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Chapter 3 Part D Cells: The Living Units © Annie Leibovitz/Contact Press Images © 2016 Pearson Education, Inc. PowerPoint® Lecture Slides prepared by Karen Dunbar Kareiva Ivy Tech Community College 3.10 Cell Cycle • Series of changes a cell undergoes from the time it is formed until it reproduces • Two major periods of cell cycle: – Interphase • Cell grows and carries on its usual activities – Cell division (mitotic phase) • Cell divides into two © 2016 Pearson Education, Inc. Interphase • Period from cell formation to cell division, when cell carries out its routine activities and prepares for cell division • During interphase, nuclear material is in uncondensed chromatin state • Interphase consists of subphases, which include the process of DNA replication © 2016 Pearson Education, Inc. Interphase (cont.) • Subphases – Interphase broken into three subphases: • G1 (gap 1): vigorous growth and metabolism – Cells that permanently cease dividing are said to be in G0 phase • S (synthetic): DNA replication occurs • G2 (gap 2): preparation for division © 2016 Pearson Education, Inc. Figure 3.28 The cell cycle. G1 checkpoint (restriction point) S Growth and DNA synthesis G1 Growth G2 Growth and final preparations for division G2/M checkpoint © 2016 Pearson Education, Inc. Interphase (cont.) • DNA replication – Prior to division, the cell makes a copy of DNA – Double-stranded DNA helices unwind and unzip • Replication fork: point where strands separate • Replication bubble: active area of replication • Each strand acts as a template for a new complementary strand – RNA starts replication by laying down short strand that acts as a primer © 2016 Pearson Education, Inc. Interphase (cont.) – DNA polymerase attaches to primer and begins adding nucleotides to form new strand • DNA polymerase synthesizes both new strands at one time (one leading and one lagging strand) – DNA polymerase works only in one direction, so leading strand is synthesized continuously; however, because lagging strand is “backwards,” it is synthesized discontinuously into segments – Another enzyme, DNA ligase, then splices short segments of discontinuous lagging strand together © 2016 Pearson Education, Inc. Interphase (cont.) • End result: two identical “daughter” DNA molecules are formed from the original • During mitotic cell division, one complete copy will be given to new cell while one is retained in original cell • Process is called semiconservative replication because each new double-stranded DNA is composed of one old strand and one new strand © 2016 Pearson Education, Inc. Figure 3.29 Replication of DNA: summary. Free nucleotides DNA polymerase Chromosome Old (parental) strand acts as a template for synthesis of new strand Leading strand Two new strands (leading and lagging) synthesized in opposite directions Old DNA Replication bubble Lagging strand Enzymes unwind the double helix and expose the bases Adenine Thymine Cytosine Guanine © 2016 Pearson Education, Inc. Replication fork (area where hydrogen bonds between base pairs are broken and DNA strands separate) DNA polymerase Old (template) strand Cell Division • Most cells need to replicate continuously for growth and repair purposes – Skeletal, cardiac, and nerve cells do not divide efficiently; damaged cells are replaced with scar tissue • M (mitotic) phase of cell cycle is phase in which division occurs; consists of 2 distinct events: – Mitosis – Cytokinesis • Control of cell division is crucial, so cells divide when necessary, but do not divide unnecessarily © 2016 Pearson Education, Inc. Figure 3.28 The cell cycle. G1 checkpoint (restriction point) S Growth and DNA synthesis G1 Growth G2 Growth and final preparations for division G2/M checkpoint © 2016 Pearson Education, Inc. Cell Division (cont.) • M phase – Mitosis is the division of nucleus, in which the duplicated DNA is distributed to new daughter cells • Four stages of mitosis ensure each cell receives a full copy of replicated DNA – Prophase – Metaphase – Anaphase – Telophase © 2016 Pearson Education, Inc. Cell Division (cont.) • Prophase can be broken into two parts: 1. Early prophase • Chromatin condenses, forming visible chromosomes • Each chromosome and its duplicate (called sister chromatids) are held together by a centromere • Centrosome and its duplicate begin synthesizing microtubules that push each centrosome to opposite poles of cell – Called the mitotic spindle – Other microtubules called asters radiate from centrosome © 2016 Pearson Education, Inc. Cell Division (cont.) • Prophase (cont.) 2. Late prophase • Nuclear envelope breaks up • Special microtubules attach to specific area on centromeres called kinetochore and serve to pull chromosomes to center (equator) of cell • Remaining nonkinetochore microtubules push against each other, causing poles of cell to move farther apart © 2016 Pearson Education, Inc. Focus Figure 3.3-1b Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it produces