How to Calculate Cotransformation Frequency with Examples?

Gene transfer in bacteria occurs by conjugation, transduction and transformation.

Transformation involves the uptake of genetic material from the surrounding medium and its incorporation into bacterial chromosome or plasmid.

Transformation is used to map bacterial genes. Cells that receive genetic material through transformation are called transformants. Genes can be mapped by looking at the rate at which two or more genes are co transformed in transformation. if two genes are close together on the same chromosomal fragment the chance of co transformation is more. 

Higher cotransformation frequency indicates a shorter distance between the two genes on the donor’s chromosome.

Cotransformation Frequency Problems

Problems 1

In a transformation experiment, donor DNA from an E.coli strain with the genotype Z’Y’ was used to transform a strain of genotype of genotype Z Y. The frequencies of transformed classes were:

Z+Y+               200

Z+Y                 400

Z-Y+                400

total               1000

what is the frequency (%) with which Y locus is co transformed with the Z locus?

Answer:

Cotransformation frequency=no. of transformed cells with Z+Y+ genotype/total number of transformed cells

that is 200/1000=0.2 or 20%

The frequency of co-transformation is 20%

Problem 2

DNA from a strain of Bacillus subtilis with the genotype trp+ tyr+ is used to
transform a recipient strain with the genotype trp tyr . The following numbers of
transformed cells were recovered:
Genotype Number of transformed cells

trp+ tyr 154
trp tyr+ 312
trp+ tyr+ 354
What do these results suggest about the linkage of the trp and tyr genes?
Answer:

Cotransformation frequency=no. of transformed cells with trp+ tyr+ genotype/total number of transformed cells

that is 354/820=0.43 or 43%

The frequency of co-transformation of trp and tyr genes are 43%

The high level of cotransformation indicates that these two genes are
closely linked.

Problem 3

DNA from a ‘c+ d+’ bacterial strain was used to transform a ‘c- d-’ strain.

The following transformants were obtained:
c+ d- : 300
c- d+ : 377
c+ d+: 223
Calculate the transformation and cotransformation frequencies ?

Answer:

Cotransformation frequency (%): Cotransformation frequency=no. of transformed cells with c+d+ genotype/total number of transformed cells x100

223/(300+377+223) X100 = 24.8%

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What are specialized cells? How do cells become specialized for different functions?

The genetic makeup of all cells in an adult body is the same. But nerve cell is different from muscle cell or cells of the eyes morphologically and functionally. How this happens?

By cell differentiation. It is the process by which genetically identical cells of an embryo become specialized or the process by which stable differences arise between cells of the embryo.

How specialization is achieved?
Let us make this concept clear by this figure given below.
All these cells have ~ 20000 genes distributed in 46 chromosomes.
Let us take the example of RBC. All these cells have haemoglobin gene in the nucleus.
This gene is not expressed in all cells… It is expressed or turned on only in RBCs where it has a role..

 

During the process of cell specialisation, in each cell only specific genes are “turned on” and transcribed to RNA and translated to proteins. Rest of the genes remain inactive. That is, genes active in the neurons may not be active in muscle cells. For instance, genes for actin and myosin filaments are present in all animal cells, but these genes are active only in muscle cells. Cell specialization involves the preferential synthesis of some specific proteins like antibodies in plasma cells or Hb in erythrocytes.

Smart genes and ‘house keeping’ genes

Remember, there are some genes that are expressed in all types of cells or genes essential for cell survival like genes making membranes. These genes are called ‘house keeping’ genes. But genes that are expressed in only certain types of cells or expressed differentially are called ‘luxury genes’ or smart genes’. Eg: IgG genes. This differential expression leads to cell specialization.

After fertilization, the first cell the zygote has nucleus of both the gametes where the cytoplasm is entirely provided by egg. Thus the zygote has only maternal effect genes contributed by egg cytoplasm only. This is conducive for the zygote development. On first division, zygotic gene are expressed that will trigger further development and differentiation.

In short: Zygote > determination > differentiation (to specialised cells and tissues).

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Types of Differentiated cells Based on their Proliferation Capacity

Cell differentiation is the process by which genetically identical cells of an embryo become specialized or the process by which stable differences arise between cells of the embryo.

Types of differentiated cells:

Differentiated cells can be categorized into three based on their proliferation and replacement capacity.

Type I: Cardiac cells, muscle cells, nerve cells, once differentiated these cells are incapable of division and cannot be replaced upon injury or cell death.

Type II: skin cells and liver cells, once differentiated enter G0 phase or resting phase of cell cycle and divide only upon injury or cell death. Mostly these cells have short life span and must be replaced by continuous division.

Type III: Blood cells once differentiated are incapable of division and undergo cell death. But these cells are replaced by the proliferation of less differentiated cells called stem cells.

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Molecular Mechanism of Cell Differentiation

Cell differentiation is the process by which genetically identical cells of an embryo become specialized or the process by which stable differences arise between cells of the embryo.

Molecular mechanism behind cell differentiation:

 Cell differentiation is achieved by one of the following mechanisms:

   a) Transcriptional control: Transcription of a particular gene in a specific cell is tissue specific. Genes that are not needed will remain inactive and are prevented from transcription.

  b) Translational control: In Xenopus oocytes, mRNAs for heat shock proteins are masked and these are translated only after fertilization. Here protein synthesis is blocked for a certain period.

 c) Gene amplification: Here specific genes are selectively amplified to attain higher rate of expression. In Oocytes of Amphibians, rRNA gene amplification occurs their by increasing the copy of that particular gene.

d) Gene rearrangement: A classic example is differentiation process of mature B-cell into plasma cell and memory cell. Here IgGs against different antigens are generated by gene rearrangement.

 e)  Transposition: sometimes under certain stimulus transfer of gene within a genome can occur which result in a stable change in the pattern of expression. At first the gene may be in a silent site and upon transposition it has reached in an expression site. This phenomenon of variation in gene expression depending on its position in the genome is referred as position effect.

      Eg: Surface antigens in yeast and trypanosomes.

Whatever be the organism, whatever is the status, big or small, all had a humble beginning from a very very small microscopic cell, the zygote.

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