How genetic material organized on a chromosome

1. Organization of
genetic material on
chromosome

2. What is Genetic material?
 The genetic material of a cell or an organism refers to those
materials found in the nucleus, mitochondria and
cytoplasm, which play a fundamental role in determining
the structure and nature of cell substances, and capable of
self-propagating and variation.
 A chromosome is a DNA molecule with part or all of the
genetic material (genome) of an organism.
 The genetic information of most living organisms is stored
in deoxyribonucleic acid (DNA).
 Chromosomes vary widely between different organisms.
 Some species such as certain bacteria, which lack histones,
also contain plasmids or other extrachromosomal DNA.

3.  Each eukaryotic chromosome contains one giant molecule of
DNA packaged into 10 nm ellipsoidal beads called nucleosomes.
 In prokaryotes and viruses , the DNA is often densely packed
and organized.
 DNA usually exists as a double helix, with two strands of
opposite polarity, held together by hydrogen bonds between
the complementary bases.
 Adenine is paired with thymine with double hydrogen bonds,
whereas guanine is paired with cytosine with triple hydrogen
bonds.
 The specific base sequence and their complementarity make it a
unique feature for each organism to store and transmit genetic
information.

4.  DNA is not only a genetic material in a cell or an organism.
 RNA also used by viruses as a genetic material.
 Ribonucleic acid (RNA) usually exists as a single-stranded
molecule containing uracil instead of thymine.
 The main function of the genetic material is to store
information required to produce an organism
 The DNA molecule does that through its base sequence.
DNA sequences are necessary for
1. Synthesis of RNA and cellular proteins
2. Proper segregation of chromosomes
3. Replication of chromosomes
4. Compaction of chromosomes

5.  Genotypic Function: Replication
 Phenotypic Function: Gene Expression
 Evolutionary: Allows for Mutation
Properties Of Genetic Material
1. Repository of genetic information
2. Info must be accessible, allow cell to respond
3. Info must be in form transmissible to progeny
4. Physical and chemical stability
5. Potential for heritable change
Functions of the Genetic Material

6.  The basic differences between the prokaryotes and the
eukaryotes is that the prokaryotes are unicellular and lack
any membrane bound organelles such as the nucleus,
mitochondria, chloroplast, lysosome etc.
 while eukaryotes on the other hand have well defined
membrane bound organelles each with its distinct
structure and function.
 In addition to the chromosomal DNA, eukaryotes contain
organelle DNA in the mitochondria (in animal cells) and
chloroplast (in plant cells).
 While Prokaryotes also contain extrachromosomal DNA
but in the form of plasmids that replicate within the same
cell.

7.  Apart from these differences, their genetic makeup (i.e.,
organization) of DNA also differs.
1. Organization of DNA in prokaryotes
2. Organization of DNA in eukaryotes
Organelle DNA: Mitochondria and Chloroplast
3. Organization of DNA in viruses

8. Organization of DNA in prokaryotes
 Prokaryotic cells do not contain a nucleus or any other
membrane-bound organelle.
 Prokaryotes generally have a small, circular (sometimes
linear) DNA present in the nucleoid region. There is a single
origin of replication.
 The genome size ranges in between 104 to 107bp with a high
gene density.
 The entire genome comprises almost of genes.
 They are polycistronic i.e. multiple genes are transcribed
through the same promoter.

9. BACTERIAL GENOME
 Bacterial chromosomal DNA is usually a circular molecule that is a
few million nucleotides in length.
Escherichia coli :: 4.6 million base pairs
Haemophilus influenzae :: 1.8 million base pairs
 A typical bacterial chromosome contains a few thousand different
genes
 Structural gene sequences (encoding proteins) account for the
majority of bacterial DNA
 The no transcribed DNA between adjacent genes are termed
intergenic regions

10. Key Features
 Most, but not all bacterial species
contain circular chromosomal DNA.
 A typical chromosomes is a few
million base pairs in length.
 Most bacterial species contain a
single type of chromosome, but it
may be present in multiple copies.
 Several thousand different genes are
interspersed throughout the
chromosome.
 One origin of replication is required
to initiate DNA replication.
 Short repetitive sequences may be
interspersed throughout the
chromosome.