two identical daughter cells. Early Prophase Early mitotic spindle Aster Centromere Chromosome consisting of two sister chromatids © 2016 Pearson Education, Inc. Focus Figure 3.3-1c Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it produces two identical daughter cells. Late Prophase Spindle pole Nonkinetochore microtubule Fragments of nuclear envelope Kinetochore © 2016 Pearson Education, Inc. Kinetochore microtubule Cell Division (cont.) • Metaphase – Centromeres of chromosomes are precisely aligned at cell’s equator – The imaginary plane midway between poles is called metaphase plate © 2016 Pearson Education, Inc. Focus Figure 3.3-2a Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it produces two identical daughter cells. Metaphase Spindle Metaphase plate © 2016 Pearson Education, Inc. Cell Division (cont.) • Anaphase – Shortest of all phases – Centromeres of chromosomes split simultaneously—each sister chromatid now becomes a separate chromosome – Chromosomes are pulled toward their respective poles by motor proteins of kinetochores • One chromosome of each original pair goes to opposite poles – Nonkinetochore microtubules continue forcing poles apart © 2016 Pearson Education, Inc. Focus Figure 3.3-2b Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it produces two identical daughter cells. Anaphase Daughter chromosomes © 2016 Pearson Education, Inc. Cell Division (cont.) • Telophase – Begins when chromosome movement stops – Each set of chromosomes (at opposite ends of cell) uncoils to form chromatin – New nuclear membranes form around each chromatin mass – Nucleoli reappear – Spindle disappears © 2016 Pearson Education, Inc. Cell Division (cont.) • Cytokinesis – Begins during late anaphase and continues through mitosis – Ring of actin microfilaments contracts to form cleavage furrow – Two daughter cells are pinched apart © 2016 Pearson Education, Inc. Focus Figure 3.3-2c Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. Together with cytokinesis, it produces two identical daughter cells. Telophase Cytokinesis © 2016 Pearson Education, Inc. Nuclear envelope forming Nucleolus forming Contractile ring at cleavage furrow Cell Division (cont.) • Control of cell division – “Go” and “Stop” signals direct when a cell should and should not divide • Go signals include: – Critical surface-to-volume ratio of cell, when area of membrane becomes inadequate for exchange – Chemicals (example: growth factors, hormones) • Stop signals include: – Availability of space; normal cells stop dividing when they come into contact with other cells » Referred to as contact inhibition © 2016 Pearson Education, Inc. Cell Division (cont.) • Two groups of proteins are crucial to cell’s ability to accomplish S phase and enter mitosis: – Cyclins: regulatory proteins that accumulate during interphase – Cdks (Cyclin-dependent kinases) that activate cyclins when they bind to them – Cyclin-Cdk complex in turn activates enzyme cascades that prepare cell for division – Cyclins are destroyed after mitotic cell division, and process begins again © 2016 Pearson Education, Inc. Cell Division (cont.) • Checkpoints are key events in the cell cycle where cell division processes are checked and, if faulty, stopped until repairs are made – G1 checkpoint (restriction point) is the most important of the three major checkpoints – If cell does not pass, it enters G0, in which no further division occurs © 2016 Pearson Education, Inc. Figure 3.28 The cell cycle. G1 checkpoint (restriction point) S Growth and DNA synthesis G1 Growth G2 Growth and final preparations for division G2/M checkpoint © 2016 Pearson Education, Inc. 3.11 Protein Synthesis • DNA is master blueprint that holds the code for protein synthesis – DNA directs the order of amino acids in a polypeptide • A segment of DNA that holds the code for one polypeptide is referred to as a gene © 2016 Pearson Education, Inc. 3.11 Protein Synthesis • The code is determined by the specific order of nitrogen bases (Adenine, Guanine, Thymine, and Cytosine) in the gene – Code consists of three sequential bases (triplet code) • Example: GGC codes for amino acid proline, whereas GCC codes for arginine – Each triplet specifies the code for a particular amino acid © 2016 Pearson Education, Inc. 3.11 Protein Synthesis • Genes are composed of exons and introns – Exons are part of gene that actually codes for amino acids – Introns are noncoding segments interspersed amongst exons © 2016 Pearson Education, Inc. The Role of RNA • RNA is the “go-between” molecule that links DNA to proteins – RNA copies the DNA code in nucleus, then carries it into cytoplasm to ribosomes • All RNA is formed in nucleus • RNA differs from DNA – Uracil is substituted for thymine in RNA – RNA has ribose instead of deoxyribose sugar • Three types of RNA: – Messenger RNA (mRNA) – Ribosomal RNA (rRNA) – Transfer RNA (tRNA) © 2016 Pearson Education, Inc. The Role of RNA (cont.) • Messenger RNA (mRNA) – Single stranded – Code from DNA template strand