Organization of genome in Prokaryotes:
The term prokaryote means “primitive nucleus”. Cell in prokaryotes have no nucleus. The prokaryotic chromosome is dispersed within the cell and is not enclosed by a separate membrane. Much of the information about the structure of DNA comes from studies of prokaryotes, because they are less complex than eukaryotes. Prokaryotes are monoploids they have only one set of genes (one copy of the genome). In most viruses and prokaryotes, the single set of genes is stored in a single chromosome (single molecule either RNA or DNA).
Organization of genome in Prokaryotes:
The term prokaryote means “primitive nucleus”. Cell in prokaryotes have no nucleus. The prokaryotic chromosome is dispersed within the cell and is not enclosed by a separate membrane. Much of the information about the structure of DNA comes from studies of prokaryotes, because they are less complex than eukaryotes. Prokaryotes are monoploids they have only one set of genes (one copy of the genome). In most viruses and prokaryotes, the single set of genes is stored in a single chromosome (single molecule either RNA or DNA). Organization of genome in Prokaryotes:
The term prokaryote means “primitive nucleus”. Cell in prokaryotes have no nucleus. The prokaryotic chromosome is dispersed within the cell and is not enclosed by a separate membrane. Much of the information about the structure of DNA comes from studies of prokaryotes, because they are less complex than eukaryotes. Prokaryotes are monoploids they have only one set of genes (one copy of the genome). In most viruses and prokaryotes, the single set of genes is stored in a single chromosome (single molecule either RNA or DNA).
3. WHAT IS A CHROMOSOME ?
A chromosome is a long DNA
molecule with part or all of the
genetic material of an organism.
Most eukaryotic chromosomes
include packaging proteins called
histones to condense the DNA
molecule to maintain its integrity.
5. GENOME IN PROKARYOTES
The term prokaryote means “primitive nucleus”.
Cell in prokaryotes have no nucleus.
The prokaryotic chromosome is dispersed within
the cell and is not enclosed by a separate
membrane.
Much of the information about the structure of
DNA comes from studies of prokaryotes, because
they are less complex than eukaryotes.
Prokaryotes are monoploids they have only one
set of genes (one copy of the genome).
The bacterial genome is confined to the nucleoid,
which is more or less compact structure without
any membrane.
The constituents of membrane-free nucleoid are
DNA (~ 60%), RNA (~30%) and Protein (~10%).
In addition to the DNA in nucleoids, they have an
extra circular DNA called plasmid.
nucleoid
6. ORGANIZATION OF DNA IN PROKARYOTES
In general, bacterial DNA ranges from 1100 µm to 1400 µm in length. A long DNA molecule is greatly folded into
loops (40-50). The DNA chain in a loop is coiled on itself to produce supercoilings called chromosomal domain.
Supercoils are induced in the loops by DNA gyrase.
7. DNA BINDING PROTEINS
• The loops in E.coli are bound by
some mechanism that may involve
proteins and or RNA but the
mechanism is not clearly
understood.
• 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). H1 (H-NS)
and P. It is suspected that HU is
involved in the nucleoid
condensation.
8. EUKARYOTIC CHROMOSOME
• Eukaryotic chromosome structure refers to
the levels of packaging from the
raw DNA molecules to the chromosomal
structures seen during metaphase in mitosis
or meiosis.
• Chromosomes contain long strands of DNA
containing genetic information.
• The components of eukaryotic
are DNA, RNA, histone, and non-histone
proteins, metallic ions, etc.
• Proteins: Two types of proteins, i.e., histone
and non-histone proteins are found
associated with DNA in chromosome.
• Histones (H1, H2A, H2B, H3 & H4) are rich
basic amino acids arginine and lysine. H1,
H2A, H2B are rich in lysine whereas H3 &
9. ORGANIZATION OF DNA IN EUKARYOTIC CHROMOSOMES
• Nucleosome Solenoid model is a scientific model
which explains the organization of DNA and
associated proteins in the chromosome.
