genetics - LECTURE 3 The Genetic Code links Genotype and...

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Unformatted text preview: LECTURE - 3 The Genetic Code links Genotype and Phenotype: - DNA, RNA and Proteins - Transcription and Translation Overview: The Flow of Genetic Information • Central Dogma: DNA RNA Proteins • The information content of DNA is in the form of specific sequences of nucleotides • The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins • Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation • The ribosome is part of the cellular machinery for translation (polypeptide synthesis) Basic Principles of Transcription and Translation • Transcription is the synthesis of RNA under the direction of DNA. • messenger RNA (mRNA) is produced by transcription ** • Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA. • Ribosomes are the sites of translation. • RNA synthesis is catalyzed by RNA polymerase • The DNA sequence where RNA polymerase attaches is called the promoter • RNA synthesis follows the same base-pairing rules as DNA, except uracil substitutes for thymine • In prokaryotes, mRNA produced by transcription is immediately translated without more processing • In a eukaryotic cell, the nuclear envelope separates transcription from translation • **In eukaryotes the primary RNA transcripts (often called pre-mRNAs) are modified through RNA processing to yield finished mRNA. LE 17-3-1 TRANSCRIPTION Prokaryotic cell DNA LE 17-3-2 TRANSCRIPTION DNA mRNA Ribosome Prokaryotic cell Polypeptide Prokaryotic cell LE 17-3-3 DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope TRANSCRIPTION Eukaryotic cell DNA LE 17-3-4 DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope TRANSCRIPTION DNA Pre-mRNA RNA PROCESSING mRNA Eukaryotic cell LE 17-3-5 DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION Polypeptide Eukaryotic cell Eukaryotic cells modify pre-mRNA after transcription • Enzymes in the eukaryotic nucleus modify pre-mRNA before they are dispatched to the cytoplasm in the form of mRNA • Primary RNA transcripts have introns and exons. Introns are spliced (cut) out during RNA processing. The exons have the “coding sequence” to make a protein. • During RNA processing, both ends of the primary transcript are also usually altered: - The 5′ end receives a modified nucleotide cap - The 3′ end gets a poly-A tail • These modifications at the two ends serve several functions: – They seem to facilitate the export of mRNA – They protect mRNA from hydrolytic enzymes – They help ribosomes attach to the 5’ end RNA Processing The Genetic Code • How are the instructions for assembling amino acids into proteins encoded in DNA? • There are 20 possible amino acids in proteins, but only four nucleotide bases in DNA • During translation, mRNA base triplets, called codons, are read in the 5′ to 3′ direction • Each codon specifies the amino acid to be placed at the corresponding position in a polypeptide Codons: Triplets of Bases • The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words • Example: AGT at a particular position on a DNA strand results in the placement of the amino acid serine at the corresponding position of the polypeptide to be produced LE 17-4 Gene 2 DNA molecule Gene 1 Gene 3 DNA strand (template) 5′ 3′ TRANSCRIPTION mRNA 5′ 3′ Codon TRANSLATION Protein Amino acid Third mRNA base (3′ end) LE 17-5 Second mRNA base Molecular Components of Translation • A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) • Molecules of tRNA are not identical: – Each carries a specific amino acid on one end – Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA LE 17-13 Amino acids Polypeptide tRNA with amino acid attached Ribosome tRNA Anticodon Codons 5′ mRNA 3′ LE 17-12 Gene DNA Exon 1 Intron Exon 2 Intron Exon 3 Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Polypeptide LE 17-26 TRANSCRIPTION DNA 3′ 5′ RNA polymerase RNA transcript RNA PROCESSING Exon RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase NUCLEUS CYTOPLASM FORMATION OF INITIATION COMPLEX Amino acid AMINO ACID ACTIVATION tRNA mRNA Growing polypeptide Activated amino acid 3′ A P E Ribosomal subunits 5′ TRANSLATION E A Codon Ribosome Anticodon Point mutations can affect protein structure and function • Mutations are changes in the genetic material of a cell or virus. • Point mutations are chemical changes in just one base pair of a gene. • The change of a single nucleotide in a DNA template strand may lead to the production of an abnormal protein. Sickle-Cell Disease: A Simple Change in Primary Structure • A slight change in primary structure can affect a protein’s conformation and ability to function. • Sickle-cell disease, an inherited blood disorder, results from a single amino acid substitution in the protein hemoglobin. LE 17-23 Wild-type hemoglobin DNA 3′ Mutant hemoglobin DNA 5′ 3′ mRNA 5′ mRNA 3′ Normal hemoglobin 5′ 5′ 3′ Sickle-cell hemoglobin LE 5-21a 10 µm Red blood Normal cells are cell shape full of individual hemoglobin molecules, each carrying oxygen. 10 µm Red blood cell shape Fibers of abnormal hemoglobin deform cell into sickle shape. LE 5-21b Sickle-cell hemoglobin Normal hemoglobin Primary structure Val His 1 2 Leu Thr 3 4 Pro Glu 5 6 Secondary and tertiary structures 7 β subunit Quaternary Normal hemoglobin structure (top view) Primary structure Secondary and tertiary structures Molecules do not associate with one another; each carries oxygen. His Leu Thr Pro Val Glu 1 2 3 4 5 6 7 Exposed hydrophobic region β subunit α Quaternary structure β Val β α Function Glu Sickle-cell hemoglobin β α Function Molecules interact with one another to crystallize into a fiber; capacity to carry oxygen is greatly reduced. β α Types of Point Mutations • Point mutations within a gene can be divided into two general categories – Base-pair substitutions – Base-pair insertions or deletions Substitutions • A base-pair substitution replaces one nucleotide and its partner with another pair of nucleotides. • Base-pair substitution can cause missense or nonsense mutations. • Missense mutations still code for an amino acid, but not the right amino acid. • Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein. • Missense mutations are more common. Why? LE 17-24 Stop Wild type mRNA 5′ Protein 3′ Amino terminus Carboxy terminus Base-pair substitutions U instead of C Silent mutation No effect on amino acid sequence Stop A instead of G Missense Stop U instead of A Nonsense Stop Third mRNA base (3′ end) LE 17-5 Second mRNA base Insertions and Deletions • Insertions and deletions are additions or losses of nucleotide pairs in a gene. • These mutations have disastrous effects on the resulting proteins more often than substitutions do. • Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation. LE 17-25 Wild type mRNA 5′ Protein 3′ Stop Carboxyl end Amino end Base-pair insertion or deletion Frameshift causing immediate nonsense Extra U Stop Frameshift causing extensive missense Missing Insertion or deletion of 3 nucleotides: no frameshift but extra or missing amino acid Missing Stop Mutagens • Spontaneous mutations can occur during DNA replication, recombination, or repair. • Mutagens are physical or chemical agents that can cause mutations. Figure 1-8-1 Figure 1-8-2 Figure 1-8-3 ...
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This note was uploaded on 10/04/2010 for the course BICD bicd 100 taught by Professor Soowal during the Spring '08 term at UCSD.

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