is copied with complementary base pairs, resulting in a strand of mRNA • Process is referred to as transcription – mRNA maintains the triplet code (codon) from DNA © 2016 Pearson Education, Inc. The Role of RNA (cont.) • Ribosomal RNA (rRNA) – Structural component of ribosomes, the organelle where protein synthesis occurs – Along with tRNA, helps to translate message from mRNA into polypeptide © 2016 Pearson Education, Inc. The Role of RNA (cont.) • Transfer RNA (tRNAs) – Carrier of amino acid – Have special areas that contain a specific triplet code (anticodon) that allows each tRNA to carry only a specific amino acid – Anticodon of tRNA will complementary base-pair with codon of mRNA at ribosome, adding its specific amino acid to growing polypeptide chain • Process is referred to as translation © 2016 Pearson Education, Inc. Protein Synthesis • Occurs in two steps: – Transcription • DNA information coded in mRNA – Translation • mRNA decoded to assemble polypeptides © 2016 Pearson Education, Inc. Figure 3.30 Simplified scheme of information flow from the DNA gene to mRNA to protein structure during transcription and translation. Nuclear envelope Transcription RNA Processing DNA Pre-mRNA mRNA Nuclear pores Ribosome Translation Polypeptide Start translation © 2016 Pearson Education, Inc. Stop; detach Transcription • Process of transferring code held in DNA gene base sequence to complementary base sequence of mRNA • Transcription factors (protein complex) activate transcription by: – Loosening histones from DNA in area to be transcribed so DNA segment can be exposed – Binding to special sequence of gene to be transcribed, called promoter (starting point) • Occurs only on DNA template strand – Mediating binding of RNA polymerase, enzyme that synthesizes mRNA, to promoter region © 2016 Pearson Education, Inc. Transcription (cont.) • Transcription is broken down into three phases: 1. Initiation • RNA polymerase separates DNA strands 2. Elongation • RNA polymerase adds complementary nucleotides to growing mRNA matching sequence of based on DNA template strand – Short, 12-base-pair segment where DNA and mRNA are temporarily bonded is referred to as DNA-RNA hybrid 3. Termination • Transcription stops when RNA polymerase reaches special termination signal code © 2016 Pearson Education, Inc. Slide 1 Figure 3.31 Overview of stages of transcription. RNA polymerase Coding strand of gene DNA Promoter Template strand of gene region containing the start point Termination signal 1 Initiation: With the help of transcription factors, RNA polymerase binds to the promoter, pries apart the two DNA strands, and initiates mRNA synthesis at the start point on the template strand. mRNA Template strand 2 Elongation: As the RNA polymerase moves along the template strand, elongating the mRNA transcript one base at a time, it unwinds the DNA double helix before it and rewinds the double helix behind it. mRNA Completed mRNA RNA polymerase Unwinding of DNA RNA nucleotides Direction of transcription mRNA 3 Termination: mRNA synthesis ends when the termination signal is reached. RNA polymerase and the completed MRNA transcript are released. © 2016 Pearson Education, Inc. Rewinding of DNA Coding strand of DNA DNA-RNA hybrid region Template strand RNA polymerase The DNA-RNA hybrid: At any given moment, 16–18 base pairs of DNA are unwound and the most recently made RNA is still bound to DNA. This small region is called the DNA-RNA hybrid. Slide 2 Figure 3.31 Overview of stages of transcription. RNA polymerase Coding strand of gene DNA Promoter Template strand of gene region containing the start point © 2016 Pearson Education, Inc. Termination signal Slide 3 Figure 3.31 Overview of stages of transcription. RNA polymerase Coding strand of gene DNA Promoter Template strand of gene region containing the start point Termination signal 1 Initiation: With the help of transcription factors, RNA polymerase binds to the promoter, pries apart the two DNA strands, and initiates mRNA synthesis at the start point on the template strand. mRNA © 2016 Pearson Education, Inc. Template strand Slide 4 Figure 3.31 Overview of stages of transcription. RNA polymerase Coding strand of gene DNA Promoter Template strand of gene region containing the start point Termination signal 1 Initiation: With the help of transcription factors, RNA polymerase binds to the promoter, pries apart the two DNA strands, and initiates mRNA synthesis at the start point on the template strand. mRNA Template strand 2 Elongation: As the RNA polymerase moves along the template strand, elongating the mRNA transcript one base at a time, it unwinds the DNA double helix before it and rewinds the double helix behind it. mRNA Rewinding of DNA Coding strand of DNA Unwinding of DNA RNA nucleotides Direction of transcription mRNA DNA-RNA hybrid region Template strand RNA polymerase The DNA-RNA hybrid: At any given moment, 16–18 base pairs of DNA are unwound and the most recently made RNA is still bound to DNA. This small region