RNA & Protein Synthesis

RNA & Protein Synthesis
Chapter 13

Chapter 13.1 RNA (pages 362-365) The Role of RNA Ribonucleic Acid
Nucleic acid consisting of a long chain of nucleotides Involved in putting the genetic code into action with DNA Genes contain coded DNA instructions Tell cells how to make proteins RNA copies part of DNA base sequence to direct production of proteins Helps determine characteristics of organisms DNA and RNA nucleotides both made up of: 5-carbon sugar, phosphate group, and a nitrogenous base

Chapter 13.1 RNA (pages 362-365) Comparing RNA & DNA
Number of Strands: RNA: usually 1 DNA: 2 Sugar: RNA: ribose DNA: deoxyribose Nitrogenous Bases: RNA: adenine, uracil, guanine, cytosine DNA: adenine, thymine, guanine, cytosine

Chapter 13.1 RNA (pages 362-365) Functions of RNA Messenger RNA (mRNA)
Carries instructions for protein synthesis from nucleus to ribosomes in the cytoplasm Code directly copied from DNA in nucleus Ribosomal RNA (rRNA) Forms important part of both subunits of ribosome Transfer RNA (tRNA) Carries amino acids to ribosome Matches amino acids to coded mRNA message

Chapter 13.1 RNA (pages 362-365) RNA Synthesis Transcription
Process in which: Segments of DNA serve as templates Produce complementary RNA molecules In prokaryotes RNA synthesis and protein synthesis occur in the cytoplasm

Chapter 13.1 RNA (pages 362-365) RNA Synthesis in Eukaryotes
RNA synthesis occurs in nucleus mRNA produced in nucleus travels to cytoplasm to play role in production of proteins Requires an enzyme RNA polymerase Similar to DNA polymerase Binds to DNA during transcription Separates DNA strands Uses one strand of DNA as template to assemble nucleotides into complementary strand of RNA Makes it possible for a single gene to produce hundreds or thousands of RNA molecules

Chapter 13.1 RNA (pages 362-365) RNA Synthesis in Eukaryotes Promoter
Region of DNA that has a specific base sequence Acts as signal in DNA molecule to show RNA polymerase exactly where to begin making mRNA Similar signals in DNA cause transcription to stop when a new mRNA molecule is completed

RNA Editing mRNA may need to be edited before it is used Pre-mRNA molecules have bits and pieces cut out before they can be used Introns Portions of pre-mRNA that are cut out and discarded Taken out while still in nucleus Exons Remaining portions of pre-mRNA that are spliced together to form final mRNA

RNA Editing Scientists don’t completely know why parts may be edited out Some pre-mRNA may be cut and spliced in different ways in different tissues Makes it possible for single gene to produce several forms of mRNA Introns and exons may also play a part in evolution Makes it possible for small changes in DNA sequences to have dramatic effects on how genes affect function in cells

Chapter 13.1 Assessment (page 365)
(a) Describe 3 main differences between RNA and DNA. RNA has the sugar ribose, while DNA has the sugar deoxyribose RNA is usually single stranded, while DNA is double stranded RNA has the nitrogenous base uracil, while DNA has the nitrogenous base thymine (b) List the 3 main types of RNA and explain what they do. Messenger RNA (mRNA) carries instructions for protein synthesis from DNA in the nucleus to ribosomes in the cytoplasm Ribosomal RNA (rRNA) forms an important part of both subunits of a ribosome, where proteins are synthesized or assembled Transfer RNA (tRNA) carries amino acids from the cytoplasm to a ribosome and matches them to the coded mRNA message

(c) Why is it important for a single gene to be able to produce hundreds or thousands of the same RNA molecule? Proteins must be continuously synthesized in the cell Instructions coded in genes must be used over and over again A single gene must be able to produce hundreds or thousands of the same RNA molecules for protein synthesis (a) Describe what happens during transcription. During transcription: Enzyme RNA polymerase binds to DNA and separates DNA strands One of the DNA strands is used as a template to assemble nucleotides into a complementary strand of mRNA Pre-mRNA may be edited before it leaves the nucleus

(b) What do you think would happen if introns were not removed from pre-mRNA? If introns were not removed: Instructions carried by mRNA for assembling amino acids into a protein might be incorrect Resulting proteins might not function properly

