Cells: The Living Units

Transcription

1 Chapter 3 Part D Cells: The Living Units Annie Leibovitz/Contact Press Images PowerPoint Lecture Slides prepared by Karen Dunbar Kareiva Ivy Tech Community College

2 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

3 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

4 Interphase (cont.) Subphases Interphase broken into three subphases: G 1 (gap 1): vigorous growth and metabolism Cells that permanently cease dividing are said to be in G 0 phase S (synthetic): DNA replication occurs G 2 (gap 2): preparation for division

5 Figure 3.28 The cell cycle. G 1 checkpoint (restriction point) G 1 Growth S Growth and DNA synthesis G 2 Growth and final preparations for division G 2 /M checkpoint

6 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

7 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

8 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

9 Figure 3.29 Replication of DNA: summary. Chromosome Free nucleotides DNA polymerase Old (parental) strand acts as a template for synthesis of new strand Old DNA Enzymes unwind the double helix and expose the bases Adenine Thymine Cytosine Guanine Replication fork (area where hydrogen bonds between base pairs are broken and DNA strands separate) Leading strand Two new strands (leading and lagging) synthesized in opposite directions Lagging strand DNA polymerase Replication bubble Old (template) strand

10 Animation: Replication of DNA

11 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

12 Figure 3.28 The cell cycle. G 1 checkpoint (restriction point) G 1 Growth S Growth and DNA synthesis G 2 Growth and final preparations for division G 2 /M checkpoint

13 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

14 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

15 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

16 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 Chromosome consisting of two sister chromatids Centromere

17 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 Kinetochore microtubule

18 Cell Division (cont.) Metaphase Centromeres of chromosomes are precisely aligned at cell s equator The imaginary plane midway between poles is called metaphase plate

19 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

20 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

21 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

22 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

23 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

24 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 Nuclear envelope forming Nucleolus forming Contractile ring at cleavage furrow

25 Animation: Mitosis

26 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

27 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

28 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 G 1 checkpoint (restriction point) is the most important of the three major checkpoints If cell does not pass, it enters G 0, in which no further division occurs

29 Figure 3.28 The cell cycle. G 1 checkpoint (restriction point) G 1 Growth S Growth and DNA synthesis G 2 Growth and final preparations for division G 2 /M checkpoint

30 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

31 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

32 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

33 Animation: DNA and RNA

34 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)

35 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

36 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

37 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

38 Protein Synthesis Occurs in two steps: Transcription DNA information coded in mrna Translation mrna decoded to assemble polypeptides

39 Figure 3.30 Simplified scheme of information flow from the DNA gene to mrna to protein structure during transcription and translation. Nuclear envelope Transcription DNA RNA Processing Pre-mRNA Translation mrna Polypeptide Ribosome Nuclear pores Start translation Stop; detach

40 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

41 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

42 Figure 3.31 Overview of stages of transcription. RNA polymerase Slide 1 DNA Coding strand of gene Promoter region containing the start point Template strand of gene 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. Rewinding of DNA Coding strand of DNA RNA nucleotides Direction of transcription Unwinding of DNA mrna mrna DNA-RNA hybrid region Template strand 3 Termination: mrna synthesis ends when the termination signal is reached. RNA polymerase and the completed MRNA transcript are released. RNA polymerase The DNA-RNA hybrid: At any given moment, 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. Completed mrna RNA polymerase

43 Figure 3.31 Overview of stages of transcription. RNA polymerase Slide 2 DNA Coding strand of gene Promoter region containing the start point Template strand of gene Termination signal

44 Figure 3.31 Overview of stages of transcription. RNA polymerase Slide 3 DNA Coding strand of gene Promoter region containing the start point Template strand of gene 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

45 Figure 3.31 Overview of stages of transcription. RNA polymerase Slide 4 DNA Coding strand of gene Promoter region containing the start point Template strand of gene 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. Rewinding of DNA Coding strand of DNA RNA nucleotides Direction of transcription Unwinding of DNA mrna mrna DNA-RNA hybrid region Template strand RNA polymerase The DNA-RNA hybrid: At any given moment, 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.