11.  To fit within the bacterial cell, the chromosomal DNA must be
compacted about a 1000-folds. This involves the formation of
loop domains. The number of loops varies according to the size of
the bacterial chromosome and the species.
 E. coli has 50-100 with 40,000 to 80,000 bp of DNA in each.

12. • DNA super coiling is a second important way to compact
the bacterial chromosome.
• Supercoiling within loops creates a more compact DNA

13. Organization of DNA in eukaryotes
 Eukaryotic species contain one or more sets of Chromosomes.
 The total amount of DNA in eukaryotic species is typically greater
than that in bacterial cells
 They have multiple linear chromosomes (2 to <50).
 The genome size ranges from 108 to 1011bp that encodes a large
number of proteins.
 Chromosomes in eukaryotes are located in the nucleus.
 They have a low gene density due to the presence of enormous
amounts of non-coding regions that include the introns
(intervening sequences within the genes, split genes),
intervening sequences (sequences between the genes)and
genome wide repeats.

14.  They are monocistronic i.e. one gene is under the control of
one promoter.
 The chromosomes have multiple origin of replication sites
throughout its length (30-40kb apart).
 Centromeres, telomeres, heterochromatic and euchromatic
regions are distinctly present on the chromosome.
 Chromosome duplication and segregation takes place during
different phases of the cell cycle which is regulated by several
checkpoints.
 Three types of RNA polymerases are present of which RNA Pol
II is involved in transcription.
 The primary transcript contains exons (coding sequences) along
with the intervening introns (non-coding sequences).

15.  Another feature of the eukaryotic genome is selective
amplification i.e. specific genes are expressed in specific cell
types.
 For synthesis of functional proteins, the introns are removed
by RNA splicing. Alternate splicing also exists that lets many
proteins to be synthesized from the same mRNA by different
combinations of exons.
 Three types of DNA sequences are required for chromosomal
replication and segregation
1. Origins of replication
2. Centromeres
3. Telomeres

16.  Eukaryotic chromosomes are usually
linear.
 A typical chromosome is tens of millions to
hundreds of millions of base pairs in
length.
 Eukaryotic chromosomes occurs in sets.
Many species are diploid. Which means
that somatic cells contains 2 sets of
chromosomes.
 Each chromosome contains a centromere
that forms a recognition site for the
kinetochore proteins.
 Telomere contains specialized sequences
located at both ends of the linear
chromosomes.
 Repetitive sequences are commonly found
near centromeric and telomeric regions.
A Typical Chromatid
Key Features

17.  DNA is about 3 meters long and it has to be packed in a
nucleus, which is only a few micrometers in a diameter.
Hence highly coiled structure is required.
 Chromatin is the material of which the chromosomes of an
organisms other than bacteria (i.e. eukaryotes) are
composed, consisting of protein, RNA, and DNA.
 The compaction of linear DNA in eukaryotic chromosomes
involves interactions between DNA and various proteins
 The DNA + histone = chromatin
 The primary function of chromatin is to compress the DNA
into a compact unit that will be less voluminous and can fit
within the nucleus.
Eukaryotic Chromatin Compaction

18. NUCLEOSOMES
• It is a basic unit of chromatin
structure , consisting of histone
octamers wrapped in 146 base pairs
(bps) of DNA
or
• The repeating structural unit within
eukaryotic chromatin is the
nucleosome.
• It is composed of double-stranded DNA wrapped around an octamer
of histone proteins.
• An octamer is composed two copies each of four different histones

19. • Structure of connected nucleosomes resembles “beads on a
string.
• This structure shortens the DNA length about seven-fold
Nucleosome showing core histone.

20. Formation of Nucleosomes
Core particle Chromatosome Nucleosome
 Core Particle:
It consist of 146 bp of DNA wrapped 1.8 times in a left handed
helix around the outside of an octamer of histone.
 Chromatosome :
The core particle interacts with one molecule of histone H1 to
form a particle containing 166bp of DNA called chromatosome
 Nucleosome :
The chromatosome links with the linker DNA forming a
nucleosome containg 200 bp of DNA

21. Chromosomal Proteins
 The chromosomes of eukaryotes are made up of DNA
and proteins.
 There are 2 major types of proteins associated with
DNA in the chromatin.

