Test: Transcription (NCERT) - NEET MCQ

mRNA has methylated region at the 5' terminus. It functions as a cap for attachment with ribosome. Cap is followed by an initiation codon (AUG) either immediately or after a small non-coding leader region. Then there is coding region followed by termination codons (UAA, UAG OF UGA). After termination codon there is a small non-coding trailer region and poly A area at the 3' terminus. These leader and trailer regions are called UTRs (Untranslated regions). Cap at 5' end and poly-A tail at 3' end also serves to protect mRNA from enzymatic decay.

NCERT Topic: mRNA structure and function

NCERT Line: "Methylated cap, Non-coding region, coding region, Poly A tail"

The RNA polymerase enzyme binds to a specific site called promoter and initiates transcription. The promoter region determines which DNA strand is to be transcribed. Thus, a promoter region has RNA polymerase recognition site and RNA polymerase binding site .

NCERT Topic: Promoter

NCERT Line: "The promoter is said to be located towards 5'-end (upstream) of the structural gene."

mRNA strand is complementary to one of the DNA strands i.e., template strand. In RNA, uracil is present instead of thymine which is complementary to adenine. Cytosine and guanine are also complementary to each other. Hence, the sequence of bases in transcript would be CGAAUCCGUU.

NCERT Topic: Transcription unit

NCERT Line: "the strand that has the polarity 3'-5' acts as a template, and is also referred to as template strand. The other strand which has the polarity (5'-3') and the sequence same as RNA (except thymine at the place of uracil)"

At 5' end of hnRNA, a cap is formed by modification of GTP into 7-methyl guanosine or 7_m(G_ppp). This process is called capping and it protects the mRNA from degradation by nucleases and also.

NCERT Topic: Capping

NCERT Line: "In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5'-end of hnRNA."

Eukaryotes have three RNA polymerases. RNA polymerase I is located in the nucleolus and transcribes for RNAs (28S, 18S and 5.8S) whereas RNA polymerase II is localised in the nucleoplasm and used for hnRNA, mRNA and RNA polymerase Ill is localised in the nucleus, possibly the nucleolar-nucleoplasm interface and transcribes for tRNA, 5S, RNA and nRNAs.

NCERT Topic: Transcription in eukaryotes

NCERT Line: "The RNA polymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA)."

In eukaryotes, the coding sequences or expressed sequences are defined as exons. Exons appear in mature or processed RNA. The exons are interrupted by introns, they do not appear in mature or processed RNA.

NCERT Topic: Structural genes in eukaryotes

NCERT Line: "The coding sequences or expressed sequences are defined as exons. Exons are said to be those sequence that appear in mature or processed RNA. The exons are interrupted by introns. Introns or intervening sequences do not appear in mature or processed RNA."

Monocistronic genes typically have interrupted coding sequences (exons), whereas polycistronic genes often have continuous coding sequences.

NCERT Topic: Monocistronic and polycistronic genes

NCERT Line: "the structural gene in a transcription unit could be said as monocistronic (mostly in eukaryotes) or polycistronic (mostly in bacteria or prokaryotes)."

Polycistronic structural genes are mostly found in prokaryotes, such as bacteria, where multiple genes are often transcribed together into a single mRNA molecule.

NCERT Topic: Polycistronic structural genes

NCERT Line: "the structural gene in a transcription unit could be said as monocistronic (mostly in eukaryotes) or polycistronic (mostly in bacteria or prokaryotes)."

As transcription proceeds the initiation and elongation steps are mediated by the RNA Polymerase enzyme (DNA dependent RNA polymerase). As a result, the RNA transcript is synthesized as a single-stranded structure and is released when the Rho factor binds to the enzyme RNA polymerase to terminate the transcription. 

NCERT Topic: Transcription in bacteria

NCERT Line: "RNA polymerase binds to promoter and initiates transcription (Initiation)."

The primary mRNA transcript is longer and localised into the nucleus, where it is also called heterogeneous nuclear RNA (hnRNA) or pre-mRNA. At the 5' end of hnRNA, a cap (consisting of 7-emthyl guanosine triphosphate or 7 mG) and a tail of poly A (Adenylate residues) at the 3' end are added. These processes are respectively called as capping and tailing. The cap is a chemically modified molecule of guanosine triphosphate (GTP). The primary mRNA are made up of two types of segments, non-coding introns and the coding exons. The introns are removed by a process called RNA splicing and the exons are joined in a defined order.

NCERT Topic: Processing of primary transcript

NCERT Line: "hnRNA undergoes additional processing called as capping and tailing. In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5'-end of hnRNA. In tailing, adenylate residues (200-300) are added at 3'-end in a template independent manner."

The process of copying genetic information from one strand of the DNA into RNA is termed as transcription. Unlike the process of replication, which once sets in, the total DNA of an organism gets duplicated, in transcription only a segment of DNA and only one of the strands is copied into RNA.

NCERT Topic: transcription

NCERT Line: "The process of copying genetic information from one strand of the DNA into RNA is termed as transcription."

The given diagram represents post-transcriptional processing resulting in the formation of mRNA. Since, introns and exons are present, it is transcription in eukaryotes.

NCERT Topic: transcription in eukaryotes

NCERT Line: "Figure 5.11 Process of Transcription in Eukaryotes"

Exons are the coding sequences in eukaryotic genes that contain the genetic information necessary for protein synthesis.

NCERT Topic: Exons in eukaryotes

NCERT Line: "In eukaryotes, the gene is split. The coding sequences, exons, are interrupted by non-coding sequences, introns."

Transcription unit is the distance between sites of initiation and termination by RNA polymerase. It may include more than one gene. RNA polymerase produces transcription unit that extends from the promoter to the termination sequences.

NCERT Topic: Transcription unit

NCERT Line: "The promoter and terminator flank the structural gene in a transcription unit."

Heterogeneous nuclear RNA or hnRNA is a term that encompasses various types and sizes of RNAs found in the eukaryotic cell nucleus. When this undergoes modifications and processing it will give rise to the mature mRNA that has the inherent code for the protein synthesis (amino acid code). The mRNA in eukaryotes is synthesized in the nucleus and hence should be transported to the cytoplasm and into the ribosome for protein synthesis by the process of translation. 

NCERT Topic: The fully processed hnRNA is called mRNA and is transported out of the nucleus into the cytoplasm for translation.

NCERT Line: "It is the fully processed hnRNA, now called mRNA, that is transported out of the nucleus for translation."