46 Figure 3.31 Overview of stages of transcription. RNA polymerase Slide 5 DNA Coding strand of gene Promoter region containing the start point Template strand of gene 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. Rewinding of DNA Coding strand of DNA RNA nucleotides Direction of transcription Unwinding of DNA mrna mrna DNA-RNA hybrid region Template strand 3 Termination: mrna synthesis ends when the termination signal is reached. RNA polymerase and the completed MRNA transcript are released. RNA polymerase The DNA-RNA hybrid: At any given moment, 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. Completed mrna RNA polymerase

47 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

48 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

49 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 4 3 = 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

50 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

51 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

52 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

53 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

54 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

55 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

56 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 ).

57 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

58 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. Slide 1 Getting Ready Making mrna (transcription) Attaching amino acid to trna trnas diffuse to ribosome Amino acid that corresponds to anticodon trna 1 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) Initiator trna bearing anticodon 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. 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. 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. A new peptide bond is formed. P E G G C A A U A C C G C U A E P A GG C G A U A U A C C G C U A Leu Amino acid corresponding to anticodon trna anticodon Complementary mrna codon New peptide bond Growing polypeptide chain A site Pre-mRNA Template strand of DNA The correct amino acid is attached to each species of trna by a synthetase enzyme (aminoacyl-trna synthetase). mrna Nucleus (site of transcription) Large ribosomal subunit E site p site Newly made (and edited) mrna leaves nucleus and travels to a ribosome for decoding. Start codon Small ribosomal subunit Cytosol (site of translation) 2c Translocation: The entire ribosome translocates, shifting its position one codon along the mrna. 3 Termination: When a stop codon (UGA, UAA, or UAG) arrives at the A site, elongation ends. The unloaded trna from the P site is now in the E site. It is released. P E C C U C U G G G A U G A Direction of ribosome movement Stop codon Release factor Released trna Release factor triggers the ribosomal subunits to separate, releasing the mrna and new polypeptide. P E G A U A C C G C U A C U C The trna that was in the A site is now in the P site. The next codon to be translated is now in the empty A site, ready for step 2a again. Polypeptide chain

59 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. Slide 2 Getting Ready Making mrna (transcription) Attaching amino acid to trna trnas diffuse to ribosome 1 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. Amino acid that corresponds to anticodon trna Methionine (amino acid) Initiator trna bearing anticodon A site The correct amino acid is attached to each species of trna by a synthetase enzyme (aminoacyl-trna synthetase). Large ribosomal subunit E site p site Start codon Small ribosomal subunit Pre-mRNA Template strand of DNA mrna Nucleus (site of transcription) Newly made (and edited) mrna leaves nucleus and travels to a ribosome for decoding. Cytosol (site of translation)

60 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. Slide 3 Getting Ready Making mrna (transcription) Attaching amino acid to trna trnas diffuse to ribosome 1 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) 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. 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. P E G G C A A U A C C G C U A Leu Amino acid corresponding to anticodon trna anticodon Complementary mrna codon Amino acid that corresponds to anticodon trna Initiator trna bearing anticodon A site The correct amino acid is attached to each species of trna by a synthetase enzyme (aminoacyl-trna synthetase). Large ribosomal subunit E site p site Start codon Small ribosomal subunit Pre-mRNA Template strand of DNA mrna Nucleus (site of transcription) Newly made (and edited) mrna leaves nucleus and travels to a ribosome for decoding. Cytosol (site of translation)

61 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. Slide 4 Getting Ready Making mrna (transcription) Attaching amino acid to trna trnas diffuse to ribosome Amino acid that corresponds to anticodon trna 1 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) Initiator trna bearing anticodon 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. 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. 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. A new peptide bond is formed. P E G G C A A U A C C G C U A E P A GG C G A U A U A C C G C U A Leu Amino acid corresponding to anticodon trna anticodon Complementary mrna codon New peptide bond Growing polypeptide chain A site The correct amino acid is attached to each species of trna by a synthetase enzyme (aminoacyl-trna synthetase). Large ribosomal subunit E site p site Start codon Small ribosomal subunit Pre-mRNA Template strand of DNA mrna Nucleus (site of transcription) Newly made (and edited) mrna leaves nucleus and travels to a ribosome for decoding. Cytosol (site of translation)

