Genetics and Genomics, Microbial genetics, Genome

1. BY
Dr. Sonia Goel
SGT University, Gurugram.

2. 1. Prokaryotic genome
 Bacterial nuclear body(E.coli)
 Bacterial chromosome: circular and supercoiled
2. Eukaryotic genome
 Eukaryotic chromosome

3.  Bacteria are very small unicellular organisms
that do not contain nuclear envelope,
mitochondria, endoplasmic reticulum, mitotic
apparatus and nucleolus etc., and divide by
fission.
 Bacteria have a rigid cell wall which surrounds
their cytoplasmic membrane.
 Their cytoplasm contains ribosomes. mesosomes
and several granular inclusions.
 About 1/5 of the cell volume is occupied by DNA,
the genetic material.

4.  Bacterial cells do not contain a typical nucleus. But
Feulgen reaction shows one, two or more discrete
nuclear bodies per cell; these are called nucleoids.
The bacterial genome is confined to this nucleoid,
which is more or less compact structure without any
membrane.
 When bacterial cells are lysed in the presence of high
salt concentration, nucleoids can be recovered
intact. The isolated nucleoid may be membrane free
or it may be associated with membrane (mesosome).
 The constituents of the membrane-free nucleoid are
DNA (~ 60%) RNA (-30%) and protein (- 10%) DNA forms
2-3% of the dry weight of a bacterial cell).

5.  Bacterial chromosome is a double-stranded
circular DNA. In general, bacterial DNA ranges
from 1100 pm to 1400 µm in length.
 An E. coli cell contains 4. 2 x 106kbp DNA which
is about 1.3 mm (1300 µm) in length.
 Such a long DNA molecule must be greatly folded
to be packaged in a small space of 1.7 x 0.65
µm.
 The bacterial chromosome is folded into loops
or domains which are about 100 in number.

7.  DNA domain: A chromosomal domain may be defined as
a discrete structural entity within which supercoiling
is independent of the other domains.
 Thus different domains can maintain different
degrees of supercoiling. The DNA chain is coiled on
itself to produce supercoiling . The ends of the loops
or domains are bound in some way which does not
allow rotational events to propagate from one
domain to another.
 If an endonuclease puts a nick in DNA strand of one
domain, this loop becomes larger due to the
uncoiling, but the other domains are not affected.
Each domain contains about 40 kbp (13 µm) of DNA.
 The loops are bound by some mechanism that may
involve proteins and/or RNA but the mechanism is not
clearly understood.

8.  DNA binding proteins: In E. coli, a number of
proteins have been isolated which have some
similarities with the eukaryotic chromosomal
proteins. These proteins are HU, IHF (integration
host factor).
 HI (H-NS) and R It is suspected that HU is
involved in the nucleoid condensation.
 The protein HI probably has effects on gene
expression.
 IHF, RNA polymerases and mRNA may help to
organise the nucleoid.

9.  In eukaryotic cells, the chromosomes are
present in a distinct, double-membrane
bound structure, the nucleus which occupies
on the average about 10% of the cell volume.
 The membrane is continuous with the
endoplasmic reticulum and is provided with
pores. The number of chromosome is
variable, but fixed for a biological species.
 Chromosomes change in their physical
characteristics during cell division. All these
features are absent in the prokaryotic cells.
 An individual eukaryotic chromosome
contains a single enormously large linear ds-
DNA molecule.

10.  The DNA-binding proteins are distinguished into two
main types — the histones and non-histone proteins.
 The histones are basic proteins, rich in basic amino
acids, like lysine and arginine. Histones have large
amount of positive charges and can bind tightly the
negatively charged DNA molecules.
 These binding results in the formation of the
characteristic structural units called the
nucleosomes.
 The long ds-DNA molecule of each chromosome is
folded in a very orderly way around the histones to
form the nucleosomes.

11.  All the eukaryotic cells have 5 types of histones :
1. H1 histone: associated with linker DNA
2. H2 histone
3. H2B histone
4. H3 histone
5. H4 histone
 The nucleosomes are basic structures from
which chromatin is made. They are further
organized into closely packed 30 nm fibres of
chromatin.

14.  Chromosomes are threadlike deeply stained
compact DNA protein complex that carry genetic
information in a linear sequence of genes.
 They are the physical basis of heredity or
hereditary vehicles as they store, replicate,
transcribe and transmit the genetic information.
 Though the scope of the term include the
bacterial nucleoid (prochromosome), organelle
genomes, viral genomes and eukaryotic nuclear
chromosome, but only the last one considered as
typical chromosome.

15.  Chromosome number:
 Each species contain a fixed number of
chromosomes (Beneden and Boveri, 1087).
However, change in chromosome number can be
seen in a species and is called as polyploidy
(euploidy and aneuploidy).
 Normally gamete or gametophyte cells contain
one set of chromosomes called genome and the
cells are called naploid. The somatic cells of
animals and sporophytes have two haploid sets
or genomes and are said to be diploid cells.

16. A typical chromosome has following parts:
(a) Centromere (Primary constriction):
A metaphase chromosome has two identical sister
chromatids, which are attached to each other at a point
called centromere.
Based on the position of centromere, chromosomes
are called:
(i) Telocentric (centromere terminal),
(ii) Acrocentric (centromere subterminal and capped
by telomere),
(iii) Sub-metacentric (centromere is submedian),
(iv) Metacentric (centromere median).

18.  Telomeres:
The terminal ends of chromosomes are called
telomeres. A telomere is a short repeated DNA
sequence (GC rich) complexed with proteins. They
are synthesized separately and later add to the
chromosomal tips.
 Ultra-structure of Chromosome:
A metaphase chromosome has two sister chromatids,
each about 700 nm in diameter. The electron
microscopic studies have revealed that, each
chromatid consists of a central non histone core
called scaffold or nuclear matrix, from which loop of
30 nm chromatin fiber out radially. Each lateral loop
is about 300nm long and has up to 100 Kb of DNA.