Genetics(4) Amany

8/7/2019 Genetics(4) Amany

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Prof. Dr. AmanyTohamy


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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.


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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


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we will focus on the organization of the

genetic material in chromosomes of

eukaryotes.

Also, the organization of individual genes

will be discussed


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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


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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.

.


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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.


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The DNA or RNA containedwithin the protein coat, or

capsid, becomes functionalonly after the injection into

a host cell


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1- organization of viral DNA and RNA


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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:-


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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.


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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


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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


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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


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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


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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.


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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.


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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


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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.


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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


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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.


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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


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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


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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


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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.

Genetics(4) Amany

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