Chapter 13.2 Ribosomes & Protein Synthesis (pages 366-371)
Genetic Code Step 1 in decoding genetic messages: Transcribe a nucleotide base sequence from DNA to mRNA Transcribed information contains a code for making proteins Proteins also called polypeptides As many as 20 different amino acids can be found in polypeptides Properties of different proteins determined by: Specific amino acids used Order in which amino acids are joined

Genetic Code RNA contains 4 nitrogenous bases: Adenine which binds only with Uracil Guanine which binds only with Cytosine These bases form a “language” based on the sequence in which they are joined This language is called the genetic code Read 3 letters at a time Each “word” is three bases long Corresponds to a single amino acid Each 3 letter “word” is called a codon Specifies which amino acid is added to the polypeptide

Genetic Code 64 possible 3-base codons possible Most amino acids can be specified by more than one codon How to read codons: Start at the middle of the circle with the first letter of the codon Move out to the second ring to find the second letter Find the third and final letter in the third ring Read the amino acid in that sector

Genetic Code Alternate forms of genetic code exist Follow the directions to read the codon: First letter on left side Second letter on top Third letter on right side Read the abbreviation for the amino acid

Genetic Code Start and Stop Codons Like punctuation marks for genetic code Start Codon Always AUG (Methionine) Serves as start codon for protein synthesis After AUG, code is read 3 letters at a time until it reaches one of three “stop” codons UGA, UAA, or UAG Do not code for amino acids Translation stops Polypeptide is complete

Quick Lab How Does a Cell Interpret Codons? A certain gene has the following base sequence: DNA sequence: GACAAGTCCACAATC From left to right, write the sequence of the mRNA molecule transcribed from this gene: mRNA sequence: CUGUUCAGGUGUUAG This is transcription

Quick Lab How Does a Cell Interpret Codons? Using the genetic code, read the mRNA codons from left to right. Then write the amino acid sequence of the polypeptide CUG-UUC-AGG-UGU-UAG Leucine-Phenylalanine-Arginine-Cysteine- Stop Repeat step 2, reading the sequence from right to left. GAU-UGU-GGA-CUU-GUC Aspartic acid-Cysteine-Glycine-Leucine- Valine

Quick Lab How Does a Cell Interpret Codons? Analyze and Conclude Why did steps 3 and 4 produce different polypeptides? The polypeptide produced in step 3 is leucine-phenylalanine-arginine-cysteine- stop In step 4, the polypeptide produced is aspartic acid-cysteine-glycine-leucine- valine They are different because the codons are read in the opposite direction

Quick Lab How Does a Cell Interpret Codons? Analyze and Conclude Do cells usually decode nucleotides in one direction only or in either direction? Cells usually decode nucleotides in one direction only Otherwise, the nucleotides could be reversed and code for a different sequence of amino acids

Transcription Process by which mRNA is made from DNA in nucleus Once mRNA is transcribed, it leaves nucleus and moves to a ribosome in cytoplasm Ribosomes are like factories where proteins are assembled Translation Process by which polypeptides are assembled On the ribosome (rRNA) Using the message from DNA (brought by mRNA from the nucleus) With tRNA bringing amino acids to the ribosome from the cytoplasm

Translation Sequence of Events Translation begins when a ribosome attaches to an mRNA molecule in the cytoplasm As each codon passes through the ribosome, tRNAs bring the proper amino acids to the growing chain of polypeptide being assembled One at a time, the ribosome attaches these amino acids to the growing chain Bonds between amino acids called peptide bonds Each tRNA molecule carries just one kind of amino acid and has 3 unpaired bases Bases on tRNA are called the anticodon Complementary to the mRNA codon

Translation Sequence of Events In the case of the tRNA molecule for methionine, the anticodon is UAC, which pairs with the methionine codon, AUG The ribosome has a second binding site for tRNA molecule for the next codon If the next codon is UUC, a tRNA molecule with an AAG anticodon fits against the mRNA molecule held in the ribosome The second tRNA molecule brings the amino acid phenylalanine into the ribosome

Translation Sequence of Events Like a worker on an assembly line, the ribosome helps form a bond between the first and second amino acids Bond called a peptide bond At the same time the bond holding the first tRNA molecule to its amino acid is broken, and that tRNA exits the ribosome The ribosome moves to the third codon, where tRNA brings it the amino acid specified by the third codon