is called the DNA-RNA hybrid. © 2016 Pearson Education, Inc. Slide 5 Figure 3.31 Overview of stages of transcription. RNA polymerase Coding strand of gene DNA Promoter Template strand of gene region containing the start point Termination signal 1 Initiation: With the help of transcription factors, RNA polymerase binds to the promoter, pries apart the two DNA strands, and initiates mRNA synthesis at the start point on the template strand. mRNA Template strand 2 Elongation: As the RNA polymerase moves along the template strand, elongating the mRNA transcript one base at a time, it unwinds the DNA double helix before it and rewinds the double helix behind it. mRNA Completed mRNA RNA polymerase Unwinding of DNA RNA nucleotides Direction of transcription mRNA 3 Termination: mRNA synthesis ends when the termination signal is reached. RNA polymerase and the completed MRNA transcript are released. © 2016 Pearson Education, Inc. Rewinding of DNA Coding strand of DNA DNA-RNA hybrid region Template strand RNA polymerase The DNA-RNA hybrid: At any given moment, 16–18 base pairs of DNA are unwound and the most recently made RNA is still bound to DNA. This small region is called the DNA-RNA hybrid. Transcription (cont.) • Processing of mRNA – Newly formed mRNA is then edited and processed before translation can begin • Before processing, it is referred to as pre-mRNA – Introns are removed by special proteins called spliceosomes, leaving only exon coding regions © 2016 Pearson Education, Inc. Translation • Step of protein synthesis where the language of nucleic acids (base sequence) is translated into the language of proteins (amino acid sequence) • Process involves: – mRNA – Genetic code – tRNA and ribosomes – Translating events – and sometimes the rough ER © 2016 Pearson Education, Inc. Translation (cont.) • Genetic code – Each three-base sequence on DNA (triplet code) is represented by a complementary three-base sequence on mRNA called codon – There are 64 possible codons • 4 bases (A, U, C, G) and 3 places, so 43 64 – There are 3 “stop” codons but rest are codons for amino acids – There are only 20 possible amino acids, so this means that some amino acids are represented by more than one codon • Redundancy helps protect against transcription errors © 2016 Pearson Education, Inc. Translation (cont.) • Role of tRNA – tRNA binds a specific amino acid at one end (stem); once amino acid is loaded onto tRNA, molecule is now called an aminoacyl-tRNA – Anticodon at other end (head) is triplet code that determines which amino acid will be bound at stem • Example: tRNA with anticodon UAU will only be able to load a methionine amino acid to its stem region © 2016 Pearson Education, Inc. Translation (cont.) – Anticodon of tRNA will bind only to codon on mRNA that is complementary • Example: if codon is AUA, only a tRNA with anticodon UAU will be able to bond – Ribosomes coordinate coupling of mRNA and tRNA – Ribosomes contain one binding site for mRNA and three binding sites for tRNA: • Aminoacyl site for incoming aminoacyl-tRNA • Peptidyl site for tRNA linked to growing polypeptide chain • Exit site for outgoing tRNA © 2016 Pearson Education, Inc. Translation (cont.) • Sequence of events in translation – Translation occurs in three phases that require ATP, protein factors, and enzymes 1. Initiation 2. Elongation 3. Termination © 2016 Pearson Education, Inc. Translation (cont.) 1. Initiation – Small ribosomal subunit binds to a special initiator tRNA (methionine) and then to the mRNA to be decoded • Ribosome scans mRNA looking for first methionine codon, which is referred to as the start codon – When anticodon of initiator tRNA binds to start codon, large ribosomal unit can then attach to small ribosomal unit forming a functional ribosome – At end of initiation, initiator tRNA is in P site of ribosome, and A and E sites are empty © 2016 Pearson Education, Inc. Translation (cont.) 2. Elongation: involves three steps: 2a. Codon recognition: tRNA binds complementary codon in A site of ribosome 2b. Peptide bond formation: Ribosomal enzymes transfer and attach growing polypeptide chain from tRNA in P site over to amino acid of tRNA in A site 2c. Translocation: ribosome shifts down three bases of mRNA, displacing tRNAs by one position tRNA in A site moves into P site tRNA in P site moves into E site tRNA in E site is ejected from ribosome © 2016 Pearson Education, Inc. Translation (cont.) 2. Elongation (cont.) – Once A site is empty, a new tRNA can enter, bringing its amino acid cargo, and whole process starts over – After a portion of mRNA is “read,” additional ribosomes may attach to already read part and start another round of translation of same mRNA • Polyribosome is a multiple ribosome-mRNA complex that produces multiple copies of same protein © 2016 Pearson Education, Inc. Figure 3.32 Polyribosome arrays. Growing polypeptides Completed polypeptide Incoming ribosomal subunits Start of mRNA Polyribosome End of mRNA Each polyribosome consists of one strand