PLEIOTROPISM

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• Pleiotropy refers to the situation in which a single gene influences more than one phenotypic trait.
• Most genes, if not all have their multiple effects and are called pleiotropic genes.
• The phenomenon of multiple effect (multiple phenotypic expressions of a single gene is called pleiotropism)
• Even though a structural gene may have many end effects, it usually has only one primary function, of producing one polypeptide. This polypeptide may give rise to different expressions at the phenotypic level.

Examples of pleiotropism

1.     In Drosophila the recessive gene for vestigial wings causes vestigial wings in homozygous condition and also effects other traits like:

Ø The tiny wing like balancer behind the wing

Ø Certain bristles

Ø The structure of reproductive organs

Ø Egg production is lowered

Ø Longevity is reduced

2.     In Drosophila the gene for white eyes may affect shape of sperm storage organs in females and also some other structures

3.     In human beings the gene for disease phenylketonuria has pleiotropic effect and produce various abnormal phenotypic traits. The affected individuals secrete large amount of an acid phynylalanine in their urine, cerebrospinal fluid and blood. They are short statured, mentally deficient with widely spaced incisors, pigmented patches on skin with excessive sweating and with non-pigmented hairs and eyes.

·      Pleiotropic genes must be more common because indirectly every gene may be involved in the expression of more than one trait. Thus most genes are pleiotropic IN NATURE

Hershey and chase experiment to prove that DNA is genetic material.

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Hershey (Alfred Hershey) and Chase (Martha Chase) performed a series of experiments in 1953 to prove that the DNA is the genetic material. The DNA was known to biologists since 1869, but still some scientists assumed that protein has some role in the inheritance. The key concept of their experiments was that Phage infection proved that DNA is the genetic material of viruses. When the DNA and protein components of bacteriophages were labelled with different radioactive isotopes, only the DNA is transmitted to the progeny phages produced by infecting bacteria.

First they labelled the bacteriophages with ³⁵S, since Sulphur is a component of protein coat of bacteriophage, the ³⁵S is incorporated to the coat of the bacteriophage. This bacteriophage was allowed to infect the bacteria. The bacteriophages infect the bacteria in a manner that their protein coat is left behind out side the bacterial cell and only the DNA of the bacteriophage enters the bacteria and incorporates with the genome of the bacteria. After the infection of the bacteriophage centrifugation is done to separate the capsids and the bacterial cells. It was found that ³⁵S was only found in the protein coat and not in the bacterial cells.

Secondly they labelled the bacteriophages with ³²P. Since Phosphorus is the essential component of the DNA. The ³²P was incorporated in the DNA of the bacteriophages. These labelled bacteriophages were allowed to infect the bacteria. The bacteriophages left their protein coat outside the bacterial cell and they injected their DNA into the bacterial cell which incorporated with the genome of bacteria. Then centrifugation was done to separate the protein coat and the bacteria. On testing ³²P was found in the bacterial cell. And not in the protein coat.

This experiment prominently proved that the DNA is the only genetic material, and the protein has nothing to do with the inheritance and genetic information.

Griffith and Avery’s experiment for the evidence that DNA is the genetic material.

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Genome consists of long sequence of nucleic acids that stores the information needed to construct the organism. The genome is the only thing that defines the hereditary nature of an organism.

The genome can be functionally divided in to genes. Each genes are nucleotide sequences that represent a particular protein. That is the genes code for the protein (Rather the genes code for an RNA which in turn may code for a specific protein.)

Experimental Evidence for DNA’s Role as Genetic material:

Griffith's experiment to show that DNA is genetic material.

The first direct evidence that the DNA transmits genetic information comes from the experiments of Griffith. In the year 1928 Griffith performed a fantastic experiment with mouse and bacteria (Streptococcus pneumoniae). The Streptococcus pneumoniae has two strains. One has smooth appearance due to presence of capsule, and are virulent to cause pneumonia, as the capsule allows the bacteria to escape destruction by the host. The other strain appears rough due to absence of the capsule, and is avirulent, because the host can easily destroy it. Avery first infected a mouse with the rough strain of bacteria, the mouse survived. Then he infected other mouse with the smooth strain, the second mouse died due to pneumonia. To the third mouse he infected it with heat killed smooth strain. The mouse survived. To the fourth mouse he infected with the mixture of heat killed smooth strain and live rough strain bacteria. Here the mouse died. Thus they concluded that the rough strain bacteria somehow transformed[1] itself to smooth strain and killed the mouse. These transformed bacteria were recovered from the dead mouse and then cultured. The result was culture of smooth strain, proofing that the transformation was permanent.

Drawbacks:

·         Griffith cannot explain the role of mouse in transformation of rough strain to smooth strain.

·         He also cannot make it clear, that which part of the smooth strain bacteria(DNA, RNA, protein etc) transformed the rough strain bacteria.

Experiment of Avery:

In 1944 Avery, C. Macleod and M. Mc Carty separated the extract of smooth, virulent bacteria into proteins, DNA, carbohydrate fractions. The incubated the rough strain of bacteria with all of these fractions. And they got the results. The only the bacteria incubated in the DNA fragment of the smooth strain bacteria, transformed to smooth strain. In other fractions there was no transformation. This experiment proved that it is the DNA which acts as genetic material.

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[1] It is a permanent, inheritable change produced in one strain of bacteria by a substance (DNA) isolated from another strain of the same kind of bacteria

An overview of polyadenylation of 3’ end: post transcriptional modification, prior to the nuclear export.

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The final RNA processing event, i.e. the polyadenylation of the 3’ end of the mRNA is linked with the termination of the transcription.

The CTD tail of the polymerase is involved in the recruiting of the enzymes necessary for the polyadenylation.
The DNA contains a poly-A signal sequence. Once the polymerase has reached the sequence and transcribed the poly-A signal sequence into the RNA, the poly-A signal sequence (these sequences once transcribed into RNA, triggers transfer of the CPSF and CstF from the CTD tail of the polymerase to the RNA, described in the text later) in the RNA triggers the transfer of polyadenylation enzymes to RNA leading to three events.
1. Cleavage
2. Addition of the “A” residues.
3. Termination of transcription.
The CTD tail of the polymerase carries two protein complexes as it reaches the end of the gene.
1. CPSF (cleavage and polyadenylation specificity factor)
2. CstF (cleavage stimulation factor)

The binding of the CPSF (cleavage and polyadenylation specificity factor) and CstF (cleavage stimulation factor) is followed by the recruitment of other proteins as well. This leads to RNA cleavage and then polyadenylation.
The poly-A polymerase mediates the process of polyadenylation and adds about 200 adenines to the RNA’s 3’ end produced by the cleavage.
The poly-A polymerase uses ATP (adenosine triphosphate) as a precursor and adds the nucleotides using the same chemistry as that of RNA polymerase, but here in the absence of a template. Thus long “A” tail is only present in RNA (and not in the DNA)
After polyadenylation (the final step in post transcriptional modification, prior to the nuclear mRNA transport) the mature mRNA is then transported from the nucleus to the cytosol.
At present, not more is known about the relation of the polyadenylation with the termination of the transcription. Two basic models have been proposed to explain the link between polyadenylation and termination.
· First: the transfer of the enzymes responsible for the polyadenylation from the CTD tail of the polymerase to the RNA triggers a conformational change in the polymerase that reduces the processivity of enzymes leading to spontaneous termination soon afterwards.
· Second: after cleavage of the RNA transcript the polymerase keeps on transcribing the DNA short while, the polymerase senses the absence of the 5’ cap on the second RNA molecule; as a result, the RNA recognizes the transcript as improper and terminates the transcription.