Chapter 15: RNA Ribonucleic Acid

I) What is RNA? RNA – Ribonucelic Acid Single stranded nucleic acid Formed from nucleotides Contains 5-carbon sugar RIBOSE 4 Nitrogenous bases – A, U, G, C Uracil replaces Thymine RNA Functions – more than just one! RNA is a temporary copy of a DNA base sequence Carries genetic information from DNA to ribosomes RNA has regulatory functions RNAi can affect gene expression RNA catalyzes peptide bond formation Synthesis of proteins at ribosome

II) The Central Dogma of DNA RNA is a complementary copy of a DNA sequence (a gene) DNA gene sequence is transcribed into a strand of RNA The RNA nucleotide sequence is translated into a polypeptide (forms protein) DNA RNA catalyzes protein synthesis transcription translation RNA PROTEIN in nucleus at ribosomes

III) Types of RNA Five types of RNA exist in cells preRNA snRNA mRNA rRNA tRNA Eukaryotes Only

III) Types of RNA Five types of RNA exist in cells preRNA snRNA mRNA rRNA tRNA Eukaryotes Only

IV) The genetic Code Code of nucleotide bases universal to all living organisms Based on a 3-base system called CODONS A codon is a 3-base sequence that determines a specific amino acid AUG = methionine GUU = valine There are 64 codons for 20 amino acid sequences Some amino acids have 2 or more codons Valine = GUU, GUC, GUA, GUG Degenerate code – multiple codons “code” for the same amino acid Reduces point mutations Reduces FRAME-SHIFT mutations Change in reading frame Ex) /CAG/CAG/CAG/CAG C/AGC/AGC/AGC/AG CA/GCA/GCA/GCA/G Four codons code function as “signaling” codons START Codon: AUG (also codes for methionine) STOP Codons: UAA, UAG, UGA (no amino acid)

V) RNA Transcription Transcription factors (proteins) attached to the DNA where a particular gene is located RNA polymerase binds to the START codon near where these transcription factors are located START codon is called the PROMOTER RNA polymerase moves along the DNA strand in the 3’  5’ direction RNA polymerase stops at the STOP codon and releases from the DNA RNA polymerase produces a strand of mRNA (preRNA in eukaryotes)

VI) RNA Processing Occurs in eukaryotes ONLY Newly transcribed preRNA is modified as follows: 1. A poly-A tail (repeating Adenines) is added to the 3’ end 2. A Methyl “cap” is added to the 5’ end (5’ cap) The cap and tail prevent enzymes from hydrolyzing the RNA 3. Splicesosomes made of snRNA cut out non-coding sections Remove introns and combine exons Spliceosomes are made of RNA units called SnRNPs (Small nuclear Ribonuclear Particles) Processed preRNA is now referred to as mRNA mRNA transcript exits the nucleus and travels to a ribosome Animation1 Animation 2

VII) The Ribosome – Protein Synthesis Consists of 2 subunits Large subunit Contains 2 amino acid binding sites Small subunit Binds to mRNA

VIII) Transfer (tRNA) Transfer RNA carries amino acids to the ribosome for proper placement in a polypeptide Contains an ANTICODON which corresponds to the codons in mRNA Ribosome aligns tRNA in proper sequence tRNA contains three “hair-pin” loops with base- pairing

IX) Protein Synthesis Occurs at the ribosome in both prokaryotes and eukaryotes Involves three steps Initiation Elongation Translocation occurs Termination Animation

XII) Mutations Changes in DNA that are passed on to the next generation of cells A) Point Mutations Changes in one base or one codon. May or may not have significant effects

Substitution – replace one base or codon with another

B) Chromosomal Mutations Mutations affecting an entire gene or chromosome Inversion – sequence of bases is reversed Deletion – one base or codon is removed

Translocation - a section of one chromosome attaches to a different chromosome Can result in trisomy and some rare cancers