It is the branch of biology that deals with the molecular basis of biological activity.

1. It is the branch of biology that deals with the molecular basis of biological activity. Molecular biology chiefly concerns itself with understanding the interactions between the various systems of a cell, including the interactions between the different types of DNA, RNA and protein biosynthesis as well as learning how these interactions are regulated.

2. DNA and RNA:The molecular basis • DNA-DeoxyriboNucleic Acid a molecule that contains the genetic code of organisms. This includes animals, plants, protists, archaea and bacteria.

3. RNA • RNA-Ribonucleic Acid –a polymer of nucleotides formed on the surface of DNA by transcription. • 3 forms of RNA-mRNA, rRNA, tRNA

4. Functions of Nucleic Acid- • Store information that determines the characteristics of the cells and organisms. • Protein synthesis. • Change the genetic characteristics that are transmitted to future generations • Replicate prior to reproduction

5. DNA and importance of Proteins • The cell’s ability to make a particular protein comes from the genetic information stored in the cell’s DNA. DNA contains genes, which are specific messages about how to construct a protein. • Most of an organism’s characteristics are direct result of proteins.

6. DNA-structure and function • In 1944,Oswald Avery provided the first evidence that DNA is the genetic molecule. • Rosalind Franklin used X-ray crystallography to determine DNA’s ,width, length and helical shape. • Finally, James Watson and Francis Crick created a model for the structure of DNA in 1952 and were awarded the Nobel prize for their work.

7. DNA structure- • Nucleic acids are large polymers made up of many repeating units called nucleotides. Each nucleotide is composed of- • a sugar molecule, a phosphate group and nitrogenous base. • Four different types of nitrogenous bases are: Adenine(A), Guanine(G), Cytosine(C), Thymine(T).

8. Sugar + Phosphate + Base=Nucleotide

10. DNA structure-Double helix • The DNA nucleotides can combine into a long linear DNA molecule that can pair with another linear DNA molecule. • The two paired strands form a double helix, with the sugars and phosphate on the outside and nitrogenous bases in the inside of the helix. • Base pair rule-It states that Adenine pairs with thymine and Guanine with Cytosine.

11. Base pairing in DNA replication-

12. Arrangement of nucleotides in DNA-In figure below notice that one strand ends with number 3’ while other with 5’, this is because the two strands run in opposite directions.

13. Adenine-ThymineGuanine-Cytosine • That is because these particular pairs fit exactly to form very effective hydrogen bonds with each other. It is these hydrogen bonds which hold the two chains together. • Adenine pairs with Thymine with double hydrogen bond. • Guanine pairs with cytosine with triple hydrogen bond.

14. A-T G-C

16. DNA Replication- • When a cell grows and divides , two new daughter cells are formed. Both daughter cells need DNA to survive, so the DNA of the parent is copied. • One copy is provided to each new cell. • DNA replication- It is the process by which a cell makes copies of its DNA. • DNA replication depends on DNA base-pair rule and many enzymes.

17. The general process of DNA replication involves following steps-1. DNA replication begins as enzymes, called helicases, attach to the DNA and separate the two strands. This forms a replication bubble and is known as “replication fork”.2. As helicases separate the two strands, another enzyme, DNA polymerase helps attach new nucleotides one at a time onto the surface of the strands. Nucleotides enter as per base-pair rule.

18. The new strands of DNA form on each of the old DNA strands.

20. DNA Repair- • Although DNA replication is highly accurate but errors and damage do occur to DNA helix. • When one strand is damaged, it is possible to rebuild this strand by using the nucleotide sequence on the other side along with the enzymes.

22. The DNA Code- • DNA stores information. • The order of the nitrogenous bases in DNA is the genetic information that codes for proteins. • The nitrogenous bases are read in sets of three. • Each sequence of three bases is a code word for a single amino acids. • Thousands of amino acids join together to form a protein. • ACC is a code word for the amino acid-Tryptophan.

23. RNA structure and Functions • Ribonucleic acid is another type of nucleic acid and is important in protein synthesis. • RNA’s nucleotides are different from DNA’s nucleotides. • Ribose sugar is present in RNA and has an –OH group where as in DNA deoxyribose sugar with –H group is present. • RNA has Uracil, Guanine, Cytosine and Adenine. • Adenine-Uracil and Guanine –Cytosine instead of Adenine-Thymine in DNA. • RNA is single stranded where as DNA –double stranded.

24. DNA Deoxyribose sugar -H group present A,T,G and C bases A-T and G-C Double stranded DNA is found in the nucleus and is the original source to make proteins. Base sequence is responsible for DNA code. DNA replication follows base pair rule. RNA Ribose sugar -OH group A,U, G and C A-U and G-C Single stranded. RNA is made in the nucleus and then moves into the cytoplasm where it is involved in protein assembly. The protein coding information in RNA comes directly from DNA. RNA sysnthesis also follows the base pair rule but different from DNA. Differences-

25. RNA structure-single strand

26. Concept Review • Genetic information is stored in what type of chemicals? • Proteins • Lipids • Nucleic acids • Sugars

27. Question • The difference between ribose and deoxyribose is- • The no. of carbon atoms. • An oxygen atom • One is sugar and one is not • No difference.

28. Questions • What is a gene? • What are the functions performed by nucleic acids? • What is a base –pair rule? • Why is DNA replication necessary? • What role does RNA play in the cell? • Describe three differences in the structure of DNA and RNA.