of mRNA being read by several ribosomes simultaneously. In this diagram, the mRNA is moving to the left and the “oldest” functional ribosome is farthest to the right. Ribosomes mRNA This transmission electron micrograph shows a large polyribosome (400,000). © 2016 Pearson Education, Inc. Translation (cont.) 3. Termination – When one of three stop codons (UGA, UAA, UAG) on mRNA enters A site, translation ends – Protein release factor binds to stop codon, causing water to be added to chain instead of another tRNA – Causes release of polypeptide chain as well as separation of ribosome subunits and degradation of mRNA – Final polypeptide product will be further processed by other cell structures into functional 3-D protein © 2016 Pearson Education, Inc. Slide 1 Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a particular polypeptide. 1 Getting Ready • Making mRNA (transcription) • Attaching amino acid to tRNA • tRNAs diffuse to ribosome 2 Elongation: Amino acids are added one at a time to the growing peptide chain via a process that has three repeating steps: 2a, 2b, 2c. Initiation: Initiation occurs when four components combine at the P site: • A small ribosomal subunit • An initiator tRNA carrying the amino acid methionine • The mRNA • A large ribosomal subunit Once this is accomplished, the next phase, elongation, begins. 2a Codon recognition: The anticodon of an incoming tRNA binds with the complementary mRNA codon (A to U and C to G) in the A site of the ribosome. Methionine (amino acid) Amino acid that corresponds to anticodon Leu P E GGC tRNA anticodon Amino acid corresponding to anticodon A A U A C C G CU A Complementary mRNA codon 2b Peptide bond formation: Initiator tRNA bearing anticodon Growing polypeptide chain The growing polypeptide bound to the tRNA at the P site is transferred to the amino acid carried by the tRNA in the A site. A new peptide bond is formed. tRNA E New peptide bond A P GG C G A U A U A C CG C U A A site The correct amino acid is attached to each species of tRNA by a synthetase enzyme (aminoacyl-tRNA synthetase). p site Large ribosomal subunit E site 2c Translocation: The entire ribosome translocates, shifting its position one codon along the mRNA. Start codon Released tRNA The unloaded tRNA from the P site is now in the E site. It is released. E P G AU Small ribosomal subunit Pre-mRNA 3 When a stop codon (UGA, UAA, or UAG) arrives at the A site, elongation ends. P Release factor CCU C U G G G A UG A Nucleus (site of transcription) © 2016 Pearson Education, Inc. Newly made (and edited) mRNA leaves nucleus and travels to a ribosome for decoding. Stop codon Cytosol (site of translation) The next codon to be translated is now in the empty A site, ready for step 2a again. Polypeptide chain Termination: E Template strand of DNA A CCG C U A C U C Direction of ribosome movement mRNA The tRNA that was in the A site is now in the P site. Release factor triggers the ribosomal subunits to separate, releasing the mRNA and new polypeptide. Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a particular polypeptide. 1 Getting Ready • Making mRNA (transcription) • Attaching amino acid to tRNA • tRNAs diffuse to ribosome Initiation: Initiation occurs when four components combine at the P site: • A small ribosomal subunit • An initiator tRNA carrying the amino acid methionine • The mRNA • A large ribosomal subunit Once this is accomplished, the next phase, elongation, begins. Methionine (amino acid) Amino acid that corresponds to anticodon Initiator tRNA bearing anticodon tRNA A site The correct amino acid is attached to each species of tRNA by a synthetase enzyme (aminoacyl-tRNA synthetase). p site Large ribosomal subunit E site Start codon Small ribosomal subunit Pre-mRNA mRNA Template strand of DNA Nucleus (site of transcription) © 2016 Pearson Education, Inc. Newly made (and edited) mRNA leaves nucleus and travels to a ribosome for decoding. Cytosol (site of translation) Slide 2 Slide 3 Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a particular polypeptide. 1 Getting Ready • Making mRNA (transcription) • Attaching amino acid to tRNA • tRNAs diffuse to ribosome 2 Elongation: Amino acids are added one at a time to the growing peptide chain via a process that has three repeating steps: 2a, 2b, 2c. Initiation: Initiation occurs when four components combine at the P site: • A small ribosomal subunit • An initiator tRNA carrying the amino acid methionine • The mRNA • A large ribosomal subunit Once this is accomplished, the next phase, elongation, begins. Methionine (amino acid) Amino acid that corresponds to anticodon Initiator tRNA bearing anticodon tRNA A site The correct amino acid is attached to each species of tRNA by a synthetase enzyme (aminoacyl-tRNA synthetase). p site Large ribosomal subunit E site Start codon Small ribosomal subunit Pre-mRNA mRNA Template strand of DNA Nucleus (site of transcription) © 2016 Pearson Education, Inc. Newly made (and edited) mRNA leaves nucleus and travels to a ribosome for decoding. Cytosol (site of translation) Leu 2a Codon recognition: The anticodon of an incoming tRNA binds with the complementary mRNA codon (A to U and C to G) in the A site of the ribosome. E P GGC tRNA anticodon A A U A C C G CU A Complementary mRNA codon Amino acid corresponding to anticodon Slide 4 Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a particular polypeptide. 