62 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. Slide 5 Getting Ready Making mrna (transcription) Attaching amino acid to trna trnas diffuse to ribosome Amino acid that corresponds to anticodon trna 1 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) Initiator trna bearing anticodon 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. 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. 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. A new peptide bond is formed. P E G G C A A U A C C G C U A E P A GG C G A U A U A C C G C U A Leu Amino acid corresponding to anticodon trna anticodon Complementary mrna codon New peptide bond Growing polypeptide chain A site The correct amino acid is attached to each species of trna by a synthetase enzyme (aminoacyl-trna synthetase). Large ribosomal subunit E site p site Start codon 2c Translocation: The entire ribosome translocates, shifting its position one codon along the mrna. The unloaded trna from the P site is now in the E site. It is released. Released trna Direction of ribosome movement P E G A U A C C G C U A C U C The trna that was in the A site is now in the P site. The next codon to be translated is now in the empty A site, ready for step 2a again. Small ribosomal subunit Pre-mRNA Template strand of DNA mrna Nucleus (site of transcription) Newly made (and edited) mrna leaves nucleus and travels to a ribosome for decoding. Cytosol (site of translation)

63 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. Slide 6 Getting Ready Making mrna (transcription) Attaching amino acid to trna trnas diffuse to ribosome Amino acid that corresponds to anticodon trna 1 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) Initiator trna bearing anticodon 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. 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. 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. A new peptide bond is formed. P E G G C A A U A C C G C U A E P A GG C G A U A U A C C G C U A Leu Amino acid corresponding to anticodon trna anticodon Complementary mrna codon New peptide bond Growing polypeptide chain A site Pre-mRNA Template strand of DNA The correct amino acid is attached to each species of trna by a synthetase enzyme (aminoacyl-trna synthetase). mrna Nucleus (site of transcription) Large ribosomal subunit E site p site Newly made (and edited) mrna leaves nucleus and travels to a ribosome for decoding. Start codon Small ribosomal subunit Cytosol (site of translation) 2c Translocation: The entire ribosome translocates, shifting its position one codon along the mrna. 3 Termination: When a stop codon (UGA, UAA, or UAG) arrives at the A site, elongation ends. The unloaded trna from the P site is now in the E site. It is released. P E C C U C U G G G A U G A Direction of ribosome movement Stop codon Release factor Released trna Release factor triggers the ribosomal subunits to separate, releasing the mrna and new polypeptide. P E G A U A C C G C U A C U C The trna that was in the A site is now in the P site. The next codon to be translated is now in the empty A site, ready for step 2a again. Polypeptide chain

64 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

65 Figure 3.33 Rough ER processing of proteins. Slide 1 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 3 An enzyme clips off the signal sequence. As protein synthesis continues, sugar groups may be added to the protein. Signal recognition particle Growing (SRP) polypeptide Receptor site Rough ER cistern Signal sequence removed Released protein Sugar group 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. 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). Cytosol Transport vesicle pinching off Protein-coated transport vesicle

66 Figure 3.33 Rough ER processing of proteins. Slide 2 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

67 Figure 3.33 Rough ER processing of proteins. Slide 3 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

68 Figure 3.33 Rough ER processing of proteins. Slide 4 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 3 An enzyme clips off the signal sequence. As protein synthesis continues, sugar groups may be added to the protein. Signal recognition particle Growing (SRP) polypeptide Receptor site Signal sequence removed Sugar group Rough ER cistern Cytosol

69 Figure 3.33 Rough ER processing of proteins. Slide 5 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 3 An enzyme clips off the signal sequence. As protein synthesis continues, sugar groups may be added to the protein. Signal recognition particle Growing (SRP) polypeptide Receptor site Signal sequence removed Sugar group 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. Released protein Rough ER cistern Cytosol

70 Figure 3.33 Rough ER processing of proteins. Slide 6 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 3 An enzyme clips off the signal sequence. As protein synthesis continues, sugar groups may be added to the protein. Signal recognition particle Growing (SRP) polypeptide Receptor site Rough ER cistern Signal sequence removed Released protein Sugar group 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. 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). Cytosol Transport vesicle pinching off Protein-coated transport vesicle

71 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

72 Figure 3.34 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 Triplets T A C G G T A G C G A T T T C C C T G C G A A A A C T 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 trna Codons A U G C C A U C G C U A A A G G G A C G C U U U U G A Anticodon U A C G G U A G C G A U U U C C C U G C G A A A Polypeptide: Amino acid sequence of the polypeptide chain Met Pro Ser Leu Lys Gly Arg Phe Start translation Stop; detach

73 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

74 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

75 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