Chp 7 DNA Structure and Gene Function Herpes virus particles and cold sore blister. 1 DNA is a Double-Stranded Helix ✶ DNA stores the information that cells use to produce proteins ✶ DNA has two strands in a double helix ✶ Base sequence on both strands is complimentary – A pairs with T, and G pairs with C Hydrogen bonds Ribbon model Partial chemical structure Space-fill model DNA molecule shown in different models 2 ✶ RNA is a single nucleic acid chain, – has ribose sugar instead of deoxyribose in DNA – has the base uracil (U) instead of thymine 3 DNA Replication Uses Base Pairing ✶ DNA replication - semi-conservative model – Parental strands separate – Each strand used to build a complimentary strand – New DNA has 1 old strand, 1 new strand Figure 10.4A DNA replication 4 5 DNA Replication Starts at Many Sites ✶ Begins at origins of replication ✶ Replication bubbles form and then merge Replication bubbles during DNA replication 6 Two Key Enzymes in DNA Replication DNA polymerase molecule 3ʹ′ 5ʹ′ Daughter strand synthesized continuously Parental DNA 5ʹ′ 3ʹ′ Daughter strand synthesized in pieces ✶ DNA polymerase – links nucleotides into a chain, moving in 3’ to 5’ direction ligase - links small fragments into a chain 3ʹ′ 5ʹ′ ✶ DNA 5ʹ′ 3ʹ′ DNA ligase Overall direction of replication Figure 10.5C Enzymes of DNA replication 7 Clicker Question #1 What is the main function of DNA? A. To encode proteins B. To produce ATP C. To speed up cell reactions D. To provide structural support to the cell E. All of these Protein Synthesis Starts with DNA ✶ Gene – a sequence of DNA that directs the synthesis of a specific protein ✶ Protein synthesis occurs in two stages – DNA gene used to make RNA copy – RNA copy used to make a polypeptide Protein synthesis in eukaryotic cells 9 Brownie Analogy for Protein Production Making brownies is a simple analogy to protein production. Transcription Uses DNA to Create RNA Let’s first look at how a cell produces an RNA copy of a gene. Transcription occurs in the nucleus. Transcription Has Three Steps a. Initiation RNA polymerase enzyme DNA DNA template strand Promoter b. Elongation DNA RNA polymerase G G C C T G DNA G G C C U G RNA C C G G A C RNA c. Termination RNA polymerase DNA Terminator RNA Transcription has three steps - Initiation - Elongation - Termination DNA template strand We will look at these steps one at a time Initiation Initiation RNA polymerase enzyme DNA Promoter DNA template strand • RNA polymerase binds to the promoter, which is the beginning of the gene • Enzymes (not shown) unzip the DNA • The DNA template strand encodes the RNA molecule Elongation RNA polymerase moves along the template strand, making an RNA copy Elongation RNA polymerase DNA RNA Notice that the RNA molecule is complementary to the DNA template strand Termination Termination RNA polymerase DNA Terminator RNA • RNA polymerase reaches the terminator, which is the end of the gene • RNA, DNA, and RNA polymerase separate • DNA becomes a double helix again Clicker Question #2 If the DNA template strand has the following sequence, what would be the nucleotide sequence of the complementary RNA molecule produced in transcription? Template strand: AGTCTT A. AGTCTT B. AGUCUU C. TCAGAA D. TCUGUU E. UCAGAA Translation Builds the Protein Now let’s look at how a ribosome uses RNA to produce a protein. Codons – A Set of Three Nucleotides DNA DNA template strand TRANSCRIPTION T T C A G T C A G A A G U C A G U C mRNA Codon Codon Codon Lysine Serine Valine TRANSLATION Protein Polypeptide (amino acid sequence) A codon is a three-nucleotide sequence that encodes one amino acid The Genetic Code: MRNA->Amino Acid The genetic code shows which mRNA codons correspond to which amino acids A A G U C A G U C mRNA Codon Codon Codon Lysine Serine Valine TRANSLATION Protein Polypeptide (amino acid sequence) The Genetic Code U U UUU UUC UUA UUG Leucine (Leu; L) CUU CUC CUA A Phenylalanine (Phe; F) Leucine (Leu; L) A UCU UAU UCC UCA Serine (Ser; S) UAC UGA Stop A UGG Tryptophan (Trp; W) G CCU CAU CCC CCA