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Cell Cycle • Defines changes from formation of cell until it reproduces • Includes: – Interphase • Cell grows and carries out functions – Cell division (mitotic phase) • Divides into two cells © 2013 Pearson Education, Inc. Interphase • Period from cell formation to cell division • Nuclear material called chromatin • Three subphases: – G1 (gap 1)—vigorous growth and metabolism • Cells that permanently cease dividing said to be in G0 phase – S (synthetic)—DNA replication occurs – G2 (gap 2)—preparation for division © 2013 Pearson Education, Inc. Figure 3.31 The cell cycle. G1 checkpoint (restriction point) S Growth and DNA synthesis G1 Growth M G2 Growth and final preparations for division G2 checkpoint © 2013 Pearson Education, Inc. Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (1 of 6) Interphase Centrosomes (each has 2 centrioles) Plasma membrane Nucleolus Chromatin Nuclear envelope © 2013 Pearson Education, Inc. DNA Replication • Prior to division cell makes copy of DNA • DNA helices separated into replication bubbles with replication forks at each end – Each strand acts as template for complementary strand • DNA polymerase begins adding nucleotides at RNA primer • DNA polymerase continues from primer – Synthesizes one leading, one lagging strand © 2013 Pearson Education, Inc. DNA Replication • DNA polymerase only works in one direction – Leading strand synthesized continuously – Lagging strand synthesized discontinuously into segments – DNA ligase splices short segments of discontinuous strand together © 2013 Pearson Education, Inc. DNA Replication • End result: two identical DNA molecules formed from original – During mitotic cell division one complete copy given to new cell; one retained in original cell • Process is called semiconservative replication – Each DNA composed of one old and one new strand © 2013 Pearson Education, Inc. Figure 3.32 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 Replication the double helix and fork expose the bases Adenine Thymine Cytosine Guanine © 2013 Pearson Education, Inc. DNA polymerase Old (template) strand DNA Replication PLAY © 2013 Pearson Education, Inc. Animation: DNA Replication Cell Division • Meiosis - cell division producing gametes • Mitotic cell division - produces clones – Essential for body growth and tissue repair – Occurs continuously in some cells • Skin; intestinal lining – None in most mature cells of nervous tissue, skeletal muscle, and cardiac muscle • Repairs with fibrous tissue © 2013 Pearson Education, Inc. Events Of Cell Division • Mitosis—division of nucleus – Four stages ensure each cell receives copy of replicated DNA • • • • Prophase Metaphase Anaphase Telophase – Cytokinesis—division of cytoplasm-by cleavage furrow © 2013 Pearson Education, Inc. Figure 3.31 The cell cycle. G1 checkpoint (restriction point) S Growth and DNA synthesis G1 Growth M G2 Growth and final preparations for division G2 checkpoint © 2013 Pearson Education, Inc. Cell Division PLAY © 2013 Pearson Education, Inc. Animation: Mitosis Prophase • Chromosomes become visible, each with two chromatids joined at centromere • Centrosomes separate and migrate toward opposite poles • Mitotic spindles and asters form © 2013 Pearson Education, Inc. Prophase • Nuclear envelope fragments • Kinetochore microtubules attach to kinetochore of centromeres and draw them toward equator of cell • Polar microtubules assist in forcing poles apart © 2013 Pearson Education, Inc. Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (2 of 6) Early Prophase Early mitotic spindle Aster Chromosome consisting of two sister chromatids © 2013 Pearson Education, Inc. Centromere Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (3 of 6) Late Prophase © 2013 Pearson Education, Inc. Spindle pole Polar microtubule Fragments of nuclear envelope Kinetochore Kinetochore microtubule Metaphase • Centromeres of chromosomes aligned at equator • Plane midway between poles called metaphase plate © 2013 Pearson Education, Inc. Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (4 of 6) Metaphase Spindle Metaphase plate © 2013 Pearson Education, Inc. Anaphase • Shortest phase • Centromeres of chromosomes split simultaneously—each chromatid becomes a chromosome • Chromosomes (V shaped) pulled toward poles by motor proteins of kinetochores • Polar microtubules continue forcing poles apart © 2013 Pearson Education, Inc. Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (5 of 6) Anaphase Daughter chromosomes © 2013 Pearson Education, Inc. Telophase • Begins when chromosome movement stops • Two sets of chromosomes uncoil to form chromatin • New nuclear membrane forms around each chromatin mass • Nucleoli reappear • Spindle disappears © 2013 Pearson Education, Inc. Cytokinesis • Begins during late anaphase • Ring of actin microfilaments contracts to form cleavage furrow • Two daughter cells pinched apart, each containing nucleus identical to original © 2013 Pearson Education, Inc. Figure 3.33 Mitosis is the process of nuclear division in which the chromosomes are distributed to two daughter nuclei. (6 of 6) Telophase © 2013 Pearson Education, Inc. Cytokinesis Nuclear envelope forming Nucleolus forming Contractile ring at cleavage furrow Control of Cell Division • "Go" signals: – Critical volume of cell when area of membrane inadequate for exchange – Chemicals (e.g., growth factors, hormones) – Availability of space–contact inhibition © 2013 Pearson Education, Inc. Control of Cell Division • To replicate DNA and enter mitosis requires – Cyclins–regulatory proteins • Accumulate during interphase – Cdks (Cyclin-dependent kinases)–bind to cyclins activated • Enzyme cascades prepare cell for division – Cyclins destroyed after mitotic cell division © 2013 Pearson Education, Inc. Control of Cell Division • "Go" signals – G1 checkpoints (restriction point) most important • If doesn't pass G0–no further division – Late in G2 MPF (M-phase promoting factor) required to enter M phase • "Other Controls" signals – Repressor genes inhibit cell division • E.g., P53 gene © 2013 Pearson Education, Inc. Protein Synthesis • DNA is master blueprint for protein synthesis • Gene - segment of DNA with blueprint for one polypeptide • Triplets (three sequential DNA nitrogen bases) form genetic library – Bases in DNA are A, G, T, and C – Each triplet specifies coding for number, kind, and order of amino acids in polypeptide PLAY © 2013 Pearson Education, Inc. Animation: DNA and RNA Protein Synthesis • Genes composed of exons and introns – Exons code for amino acids – Introns–noncoding segments • Role of RNA – DNA decoding mechanism and messenger – Three types–all formed on DNA in nucleus • Messenger RNA (mRNA); ribosomal RNA (rRNA); transfer RNA (tRNA) • RNA differs from DNA – Uracil is substituted for thymine © 2013 Pearson Education, Inc. Roles of the Three Main Types of RNA • Messenger RNA (mRNA) – Carries instructions for building a polypeptide, from gene in DNA to ribosomes in cytoplasm © 2013 Pearson Education, Inc. Roles of the Three Main Types of RNA • Ribosomal RNA (rRNA) – Structural component of ribosomes that, along with tRNA, helps translate message from mRNA © 2013 Pearson Education, Inc. Roles of the Three Main Types of RNA • Transfer RNAs (tRNAs) – Bind to amino acids and pair with bases of codons of mRNA at ribosome to begin process of protein synthesis © 2013 Pearson Education, Inc. Figure 3.34 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 © 2013 Pearson Education, Inc. Protein Synthesis • Occurs in two steps – Transcription • DNA information coded in mRNA – Translation • mRNA decoded to assemble polypeptides © 2013 Pearson Education, Inc. Transcription • Transfers DNA gene base sequence to complementary base sequence of mRNA • Transcription factors–gene activators – Loosen histones from DNA in area to be transcribed – Bind to promoter-DNA sequence specifying start site of gene on template strand – Mediate binding of RNA polymerase (enzyme synthesizing mRNA) to promoter © 2013 Pearson Education, Inc. Transcription • Three phases – Initiation • RNA polymerase separates DNA strands – Elongation • RNA polymerase adds complementary nucleotides – Termination • Termination signal indicates "stop" © 2013 Pearson Education, Inc. Processing of mRNA • mRNA edited and processed before translation – Introns removed by spliceosomes – mRNA complex proteins associate to guide export, ensure accuracy for translation © 2013 Pearson Education, Inc. Figure 3.35 Overview of stages of transcription. Slide 1 RNA polymerase Coding strand DNA Promoter region Termination signal Template strand 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 Coding strand of DNA 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. Rewinding of DNA Unwinding of DNA RNA nucleotides Direction of transcription mRNA transcript mRNA 3 Termination: mRNA synthesis ends when the termination signal is reached. RNA polymerase and the completed mRNA transcript are released. Completed mRNA transcript © 2013 Pearson Education, Inc. RNA