Translation Sequence of Events The polypeptide chain continues to grow until the ribosome reaches a “stop” codon on the mRNA molecule When the ribosome reaches a stop codon, it releases both the newly formed polypeptide and the mRNA molecule, completing translation

Roles of tRNA & rRNA in Translation All three major forms of RNA come together in the ribosome during translation mRNA carries the coded message that directs the process tRNA delivers exactly the right amino acid coded for by the mRNA codon tRNA anticodon is complementary to mRNA codon rRNA is part of the ribosome along with about 80 proteins. rRNA helps hold the proteins in place rRNA helps locate the beginning of the mRNA message May also carry out the chemical reaction that joins amino acids together Form peptide bonds

Molecular Basis of Heredity Most genes contain instructions for assembling proteins Proteins are microscopic tools Each specifically designed to build or operate a component of a living cell Many proteins are enzymes Catalyze and regulate chemical reactions A gene that codes for an enzyme to produce pigment can control the color of a flower Another gene produces proteins that regulate patterns of tissue growth in a leaf Yet another may trigger the female or male pattern of development in an embryo

Molecular Basis of Heredity Molecular Biology Seeks to explain living organisms by studying them at the molecular level DNA & RNA are molecules Central Dogma of Molecular Biology: Information is transferred from DNA to RNA to protein Many exceptions Viruses can transfer from RNA to DNA

Gene Expression The way in which DNA, RNA, and proteins are involved in putting genetic information into action in living cells DNA carries information for specifying the traits of an organism Cells use base sequences in DNA as template for making mRNA Codons of mRNA specify sequence of amino acids in protein Proteins play a key role in producing an organism’s traits

Gene Expression One of the most interesting discoveries of molecular biology: Near-universal nature of genetic code Some organisms show slight variations in the amino acids assigned to particular codons Genetic code is always read three bases at a time Read in the same direction Despite enormous diversity in form and function, living organisms display remarkable unity at life’s most basic level Molecular biology of the gene

(a) How does a cell interpret the genetic code? The genetic code is read on codon, or 3 bases at a time Each codon codes for an amino acid Except the stop code (b) What are codons and anticodons? Codons are 3 letter “words” in mRNA that specify amino acids Anticodons are 3 unpaired bases in tRNA Complementary to mRNA codons

(c) Using the table (Figure 13-6), identify the amino acids specified by codons: UGG, AGG, and UGC. UGG: Tryptophan AGG: Arginine UGC: Cysteine (a) What happens during translation? A ribosome uses the sequence of codons in mRNA to assemble amino acids into a polypeptide chain The correct amino acids are brought to the ribosome by tRNA

(b) How is protein synthesis different from DNA replication? In DNA replication DNA unzips New complementary strand of DNA is produced on each strand of DNA In opposite directions Using enzyme DNA polymerase DNA never leaves the nucleus. In transcription mRNA is produced from a specific area of DNA using enzyme RNA polymerase Message is taken from nucleus out to ribosomes in cytoplasm.

(b) How is protein synthesis different from DNA replication? In translation mRNA gives DNA message to rRNA In codons tRNA brings appropriate amino acid to ribosome rRNA attaches amino acids to other amino acids to make new protein molecule (a) Why is the genetic code considered universal? In all organisms, the code is read 3 bases at a time and in the same direction In most organisms, the same amino acids are assigned to particular codons (b) What does the term gene expression mean? It refers to the way in which DNA, RNA, and proteins are involved in putting genetic information into action in living cells

(c) In what way does controlling the proteins in an organism control the organism’s characteristics? Proteins build or operate components of cells Proteins play a key role in producing an organism’s characteristics For example: Enzymes catalyze and regulate chemical reactions in cells Other proteins regulate growth patterns or embryonic development

Chapter 13.3 Mutations (pages 372 - 376)
A heritable (inheritable) change in genetic information Mistake in copying DNA Mistake inserting the wrong base Skipping a base as a strand is put together From the Latin word “mutare” “to change” Types of Mutations 2 major types: Gene Mutations Produce changes in a single gene Chromosomal Mutations Produce changes in an entire chromosome

