Upload
hosny2020
View
228
Download
0
Embed Size (px)
Citation preview
8/7/2019 Genetics(4) Amany
1/32
Prof. Dr. AmanyTohamy
8/7/2019 Genetics(4) Amany
2/32
ORGANIZATION OF THE GENETIC
MATERIALThe elucidation of how DNA is organized inchromosomes is currently an extremely active area of
research.
There has been much interest in this topic because it
is believed that the determination of the arrangement
of the genetic material and associated molecules willprovide valuable information about other aspects of
genetics.
8/7/2019 Genetics(4) Amany
3/32
For example, how the genetic
information is stored, expressed, and
regulated must be related to its
organization.
In eukaryotes, how the chromatin
fibers characteristic ofinterphase are
condensed into chromosome
structures visible during mitosis and
meiosis is also of great interest
8/7/2019 Genetics(4) Amany
4/32
we will focus on the organization of the
genetic material in chromosomes of
eukaryotes.
Also, the organization of individual genes
will be discussed
8/7/2019 Genetics(4) Amany
5/32
I. Viral and bacterial chromosomes :
In comparison with higher form, the
chromosomes of viruses and bacteria are much
less complicated.
They consist of a single nucleic acid molecule
which is often, but not always, circular in form.
Such chromosome, much less genetic
information is stored in viruses and bacteria
than in the multiple units of higher forms
8/7/2019 Genetics(4) Amany
6/32
8/7/2019 Genetics(4) Amany
7/32
1- organization of viral DNA and RNA
The chromosomes of viruses consist
of a single nucleic acid molecule-either DNA or RNA - which is either
single or double stranded.
.
8/7/2019 Genetics(4) Amany
8/32
Viruses cannot reproduce in the
absence of their host cell,becausethey depend on the host's
biosynthetic machinery to
duplicate their genetic material
and to synthesize their protein
coat.
8/7/2019 Genetics(4) Amany
9/32
The DNA or RNA containedwithin the protein coat, or
capsid, becomes functionalonly after the injection into
a host cell
8/7/2019 Genetics(4) Amany
10/32
1- organization of viral DNA and RNA
8/7/2019 Genetics(4) Amany
11/32
Viral nucleic acid molecules may exist as
ring structures,
or they may take the form of linear
molecules, according to the shape of theDNA molecule within the viral protein coat
and within the host cell,
we have three different groups of viruses:-
8/7/2019 Genetics(4) Amany
12/32
8/7/2019 Genetics(4) Amany
13/32
a- The single - stranded DNA of
the X174 bacteriophage and thedouble - stranded DNA of the
polyoma virus are ring - shaped
nucleic and molecules both within
the protein coat of the mature virus
and within the host cell.
8/7/2019 Genetics(4) Amany
14/32
8/7/2019 Genetics(4) Amany
15/32
8/7/2019 Genetics(4) Amany
16/32
b- The baceriophage lambda ( ) , possesses a linear
double - stranded DNA molecule prior to infection,
which closes to form a ring upon infection of the host
cell
8/7/2019 Genetics(4) Amany
17/32
c- Other bacteriophages, such as the T-even series, have linear,
double - stranded chromosomes of DNA, which do not from circles
inside the bacterial host chromosomes are largely devoid of associated
proteins, which are characteristic of eukaryotes, because there is only asingle
8/7/2019 Genetics(4) Amany
18/32
DNA packing
One constant feature shared by viruses,
bacteria and eukaryotic cells is the ability to
package a long DNA molecule into a relatively
small volume.
In lambda, the DNA is 17 um long and must fit
into the phage head, which is less than 0.1 umon any side
8/7/2019 Genetics(4) Amany
19/32
In single stranded RNA viruses like the tobacco mosaic virus (RMV),
the capsid is assembled around the chromosome.
The protein subunits of the capsid are thought to interact with theRNA molecule and thus play a role in the packing phenomena
8/7/2019 Genetics(4) Amany
20/32
On the other hand, DNA viruses
with nearly sphericel heads, such
as the T - even group, first
assemble the capsid.
Then, the chromosome is
inserted into the head which,
when filled, is sealed by addition
of a tail.
Once packed within the head, the
genetic material becomes
genetically inactive until released
into a host cell.
8/7/2019 Genetics(4) Amany
21/32
8/7/2019 Genetics(4) Amany
22/32
8/7/2019 Genetics(4) Amany
23/32
2- Organization of bacterial DNA :
Bacterial chromosomes are also relatively simplein form compared with those of eukaryotic cells.
They always consist of a double - stranded DNA
molecule occupying special region of the cell
called nucleotide
. The nucleotide, which may occupy more thanhalf the volume of the cell, is found near the
periphery, where part of the chromosome is
associated with the cell membrance.
8/7/2019 Genetics(4) Amany
24/32
Bacterial Chromosomes Commonly circular DNA
E. coliabout 1.2 mm in circumference
Found in nucleoid region
Associated with DNA-binding proteins
Small positively charged, similar to histones
HU and H
8/7/2019 Genetics(4) Amany
25/32
2- Organization of bacterial DNA :
Bacterial chromosomes are also relatively simple in form compared with those of
eukaryotic cells.
They always consist of a double - stranded DNA molecule occupying special region of
the cell called nucleotide
. The nucleotide, which may occupy more than half the volume of the cell, is found near
the periphery, where part of the chromosome is associated with the cell membrance.
8/7/2019 Genetics(4) Amany
26/32
8/7/2019 Genetics(4) Amany
27/32
Echerichia coli (E.coli), the most extensively studied bacterium, has
a large circular chromosome, measuring approximately 1200 um
(1.2mm) in length.
When the cell is lysed and the chromosome released, the DNA is
thought to be associated with several types of proteins, including
those called HU and H They are small but abundant in the cell and
contain high percentage of positively charged amino acids that can
bond electro statistically to the negative charges of the phosphategroups in DNA
8/7/2019 Genetics(4) Amany
28/32
As we will soon see, these proteins resemble structurally
similar molecules called histones that are found associated
with eukaryotic DNA.
It seems likely that these bacterial DNA proteins play a
role in packaging the chromosome within the nucleoid
region.
In spite of the compacted condition of the bacterial
chromosome, transcription readily occurs.
The DNA molecule in this bacterium contains sufficient
genetic information to code 2000 to 3000 different
proteins.
8/7/2019 Genetics(4) Amany
29/32
In addition to a chromosome,
certain bacteria that develop
resistance to antibiotics contain a
small extrachromosomal circularDNA called a plasmid
In some cases, plasmids can
integrate into the bacterial
chromosome and they are
called episomes
8/7/2019 Genetics(4) Amany
30/32
Other plasmids maintain an independent
existence in the bacterial cytoplasm andmultiple copies may be present in a single
cell.
Each may contain as few as three or four
genes or as many as several hundred genes
8/7/2019 Genetics(4) Amany
31/32
Bacterial Chromosomes Commonly circular ds DNA
E. coliabout 1.2 mm in circumference
Found in nucleoid region
Associated with DNA-binding proteins
Small positively charged, similar to histones
HU and H
8/7/2019 Genetics(4) Amany
32/32
Interest in plasmids has been enhanced by the
phenomenon of recombinant DNA which is one aspect of
the more general topic of genetic engineering.
In genetic engineering, specific genes from a variety of
organisms may be inserted into plasmids by special
experimental procedures.
Then these genes may increase in number as the plasmid
replicates and copies of these genes are distributed to
progeny cells.
This process is known as DNA cloning.