NUCLEIC ACIDS AND PROTEIN SYNTHESIS Ch. 10

Ch. 10-1 DNA

Structure of DNA DNA made of repeating subunits called nucleotides Each DNA molecule consists of two long chains of nucleotides DNA nucleotide has three main parts: Sugar molecule (Deoxyribose) Phosphate group Nitrogen-containing base Deoxyribose & phosphate group are identical in ALL DNA nucleotides Nitrogen-containing base may be one of four different kinds: Adenine (A) Guanine (G) Cytosine (C) Thymine (T) Nitrogen-Containing Base Deoxyribose

Structure of DNA Adenine & Guanine = PURINES Have two rings of carbon Thymine & Cytosine = PYRIMIDINES Have one ring of carbon

The Double Helix 1953: James Watson & Francis Crick suggested model for DNA Rosalind Franklin & Maurice Wilkins X-ray photographed DNA 1962 Watson, Crick & Wilkins received Nobel Prize Covalent bonds btwn deoxyribose sugar & phosphate molecules Sugar & phosphate form a “backbone” where nitrogen-containing bases attach Hydrogen bonds btwn 2 nucleotides

-Bases on one chain of DNA face bases on other chain of DNA forming hydrogen bonds btwn them -Hydrogen bonds are relatively weak -Adenine pairs with Thymine -Cytosine pairs with Guanine Adenine --------------- Thymine (purine) (pyrimidine) Cytosine --------------- Guanine (pyrimidine) (purine)

Complementary Base Pairing Pairing of one base to specific other base = complementary base pairing Two base-pairing rules: Adenine ---- Thymine Cytosine ---- Guanine

Replication of DNA Copying of DNA: Replication Two nucleotide chains unwind and each chain serves as template for new nucleotide chain point at which two chains separate: Replication fork Two chains are separated by enzyme Helicase as helicase moves along DNA, they break hydrogen bonds btwn bases DNA polymerase: bind to separated chains of DNA, move along chains & assemble nucleotides complementary to existing DNA chains Complementary bases are bonded by hydrogen bonds DNA polymerases begin replication simultaneously at many points along the separated nucleotide chains (~6000 sites at same time in fruit fly DNA) When replication complete: two new exact copies of original DNA molecule are produced and cell is ready to undergo cell division Example: DNA Molecule of ATTCCG would have complementary strand of TAAGGC

Accuracy and Repair DNA rep occurs with high degree of accuracy—1 error in every 10,000 paired nucleotides Change in nucleotide sequence at even one location: mutation: can have serious effects in new cells DNA proofreading & repair processes help keep error rate to one error/1 billion nucleotides!

Review Questions What are the main functions of DNA? Identify the types and locations of covalent bonds and hydrogen bonds in a DNA molecule. List the base-pairing rules. What roles do enzymes play in DNA replication? How would the deoxyribose sugar-phosphate backbone of nucleotide chains look if purines paired with only purines and pyrimidines paired with only pyrimidines?

Review Questions: Answers What are the main functions of DNA? Stores and transmits genetic information Identify the types and locations of covalent bonds and hydrogen bonds in a DNA molecule: Covalent bonds exist between the deoxyribose sugar and phosphate molecules. Hydrogen bonds join the nitrogen bases List the base-pairing rules: Guanine and cytosine pair; thymine and adenine pair What roles do enzymes play in DNA replication? Helicase attaches to DNA and breaks hydrogen bonds between the complementary bases and the chains separate. DNA polymerases bind to the separated chains of DNA. DNA polymerase identifies an unpaired base on a template chain, locates a nucleotide that has the complementary base in the surrounding medium, and attaches the free nucleotide to the new nucleotide chain that is being formed. Other enzymes are responsible for identifying and repairing errors in replicated DNA. How would the deoxyribose sugar-phosphate backbone of nucleotide chains look if purines paired with only purines and pyrimidines paired with only pyrimidines? The backbone would appear uneven, similar to the appearance of a roller coaster.

Ch. 10-2 RNA

Structure of RNA Sugar = Ribose (not deoxyribose) Single strand of repeating nucleotides Adenine pairs with Uracil (not thymine) Adenine----Uracil; Cytosine----Guanine

Types of RNA mRNA (Messenger RNA): carries genetic information from DNA in nucleus  cytosol tRNA (Transfer RNA): binds to specific amino acids; 45 varieties rRNA (Ribosomal RNA): most abundant form; make up ribosomes where proteins are made

Transcription Process by which genetic information is copied from DNA to RNA: Transcription RNA polymerase: primary transcription enzyme, make RNA copies from DNA sequences Initiates transcription by binding t specific regions of DNA called promoters: mark beginning of the DNA chain that will be transcribed (beginning of gene) When RNA polymerase binds to promoter, DNA molecule separates—only one of separated DNA chains—template—is used for transcription

Steps of Transcription RNA polymerase attaches to first DNA nucleotide of template chain It begins adding complementary RNA nucleotides to newly forming RNA molecule Same base-pairing rules, except one exception: C— G, A—U Transcription continues until RNA polymerase reaches DNA region called termination signal: specific sequence of nucleotides that marks end of gene ( in eukaryotes) At termination signal, RNA polymerase releases both DNA molecule and newly formed RNA molecule

Products of Transcription Following transcription, mRNA moves through pores of nuclear membrane into cytosol of cell where it will direct synthesis of proteins

Review Questions Define transcription. List the main steps involved in this process. In what ways does the structure of RNA differ from that of DNA? Describe the structure and function of each of the three types of RNA. List three roles of RNA polymerase in transcription. What basic principle ensures that the transcribed RNA molecule is carrying the right genetic message? Does it matter which of the separated DNA chains is used for transcription? Why?