Gene Mutations Point Mutations Involve changes in one or a few nucleotides Occur at a single point in the DNA Generally occur during replication If a gene is altered, the change can be passed on to every cell that develops from that cell Substitution One base is changed to a different base Usually affects no more than a single amino acid Sometimes no effect at all

Gene Mutations Point Mutations Insertions and Deletions One base is added to or removed from the DNA sequence Effects of these changes can be dramatic Since genetic code is read 3 bases at a time, if a base is added or deleted Bases still read 3 at a time Groupings shift in every codon following the mutation Called frameshift mutations Shift the reading frame Can change any amino acid after the point of mutation

Gene Mutations Involve changes in one or a few nucleotides Substitutions May affect only one amino acid Insertions and Deletions Affect every codon after point of mutation Called frameshift mutations Effects may be dramatic

Chromosomal Mutations Involves changes in the number or structure of chromosomes Can change the location of genes on chromosomes Can change the number of copies of some genes Deletion Involves the loss of all or part of a chromosome Duplication Produces an extra copy of all or part of a chromosome Inversion Reverses the direction of parts of a chromosome Translocation Occurs when part of one chromosome breaks off and attaches to another chromosome

Effects of Mutations Genetic material can be altered by natural events or by artificial means Resulting mutations may or may not affect an organism Some mutations that affect individual organisms can also affect a species or an entire ecosystem Many mutations are caused by errors in genetic processes For Example: Point mutations can be caused by errors in DNA replication Roughly once in every 10 million bases Small changes can accumulate over time

Effects of Mutations Stressful environmental conditions may cause some bacteria to increase mutation rates Can be helpful to organism Mutations can sometimes give bacteria new traits Ability to consume new food source Ability to resist a poison in the environment Ability to resist an antibiotic

Mutagens Chemical or physical agents in the environment that can cause mutations to occur Physical Mutagens Electromagnetic radiation X-rays, UV radiation Chemical Mutagens Pesticides, plant alkaloids, tobacco smoke, environmental pollutants Other types of Mutagens Viruses and other infectious agents

Mutagens Agents interact with DNA Can produce mutations at high rates Cells may repair damage If cells do not repair damage DNA changes permanently Some agents interfere with base- pairing Increases error rate in DNA replication Some agents weaken the DNA strand Cause breaks and inversions that produce chromosomal mutations

Harmful & Helpful Mutations Effects of mutations on genes vary widely Some have little or no effect Some produce beneficial variations Some negatively disrupt gene functions

Harmful Effects Some of the most harmful mutations can dramatically change protein structure or gene activity Defective proteins can: Disrupt normal biological activities Result in genetic disorders Result in cancer Product of mutations that cause uncontrolled growth of cells

Harmful Effects Sickle Cell disease is a genetic disorder associated with changes in shape of red blood cells (RBC’s) Caused by point mutation in one of the proteins in hemoglobin Blood’s principal oxygen-carrying protein Symptoms include: Anemia (shortage of red blood cells) Severe pain Frequent infections Stunted growth When a person with sickle cell disease gets out of breath, their abnormal hemoglobin crystallizes irreversibly Makes RBC’s unable to carry oxygen

Beneficial Mutations Mutations often produce proteins with new or altered functions that can be useful to organisms in different or changing environments Helped insects to resist chemical pesticides Beneficial to insects, bad for us Enabled microorganisms to adapt to new chemicals in the environment Example: Bacteria become resistant to antibiotics Human mutations increased bone strength and density Makes fractures less likely Human mutations increase resistance to HIV, the virus that causes AIDS

Beneficial Mutations Plant and animal breeders can make use of “good” mutations When a complete set of chromosomes fails to separate during meiosis gametes may result that produce triploid (3N) or tetraploid (4N) organisms Organism with extra sets of chromosomes has polyploidy

Chapter 13.3 Assessment (pages 386 - 389)
(a) The process by which the genetic code of DNA is copied into a strand of RNA is called ____. B. Transcription Which of the following describes RNA? B. RNA is usually single-stranded and contains the base uracil Describe the function of each of the three types of RNA. Messenger RNA (mRNA) carries the instructions for protein synthesis from DNA to the cytoplasm Ribosomal RNA (rRNA) makes up ribosomes, where proteins are made Transfer RNA (tRNA) carries amino acids to the ribosome and matches them to the coded mRNA message

