MBI- transcription Flashcards
define transcription
The process of copying genetic information from one
strand of the DNA into RNA is termed as
transcription. Here also, the principle of
complementarity governs the process of transcription, except the adenosine
complements now forms base pair with uracil instead of thymine.
how is transcription diff from replication
in transcription only a segment of DNA and
only one of the strands is copied into RNA. This necessitates defining the
boundaries that would demarcate the region and the strand of DNA that
would be transcribed.
why are both sides/strands of a segment not transcripted
First, if both strands act as a template, they would code
for RNA molecule with different sequences (Remember complementarity
does not mean identical), and in turn, if they code for proteins, the sequence
of amino acids in the proteins would be different. Hence, one segment of
the DNA would be coding for two different proteins, and this would
complicate the genetic information transfer machinery. Second, the two
RNA molecules if produced simultaneously would be complementary to
each other, hence would form a double stranded RNA. This would prevent
RNA from being translated into protein and the exercise of transcription
would become a futile one.
segments of a transcription unit
A transcription unit in DNA is defined primarily by the three regions in
the DNA:
(i) A Promoter
(ii) The Structural gene
(iii) A Terminator
what is a template strand
Since the two strands have opposite
polarity and the DNA-dependent RNA polymerase also catalyse the
polymerisation in only one direction, that is, 5’→3’, the strand that has
the polarity 3’→5’ acts as a template, and is also referred to as template
strand
what is coding strand
. The other strand which has the polarity (5’→3’) and the sequence
same as RNA (except thymine at the place of uracil), is displaced during
transcription. Strangely, this strand (which does not code for anything)
is referred to as coding strand
IMPORTANT POINT
All the reference point while defining a
transcription unit is made with coding strand.
what is the promoter
The promoter and terminator flank the structural gene in a
transcription unit.
The promoter is said to be located towards 5’ -end
(upstream) of the structural gene (the reference is made with respect to
the polarity of coding strand). It is a DNA sequence that provides binding
site for RNA polymerase, and it is the presence of a promoter in a
transcription unit that also defines the template and coding strands.
By
switching its position with terminator, the definition of coding and template
strands could be reversed.
what is terminator
The terminator is located towards 3’ -end
(downstream) of the coding strand and it usually defines the end of the
process of transcription (Figure 5.9). There are additional regulatory
sequences that may be present further upstream or downstream to the
promoter.
- Leader sequences- DNA sequences at the 5’ end, before the Promoter
- Trailer sequences- DNA sequences at the 3’ end after the Terminator
define:
i) gene
ii) cistron
i) Gene- it is a segment of dna which codes for an RNA
ii) It is a segment of dna which codes for an m-rna and hence protein. also c/a structural gene
what is
i) monocistronic
ii) poly cistronic arrangement
- Monocistronic – a segment of DNA/RNA having info for only one protein, found in
eukaryotes and prokaryotes, separate promoter and terminator for each gene - Polycistronic- a segment of DNA/RNA having info for two or more proteins, in a
continuous manner, found only in prokaryotes, common promoter and terminator for
all genes present in this segment of DNA
what is split gene arrangement
exons, introns, a segment of DNA having info for a protein split into parts,
coding sequences called exons and non-coding sequences called introns, found only in
eukaryotes
what are the three types of rna
In bacteria, there are three major types of RNAs: mRNA (messenger RNA),
tRNA (transfer RNA), and rRNA (ribosomal RNA). All three RNAs are
needed to synthesise a protein in a cell. The mRNA provides the template,
tRNA brings aminoacids and reads the genetic code, and rRNAs play
structural and catalytic role during translation.
how does rna polymerase start, elongate and end the chain
An intriguing question is that how is the RNA polymerases able
to catalyse all the three steps, which are initiation, elongation and
termination. The RNA polymerase is only capable of catalysing the
process of elongation. It associates transiently with initiation-factor (σ)
and termination-factor (ρ) to initiate and terminate the transcription,
respectively. Association with these factors alter the specificity of the
RNA polymerase to either initiate or terminate
explain function of rna polymerase in bacteria
There is single
DNA-dependent RNA polymerase that catalyses transcription of all types
of RNA in bacteria.
RNA polymerase binds to promoter and initiates
transcription (Initiation). It uses nucleoside triphosphates as substrate and polymerises in a template depended fashion following the rule of
complementarity.
It somehow also facilitates opening of the helix and
continues elongation. Only a short stretch of RNA remains bound to the
enzyme.
Once the polymerases reaches the terminator region, the nascent
RNA falls off, so also the RNA polymerase. This results in termination of
transcription.