1 Getting Ready • Making mRNA (transcription) • Attaching amino acid to tRNA • tRNAs diffuse to ribosome 2 Elongation: Amino acids are added one at a time to the growing peptide chain via a process that has three repeating steps: 2a, 2b, 2c. Initiation: Initiation occurs when four components combine at the P site: • A small ribosomal subunit • An initiator tRNA carrying the amino acid methionine • The mRNA • A large ribosomal subunit Once this is accomplished, the next phase, elongation, begins. Methionine (amino acid) Amino acid that corresponds to anticodon Leu 2a Codon recognition: The anticodon of an incoming tRNA binds with the complementary mRNA codon (A to U and C to G) in the A site of the ribosome. E P GGC tRNA anticodon A A U A C C G CU A Complementary mRNA codon 2b Peptide bond formation: Initiator tRNA bearing anticodon Growing polypeptide chain The growing polypeptide bound to the tRNA at the P site is transferred to the amino acid carried by the tRNA in the A site. A new peptide bond is formed. tRNA E P A GG C G A U A U A C CG C U A A site The correct amino acid is attached to each species of tRNA by a synthetase enzyme (aminoacyl-tRNA synthetase). p site Large ribosomal subunit E site Start codon Small ribosomal subunit Pre-mRNA mRNA Template strand of DNA Nucleus (site of transcription) © 2016 Pearson Education, Inc. Newly made (and edited) mRNA leaves nucleus and travels to a ribosome for decoding. Amino acid corresponding to anticodon Cytosol (site of translation) New peptide bond Slide 5 Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a particular polypeptide. 1 Getting Ready • Making mRNA (transcription) • Attaching amino acid to tRNA • tRNAs diffuse to ribosome 2 Elongation: Amino acids are added one at a time to the growing peptide chain via a process that has three repeating steps: 2a, 2b, 2c. Initiation: Initiation occurs when four components combine at the P site: • A small ribosomal subunit • An initiator tRNA carrying the amino acid methionine • The mRNA • A large ribosomal subunit Once this is accomplished, the next phase, elongation, begins. Methionine (amino acid) Amino acid that corresponds to anticodon Leu 2a Codon recognition: The anticodon of an incoming tRNA binds with the complementary mRNA codon (A to U and C to G) in the A site of the ribosome. E P GGC tRNA anticodon Amino acid corresponding to anticodon A A U A C C G CU A Complementary mRNA codon 2b Peptide bond formation: Initiator tRNA bearing anticodon Growing polypeptide chain The growing polypeptide bound to the tRNA at the P site is transferred to the amino acid carried by the tRNA in the A site. A new peptide bond is formed. tRNA E P New peptide bond A GG C G A U A U A C CG C U A A site The correct amino acid is attached to each species of tRNA by a synthetase enzyme (aminoacyl-tRNA synthetase). p site Large ribosomal subunit E site Start codon 2c Translocation: The entire ribosome translocates, shifting its position one codon along the mRNA. Released tRNA The unloaded tRNA from the P site is now in the E site. It is released. E Small ribosomal subunit Pre-mRNA Template strand of DNA Nucleus (site of transcription) © 2016 Pearson Education, Inc. Newly made (and edited) mRNA leaves nucleus and travels to a ribosome for decoding. G AU The tRNA that was in the A site is now in the P site. A CCG C U A C U C Direction of ribosome movement mRNA P Cytosol (site of translation) The next codon to be translated is now in the empty A site, ready for step 2a again. Slide 6 Focus Figure 3.4 Translation is the process in which genetic information carried by an mRNA molecule is decoded in the ribosome to form a particular polypeptide. 1 Getting Ready • Making mRNA (transcription) • Attaching amino acid to tRNA • tRNAs diffuse to ribosome 2 Elongation: Amino acids are added one at a time to the growing peptide chain via a process that has three repeating steps: 2a, 2b, 2c. Initiation: Initiation occurs when four components combine at the P site: • A small ribosomal subunit • An initiator tRNA carrying the amino acid methionine • The mRNA • A large ribosomal subunit Once this is accomplished, the next phase, elongation, begins. 