1/27/19 Learning Objectives RNA § Describe how RNA differs from DNA. § Explain how the cell makes RNA. Comparing RNA and DNA Types of RNA §The sugar in RNA The three main types of RNA are: is ribose instead of deoxyribose. §RNA is generally single-stranded, not double-stranded. §RNA contains uracil in place of thymine. Messenger RNA Ribosomal RNA § An mRNA molecule is a copy of the portion of DNA that will be § Protein synthesis occurs on ribosomes, which are made up of used to make a protein. two subunits. § After being made in the nucleus, mRNA travels to the § These subunits consist of several molecules of ribosomal RNA ribosomes in the cytoplasm, the site of protein synthesis. (rRNA) and as many as 80 different proteins. 1 1/27/19 Transfer RNA RNA Synthesis: Transcription During protein synthesis, transfer RNA molecules (tRNA) In transcription, segments of DNA serve as templates to carry amino acids from the cytoplasm to the mRNA at the produce complementary mRNA molecules. ribosome. RNA Synthesis: Promoters RNA Synthesis: RNA Editing • RNA polymerase reads the DNA sequence on the coding New RNA molecules sometimes require a bit of editing strand of the DNA and adds the complementary RNA before they are ready to be read. nucleotides to the mRNA • RNA polymerase binds only to regions of DNA that have specific base sequences. These regions are called promoters. Introns Exons Cap Tail RNA Synthesis: RNA Editing • In eukaryotes genes contain both introns and exons • Introns – non-coding sequences • Exons – coding sequences • The introns are cut out and the exons are spliced together to make a mature mRNA • This allows different exons to be put together to make different proteins from the same gene • A methyl cap and poly-A tail are added to protect the mRNA as it moves into the cytoplasm 2.