Adv Biology

Transcription  There are four main steps in making a protein. Transcription RNA Processing Translation Protein Processing

Transcription cont’d  Transcription-the transfer of information from a DNA molecule into an RNA molecule. DNA is used as a template for making RNA. DNA cannot leave the nucleus.

Transcription cont’d  RNA (Ribonucleic acid) Is single stranded Uses the sugar ribose Has the base uracil (U) in place of thymine.

Transcription cont’d  Transcription begins when RNA polymerase binds to the promoter of a gene. Promoter-a specific nucleotide sequence in DNA, flanking the start of a gene; instructs RNA polymerase where to start transcription.

Transcription cont’d  RNA polymerase reads the DNA sense strand. Nonsense strand-the complementary strand of DNA not read during transcription. The sense strand is read 3’ to 5’. Reads only short segments at a time.

Transcription cont’d  RNA polymerase pairs RNA nucleotides with the DNA sense strand. Uracil pairs with DNA’s adenine. RNA polymerase bonds RNA nucleotides into a strand. RNA is made 5’ to 3’

Transcription cont’d  RNA polymerase stops when it reaches a terminator. Terminator-a special sequence of nucleotides in DNA that marks the end of a gene. ○ RNA polymerase releases the DNA and RNA strands. ○ DNA closes and the helix rewinds.

RNA Processing  RNA processing-modification of RNA before it leaves the nucleus. Pares the RNA into a more concise message. Occurs only in eukaryotes.

RNA Processing cont’d  Sections of the RNA (introns) are removed. Intron-a nonexpressed portion of a gene that is excised from the RNA transcript. Exon-the coding portion of a gene.  Exons are spliced together to make a shorter strand of RNA (mRNA). mRNA-messenger RNA.

RNA Processing  A single piece of pre-mRNA can be used to make different mRNA. Different introns can be removed. A single gene can code for different proteins.  After RNA processing, the mRNA leaves the nucleus. mRNA is used as a guide for making a protein.