2a Codon recognition: The anticodon of an incoming tRNA binds with the complementary mRNA codon (A to U and C to G) in the A site of the ribosome. Methionine (amino acid) Amino acid that corresponds to anticodon Leu P E GGC tRNA anticodon Amino acid corresponding to anticodon A A U A C C G CU A Complementary mRNA codon 2b Peptide bond formation: Initiator tRNA bearing anticodon Growing polypeptide chain The growing polypeptide bound to the tRNA at the P site is transferred to the amino acid carried by the tRNA in the A site. A new peptide bond is formed. tRNA E New peptide bond A P GG C G A U A U A C CG C U A A site The correct amino acid is attached to each species of tRNA by a synthetase enzyme (aminoacyl-tRNA synthetase). p site Large ribosomal subunit E site 2c Translocation: The entire ribosome translocates, shifting its position one codon along the mRNA. Start codon Released tRNA The unloaded tRNA from the P site is now in the E site. It is released. E P G AU Small ribosomal subunit Pre-mRNA 3 When a stop codon (UGA, UAA, or UAG) arrives at the A site, elongation ends. P Release factor CCU C U G G G A UG A Nucleus (site of transcription) © 2016 Pearson Education, Inc. Newly made (and edited) mRNA leaves nucleus and travels to a ribosome for decoding. Stop codon Cytosol (site of translation) The next codon to be translated is now in the empty A site, ready for step 2a again. Polypeptide chain Termination: E Template strand of DNA A CCG C U A C U C Direction of ribosome movement mRNA The tRNA that was in the A site is now in the P site. Release factor triggers the ribosomal subunits to separate, releasing the mRNA and new polypeptide. Translation (cont.) • Role of rough ER in protein synthesis – A short amino acid segment, called the ER signal sequence, present on a growing polypeptide chain, signals associated ribosome to dock on rough ER surface – Signal-recognition particle (SRP) on ER directs mRNA–ribosome complex where to dock – Once docked, forming polypeptide enters ER • Sugar groups may be added to protein, and its shape may be altered • Protein is then enclosed in vesicle for transport to Golgi apparatus © 2016 Pearson Education, Inc. Slide 1 Figure 3.33 Rough ER processing of proteins. 1 The SRP directs the 2 Once attached to the ER, the SRP is mRNA-ribosome complex to the rough ER. There the SRP binds to a receptor site. released and the growing polypeptide snakes through the ER membrane pore into the cistern. ER signal sequence 3 An enzyme clips off the signal Ribosome sequence. As protein synthesis continues, sugar groups may be added to the protein. mRNA 4 In this example, the completed protein Signal Signal recognition sequence particle Growing (SRP) polypeptide removed Receptor site is released from the ribosome and folds into its 3-D conformation, a process aided by molecular chaperones. Sugar group Released protein 5 The protein is enclosed within a protein-coated transport vesicle. The transport vesicles make their way to the Golgi apparatus, where further processing of the proteins occurs (see Figure 3.17). Rough ER cistern Cytosol © 2016 Pearson Education, Inc. Transport vesicle pinching off Protein-coated transport vesicle Figure 3.33 Rough ER processing of proteins. 1 The SRP directs the mRNA-ribosome complex to the rough ER. There the SRP binds to a receptor site. ER signal sequence Ribosome mRNA Signal recognition particle (SRP) Receptor site Rough ER cistern Cytosol © 2016 Pearson Education, Inc. Slide 2 Slide 3 Figure 3.33 Rough ER processing of proteins. 1 The SRP directs the 2 Once attached to the ER, the SRP is mRNA-ribosome complex to the rough ER. There the SRP binds to a receptor site. released and the growing polypeptide snakes through the ER membrane pore into the cistern. ER signal sequence Ribosome mRNA Signal recognition particle Growing (SRP) polypeptide Receptor site Rough ER cistern Cytosol © 2016 Pearson Education, Inc. Slide 4 Figure 3.33 Rough ER processing of proteins. 1 The SRP directs the 2 Once attached to the ER, the SRP is mRNA-ribosome complex to the rough ER. There the SRP binds to a receptor site. released and the growing polypeptide snakes through the ER membrane pore into the cistern. ER signal sequence 3 An enzyme clips off the signal Ribosome sequence. As protein synthesis continues, sugar groups may be added to the protein. mRNA Signal Signal recognition sequence particle Growing (SRP) polypeptide removed Receptor site Rough ER cistern Cytosol © 2016 Pearson Education, Inc. Sugar group Slide 5 Figure 3.33 Rough ER processing of proteins. 1 The SRP directs the 2 Once attached to the ER, the SRP is mRNA-ribosome complex to the rough ER. There the SRP binds to a receptor site. released and the growing polypeptide snakes through the ER membrane pore into the cistern. ER signal sequence 3 An enzyme clips off the signal Ribosome sequence. As protein synthesis continues, sugar groups may be added to the protein. mRNA 4 In this example, the completed protein Signal Signal recognition sequence particle Growing (SRP) polypeptide removed Receptor site is released from the ribosome and folds into its 3-D conformation, a process aided by molecular chaperones. Sugar group Released protein Rough ER cistern Cytosol © 2016 Pearson Education, Inc. Slide 6 Figure 3.33 Rough ER processing of proteins. 