The document discusses several key concepts related to DNA packaging and gene expression. It describes the central dogma where genetic information flows from DNA to RNA to proteins. It also discusses how DNA is packaged in cells through nucleosomes and chromatin, and how chromatin exists in two forms - euchromatin which is loosely packed and transcriptionally active, and heterochromatin which is tightly packed and transcriptionally inactive. It provides information on calculating DNA length and the number of base pairs.

1. • Central dogma
• Packaging of DNA Helix
• Nucleosome
• Euchromatin and Heterochromatin..

2. Central DOGMA

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

4. • 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

5. Packaging of DNA Helix

6. • 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 ?

7. 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

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

10. In prokaryotes
• 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.

11. In eukaryotes
• 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.

12. • Histones are rich in the basic amino acid residues lysines
and arginines.
• Both the amino acid residues carry positive charges in
their side chains.

14. • Histones are organised to form a unit of eight molecules
called as histone octamer.
• The negatively charged DNA is wrapped around the
positively charged histone octamer to form a structure
called nucleosome..

16. • A typical nucleosome contains 200 bp of DNA helix.
• Nucleosomes constitute the repeating unit of a
structure in nucleus called chromatin.
•

17. • Chromatin thread-like stained (coloured) bodies seen in
nucleus.
• The nucleosomes in chromatin are seen as ‘beads-on-
string’ structure when viewed under electron
microscope (EM) ..
•

18. • The beads-on-string structure in chromatin is packaged
to form chromatin fibers..
• Chromatin fibers are further coiled and condensed at
metaphase stage of cell division to form chromosomes.

20. Non-histone Chromosomal (NHC) proteins.
• The packaging of chromatin at higher level requires
additional set of proteins that collectively are referred to
as Non-histone Chromosomal (NHC) proteins.

21. Euchromatin and heterochromatin

22. Euchromatin
• In a typical nucleus, some region of chromatin are
loosely packed (and stains light) and are referred to as
euchromatin.
• Euchromatin is said to be transcriptionally active
chromatin.

23. Heterochromatin
• The chromatin that is more densely packed and stains
dark are called as Heterochromatin.
• Heterochromatin is transcriptionally inactive..

24. • Shine dalgarno sequence:

25. • A sequence of five to nine (typically seven)
nucleotides length.
• Present before the start codon in prokaryotic
messenger RNA (mRNA) that is recognized by the
small subunit of ribosome during initiation of
translation.

26. P and Q arm of chromosome

28. Punctuation codons
• There are two punctuation marks in the genetic
code called the START and STOP codons ..

29. No. of nucleosome in Diploid human cell
• A diploid human cell contain approximately 30
million nucleosomes..

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