MOLECULAR GENETICS 9

 Transcription

• The process of copying genetic information from one strand of the DNA into RNA is termed as transcription.

• In transcription only a segment of DNA and only one of the strands is copied into RNA because
• if both strands act a template, they would code for RNA molecule with different sequences and the sequences of amino acids in the coded protein would be different.
• the two RNA molecules would be complementary to each other and would form a double stranded RNA which would prevent translation.

 

Transcription unit

A  transcription unit consists of

• A Promoter
• The Structural gene
• A Terminator

• The two strands of the DNA in the structural gene of a transcription unit is termed as template strand and coding strand.
• The strand that has the polarity 3'→5' acts as a template, and is referred as template strand.

• The other strand which has the polarity (5'→3') is referred as coding strand.
• The promoter and terminator flank the structural gene in a transcription unit.
• The promoter is located towards 5'-end (upstream) of the structural gene which provides binding site for RNA polymerase.
• The terminator is located towards 3'-end (downstream) of the coding strand which defines the end of the process of transcription.

Structure of a gene

• A gene is defined as the functional unit of inheritance.
• A gene also referred as a cistron can be defined as a segment of DNA coding for a polypeptide.
• The structural gene in a transcription unit could be said as monocistronic mostly in eukaryotes or polycistronic mostly in bacteria or prokaryotes.

• Exons are the coding sequences or expressed sequences that appear in mature or processed RNA.
• Introns are the intervening sequences which interrupt exons and do not appear in mature or processed RNA.

Types of RNA

• There are three major types of RNAs
• mRNA (messenger RNA),
• tRNA (transfer RNA),
• rRNA (ribosomal RNA).

 

• All three RNAs are needed to synthesize a protein in a cell.
• The mRNA provides the template, tRNA brings amino acids and reads the genetic code, and rRNAs play structural and catalytic role during translation

Structure of tRNA

• The tRNA, also called as sRNA (soluble RNA) has a role as an adapter molecule.

• tRNA has an anticodon loop that has bases complementary to the code.

• It has an amino acid accepter end to which it binds to amino acids.
• The secondary structure of tRNA looks like a clover-leaf.
• In actual structure, the tRNA is compact molecule which looks like inverted L.

rRNA-