Gene Expression

Gene expression and its regulation

Gene expression or gene regulation is the ability of a gene of an organism to express phenotype at the molecular level. In other words, gene expression is the mechanism at the molecular level by which a gene is able to express itself in the phenotype of an organism. The mechanism of gene expression involves biochemical genetics. Biochemically, a gene is a segment of DNA with a specific sequence of nitrogenous base. It consists of the synthesis of RNA, polypeptide, structural proteins, enzymes, and hormones that control the structure and function (metabolic activities)of specific traits. The molecular structure of gene is expressed in following steps:

DNA (gene) {transcription}→ RNA {translation}→ Polypeptide {expression} → Proteins {enzymes or hormones}

One gene one enzyme hypothesis

Beadle and Tatum (1948) proposed one gene-one enzyme hypothesis from their work on Neurospora.They defined the gene as the unit of hereditary material that specifies a single enzyme. It means biosynthetic pathway is composed of several steps and each step is controlled by a specific enzyme which is synthesized by a specific gene.

Later this hypothesis was replaced by one gene one polypeptide hypothesis because it was found that the functional unit at the genetic level is the polypeptide.

As we know that all genes are not responsible for the expression of character. Cistron helps in the expression of character. So one gene one polypeptide hypothesis is modified as one cistron one polypeptide hypothesis.

Gene expression in virus

The virus is a nucleoprotein entity that lives as an intracellular obligate parasite as it is able to utilize the synthetic machinery of the host cell for it multiplication without involving growth and division. It is inert in the absence of host cell. It is the most primitive and simplest living entity. The 1^stvirus to be discovered was Tobacco Mosiac Virus.

A virus consists of few genes in its nucleoid. The expression of viral gene possible only inside the host cell by viral reproduction. The bacteriophage is the best-known virus in the term of their structure and expression. The bacteriaE.coliis a typical host that can be infected by bacteriophage. Virus exhibits two types of the life cycle. They are lytic and lysogenic.

A) Lytic life cycle of virus

In this life cycle, virus undergoes replication in the host cell and causes the lysis (death) of the host when the virus is released. It is the reproductive cycle of T₂ and T₄ bacteriophage or virulent phages. It can be described in following steps:

1) Adsorption:The bacteriophage virus is adsorbed to the wall of the bacterium (host) with the help of its tail fibers.

2) Penetration:The tail sheath contracts and enzyme lysosome present in the tail comes to the cell wall of the bacterium and make a hole. Then, viral DNA only enters into the host cell by rhythmic contraction and expansion of contractile tail. The protein coat and the tail remain outside of the host.

3) Formative phase:The viral DNA inside the host cell produces enzyme nuclease that breaks down host DNA, Viral DNA replicates itself and forms all components of the virus inside the host cell separately.

4) Maturation:The new viruses are made by the wrapping of the viral genome by a protein coat. The period between the entry of viral genome inside the bacterium up to the formation of the first new virus is called eclipse period. It is 13 minutes in T₂ bacteriophage.

5) Lysis:After maturation, the lysozyme made by the viral DNA brings about the breakdown of the host cell releasing the new viruses. These new viruses are ready to infect other bacteria. The period between entry of viral DNA into the host cell and bursting of the host cell to release new viruses is called latent period. It is 18 minutes in T₂ bacteriophage.

B) Lysogenic life cycle:

In this life cycle, phage may inject its DNA to a bacterium and does not exert any influence over the bacterial cell. The viral DNA becomes integrated into bacterial DNA at a specific site with the help of enzyme integrase. In this form, the viral DNA is called prophage.

Prophage produces a repressor protein that keeps the phage genes in the repressed stage and becomes non-virulent. In the way, phage DNA may keep multiplying for the generation to generation without causing any damage. The bacterial cell carrying prophage is called lysogenic cell and the phenomenon of existence of viral DNA in prophage state along with host DNA is termed as lysogeny.

Sometimes, the prophage may get activated by chemicals, high energy radiations and other adverse conditions. This causes the inhibition of the synthesis of the repressor protein. In such case, phage DNA again becomes active i:e. non-virulent DNA becomes virulent or lytic. In this condition, prophage undergoes the lytic cycle to produce more phages which are released by bursting of the cell.

Reverse transcription:

Some RNA viruses have a gene that codes for enzymes reverse transcriptase (RNA-dependent DNA polymerase). This enzyme helps in the synthesis of double-stranded DNA. This phenomenon is called reverse transcription or timinism. This was suggested by Temin and Baltimore in 1970 A.D. This DNA later forms protein by transcription which is called reverse central dogma.

RNA (reverse transcription)→ DNA (transcription)→ mRNA (translation) → Protein

Retroviruses:Viruses which are capable of reverse transcription are called retroviruses. Eg- Cancer causing viruses, hepatitis- B virus, AIDS-causing HIV virus.

Gene expression in prokaryotes

Gene expression is the mechanism at the molecular level by which a gene is able to express itself in the phenotype of an organism. Most of the important work on genetics was done initially on the colonBacillusbacteria calledEscherichia coli.

Bacterial genome: Bacterial or prokaryotic genome is represented by circular double stranded DNA that lies freely inside the cytoplasm. The bacterial DNA remains highly folded without a nuclear membrane. Histone proteins are not associated with the DNA. Therefore DNA of bacteria or prokaryotes is naked. The DNA ofE.colicontains 2000-3000 genes.

• Gene expression in Bacteria:

Bacteria usually reproduces by the asexual method without any recombination of genetic material. However, some bacteria exhibits a primitive form of sexual reproduction to exchange genetic material between two cells. It is also called parasexual methods of reproduction. The part of DNA from the donor bacterial cell is transferred to the recipient bacterial cell. The DNAs of both combine together to from recombinant DNA containing genes of both donor and recipient cells. The recombinant DNA will show variation as a result of the mixing of genes. There are three modes of exchange of genetic materials or genetic recombination. They are:

1) Transformation

2) Transduction

3) Conjugation

1) Transformation:It is the method if introducing of foreign DNA into the bacterial cell. In it, the recipient and donor cells do not come in contact. A short piece of DNA is released by donor cell which is actively taken up by the DNA of recipient cell in which it replaces one strand of recipient DNA. So, donor DNA and recipient DNA combines and forms recombinant DNA. In this way, DNA of donor cell expresses some of its properties into the recipient cell. Such cell is called transformant. Griffith confirmed it with experiment on Diplococcus pneumonia,

2) Transduction:It is a process of exchange of genetic material with the help of bacteriophage. It was discovered by Lederberg and Zinder in 1952 inSalmonella Typhimurium.During this process, the bacteriophage is used as plasmid to transfer a small piece of double stranded DNA from one bacterium cell (so-called donor) to another cell (so-called recipient cell). A recombinant or hybrid DNA has formed that consists of the genome of both bacteria and thus expresses both the properties. The virus which takes part in this process is never virulent.

3) Conjugation:The direct transfer of genetic material (DNA) from one bacterium to another bacterium through a sexual mating is called conjugation. Lederberg and Tatum first demonstrated conjugation inE.coliin 1946. During conjugation, the sex pili of donor cell form conjugation tube. Then plasmid present in donor cell elongates and divides into two. One plasmid present in donor cell and other passes into the recipient cell through conjugation tube. Now, recipient cell after getting plasmid from donor cell forms pili and changes into donor cell. This process of changing recipient cell into donor cell by the process of conjugation is called sex-duction.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition. Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.