DNA  RNA  protein Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 17-UN3 mRNA Ribosome Polypeptide

Fig. 17-3a-2 (a) Bacterial cell TRANSCRIPTION DNA mRNA TRANSLATION Ribosome Polypeptide

Basic Principles of Protein Synthesis 1. DNA unzips by the enzyme RNA Polymerase 2. Transcription synthesis of RNA under the direction of DNA which occurs in the nucleus DNA the template produces messenger RNA (mRNA) mRNA is the complement to DNA 3. mRNA carries the codons from DNA to the ribosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 17-3a-1 TRANSCRIPTION DNA mRNA (a) Bacterial cell

Cracking the Code Codon- three bases, triplets 4 bases make the 64 codons they were deciphered by the mid-1960s 3 triplets are “stop” signals to end translation Codons must be read in the correct order called a reading frame Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig Second mRNA base First mRNA base (5 end of codon) Third mRNA base (3 end of codon)

4. mRNA binds to rRNA r=ribosomal Ribosomes facilitate translation coupling 5. Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA Ribosomes are the sites of translation

Fig a Growing polypeptide Exit tunnel tRNA molecules Large subunit Small subunit (a) Computer model of functioning ribosome mRNA E P A 5 3

Molecular Components of Translation 6. tRNA has the code book, transfer RNA 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings BioFlix: Protein Synthesis BioFlix: Protein Synthesis

The Structure and Function of Transfer RNA A C C a tRNA molecule looks like a cloverleaf Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig a Amino acid attachment site (a) Two-dimensional structure Hydrogen bonds Anticodon 3 5

7. The anticodon picks up the amino acids and helps forms the protein chain In prokaryotes transcription and translation occur simulataneously

Fig Polypeptide Ribosome Amino acids tRNA with amino acid attached tRNA Anticodon Trp Phe Gly Codons 3 5 mRNA

Fig b Amino acid attachment site Hydrogen bonds Anticodon (b) Three-dimensional structure (c) Symbol used in this book

Fig DNA molecule Gene 1 Gene 2 Gene 3 DNA template strand TRANSCRIPTION TRANSLATION mRNA Protein Codon Amino acid

Fig. 17-3b-3 (b) Eukaryotic cell TRANSCRIPTION Nuclear envelope DNA Pre-mRNA RNA PROCESSING mRNA TRANSLATION Ribosome Polypeptide

Completing and Targeting the Functional Protein Often translation is not sufficient to make a functional protein Polypeptide chains are modified after translation Completed proteins are targeted to specific sites in the cell Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Concept 17.5: 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 can lead to the production of an abnormal protein Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig Wild-type hemoglobin DNA mRNA Mutant hemoglobin DNA mRNA CCTT T T G G A A A A AA A GG U Normal hemoglobinSickle-cell hemoglobin Glu Val

Types of Point Mutations Point mutations within a gene can be divided into two general categories – Base-pair substitutions – Base-pair insertions or deletions Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig Wild-type 3 DNA template strand Stop Carboxyl end Amino end Protein mRNA A instead of G U instead of C Silent (no effect on amino acid sequence) Stop T instead of C A instead of G Stop Missense A instead of T U instead of A Stop Nonsense No frameshift, but one amino acid missing (3 base-pair deletion) Frameshift causing extensive missense (1 base-pair deletion) Frameshift causing immediate nonsense (1 base-pair insertion) Stop missing Stop Extra U Extra A (a) Base-pair substitution(b) Base-pair insertion or deletion

Substitutions A base-pair substitution replaces one nucleotide and its partner with another pair of nucleotides Silent mutations have no effect on the amino acid produced by a codon Missense mutations still code for an amino acid, but not necessarily the right amino acid Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Insertions and Deletions Insertions and deletions are additions or losses of nucleotide pairs in a gene Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation These mutations have a disastrous effect on the resulting protein more often than substitutions do Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Mutagens Spontaneous mutations can occur during DNA replication, recombination, or repair Mutagens are physical or chemical agents that can cause mutations Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig TRANSCRIPTION RNA PROCESSING DNA RNA transcript 3 5 RNA polymerase Poly-A RNA transcript (pre-mRNA) Intron Exon NUCLEUS Aminoacyl-tRNA synthetase AMINO ACID ACTIVATION Amino acid tRNA CYTOPLASM Poly-A Growing polypeptide 3 Activated amino acid mRNA TRANSLATION Cap Ribosomal subunits Cap 5 E P A A Anticodon Ribosome Codon E

In summary, a gene can be defined as a region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 17-UN6

Fig. 17-UN8