2/22/12 Objective: Recognize the central dogma of genetics Describe the process of transcription Describe the structure of messenger RNA Warm-Up: What does the term “DNA sequence” refer to?

Protein Synthesis Protein synthesis is the complex, multi-step process of using the information encoded in DNA to build protein The sequence of nucleotides in the DNA determines the amino acid sequence of polypeptides Recall from 1st quarter that a polypeptide is a string of amino acids bonded together by (covalent) peptide bonds There are two main parts of protein synthesis: transcription and translation A gene that is transcribed and translated is said to be expressed

Transcription During transcription, DNA is used as a template for the synthesis of messenger RNA (mRNA) An enzyme complex called RNA polymerase is responsible for transcription RNA polymerase “reads” the DNA and builds a single strand of messenger RNA

2/23/12 Objective: Understand reverse transcription and RNA processing Warm-Up: Contrast DNA and messenger RNA Homework: Transcription Practice Due Friday 24 Feb, 10 pts FYI: Interim grades close Friday (tomorrow) Please be sure your composition book is in the classroom Friday afternoon so I can update scores

Misconception! “It doesn’t matter which side of the DNA you use to make the messenger RNA because you end up using both” Not true! It makes a big difference! The two strands of a double helix are complementary, not identical. This means that each strand will yield a different messenger RNA sequence. mRNAs that are different carry completely different messages to the ribosomes in the cytoplasm.

RNA Structure It is a nucleic acid Single stranded Ribose instead of deoxyribose Uracil instead of thymine There are several different varieties of RNA; we will cover messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

Retroviruses Retroviruses are unique because they carry an RNA genome, not a DNA genome HIV When the viral RNA genome is introduced into a host cell, the enzyme reverse transcriptase “reads” the viral RNA and synthesizes DNA The newly synthesized viral DNA is then integrated into the host’s DNA genome Once there, the viral DNA sequence is expressed along with the host’s own genome

messenger RNA Processing Your DNA has coding regions and non-coding regions Non-coding regions do not contain sequence that codes for amino acids Coding regions are exons Non-coding regions are introns When messenger RNA is synthesized, it contains both exons and introns The introns are spliced (cut) out of the mRNA during processing

mRNA processing After splicing, there are two important modifications made to the messenger RNA strand A specialized nucleotide called GTP is added to the “head” of the mRNA This is called a GTP cap A long string of adenines is added to the “tail” of the mRNA This is called a poly-A tail Both the GTP cap and the poly-A tail protect the mRNA from being degraded in the cytoplasm