1 The SRP directs the 2 Once attached to the ER, the SRP is mRNA-ribosome complex to the rough ER. There the SRP binds to a receptor site. released and the growing polypeptide snakes through the ER membrane pore into the cistern. ER signal sequence 3 An enzyme clips off the signal Ribosome sequence. As protein synthesis continues, sugar groups may be added to the protein. mRNA 4 In this example, the completed protein Signal Signal recognition sequence particle Growing (SRP) polypeptide removed Receptor site is released from the ribosome and folds into its 3-D conformation, a process aided by molecular chaperones. Sugar group Released protein 5 The protein is enclosed within a protein-coated transport vesicle. The transport vesicles make their way to the Golgi apparatus, where further processing of the proteins occurs (see Figure 3.17). Rough ER cistern Cytosol © 2016 Pearson Education, Inc. Transport vesicle pinching off Protein-coated transport vesicle Summary: From DNA to Proteins • Complementary base pairing directs transfer of genetic information in DNA into amino acid sequence of protein – DNA triplets are coded to mRNA codons – mRNA codons are base-paired with tRNA anticodons to ensure correct amino acid sequence • Anticodon sequence of tRNA is identical to DNA sequence, except uracil is substituted for thymine © 2016 Pearson Education, Inc. Figure 3.34 Information transfer from DNA to RNA to polypeptide. DNA molecule Gene 2 Gene 1 Gene 4 Triplets DNA: DNA base sequence (triplets) of the gene codes for synthesis of a particular polypeptide chain mRNA: Base sequence (codons) of the transcribed mRNA tRNA: Consecutive base sequences of tRNA anticodons recognize the mRNA codons calling for the amino acids they transport Polypeptide: Amino acid sequence of the polypeptide chain 1 T A C G G Codons 1 A U G 3 T A G 2 4 C G A 5 T T T 6 C 3 4 C G C U A A A U U U C C C A U G G U A G C C 5 A T G C 6 G G A A G G A C U G C G G C A 8 7 C 9 8 7 A C T 9 U U U A A U G A Anticodon U A C C G C A tRNA Met Start translation © 2016 Pearson Education, Inc. 2 Pro Ser Leu Lys Gly Arg Phe Stop; detach Other Roles of DNA • DNA codes for other types of RNA: – MicroRNA (miRNA) • Small RNAs that can bind to and silence mRNAs made by certain exons – Riboswitches • Folded RNAs that act as switches that can turn protein synthesis on or off in response to certain environmental conditions – Small interfering RNAs (siRNA) • Similar to miRNA, but can also be made to silence mRNA from pathogenic sources such as viruses © 2016 Pearson Education, Inc. 3.12 Apoptosis, Autophagy, and Proteasomes • Cells that have become obsolete or damaged need to be taken out of system • Autophagy (self-eating) is the process of disposing of nonfunctional organelles and cytoplasmic bits by forming autophagosomes, which can then be degraded by lysosomes • Unneeded proteins can be marked for destruction by ubiquitins – Proteasomes disassemble ubiquitin-tagged proteins, recycling the amino acids and ubiquitin © 2016 Pearson Education, Inc. 3.12 Apoptosis, Autophagy, and Proteasomes • Apoptosis, also known as programmed cell death causes certain cells (examples: cancer cells, infected cells, old cells) to neatly selfdestruct – Process begins with mitochondrial membranes leaking chemicals that activate enzymes called caspases – Caspases cause degradation of DNA and cytoskeleton, which leads to cell death – Dead cell shrinks and is phagocytized by macrophages © 2016 Pearson Education, Inc. Developmental Aspects of Cells • All cells of body contain same DNA, but not all cells are identical or carry out same function • Chemical signals in embryo channel cells into specific developmental pathways by turning some genes on and others off • Development of specific and distinctive features in cells is called cell differentiation © 2016 Pearson Education, Inc. Cell Destruction and Modified Rates of Cell Division • Organs are well formed and functional before birth, but we need cell division for growth • Cell division in adults is needed to replace shortlived cells and repair wounds • Hyperplasia is accelerated growth that increases cell numbers when needed • Atrophy is a decrease in size that results from loss of stimulation or use © 2016 Pearson Education, Inc. Cell Aging • The mechanism of aging is a mystery, but there are several theories: – Wear and tear theory: a lifetime of chemical insults and free radicals have cumulative effects – Mitochondrial theory of aging: free radicals in mitochondria diminish energy production – Immune system disorders: autoimmune responses, as well as progressive weakening of immune response © 2016 Pearson Education, Inc. Cell Aging (cont.) – Genetic theory: cessation of mitosis and cell aging are programmed into genes • Telomeres are strings of nucleotides that protect ends of chromosomes (like caps on shoestrings) • Everytime a cell divides, the telomere shortens, so telomeres may act like an hour-glass on how many times a cell can divide • Telomerase is an enzyme that lengthens telomeres – Found in germ cells of embryos but absent in adult cells, except for cancer cells © 2016 Pearson Education, Inc.