22.  Histone proteins are basic.
 There are five types of histones
 1. H1 3. H2B
2. H2A & 4. H3 5.H4
 Two of each make up the octamer
 Each histone consist of a :
N - terminal , which is hydrophobic.
C – terminal , which is hydrophilic.
 Central globular structure , which forms the central
molecule.
 H1 is the linker histone Binds to linker DNA also binds to
nucleosomes but not as tightly as are the core histones

23.  Histones lack tryptophan.
 At normal pH of the cell the histones have net positive
charge that facilitates their binding to the negatively
charged DNA
 This positive charge is found mainly on the amino group of
the side chains of the basic amino acids.
 They contain many positively-charged amino acids
1. Lysine
2. Arginine
 These bind with the phosphates along the DNA backbone

24. Histone type Common basic
residues
Molecular weight
H1 Lysine rich 23 kDa Protein
H2A Slightly serine rich
more lysine rich
13,960 Da protein
H2B Slightly serine rich
more lysine rich
13,744 Da protein
H3 Arginine rich 15,242 Da protein
H4 Arginine rich 11,282 Da protein

25. Non Histones
 They are all the proteins associated
with the DNA apart from the
histones.
 They are very different from
histones.
 They are acidic proteins
i.e. have a net negative charge and
likely to bind the positive charged
histones.
 Play a role in the organization and
compaction of the chromosomes.
Nucleosomes showing linker
histones and non histones
proteins.

26. Solenoid model of Nucleosomes
 According to this model , the 10 nm fiber of nucleosomes gets coiled upon
itself to form 30nm wide helix.
 This 30 nm structure is called as solenoid. It has 5 or 6 nucleosomes per
helix.
 The histone N-terminal tails direct the DNA to wrap around the histone
octamer disc.
 These N-terminals are thus required for the formation of 30 nm fiber as they
interact with adjacent nucleosomes by making multiple H-bonds and thus
stabilizing the 30 nm fiber.

27. DNA Packaging

28. Organelle DNA: Mitochondria and Chloroplast
 In eukaryotic cells mitochondria (in animal) and chloroplasts (in
plants) contain organelle DNA in addition to the nuclear DNA.
 mtDNA is circular loop with a size of 16.5 kb. It is not associated
with any histone proteins and is susceptible to somatic mutations.
 The DNA has four main regions – D loop, r RNA genes, t RNA
genes and the protein coding genes.
 Chloroplast DNA on the other hand consists of closed circular
DNA with an approximate size of 1,21,024bp. It contains nearly
128 genes. The number of intervening sequences is very low.
 According to the Endosymbiotic Theory proposed by Lynn
Margulis, mitochondria and chloroplast were a result of
endocytosis of aerobic bacteria and photosynthetic bacteria
respectively.

29. Organization of DNA in viruses
 Viruses do not have cells. A viral particle is made up of
nucleic acids in the core and is enclosed within a protein
capsule (or capsid).
 A viral genome is a term used as in whole of the genetic
material. Also termed the viral chromosome
 Their genomes may have DNA or RNA in single stranded
(ss) or double stranded (ds) & Circular or linear in form as
the genetic material.
 The topology of the genetic content can be linear, circular
or even in segmented form.

30.  DNA viruses are also categorized into small and large DNA
viruses.
 They have a genome organization with many features
similar to eukaryotic genomes.
 DNA is associated with histone proteins.
 Since viruses are obligate intracellular parasites, their
genetic code matches that of the host machinery.
 The viral genome contains all the essential genes that the
organism requires for perpetuation which includes genes
for synthesis of RNA, proteins, replication, compaction to
fit within the capsid and segregation of the chromosomes.

31. GENERAL STRUCTURE OF VIRUSES
Lipid bilayer Picked up
when virus leaves host cell

32.  Bacteriophage is very common virus consists of Nucleic Acid+ protein
which is usually enclosed by 3 types of capsid structure.
• Icosahedral
• Filamentous
• Head and tail
Phage Host Shape Genome Genome
Size (kb)
MS² E.coli Icosahedral SS linear
RNA
3.6
M13 E.coli Filamentous SS linear
RNA
6.7
T₇ E.coli Head and
tail
DS linear
DNA
39.9