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MCAT TOPIC REVIEWS & RESOURCES

Biological and Biochemical Foundations of Living Systems

BIO
Transcription

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Transfer RNA (tRNA); ribosomal RNA (rRNA)

The process of transcription produces RNA transcripts following from a DNA template. Some of these RNA products are themselves functional as machinery within the cell. Two specific types of RNAs that go on to assist in the translation process are transfer RNAs and ribosomal RNAs. tRNA houses the anticodon, a region for binding to mRNA codons, and binds a specific amino acid to be transferred to an elongating polypeptide. rRNA plays a role in the ribosomal subunits that provide the active sites for translation.

Mechanism of transcription

Transcription proceeds through the stages of initiation, elongation, and termination.

Initiation

The portion of DNA to be transcribed is identified and regulated by the presence of a promoter, a specific preceeding sequence in the DNA. A transcription initiation complex is formed around the promoter, composed of transcription factors (DNA binding proteins) and the enzyme RNA Polymerase.

Elongation

Similarly to DNA replication, the double strands of the DNA are unzipped by the RNA Polymerase to form a transcription bubble, where one strand of the DNA is read as a template to form a complementary strand of RNA.

The strand being read is called the template strand. Being complementary to the RNA transcript it is also referred to as the (–) antisense strand. The DNA strand not being read will therefore be similar to the RNA transcript (aside from the presence of U instead of T in the RNA). This partner to the template strand is called the coding strand or, on account of its similarity to the RNA transcript, the (+) sense strand.

The RNA transcript must be elongated in the 5' → 3' direction, therefore the DNA template strand is read by the RNA Polymerase in the 3' → 5' direction without proofreading.

Termination

Once the RNA Polymerase reaches a termination sequence, the RNA Polymerase detaches from the DNA, and the RNA transcript is ended.

mRNA processing in eukaryotes, introns, exons

RNA transcripts can be modified by post-transciptional processing. For mRNA specifically, whether the transcript is modified differs between prokaryotes and eukaryotes. There is generally no modification in prokaryotes where, without a nucleus separating transcription and translation machinery, translation of an elongating mRNA transcript may begin while the strand is still being transcribed. Comparatively, in eukaryotes this primary transcript or pre-mRNA is modified before translation.

Addition of 5' Cap and Poly A Tail

The additions to the end of the RNA transcript help protect it from exonuclease degradation.

Splicing

The process of splicing removes portions of the RNA transcript. The portions removed are called introns (and stay in the nucleus), while the portions that remain are called exons (and exit the nucleus as part of the transcript). Some exons may also be removed during alternative splicing, which allows for a single primary transcript to be used for producing a variety of mature transcripts based on the exons chosen for the final product.

Ribozymes, spliceosomes, small nuclear ribonucleoproteins (snRNPs), small nuclear RNAs (snRNAs)

A ribozyme is an enzyme which is not a protein but comprised of RNA instead. A class of ribozymes called small nuclear RNAs (snRNAs) combine with various proteins to form small nuclear ribonucleoproteins (snRNPs), which assist in splicing. The snRNPs and other associated proteins together form a spliceosome that performs splicing by pinching off introns from the RNA transcript into a loop that is removed and joining the ends of the remaining exons.

Functional and evolutionary importance of introns

Introns are non-coding regions. They offer support to the transcript as it is transported out of the nucleus and assist in control of gene expression by influencing a variety of protein products dependent on splicing sites and the inclusion or omission of particular exons.

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