The specificity of the PCR depends crucially upon the primers. The following factors are important in choosing effective primers

Primers should be 17 to 30 nucleotides in length

A GC content of about 50% is ideal. For primers with a low GC content, it is desirable to choose a long primer to avoid a low melting

 temperature.

Sequences with long runs (i.e., more than three or four) of a single nucleotide should be avoided.

TLS Online TPP Program

#Id: 4050

#Unit 2. Cellular Organization

The interphase nuclei of some tissues of the larvae of dipteran flies contain chromosomes that are greatly enlarged relative to their usual condition. They possess both increased diameter and greater length. shows an example of a chromosome set from the salivary gland of D. melanogaster. The members of this set are called polytene chromosomes.

TLS Online TPP Program

#Id: 4051

#Unit 2. Cellular Organization

The centromeres of all four chromosomes of D. melanogaster aggregate to form a chromocenter that consists largely of heterochromatin.

TLS Online TPP Program

#Id: 4052

#Unit 2. Cellular Organization

Each is produced by the successive replications of a synapsed diploid pair of chromosomes. The replicas do not separate, but instead remain attached to each other in their extended state, a process known as endoreduplication.

At the start of the process, each synapsed pair has a DNA content of 2C (where C represents the DNA content of the individual chromosome). This amount then doubles up to nine times, at its maximum giving a content of 1024C. The number of doublings is different in the various tissues of the D. melanogaster larva.

TLS Online TPP Program

#Id: 6359

#Unit 2. Cellular Organization

Operon consist

Physiology/Mechanism

Molecular biology/Biochemistry of component

Effect of mutations

TLS Online TPP Program

#Id: 6360

#Unit 2. Cellular Organization

Repressors impede access of RNA polymerase to the promoter and Activators enhance the RNA polymerase–promoter interaction.

TLS Online TPP Program

#Id: 6361

#Unit 2. Cellular Organization

Repressors bind to specific sites on the DNA. In bacterial cells, such binding sites, called operators

Regulation by means of a repressor protein that blocks transcription is referred to as negative regulation. Activators provide a molecular counterpoint to repressors; they bind to DNA and enhance the activity of RNA polymerase at a promoter; this is positive regulation.Repressor binding to DNA is regulated by a molecular signal called effector