Dr. Israa ayoub alwan Lec -7- AL-Ma’moon University College Medical Laberatory techniques Department Molecular biology/ Second stage Dr. Israa ayoub alwan Lec -7-

Steps of Transcription

Steps of Transcription Transcription described in three steps: 1.In transcription initiation transcription factors and RNA polymerase are attracted to a promoter, which is a special sequence that signals the start of the gene. The first transcription factor to bind, called a TATA binding protein, is chemically attracted to a DNA sequence called a TATA box—the base sequence TATA surrounded by long stretches of G and C. Once the first transcription factor binds, it attracts others in groups. Finally RNA polymerase joins the complex, binding just in front of the start of the gene sequence. The assembly of these components is transcription initiation.

2.transcription elongation enzymes unwind the DNA double helix locally, and free RNA nucleotides bond with exposed complementary bases on the DNA template strand . RNA polymerase adds the RNA nucleotides in the sequence the DNA specifies, moving along the DNA strand in a 3 ′ to 5 ′ direction, synthesizing the RNA molecule in a 5 ′ to 3 ′ direction. A terminator sequence in the DNA indicates where the gene’s RNA-encoding region ends. When this spot is reached,

3.transcription termination   3.transcription termination A typical rate of transcription in humans is 20 bases per second. RNA is typically transcribed using only a gene’s template strand. However, different genes on the same chromosome may be transcribed from different strands of the double helix. The coding strand of the DNA is so-called because its sequence is identical to that of the RNA, except with thymine (T) in place of uracil (U). Several RNAs may be transcribed from the same DNA template strand simultaneously due to the —OH at the 2 ′ position in ribose. Since mRNA is short-lived, with about half of it degraded every 10 minutes, a cell must constantly transcribe certain genes to maintain supplies of essential proteins.

To determine the sequence of RNA bases transcribed from a gene, write the RNA bases that are complementary to the template DNA strand, using uracil opposite adenine. For example, a DNA template strand that has the sequence 3 C C T A G C T A C 5 is transcribed into RNA with the sequence 5 G G A U C G A U G 3 and the coding DNA sequence is 5 G G A T C G A T G 3

RNA Processing

RNA Processing In eukaryotic cells, mRNA must first exit the nucleus to enter the cytoplasm, where ribosomes are located. Messenger RNA is altered before it is translated in these more complex cells. The modifications take several steps. First, after mRNA is transcribed, a short sequence of modified nucleotides, called a cap, is added to the 5 ′ end of the molecule. The cap consists of a backwardly inserted guanine (G), which attracts an enzyme that adds methyl groups (CH 3 ) to the G and one or two adjacent nucleotides. This methylated cap is a recognition site for protein synthesis. At the 3 ′ end, a special polymerase adds about 200 adenines, forming a “poly A tail.” The poly A tail is necessary for protein synthesis to begin, and may also stabilize the mRNA so that it stays intact longer.

Second, occur the capped, poly A tailed mRNA before it is translated into protein. Parts of mRNAs called introns (short for “intervening sequences”) that were transcribed are removed. The ends of the remaining molecule are spliced together before the mRNA is translated. The parts of mRNA that remain and are translated are called exons Once introns are spliced out, enzymes check, or proofread, the remaining mRNA. Messenger RNAs that are too short or too long may be held in the nucleus. Proofreading also monitors tRNAs, ensuring that they assume the correct conformation. Prior to intron removal, the mRNA is called pre-mRNA. Introns control their own removal. They associate with certain proteins to form small nuclear ribonucleoproteins (snRNPs), or “snurps.” Four snurps form a structure called a spliceosome that cuts introns out and attaches exons to form the mature mRNA that exits the nucleus. The introns cut themselves out of the RNA.

THANK YOU