RNA & Transcription

What are the types of RNA & their functions? 1) Messenger RNA (mRNA) – brings DNA code for proteins, from the nucleus to cytoplasm ribosomes, and directs amino acid sequence. It is a single strand without loops. It contains nucleotide sequences called codons. mRNA Modern Biology Text Book (Holt)

2) Transfer RNA (tRNA) binds to amino acids and carries them to the proper locations within the growing polypeptide chain. It has a looped structure & contains an amino acid attachment site, as well as a sequence of nucleotides known as the anticodon.

3) Ribosomal RNA (rRNA) combines with protein to form ribosomes that are the site of protein synthesis. Modern Biology Text Book (Holt)

Transcription: RNA synthesis In Eukaryotic cells, the DNA does not leave the nucleus to direct the building of proteins in the cytosol.

All Ribonucleic Acid , mRNA, tRNA, rRNA, even snRNA, are assembled according to instructions in the nuclear DNA. Then, the RNA can leave the nucleus to fulfill its function in the cytosol. Modern Biology Text Book (Holt)

Transcription may be divided into three stages: Initiation Elongation Termination

Stage 1: Initiation An enzyme called RNA polymerase which binds to the DNA strand at the beginning of a gene called the promoter region . The DNA is transcribed from the template of the the coding strand. It is used to direct synthesis of RNA.

2) The DNA strands separate as DNA “unwinds” or “unzips”. RNA synthesis starts with the unwinding of DNA exposing the template of the coding strand.

Stage 2: Elongation 3) As the RNA polymerase leaves the promoter region and moves along the template of DNA coding strand RNA nucleotides, matching the corresponding DNA nucleotides bond to form an RNA strand. A single complimentary strand is synthesized becoming longer as the RNA polymerase moves along the template of the coding strand.

Stage Three: Termination 4) The RNA is complete when the RNA polymerase reaches the terminator region at the end of the DNA strand. (termination). Transcription is complete. The RNA is released, & the RNA polymerase falls off.

At the end of RNA transcription, the DNA molecule “rewinds” as the strands “rebond” once again.

mRNA processing: The newly synthesized RNA is called the primary RNA transcript (preRNA). It may average 200,000 nucleotides (the avg. for human cells is only 1000.) Studies show that RNA molecules undergo extensive processing before leaving the nucleus. Processing involves the addition and chemical modification of certain nucleotides as well as the removal of some segments from RNA.

When transcription ends editing begins. 5) RNA IS EDITED: sections removed are called Introns while the parts that stay are called exons. The parts of the primary transcript called introns are cut out. Introns appear to match noncoding regions of DNA. In order for this to happen, “Snurps” (snRNA & Protein complexes) bind to form spliceosome to cut out the introns, and bond the remaining portions called exons.

A methyl-guanine or (mG cap) is placed on the starting end of the mRNA. Then 100 to 200 adenine nucleotides are added to the other end to form a “poly-Adenine-tail” (poly-A tail)

The cap and tail protect the mRNA from enzymes in the cytosol. Tail may aid in transport through the nuclear pore.