NUCLEIC ACIDS BY DR. MARYJANE

Are of two types: DNA RNA

DEOXYRIBONUCLEIC ACID (DNA) DNA is a major component of chromosomes found primarily in the nucleus, although a small amount is found in mitochondria and chloroplasts. Human DNA consists of 2 strands (chains) of polynucleotides. Each nucleotide is composed of base, sugar and phosphate. The bases in DNA are: guanine, adenine, cytosine and thymine The sugar is deoxyribose. The nucleotides are arranged in chains linked together by phosphodiester bond between C₅ of deoxyribose of one nucleotide and C₃ of the next one.

Deoxyribose and ribose sugar

PHOSPHODIESTER BOND Is a strong covalent bond between a phosphate group and 5-carbon ring carbohydrate pentoses. In DNA and RNA, the phosphodiester bond is the linkage between the 3′ carbon atom of one sugar molecule and the 5′ carbon atom of another with sugar molecules being deoxyribose in DNA and ribose in RNA The 3′ carbon atom bears the –OH group and the 5′ carbon bears the phosphate group.

PHOSPHODIESTER BOND

CHARGAFF’S PAIRING It was found that in DNA molecule: a. Adenine is paired with thymine (A=T) by 2 hydrogen bonds. b. Guanine is paired only with cytosine (G≡C) by 3 hydrogen bonds. Thus in double stranded DNA molecule, the content of adenine equals that of thymine and the content of guanine equals that of cytosine. Also the G≡C bond (3 hydrogen bonds) is much more stronger than A=T (2 hydrogen bonds).

DNA ORGANIZATION 1. CHROMOSOMES: a. these are nucleoproteins formed mainly of DNA and basic proteins. b. In man, they are 46 in number. c. they bear genes and act as a functional unit of heredity. d. they are capable of reproducing its physical and chemical structure through successive cell division.

2. CHROMATIN: is the condensed DNA-protein complex. Chromatin consists of: i. very long double stranded DNA molecule. ii. Histones which are basic proteins iii. Protamines iv. Small quantity of RNA.

3. HISTONES: are small proteins and are positively charged at physiological pH due to their high content of lysine and arginine. There are 5 major classes of histones: H1, H2A, H2B, H3 and H4. These histones are arranged in structural octamer units called nucleosomes. 4. GENE:can be defined as a region of DNA that controls a hereditary characteristic 5. GENOME: is the total chromosomal (DNA) content of a cell.

DENATURATION OF DNA Involves separation of the double helix of DNA into single strands when hydrogen bonds between the bases are disrupted. Factors that are responsible for denaturation of DNA includes: ↑temperature, ↓pH. Because there are 3 bonds between G and C but only 2 between A and T, DNA that contains high concentration of A and T will denaturate at a lower temperature than G and C rich DNA.

RIBONUCLEIC ACIDS (RNA) There are 3 types of RNA: a. messenger RNA (mRNA) b. ribosomal RNA (rRNA) c. transfer RNA (tRNA) All RNA molecules are formed in the nucleus under the control of DNA and the enzyme RNA polymerase. All RNA are formed of one strand only. Sugar: ribose Bases: adenine, guanine, cytosine and uracil.

Messenger RNA (mRNA): is a one stranded nucleic acid and forms about 5% of cellular RNA. It is the most abundant FUNCTION: mRNA carries the genetic information from DNA of the nucleus to the ribosome.

Ribosomal RNA (rRNA): is the nucleic acid present in ribosome. It is the largest Functions for synthesis of protein In the nucleus, it is divided into 2 subunits, a large subunit (60S or 70S) is the binding site for tRNA and a small subunit (30S or 40S) is the binding site for mRNA.

Transfer RNA (tRNA): transports amino acids to the ribosome for synthesis of protein. It forms 15% of the cellular RNA and the tiniest.