of the DNA double helix. (A) A-DNA is a short, wide, right-handed helix. (B) B-DNA, the structure proposed by Watson and Crick, is the most common conformation in most living cells. (C) Z-DNA, is a left-handed helix unlike A- and B-DNA, is a left-handed helix.
Fig. 9-10, p.224
Fig. 9-11, p.225
Fig. 9-12, p.225
Fig. 9-14a, p.227
Fig. 9-14b, p.227
Fig. 9-15, p.228
Chromatin is comprised of histones and DNA: 147 base pairs of DNA wraps around the 8 core histones to form the basic chromatin unit, the nucleosome The function of chromatin is to efficiently package DNA into a small volume to fit into the nucleus of a cell and protect the DNA structure and sequence Fig. 9-16, p.229
Fig. 9-17, p.229
Fig. 9-18, p.231
END