polymerase 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. Figure 3.35 Overview of stages of transcription. Slide 2 RNA polymerase Coding strand DNA Promoter region © 2013 Pearson Education, Inc. Template strand Termination signal Figure 3.35 Overview of stages of transcription. Slide 3 RNA polymerase Coding strand DNA Promoter region Template strand 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 © 2013 Pearson Education, Inc. Template strand Figure 3.35 Overview of stages of transcription. Slide 4 RNA polymerase Coding strand DNA Promoter region Template strand 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 Coding strand of DNA 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. Rewinding of DNA Unwinding of DNA RNA nucleotides Direction of transcription mRNA transcript 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. © 2013 Pearson Education, Inc. Figure 3.35 Overview of stages of transcription. Slide 5 RNA polymerase Coding strand DNA Promoter region Termination signal Template strand 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 Coding strand of DNA 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. Rewinding of DNA Unwinding of DNA RNA nucleotides Direction of transcription mRNA transcript mRNA 3 Termination: mRNA synthesis ends when the termination signal is reached. RNA polymerase and the completed mRNA transcript are released. Completed mRNA transcript © 2013 Pearson Education, Inc. RNA polymerase 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. Translation • Converts base sequence of nucleic acids into amino acid sequence of proteins • Involves mRNAs, tRNAs, and rRNAs © 2013 Pearson Education, Inc. Genetic Code • Each three-base sequence on DNA (triplet) represented by codon – Codon—complementary three-base sequence on mRNA – Some amino acids represented by more than one codon © 2013 Pearson Education, Inc. Figure 3.36 The genetic code. SECOND BASE C UUU A UCU UAU Phe U G U UGU Tyr Cys UAC UGC C UCA UAA Stop UGA Stop A UUG UCG UAG Stop UGG Trp G CUU CCU CAU CGU U CUC CCC CAC UUC UCC UUA Ser Leu His C Leu FIRST BASE CUA CCA C CGC Pro Arg CAA CGA A CGG G Gln CUG CCG CAG AUU ACU AAU A AUC lle ACC U AGU Ser Asn AAC AGC Thr AUA AUG ACA Met or Start GUU AAA AGA Arg Lys ACG AAG GCU GAU GUC GCC Val GAC G GGU U Gly GCA GAA GUG GCG GAG GGA A GGG G Glu © 2013 Pearson Education, Inc. C GGC Ala GUA A AGG Asp G C THIRD BASE U Role of tRNA • 45 different types • Binds specific amino acid at one end (stem) • Anticodon at other end (head) binds mRNA codon at ribosome by hydrogen bonds – E.g., if codon = AUA, anticodon = UAU • Ribosome coordinates coupling of mRNA and tRNA; contains three sites – Aminoacyl site; peptidyl site; exit site © 2013 Pearson Education, Inc. Sequence of Events in Translation • Three phases that require ATP, protein factors, and enzymes – Initiation – Elongation – Termination © 2013 Pearson Education, Inc. Translation: Initiation • Small ribosomal subunit binds to initiator tRNA and mRNA to be decoded; scans for start codon • Large and small ribosomal units attach, forming functional ribosome • At end of initiation – tRNA in P site; A site vacant © 2013 Pearson Education, Inc. Translation: Elongation • Three steps – Codon recognition • tRNA binds complementary codon in A site – Peptide bond formation • Amino acid of tRNA in P site bonded to amino acid of tRNA in A site – Translocation • tRNAs move one position–A P; P E © 2013 Pearson Education, Inc. Translation: Elongation • New amino acids added by other tRNAs as ribosome moves along mRNA • Initial portion of mRNA can be "read" by additional ribosomes – Polyribosome • multiple ribosome-mRNA complex – Produces multiple copies of same protein © 2013 Pearson Education, Inc. Figure 3.38 Polyribosome arrays. Growing polypeptides Completed polypeptide Incoming ribosomal subunits Ribosomes Polyribosome Start of mRNA mRNA 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. © 2013 Pearson Education, Inc. This transmission electron micrograph shows a large polyribosome (400,0003). Translation: Termination • When stop codon (UGA, UAA, UAG) enters A site – Signals end of translation – Protein release factor binds to stop codon water added to chain release of polypeptide chain; separation of ribosome subunits; degradation of mRNA – Protein processed into functional 3-D structure © 2013 Pearson Education, Inc. Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the ribosome to form a particular polypeptide. Slide 1 2 Elongation. Amino acids are added one at a time to the growing peptide chain via a process that has three repeating steps. Template strand of DNA Amino acid Met corresponding to anticodon Pre-mRNA tRNA P A GGC AUACCGCUA mRNA Met 1 Initiation. Initiation occurs when four components combine: • A small ribosomal subunit • An initiator tRNA that carries the amino acid methionine • The mRNA • A large ribosomal subunit Once this is accomplished, the next phase, elongation, begins. Cytosol (site of translation) Met P site Large ribosomal subunit E site © 2013 Pearson Education, Inc. Aminoacyl-tRNA synthetase Initiator tRNA bearing anticodon Newly made (and edited) mRNA leaves nucleus and travels to a free or attached ribosome for decoding. A site Start codon Ile Pro E Nucleus (site of transcription) Methionine (amino acid) Amino acid corresponding to anticodon The correct amino acid is attached to each species of tRNA by a synthetase enzyme. Small ribosomal subunit Polypeptide tRNA anticodon New peptide Ile bond Pro Leu Released tRNA Ile Pro Leu P A E Complementary GGC GAU mRNA codon AUACCG CUA 2a Codon recognition. P A E The anticodon of an GAU incoming tRNA binds with CCGCUA CUC 2b Peptide bond the complementary mRNA codon (A to U and C to G) formation. The growing Direction of in the A site of the polypeptide bound to the ribosome movement ribosome. tRNA at the P site is 2c Translocation. As the transferred to the amino entire ribosome translocates, it acid carried by the tRNA shifts by one codon along the mRNA: in the A site, and a new • The unloaded tRNA in the P peptide bond is formed. site is moved to the E site and then released. • The tRNA in the A site moves to the P site. • The next codon to be translated is now in the empty A site ready for step 2a again. P E Release factor CCU Polypeptide CUGGGA UGA Stop codon 3 Termination. When a stop codon (UGA, UAA, or UAG) arrives at the A site, elongation ends. Release of the newly made polypeptide is triggered by a release factor and the ribosomal subunits separate, releasing the mRNA. Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the ribosome to form a particular polypeptide. Template strand of DNA Pre-mRNA Met Amino acid corresponding to anticodon The correct amino acid is attached to each species of tRNA by a synthetase enzyme. tRNA mRNA Nucleus (site of transcription) Methionine Newly made (amino acid) (and edited) Met mRNA leaves nucleus and travels to a free or attached ribosome for decoding. Aminoacyl-tRNA synthetase Initiator tRNA bearing anticodon Cytosol (site of translation) Met P site Large ribosomal subunit © 2013 Pearson Education, Inc. E site A site Start codon Slide 2 Small ribosomal subunit 1 Initiation. Initiation occurs when four components combine: • A small ribosomal subunit • An initiator tRNA that carries the amino acid methionine • The mRNA • A large ribosomal subunit Once this is accomplished, the next phase, elongation, begins. Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the ribosome to form a particular polypeptide. 2 Elongation. Amino acids are added one at a time to the growing peptide chain via a process that has three repeating steps. Amino acid corresponding to anticodon lle Pro E P A GGC AUA CCG CUA tRNA anticodon Complementary mRNA codon © 2013 Pearson Education, Inc. 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. Slide 3 Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the ribosome to form a particular polypeptide. Polypeptide New peptide Ile Pro bond Leu E P A GG C G A U A UA CCG C UA © 2013 Pearson Education, Inc. 2b Peptide bond formation. 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, and a new peptide bond is formed. Slide 4 Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the ribosome to form a particular polypeptide. Slide 5 Released tRNA Ile Pro Leu E P A G AU C CG C U A C UC Direction of ribosome movement 2c Translocation. As the entire ribosome translocates, it shifts by one codon along the mRNA: • The unloaded tRNA in the P site is moved to the E site and then released. • The tRNA in the A site moves to the P site. • The next codon to be translated is now in the empty A site ready for step 2a again. © 2013 Pearson Education, Inc. Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the ribosome to form a particular polypeptide. E P CC U CUGGGA UGA Release factor Stop codon 3 Termination. When a stop codon (UGA, UAA, or UAG) arrives at the A site, elongation ends. Release of the newly made polypeptide is triggered by a release factor and the ribosomal subunits separate, releasing the mRNA. © 2013 Pearson Education, Inc. Slide 6 Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the ribosome to form a particular polypeptide. E P CC U CUGGGA UGA Release factor Polypeptide Stop codon 3 Termination. When a stop codon (UGA, UAA, or UAG) arrives at the A site, elongation ends. Release of the newly made polypeptide is triggered by a release factor and the ribosomal subunits separate, releasing the mRNA. © 2013 Pearson Education, Inc. Slide 7 Figure 3.37 Translation is the process in which genetic information carried by an mRNA is decoded in the ribosome to form a particular polypeptide. Slide 8 2 Elongation. Amino acids are added one at a time to the growing peptide chain via a process that has three repeating steps. Template strand of DNA Amino acid Met corresponding to anticodon Pre-mRNA tRNA P A GGC AUACCGCUA mRNA Met 1 Initiation. Initiation occurs when four components combine: • A small ribosomal subunit • An initiator tRNA that carries the amino acid methionine • The mRNA • A large ribosomal subunit Once this is accomplished, the next phase, elongation, begins. Cytosol (site of translation) Met P site Large ribosomal subunit E site © 2013 Pearson Education, Inc. Aminoacyl-tRNA synthetase Initiator tRNA bearing anticodon Newly made (and edited) mRNA leaves nucleus and travels to a free or attached ribosome for decoding. A site Start codon Ile Pro E Nucleus (site of transcription) Methionine (amino acid) Amino acid corresponding to anticodon The correct amino acid is attached to each species of tRNA by a synthetase enzyme. Small ribosomal subunit Polypeptide tRNA anticodon New peptide Ile bond Pro Leu Released tRNA Ile Pro Leu P A E Complementary GGC GAU mRNA codon AUACCG CUA 2a Codon recognition. P A E The anticodon of an GAU incoming tRNA binds with CCGCUA CUC 2b Peptide bond the complementary mRNA codon (A to U and C to G) formation. The growing Direction of in the A site of the polypeptide bound to the ribosome movement ribosome. tRNA at the P site is 2c Translocation. As the transferred to the amino entire ribosome translocates, it acid carried by the tRNA shifts by one codon along the mRNA: in the A site, and a new • The unloaded tRNA in the P peptide bond is formed. site is moved to the E site and then released. • The tRNA in the A site moves to the P site. • The next codon to be translated is now in the empty A site ready for step 2a again. P E Release factor CCU Polypeptide CUGGGA UGA Stop codon 3 Termination. When a stop codon (UGA, UAA, or UAG) arrives at the A site, elongation ends. Release of the newly made polypeptide is triggered by a release factor and the ribosomal subunits separate, releasing the mRNA. Role of Rough ER in Protein Synthesis • mRNA–ribosome complex directed to rough ER by signal-recognition particle (SRP) • Forming protein enters ER • Sugar groups may be added to protein, and its shape may be altered • Protein enclosed in vesicle for transport to Golgi apparatus © 2013 Pearson Education, Inc. Figure 3.39 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 Slide 1 2 Once attached to the ER, the SRP is released and the growing polypeptide snakes through the ER membrane pore into the cistern. 3 An enzyme clips off the signal sequence. As protein synthesis continues, sugar groups may be added to the protein. Ribosome mRNA Signal Signal recognition sequence particle removed (SRP) Receptor site Growing polypeptide 4 In this example, the completed protein 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 3.19). Rough ER cistern Cytosol © 2013 Pearson Education, Inc. Transport vesicle pinching off Protein-coated transport vesicle Figure 3.39 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 © 2013 Pearson Education, Inc. Slide 2 Figure 3.39 Rough ER processing of proteins. 1 The SRP directs the U 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 Growing polypeptide Rough ER cistern Cytosol © 2013 Pearson Education, Inc. 2 Once attached to the ER, the SRP is released and the growing polypeptide snakes through the ER membrane pore into the cistern. Slide 3 Figure 3.39 Rough ER processing of proteins. 1 The SRP directs the U mRNA-ribosome complex to the rough ER. There the SRP binds to a receptor site. ER signal sequence Slide 4 2 Once attached to the ER, the SRP is released and the growing polypeptide snakes through the ER membrane pore into the cistern. 