Review Questions: Answers Define transcription. List the main steps involved in this process: The process of producing RNA from DNA In what ways does the structure of RNA differ from that of DNA? RNA usually consists of a single chain of nucleotides, has ribose instead of deoxyribose as its sugar, and contains uracil rather than thymine. Describe the structure and function of each of the three types of RNA: mRNA consists of RNA nucleotides in the form of a single uncoiled chain. mRNA carriers genetic information from the DNA in the nucleus to the cytosol of a eukaryotic cell. rRNA is a single chain of about 89 nucleotides folded into a hairpin shape that binds to a specific amino acid. rRNA consists of RNA nucleotides in a globular form and makes up the ribosomes where proteins are made. List three roles of RNA polymerase in transcription: RNA polymerase initiates transcription by binding to promoters. When RNA polymerase binds to a promoter, the DNA molecule in that region separates. RNA polymerase attaches to the first DNA nucleotide of the template chain. Then it begins adding complementary RNA nucleotides to the newly forming RNA molecule. At the termination signal, RNA polymerase releases DNA and RNA. What basic principle ensures that the transcribed RNA molecule is carrying the right genetic message? Accuracy of the genetic message is assured by complementary base pairing. Does it matter which of the separated DNA chains is used for transcription? Why? Yes, because templates are complementary, they do not contain identical sequences of nucleotides A sequence complementary to the template will code for different information.

Ch. 10-3 PROTEIN SYNTHESIS

Protein Structure and Composition Proteins are polymers which are made of one or more polypeptides which are made of amino acids linked to each other by peptide bonds 20 different amino acids Polypeptides consist of hundreds or thousands of the 20 AA arranged in particular sequence sequence will determine the function of protein amino acids polypeptides polymers

The Genetic Code During protein synthesis (translation), sequence of nucleotides in mRNA is translated into sequence of amino acids Genetic code—used by most organisms to translate mRNA into proteins; correlation btwn nucleotide sequence & AA sequence Combination of 3 mRNA nucleotides = CODON Each codon codes for specific amino acid There are 64 different codons that code for the 20 different amino acids The near-universality of the genetic code supports idea that all organisms are evolutionarily related

CODON TABLE ANSWER THE FOLLOWING: 1) CCA= 2) AUU= 3) GUC= 4) AUG= 5) CGA=

Start codon: (AUG); engages ribosomes to start translating an mRNA molecule; codes for amino acid methionine Stop codon: (UAA, UAG, UGA); cause ribosome to stop translating an mRNA

Translation Translation: process of assembling polypeptides from information encoded in mRNA Begins when mRNA leave nucleus and migrates to ribosomes where proteins are made

tRNA and Anticodons tRNA carry AA from cytosol to ribosomes tRNA has region that bonds to specific AA Opposite side of tRNA has sequence of 3 nucleotides called anticodon: complementary to and pairs with its corresponding mRNA codon tRNA with anticodon of AAA would bind to mRNA codon sequence of UUU and would be specific for AA phenylalanine ensures that AA are added to growing polypeptide in order prescribed by mRNA transcript

Ribosomes Made of both rRNA and proteins Can be free floating or attached to RER Membrane proteins & proteins that will be exported for outside use are produced by ribosomes on RER Ribosomes have 3 binding sites: One site holds an mRNA so that its codons are accessible to rRNA Other two sites hold tRNAs whose anticodons pair with the mRNA codons

Protein Assembly Ribosomes attaches to start codon (AUG) on mRNA start codon pairs with anticodon UAC on a tRNA (because tRNA that has UAC anticodon carries AA methionine, the first AA in every polypeptide is methionine) it may be removed later Ribosome moves along mRNA and each codon is paired with its tRNA anticodon this causes specified AA to attach to previously translated AA with a covalent bond called a peptide bond

Review Questions Compare transcription with translation. Distinguish a codon from an anticodon, and explain the significance of each. How does the structure of tRNA relate to its function in translation. Explain the significance of the start codon and stop codons. Do all polypeptides begin with the amino acid coded for by the start codon? What would translation of the mRNA transcript UAACAAGGAGCAUCC produce?

Review Questions: Answers Compare transcription with translation: Transcription is the process of producing RNA from DNA. Translation is the process by which polypeptides are assembled from the information encoded in mRNA. Distinguish a codon from an anticodon, and explain the significance of each: A codon is a series of three nucleotides in mRNA that code for a specific amino acid. An anticodon on tRNA is a set of three bases complementary to the codon. How does the structure of tRNA relate to its function in translation: The structure of tRNA allows an amino acid at one end to be positioned in a polypeptide when the anticodon at the other end of the tRNA pairs with a codon. Explain the significance of the start codon and stop codons. Do all polypeptides begin with the amino acid coded for by the start codon? The start codon (AUG) engages a ribosome to start translating an mRNA molecule. Stop codons (UAA, UAG, UGA) cause the ribosome to stop translating mRNA. All polypeptides do not begin with the amino acid coded for the by start codon. What would translation of the mRNA transcript UAACAAGGAGCAUCC produce? No protein would be produced because the mRNA begins with a stop codon.