From Genes to Protein Chapter 17

The Flow of Genetic Information The information content of DNA is in the form of specific nucleotides along the DNA strands Review: nucleotides-sugar, phosphate, nitrogenous base DNA inherited by organisms leads to specific traits by dictating the synthesis of proteins Protein synthesis occurs in two stages: transcription and translation

The Central Dogma DNA mRNA Proteins Transcription Translation

The Genetic Code of Life Genes Specify Enzymes Beadle and Tatum: Experiments on the fungus Neurospora crassa (bread mold) Proposed that each gene specifies the synthesis of one enzyme One gene-one enzyme hypothesis Function of a gene is to dictate the production of a specific enzyme

Transcription RNA is a polymer of RNA nucleotides Types of RNA RNA nucleotides contain the sugar ribose instead of deoxyribose RNA nucleotides are of four types: uracil (U), adenine (A), cytosine (C), and guanine(G) Uracil (U) replaces thymine (T) of DNA Types of RNA Messenger (mRNA) - Takes a message from DNA in the nucleus to ribosomes in the cytoplasm Ribosomal (rRNA) - Makes up ribosomes, which read the message in mRNA Transfer (tRNA) - Transfers the appropriate amino acid to the ribosomes for protein synthesis

Transcription Transcription is the process by which DNA makes RNA Occurs in the nucleus Contains 3 stages: initiation, elongation and termination

Transcription

Stage 1: Initiation RNA polymerase recognizes and binds to DNA at th promoter region Transcription factors (proteins) that bind with RNA polymerase Recognizes TATA box

Stage 2: Elongation RNA polymerase advances 3’5’ down template strand making pre-RNA in the 5’3’ direction Antisense vs sense strands RNA polymerase untwist the double helix and continues to add nucleotides following the base pairing rules Transcription unit

Stage 3: Termination Final stage of transcription RNA polymerase recognizes the termination sequence- AAUAAAA

RNA Processing Pre-mRNA is modified before leaving the eukaryotic nucleus. Modifications to the ends of the primary transcript: Cap on the 5′ end The cap is a modified guanine (G) nucleotide Helps a ribosome determine where to attach when translation begins Poly-A tail of 150-200 adenines on the 3′ end Facilitates the transport of mRNA out of the nucleus Inhibits degradation of mRNA by hydrolytic enzymes Helps ribosome attach to the RNA

RNA Processing: Final Step Introns (intervening sequences)- Noncoding regions of the mRNA Exons- expressed sequence of the mRNA Introns are removed by snRNPs (small nuclear ribonucleoprotein) and splicesomes

Messenger RNA Processing in Eukaryotes exon exon exon DNA intron intron transcription exon exon exon pre-mRNA 5 intron intron 3 exon exon exon 5 3 cap intron intron poly-A tail spliceosome exon exon exon 5 3 cap poly-A tail pre-mRNA splicing intron RNA mRNA 5 3 13 cap poly-A tail

Cracking the Code Genetic information is encoded as a sequence of 3 nitrogenous bases called a codon 3 bases = 1 amino acid 20 total amino acids Codons must be read in the correct reading frame

Genetic Code The genetic code is nearly universal Shared by most organisms 64 codons 61 code for AA AUG-start UAA-stop UGA-stop UAG-stop Wobble

Translation Translation-process by which the codons of an mRNA are changed into an amino acid sequence Occurs in the cytoplasm at the ribosome (rRNA) Contains 3 stages: initiation, elongation and termination tRNA carries amino acids to the rRNA (ribosome)

tRNA tRNA molecules come in 64 different kinds All are very similar except that One end bears a specific triplet (of the 64 possible) called the anticodon The other end binds with a specific amino acid type All tRNA molecules with a specific anticodon will always bind with the same amino acid

Structure of a tRNA Molecule amino acid leucine 3’ 5’ hydrogen bonding amino acid end anticodon anticodon end G A A C A G U C C U U C C U C mRNA 5’ 3’ codon a. b.

Ribosomes Ribosomes Ribosomal RNA (rRNA): Produced from a DNA template in the nucleolus of a nucleus Combined with proteins into large and small ribosomal subunits A completed ribosome has three binding sites to facilitate pairing between tRNA and mRNA The E (for exit) site The P (for peptide) site, and The A (for amino acid) site

Stage 1: Initiation Initiation begins when mRNA becomes attached to a subunit of the ribosome First codon is always AUG (methionine-met)

Stage 2: Elongation Continues as tRNA brings amino acids to the ribosome and polypeptide chain is formed

Stage 3: Termination Translation is stopped when a ribosome reaches one of the 3 termination or stop codons. Release factor breaks the bond between the tRNA and the last amino acid mRNA is broken down

Summary of Protein Synthesis in Eukaryotes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. TRANSCRIPTION TRANSLATION 1. DNA in nucleus serves as a template for mRNA. DN A 3. mRNA moves into cytoplasm and becomes associated with ribosomes. 2. mRNA is processed before leaving the nucleus. large and small ribosomal subunits 5 mRNA introns pre-mRN A 3 mRN A amino acids 4. tRNAs with anticodons carry amino acids to mRNA. nuclear pore peptide ribosome tRNA U A C U A C 5 A U G 3 anticodon codon 5. During initiation, anticodon-codon complementary base pairing begins as the ribosomal subunits come together at a start codon. 8. During termination, a ribosome reaches a stop codon; mRNA and ribosomal subunits disband. C C C 5 C C C U G G U U U G G G A C C A A A G U A 3 6. During elongation, polypeptide synthesis takes place one amino acid at a time. 7. Ribosome attaches to rough ER. Polypeptide enters lumen, where it folds and is modified.