B- DNA Nucleic acid discovery

1. Molecular Basis of
Inheritance
• How to calculate length of DNA?
• B-DNA by Watson and Crick
• Chargaff's rules

2. DNA and RNA are Genetic materials
• Deoxyribonucleic acid (DNA) and Ribonucleic acid
(RNA) are the two types of nucleic acids found in
living systems…
• DNA acts as the genetic material in most of the
organisms.
• RNA also acts as a genetic material in some viruses….

3. RNA has additional roles..
• Mostly RNA functions as a messenger RNA (m-RNA).
• RNA functions as adapter (t-RNA/transfer RNA).
• RNA functions as Structural RNA (r-RNA/ Ribosomal RNA).
• In some cases function as a catalytic molecule
(Enzyme/Ribozyme).

4. Nucleic acids (DNA/RNA) are polymers
nucleotides
• Nucleotides  DNA DNA+ Histone proteins 
Nucleosomes Chromatin  Chromosomes Genome

5. In this chapter we are going to discuss
• the structure of DNA.
• DNA replication,
• the process of making RNA from DNA (transcription).
• the genetic code that determines the sequences of amino
acids in proteins.
• the process of protein synthesis (translation).
• Gene regulation.

6. THE DNA
• DNA is a long polymer of deoxyribonucleotides.
• A nucleotide has three components:
• a nitrogenous base,
• a pentose sugar (ribose in case of RNA, and deoxyribose
for DNA),
• A phosphate group.

7. The length of the DNA
• The length of DNA is usually defined as number of
nucleotides..
• or a pair of nucleotide referred to as base pairs present in
it.

8. Single strand DNA
• The length of ss DNA is usually
defined as number of
nucleotides..

9. • The length of ds DNA is
usually defined as
number of nucleotides..

10. A Bacteriophage
known as φ ×174 has
5386 nucleotides
Bacteriophage lambda
has
48502 base pairs (bp)

11. Haploid content of human DNA is
3.3 × 109 bp.
Escherichia coli has 4.6 × 106 bp

12. Structure of Polynucleotide Chain
• DNA is a polynucleotide chain.
• A nucleotide has three components:
1. a nitrogenous base,
2. a pentose sugar (ribose in case of RNA, and
deoxyribose for DNA),
3. A phosphate group.

13. There are two types of nitrogenous bases
• Thymine is present in DNA. Uracil is present in RNA at
the place of Thymine.
Purines:
(Adenine and Guanine):
Pyrimidines
(Cytosine, Uracil and
Thymine).

14. • In DNA
• A=T
• G=C
• In RNA
• A=U
• G=C
• Thymine also called as 5-methyl uracil…

15. Deoxyribose sugar and Ribose sugar

16. N-glycosidic
linkage
Phosphate group is linked to 5' -OH of a
nucleoside through phosphoester
linkage,

17. Uracil Uridine Uridylic acid
Nitogen base +Sugar +Phosphate

18. • Two nucleotides are linked through 3'-5' phosphodiester
linkage to form a dinucleotide.
• More nucleotides can be joined in such a manner to form
a polynucleotide chain..

19. • One end of polymer is called 5’-end contain free
phosphate moiety at 5' -end of ribose sugar.
• The other end of the polymer the ribose has a free 3' -OH
group which is referred to as 3'-end of the polynucleotide
chain.

20. • The backbone in a
polynucleotide
chain is formed
due to sugar and
phosphates.

21. • DNA as an acidic substance present in nucleus was first
identified by Friedrich Meischer in 1869.
• He named it as ‘Nuclein’.
Who DNA First discovered?

22. • In 1953 that James Watson and Francis Crick, proposed a
famous Double Helix model of DNA based on the X-ray
diffraction data.
• X-ray diffraction data. produced by Maurice Wilkins
and Rosalind Franklin.
Who proposed Secondary
helical structure of DNA?

23. • One of the hallmarks of Watson and crick proposition
was base pairing between the two strands of
polynucleotide chains.
• However, this base pairing proposition was also based on
the observation of Erwin Chargaff rule.

24. • Erwin Chargaff that for a double stranded DNA.
• The ratios between Adenine and Thymine and Guanine
and Cytosine are constant and equals one.
Erwin Chargaff rule

25. • The base pairing confers a very unique property to the
polynucleotide chains..
• DNA strands are complementary to each other.

26. • During replication each DNA
strand acts as a template for
synthesis of a new strand.
• After replication the two double
stranded DNA produced would
be identical to the parental DNA
molecule.

27. The salient features of the Double-helix structure of
DNA

28. • The two chains are complementary and anti-parallel
polarity each other.
• It means, if one chain has the polarity 5‘3', the other
has 35‘.

29. • Hydrogen bonds generates uniform distance between the
two strands of the helix..
• The plane of one base pair stacks over the other in
double helix.
• This, in addition to H-bonds, confers stability of the
helical structure..

30. If DNA contains 10000 base pairs then the length of this DNA?
is?
• Ans: 3.4 Å × 10,000 = 34,000 Å = 3.4 µm.

31. The length of ds DNA is 136 Å, How many base pairs are
present?

32. Central dogma
• Francis Crick proposed the Central dogma.
• The flows of genetic information from DNA to RNA to proteins
is called Central dogma.

33. • In some viruses (RNA viruses) the flow of information is
in reverse direction, that is, from RNA to DNA.
RNA  DNA
• Synthesis of RNA form DNA is called Reverse transcription.
• Enzyme involved in this process is Reverse transcriptase..
• or RNA dependent DNA Polymerase enzyme..
Reverse transcription

35. Packaging of DNA Helix

36. • The length of DNA is calculated by multiplying the total
number of bp with distance between two consecutive bp.
• The length of DNA = Total no.of base pairs × Distance
between two base pairs.
• For example: 6.6 × 10 9 bp × 0.34 × 10-9 m/bp = 2.2 metres.
How to calculate length of DNA ?

37. If the length of E. coli DNA is 1.36 mm, can you calculate the
number of base pairs in E.coli?
– 1.36mm is DNA length = 1.36 × 10-3 M
– 1.36 × 10-3 M / 0.34 × 10-9 M
– 4.6 × 10 6 base pairs
Total no of base pairs= Total length of DNA / Distance between two base pairs

38. Prokaryotes DNA
• In prokaryotes, such as, E. coli, though they do not have
a defined nucleus.
• The DNA is not scattered throughout the cell.

39. • DNA (being negatively charged) is held with some
proteins (that have positive charges) in a region termed
as ‘nucleoid’.
• The DNA in nucleoid is organised in large loops held by
proteins.

40. . How is such a long polymer packaged inside
nucleus?
• The length of DNA greater than the dimension of a
typical nucleus (approximately 10-6 m)…
• A set of positively charged, basic proteins called
histones involved in packaging of DNA.
• Histones are rich in the basic amino acid residues lysines
and arginines.

41. Dr. HarinathaReddy Aswartha
Assistant professor
Department of Microbiology
ANDHRAPRADESH
INDIA