3 An enzyme clips off the signal sequence. As protein synthesis continues, sugar groups may be added to the protein. Ribosome mRNA Signal Signal recognition sequence particle removed (SRP) Receptor site Growing polypeptide Rough ER cistern Cytosol © 2013 Pearson Education, Inc. Sugar group Figure 3.39 Rough ER processing of proteins. 1 The SRP directs the U mRNA-ribosome complex to the rough ER. There the SRP binds to a receptor site. ER signal sequence Slide 5 2 Once attached to the ER, the SRP is released and the growing polypeptide snakes through the ER membrane pore into the cistern. 3 An enzyme clips off the signal sequence. As protein synthesis continues, sugar groups may be added to the protein. Ribosome mRNA Signal Signal recognition sequence particle removed (SRP) Receptor site Growing polypeptide 4 In this example, the completed protein 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 © 2013 Pearson Education, Inc. Figure 3.39 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 Slide 6 2 Once attached to the ER, the SRP is released and the growing polypeptide snakes through the ER membrane pore into the cistern. 3 An enzyme clips off the signal sequence. As protein synthesis continues, sugar groups may be added to the protein. Ribosome mRNA Signal Signal recognition sequence particle removed (SRP) Receptor site Growing polypeptide 4 In this example, the completed protein 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 3.19). Rough ER cistern Cytosol © 2013 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 mRNA codons – Complementary base pairing of mRNA codons with tRNA anticodons ensures correct amino acid sequence – Anticodon sequence identical to DNA sequence except uracil substituted for thymine © 2013 Pearson Education, Inc. Figure 3.40 Information transfer from DNA to RNA to polypeptide. DNA molecule Gene 2 Gene 1 Gene 4 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 Triplets 1 2 3 4 5 6 7 8 9 Codons 1 2 3 4 5 6 7 8 9 Anticodon tRNA Start translation © 2013 Pearson Education, Inc. Stop; detach Other Roles of DNA • Intron ("junk") regions of DNA code for other types of RNA: – Antisense RNA • Prevents protein-coding RNA from being translated – MicroRNA • Small RNAs that silence mRNAs made by certain exons – Riboswitches • Folded RNAs that act as switches regulating protein synthesis in response to environmental conditions © 2013 Pearson Education, Inc. Cytosolic Protein Degradation • Autophagy – Cytoplasmic bits and nonfunctional organelles put into autophagosomes; degraded by lysosomes • Ubiquitins – Tag damaged or unneeded soluble proteins in cytosol – Digested by soluble enzymes or proteasomes © 2013 Pearson Education, Inc. Extracellular Materials • Body fluids-interstitial fluid, blood plasma, and cerebrospinal fluid • Cellular secretions-intestinal and gastric fluids, saliva, mucus, and serous fluids • Extracellular matrix–most abundant extracellular material – Jellylike mesh of proteins and polysaccharides secreted by cells; acts as "glue" to hold cells together © 2013 Pearson Education, Inc. Developmental Aspects of Cells • All cells of body contain same DNA but cells not identical • 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 called cell differentiation © 2013 Pearson Education, Inc. Apoptosis and Modified Rates of Cell Division • During development more cells than needed produced (e.g., in nervous system) • Eliminated later by programmed cell death (apoptosis) – Mitochondrial membranes leak chemicals that activate caspases DNA, cytoskeleton degradation cell death – Dead cell shrinks and is phagocytized © 2013 Pearson Education, Inc. Apoptosis and Modified Rates of Cell Division • Organs well formed and functional before birth • Cell division in adults to replace short-lived cells and repair wounds • Hyperplasia increases cell numbers when needed • Atrophy (decreased size) results from loss of stimulation or use © 2013 Pearson Education, Inc. Theories of Cell Aging • Wear and tear theory-Little chemical insults and free radicals have cumulative effects • Mitochondrial theory of aging–free radicals in mitochondria diminish energy production • Immune system disorders-autoimmune responses; progressive weakening of immune response; C-reactive protein of acute inflammation causes cell aging © 2013 Pearson Education, Inc. Theories of Cell Aging • Most widely accepted theory – Genetic theory-cessation of mitosis and cell aging programmed into genes • Telomeres (strings of nucleotides protecting ends of chromosomes) may determine number of times a cell can divide • Telomerase lengthens telomeres – Found in germ cells; ~ absent in adult cells © 2013 Pearson Education, Inc.