• The model was proposed by Roger Kornberg in
1974 and is the most accepted model of
chromatin organization.
• In eukaryotes, DNA is tightly bound to an equal
mass of histones, which serve to form a
array of DNA-protein particles, called
nucleosomes.
• If it was stretched out, the DNA double-helix in
each human chromosome would form very thin
thread, about 6 feet (2 meters) long.
• Histones play a crucial role in packing this very
long DNA molecule in an orderly way (i.e.,
nucleosome) in nucleus of only a few
in diameter.
12. TELOMERE
Telomeres are the region of DNA at the end of
the linear eukaryotic chromosome that are
required for the replication and stability of the
chromosome.
The ends of broken chromosomes are sticky,
whereas the normal ends are not sticky.
CENTROMERE
The region where two sister chromatids of a
chromosome appear to be joined or “held together”
during mitotic metaphase is called Centromere.
Within the centromere region, most species have
several locations where spindle fibers attach, and
these sites consist of DNA as well as protein.
The actual location where the attachment occurs is
called the kinetochore and is composed of both
DNA and protein.
The DNA sequence within these regions is called
CEN DNA. Typically, CEN DNA is about 120 base
pairs long and consists of several sub-domains, CDE-
I, CDE-II and CDE-III.
13. GENE STRUCTURE
• Genes are made of DNA, where the particular DNA sequence determines
the function of the gene.
• A gene is transcribed (copied) from DNA into RNA, which can either be
non-coding (ncRNA) with a direct function, or an intermediate messenger
(mRNA) that is then translated into protein.
• Each of these steps is controlled by specific sequence elements, or regions,
within the gene.
• These regions may be as short as a few base pairs, up to many thousands
of base pairs long.
16. GENE STRUCTURE IN PROKARYOTES
A transcription unit is that stretch of
DNA that is transcribed into a single
RNA molecule. A typical transcription
unit has.
1) A promoter at its beginning
2) A start point
3) A coding region and
4) A terminator sequence at its end
In case of prokaryotes, genes encoding
the enzymes for a single biosynthetic
pathway are usually clustered together
into a single regulatory unit called
operon. Each operon functions as a
single transcription unit.
20. LAC OPERON
• The lac operon consists of 3 structural genes, and a promoter, a terminator, regulator, and an operator. The three
structural genes are: lacZ, lacY, and lacA.
• LacZ: It encodes the enzyme β-galactosidase which converts lactose into allolactose (isomeric form) and also
breaks a β-galactoside (lactose) into glucose and galactose
• LacY: It encodes the enzyme galactoside permease. It is transporter protein which facilitates the entry of lactose
into cell
• LacA: It encodes the enzyme β-galactoside transacetylase. It modify toxic galactoside by transferring an acetyl
group from acetyl-CoA to β-galactosides and remove out of cell.
• Regulatory gene include Lac I along with Promoter and Operator sequence. Regulatory gene regulate transcription
of structural gene.
Lac I: It encodes Repressor protein. The repressor protein binds to operator near promotor.
Promoter: It is the binding site for RNA polymerase.
Operator: It is the binding site for repressor protein.
24. EUKARYOTIC PROMOTER
• Eukaryotic promoters are defined as regions that can support transcription at normal efficiency and with proper control. The
promoters for RNA polymerase II usually have the following modules or functional sequences.
• Initiator: It is the region that contains startpoint. It is recognized by RNA polymerase II. The choice of startpoint seems to
depend on the location of TATA box.
• TATA Box: It is a consensus sequence of 8 bases TATAAAAA, having only A.T base pairs and is the only consensus sequence
that occupies a fixed position in the promoter at -25. It is usually surrounded by a G.C rich sequence.
• CAAT Box: This is sequence is located ~ 80 bp upstream of the start point, has the consensus sequence GGCCAATCT and it
increases promoter strength.
• GC Box: It has the consensus sequence GGGCGG, is usually located at -90 and may be present in several copies in a promoter.
