Albia Dugger Miami Dade College Cecie Starr Christine Evers Lisa Starr Chapter 9 From DNA to Protein (Sections )

9.4 RNA and the Genetic Code Three types of RNA interact to translate DNA’s information into a protein: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)

Genetic Code The protein-building information in mRNA consists of a sequence of three mRNA bases (codon); each is a code for a particular amino acid The four bases A, C, G, and U can be combined into 64 different codons, which constitute the genetic code Example: AUG codes for the amino acid methionine (met), and UGG codes for tryptophan (trp)

Key Terms codon In mRNA, a nucleotide base triplet that codes for an amino acid or stop signal during translation genetic code Complete set of sixty-four mRNA codons

Codons and Amino Acids There are only twenty kinds of amino acids found in proteins, so some amino acids are specified by more than one codon Some codons signal the beginning and end of a protein- coding sequence: AUG (methionine) start translation UAA, UAG, and UGA are stop codons The order of mRNA codons determines the order of amino acids in the polypeptide that will be translated from it

The Genetic Code

Animation: Genetic Code

DNA, mRNA, and Proteins

Fig. 9.8, p. 143 serine (ser) codon translation transcription codon a gene region in DNA mRNA methionine (met) tyrosine (tyr) amino acid sequence DNA, mRNA, and Proteins

rRNA and tRNA: The Translators Ribosomes (containing rRNA and structural proteins) and tRNAs interact to translate an mRNA into a polypeptide One large and one small ribosomal subunit join with mRNA, and rRNA enzymatically catalyzes the formation of a peptide bond between amino acids Transfer RNAs deliver amino acids to ribosomes in the order specified by mRNA

Ribosome Structure A polypeptide chain threads through the tunnel in the large subunit as it is being assembled by the ribosome

Fig. 9.9, p. 143 large subunit tunnel intact ribosomesmall subunit Ribosome Structure

ANIMATION: Structure of a ribosome To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

tRNA Structure Each tRNA has two attachment sites: A triplet of nucleotides (anticodon) base-pairs with an mRNA codon Another attachment site binds to the amino acid specified by the codon Transfer RNAs with different anticodons carry specific amino acids to a ribosome during translation of an mRNA

tRNA Structure

Fig. 9.10, p. 143 amino acid attachment site anticodon AB tRNA Structure

ANIMATION: Structure of a tRNA To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

Key Concepts RNA Messenger RNA (mRNA) carries DNA’s protein-building instructions Its nucleotide sequence is read three bases at a time Sixty-four mRNA base triplets—codons—represent the genetic code Two other types of RNA interact with mRNA during translation of that code

9.5 Translating the Code: RNA to Protein Translation converts the information carried by an mRNA into a polypeptide Translation occurs in the cytoplasm Translation proceeds in three stages: initiation, elongation, and termination

6 Steps in Translation (1) Translation initiates when ribosome subunits and an initiator tRNA (attached to a start codon) converge on an mRNA A second tRNA binds to the second codon, and so on…

6 Steps in Translation (2) The ribosome catalyzes formation of a peptide bond between the first two amino acids

6 Steps in Translation (3) The first tRNA is released and the ribosome moves to the next codon. A third tRNA binds to the third codon

6 Steps in Translation (4) A peptide bond forms between the second and third amino acids

6 Steps in Translation (5) The second tRNA is released and the ribosome moves to the next codon A fourth tRNA binds the fourth codon

6 Steps in Translation (6) A peptide bond forms between the third and fourth amino acids Process repeats until the ribosome encounters a stop codon

Fig , p Ribosome subunits and an initiator tRNA converge on an mRNA. A second tRNA binds to the second codon. first amino acid of polypeptide start codon (AUG) initiator tRNA 6 Steps in Translation

Fig , p. 144 A peptide bond forms between the first two amino acids. peptide bond 2 6 Steps in Translation

Fig , p. 144 The first tRNA is released and the ribosome moves to the next codon. A third tRNA binds to the third codon. 3 6 Steps in Translation

Fig , p. 144 A peptide bond forms between the second and third amino acids. 4 6 Steps in Translation

Fig , p. 144 The second tRNA is released and the ribosome moves to the next codon. A fourth tRNA binds the fourth codon. 5 6 Steps in Translation