Proline (Pro; P) CAC CAA AAU ACC ACA Proline (Pro; P) AAC AAA AUG Start Methionine (Met; M) ACG AAG GCU GAU GUA GUG Valine (Val; V) C Stop ACU GUC UGC U Cysteine (Cys; C) Stop AUU G GUU UGU UAG CAG Isoleucine (Ile; I) Tyrosine (Tyr; Y) UAA CCG AUC G UCG CUG AUA C GCC GCA GCG Proline (Pro; P) GAC GAA GAG Histidine (His; H) Glutamine (Gln; Q) Asparagine (Asn; N) Lysine (Lys; K) Aspartic acid (Asp; D) Glutamic acid (Glu; E) CGU CGC CGA U Arginine (Arg; R) CGG AGU AGC AGA AGG GGA GGG A G Serine (Ser; S) Arginine (Arg; R) GGU GGC C U C A G U Glysine (Gly; G) C A G Third letter of codon First letter of codon C Second letter of codon Transfer RNA are the “Translators” Transfer RNA (tRNA) molecules translate the genetic code. tRNA binds to an mRNA codon here, at the anticodon… and binds to the corresponding amino acid here. Section 7.4 Figure 7.8 Translation Builds the Protein Translation also occurs in three steps: -Initiation -Elongation -Termination All of these steps happen at ribosomes. Initiation • Small ribosomal subunit binds to mRNA • Large ribosomal subunit binds • First tRNA molecule binds Initiation • tRNA complementary base pairs to mRNA • tRNA already carries an amino acid (Met). Elongation • The second tRNA enters the ribosome next to the first tRNA • Amino acids covalently bond Covalent bond Elongation • The first tRNA leaves • The ribosome moves to the right, and a third tRNA comes in Elongation • But, notice that the amino acids remain bonded together! • This process continues and the protein grows Linked amino acids Termination • The ribosome reaches the stop codon • A release factor binds • The polypeptide detaches from the mRNA and folds into a functional protein Translation is efficient when multiple ribosomes attach to an mRNA molecule simultaneously. mRNA Ribosome Polypeptide SEM (false color) 50 nm Clicker Question #3 Look at the image below. Which ribosome has been on the mRNA the longest? A. The one on the far right. B. The one of the far left. Polypeptide (purple) Ribosome (green) Mastering Concepts What are the steps of translation? Where in the cell does translation occur? Flow of Genetic Info in Living Cells: DNA → RNA → Protein – Which represents translation? – DNA → RNA, or RNA → protein – Where does the info for making a protein originate? – DNA, or RNA – Which has a linear sequence of codons? – rRNA, mRNA, or tRNA – Which directly produces observable traits? – DNA, RNA, or protein 32 Mutations Change DNA ✶ Mutation = any change in a cell’s DNA sequence ✶ – Can be caused by errors in DNA replication, or – By mutagens (highenergy radiation, or chemicals) A mutation in one gene causes a fly to develop legs where its antenna should be! 33 A point mutation changes one, or a few base pairs in a gene The table to the left uses sentences to show a few examples of point mutations • Remember that codons are sequences of three nucleotides • Each word in the sentences above represents one codon “Frameshift” mutations affect multiple codons. Insertion of one nucleotide changes every codon after the insertion. Sickle Cell Anemia Normal red blood cells G G A C T C C T T C C U G A G G A A No aggregation of hemoglobin molecules SEM 6 µm Pro Glu Glu Sickled red blood cells Abnormal G G A C A C C T T aggregation C C U G U G G A A of hemoglobin molecules Pro Val Glu SEM 6 µm A single base substitution in a hemoglobin gene causes blood cells to form abnormally, leading to sickle cell disease Types of mutations and their effects But mutations are not always harmful! Mutations create different versions of alleles, which are alternative versions of the same gene. Genetic variation is important for evolution. Plant breeders even induce mutations to create new varieties of plants. Mastering Concepts 1. Describe the components of DNA and its three-dimensional structure 2. What is the relationship between a gene and a protein? 3. What are the steps of translation? 4. Where in the cell does translation occur? 5. What are the types of mutations, and how does each alter the encoded protein?