How does the enzyme that makes RNA know where to start transcribing the DNA? The enzyme knows where to start transcribing DNA at a promoter Promoter is a region of DNA that has specific base sequences so that the RNA polymerase knows where to start transcription Compare introns and exons. Introns are sections of mRNA that are not needed for protein synthesis Cut out of mRNA before it leaves the nucleus Exons are sections of mRNA that are needed for protein synthesis

Suppose you start with the DNA strand ACCGTCAC. Use the rules of base pairing to list the bases on a messenger RNA strand transcribed from this DNA strand. DNA sequence: ACCGTCAC mRNA sequence: UGGCAGUG tRNA sequence: ACCGUCAV (anticodon information) In messenger RNA, each codon specifies a particular ____. C. Amino acid The number of codons in the genetic code is ____. D. 64 Which of the following statements about the genetic code is true? C. Some codons specify the same amino acid

The process of making proteins on the ribosome based on instructions from messenger RNA is called ____. C. Translation What is a codon? A codon is a three-base code “word” in the genetic code A codon specifies a particular amino acid, start, or stop How do anticodons function? At the ribosome, anticodons in tRNA form bonds with the complementary codons in mRNA. tRNA adds its amino acid to the polypeptide chain when the anticodon and the codon are complementary

If a code on a DNA molecule for a specific amino acid is CTA, what would the messenger RNA codon be? The tRNA anticodon? Messenger RNA codon: GAU (remember U replaces T in RNA) Transfer RNA anticodon: CUA Changes in DNA sequences that affect genetic information are known as ____. B. Mutations A single-base mutation in a messenger RNA molecule could transcribe the DNA sequence CAGTAT into____. D. GUAAUA A substance that can cause a change in the DNA code of an organism is called a ____. B. mutagen

Name and give examples of two major types of mutations. What do they have in common? How are they different? Two main types: gene mutations and chromosomal mutations Examples of gene mutations: (affect a single gene) Substitutions in which one base is changed to a different base Insertions in which an extra base is inserted into a codon Deletions in which a base is left out of a codon Examples of chromosomal mutations: (affect all or part of a chromosome) Deletions which involves the loss of all or part of a chromosome Duplication in which an extra copy of all or part of a chromosome is produced Inversion which reverses the direction of parts of a chromosome Translocation which occurs when part of one chromosome breaks off and attaches to another Gene and chromosomal mutations both involve a change in the DNA sequence

How does a deletion mutation differ from a substitution mutation? A deletion mutation occurs when a base is lost from a codon This shifts the reading frame so all the codons after the point of deletion are affected A substitution mutation occurs when a single base is replaced by a different base This does NOT affect the reading frame It usually only affects one amino acid (and sometimes no amino acids) Can mutations have a positive effect? Yes A mutation could produce a protein with a new or altered function that might be useful to an organism in a changing environment

How does the possible impact of a chromosomal mutation that occurs during meiosis differ from that of a similar event that occurs during mitosis of a body cell that is not involved in reproduction? A chromosomal mutation that occurs during meiosis will be carried by some of the organism’s gametes and possibly the organism’s offspring A mutation that occurs during mitosis in a body cell will be passed on to that cell’s daughter cell but not to the organism’s offspring A mutation in the DNA of an organism changes one base sequence in a protein- coding region from CAC to CAT. What is the effect of the organism on the final protein? Explain your answer. The mutation in the DNA changes the codon in mRNA from GUG to GUA. Both of these codons code for the amino acid valine The final protein would not be affected

Chapter 13.3 Standardized Test Prep (page 389)
How does RNA differ from DNA? C. RNA contains uracil and ribose How would the DNA sequence GCTATA be transcribed to mRNA? C. CGAUAU Use the chart to answer the questions: Which of the following codons signifies the end of translation? UGA (also UAA and UAG) Which of the chains of amino acids corresponds to the nucleotide sequence UCAAGCGUA? D. Ser-ser-val

Chapter 13.3 Standardized Test Prep (page 389)
Mutant 1 is a(n) ____. C. Inversion Mutant 2 is a(n) ____. D. Duplication Normal chromosome M N O P Q R S Mutant 1 Mutant 2

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