Molecular Biology Tutorials

Photoreactivation : Method of DNA Repair for the Recovery of UV Induced DNA Damages by Phytolyase Enzyme and Visible Light


Science is beautiful when it makes simple explanations of phenomena or
connections between different observations.

Stephen W. Hawking, 2011

What is Photoreactivation?

Nobel Prize in Chemistry 2015

Aziz Sancar

Photoreactivation is a type of DNA repair mechanism present in prokaryotes, archaea and in many eukaryotes. It is the recovery of ultraviolet irradiated damages of DNA by visible light. As the name suggests, it is a light dependent process. In this DNA repair method cells recovers its DNA after UV exposure induced damages. The UV light is lethal to cellular DNA since it induces structural lesions in the DNA by the formation of pyrimidine dimer. Photoreactivation process removes the pyrimidine dimers without altering other nucleotides in the DNA by using energy obtained from visible light. Photoreactivation is the first discovered DNA repair mechanism in the cell. Major credits for the discovery of Photoreactivation goes to Professor Aziz Sancar for which he was shared the 2015 Nobel Prize in chemistry along with Thomas Lindhal and Paul Modrich.

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Molecular Biology Tutorials

DNA Repair Mechanism – Part I Introduction (DNA Damaging Agents, DNA Damages and Recovery of DNA Damages)


Introduction to DNA Repair Mechanisms

“Genetic variation is important for evolution, but the survival of the individuals
demand genetic stability”

What is DNA repair?

DNA is the genetic information carrier molecule in the cell and thus it is very essential to keep the genetic information intact. Even though DNA holds a prime position, it is one of the highly susceptible molecules in the cells because DNA can be damaged by a number of factors both internal and external in origin. It is very surprising to know that, our cells lose approximately 5000 nucleotides every day due to different damages of the DNA. If these damages are not rectified properly, our cells will be subjected to severe mutations and that will be fatal for the survival of the individual cells and the organism itself. DNA replication process in the cell which ensures the production of exact copy of the genetic information is very accurate due to the high fidelity of DNA polymerase enzyme. However, the process of DNA replication is not 100 percent error free. DNA polymerase enzyme sometimes accidentally introduces wrong bases which will disrupt the normal Watson-Crick base paring of the DNA. There are also many possibilities of DNA damage during genetic recombination happens during gametogenesis by meiotic cell division. If the damages or errors in the DNA are not corrected in the somatic cells, it may leads to the development of cancer or it results in the loss of function of genes. More than that, if DNA damages occur in the gametes is not rectified, it will be carried over to next generation through progenies. Thus, damage to the genetic materials is a major threat to all organisms. In order to counteract these threats, cells has evolved many methods to overcome and rectify different types DNA damages. All these methods are collectively termed as DNA REPAIR mechanisms. Similar to DNA replication, transcription and translation, the process of DNA repair is also a prime molecular event in the cells which is very essential for the ultimate survival of the cells and also for the survival of the organism.

DNA Repair and Nobel Prize in Chemistry (2015)

DNA Repair Mechanism

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Lecture notes in Microbiology

Difference Between Prokaryotic and Eukaryotic Translation


Prokaryotic and Eukaryotic Translation

Translation is the process of protein synthesis in the cells. It is the second step in genetic expression in which the ribosomes decodes the information present in mRNA to synthesize proteins according the sequence of codons present in them with different amino acids. Translation is a universal process occurs in both prokaryotes and eukaryotes. The fundamental process of translation is same in prokaryotes and eukaryotes. Members of both groups uses information present in mRNA, which is came from the DNA by transcription, to synthesize proteins with ribosome as the machinery. This post summarized the similarities and differences between prokaryotic and eukaryotic translation process in an EASY but in-depth way.

Similarities of prokaryotic and eukaryotic translation:

@. Both groups uses mRNA template

@. In both groups mRNA is synthesized from the genetic molecule, DNA

@. Ribosome is the protein synthesis machinery in both groups

@. All the 20 amino acids are same in both groups

@. All the 61 codons are similar in both groups

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Lecture notes in Microbiology

Difference between Prokaryotic & Eukaryotic Transcription

Transcription in Prokaryotes and Eukaryotes

Transcription is a process by which the genetic information present in the DNA is copied to an intermediate molecule (RNA). The sequence in the RNA is complementary to that of the gene which is transcribed and thus the RNA retains the same information as the gene itself. Transcription is a universal process in the living word and it occurs both in prokaryotes and eukaryotes.  Even though the overall process of transcription is similar in both prokaryotes and eukaryotes, there do exists some fundamental differences between these groups. This post summarizes the overall similarities and differences between the Prokaryotic and Eukaryotic transcription in a detailed but easy way.

Similarities between prokaryotic and eukaryotic transcription

1.      In both groups DNA acts as the template for RNA synthesis

2.      In both groups transcription produces RNA molecule

3.      Chemical composition of transcript is similar in both groups

4.      Transcription is facilitated by the enzyme RNA polymerase in both groups

5.      In both groups, one strand of the DNA duplex acts as the template

Difference between prokaryotic and eukaryotic transcription

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Lecture notes in Microbiology

Prokaryotes Vs Eukaryotes: (Similarities and Differences Between Prokaryotes & Eukaryotes)


Difference between Prokaryotes and Eukaryotes

.Prokaryotes and Eukaryotes are the two fundamental or basic classes of cells. They are mainly distinguished by their size and internal structures. The existence of two classes of cells without any intermediate type represents one of the most fundamental evolutionary separations in the living world. Prokaryotes are structurally and evolutionarily more primitive to eukaryotes. Even though they are two separate classes of living organisms, they share many common characteristics. The shared properties reflect the fact that eukaryotic cells most certainly evolved from prokaryotic ancestors.

Shared common characters of prokaryotes and eukaryotes are as follows:

(1).   Cellular contents in both prokaryotes and eukaryotes are limited by plasma membrane

(2).   In both groups, plasma membrane composed of lipid bilayer with intermittent protein molecules

(3).   DNA is the genetic information carrier in both the groups

(4).   Chemical composition of nucleic acids (DNA and RNA), their structure and their organization are similar in both groups

(5).  Even though the mechanism slightly varies, the process of transcription, translation and DNA replication occurs in both prokaryotes and eukaryotes

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