1 Ribosome subunits and an initiator tRNA converge on an mRNA. A second tRNA binds to the second codon. first amino acid of polypeptide start codon (AUG) initiator tRNA Stepped Art Fig. 9.11, p. 144 A peptide bond forms between the first two amino acids. peptide bond 2 The first tRNA is released and the ribosome moves to the next codon. A third tRNA binds to the third codon. 3 A peptide bond forms between the second and third amino acids. 4 The second tRNA is released and the ribosome moves to the next codon. A fourth tRNA binds the fourth codon. 5 A peptide bond forms between the third and fourth amino acids. The process repeats until the ribosome encounters a stop codon in the mRNA. 6 6 Steps in Translation

ANIMATION: Translation To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

Overview of Translation

Fig. 9.12, p. 145 polysomes Transcription Translation polypeptide mRNA Convergence of RNAs tRNA ribosome subunits polysomes RNA transport newly forming polypeptidemRNA Overview of Translation

Key Concepts RNA to Protein: Translation Translation is an energy-intensive process by which a sequence of codons in mRNA is converted to a sequence of amino acids in a polypeptide chain Transfer RNAs deliver amino acids to ribosomes, which catalyze the formation of peptide bonds between the amino acids

9.6 Mutated Genes and Their Protein Products If a mutation (change in genetic code) changes the genetic instructions encoded in the DNA, an altered gene product may result Example: Hemoglobin consists of four polypeptides (globins) folded around a heme (iron-containing cofactor) Various defects in the polypeptides can cause anemia

Hemoglobin Hemoglobin consists of 4 polypeptides: 2 alpha globins (blue) and 2 beta globins (green) Oxygen molecules bind to the iron atom at the center of each heme

Defects in Hemoglobin Frameshift mutations: A deletion from DNA of the beta globin gene causes a type of anemia called beta thalassemia An insertion mutation can also alter polypeptides deletion Mutation in which one or more base pairs are lost insertion Mutation in which one or more base pairs are added

Frameshift Mutations A frameshift garbles the genetic message like adding or deleting a letter garbles the meaning of a sentence: The cat ate the rat

Base-Pair Deletion

Defects in Hemoglobin (cont.) Other types of mutations do not cause frameshifts: In base-pair-substitution, a nucleotide and its partner are replaced by a different base pair Sickle-cell anemia results from a substitution of valine for glutamic acid base-pair substitution Type of mutation in which a single base-pair changes

Base-Pair Substitution

Fig. 9.13, p. 146 Stepped Art B Part of the DNA (blue), mRNA (brown), and amino acid sequence (green) of human beta globin. C A base-pair deletion causes the reading frame for the rest of the mRNA to shift, so a completely different protein product forms. The mutation shown results in a defective beta globin. The outcome is beta thalassemia, a genetic disorder in which a person has an abnormally low amount of hemoglobin. D A base-pair substitution replaces a thymine with an adenine. When the altered mRNA is translated, valine replaces glutamic acid as the sixth amino acid of the polypeptide. Hemoglobin with this form of beta globin is called HbS, or sickle hemoglobin. Base-Pair Substitution

ANIMATION: Base-pair substitution To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

Sickle-Cell Anemia Substitution of valine for glutamic acid causes HbS protein to clump Normally round red blood cells are distorted into sickle shapes

What Causes Mutations? There are many causes of mutations: Transposable elements can cause insertion mutations Mistakes occur during DNA replication Environmental agents can damage DNA Natural or synthetic chemicals can cause mutations transposable element Segment of DNA that can spontaneously move to a new location in a chromosome

Environmental Factors in Mutation Ionizing radiation (such as x-rays) breaks chromosomes into pieces that get lost during DNA replication, or forms destructive free radicals Nonionizing radiation (such as UV light) can form thymine dimers (two adjacent thymine bases covalently bonded to one another) that kink DNA Chemicals in cigarette smoke can cause mispairing during replication, or stop replication entirely

Key Concepts Mutations Small-scale, permanent changes in the nucleotide sequence of DNA may result from replication errors, the activity of transposable elements, or exposure to environmental hazards Such mutations can change a gene’s product