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Download DNA Replication Replication begins simultaneously on several
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DNA Replication Replication begins simultaneously on several chromatin threads & continues until all DNA has been replicated Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Steps in DNA Replication (cont.) Steps in DNA Replication 1) DNA helices unwind from the nucleosomes 2) Helicase untwists the double helix into 2 complementary nucleotide chains exposing the nitrogenous bases 3) Each nucleotide strand serves as a template for building a new complementary strand. Occurs at the replication fork 4) The replisome uses RNA primers to begin DNA synthesis of 2nd strand 5) DNA polymerase III continues from the primer and covalently adds complementary nucleotides to the template 6) DNA ligase splices the short segments together Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings DNA Replication Since DNA polymerase only works in one direction: A continuous leading strand is synthesized A discontinuous lagging strand is synthesized DNA ligase splices together the short segments of the discontinuous strand Two new telomeres are also synthesized This process is called semiconservative replication 7) After replication, histones associate w/ the DNA, chromatin strands condense forming chromatids, and are held together by the centromere until anaphase when they are distributed to each daughter cell Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Cell Division Mitosis Essential for body growth and tissue repair DNA from the mother cell Mitosis – nuclear division The phases of mitosis are: Cytokinesis – division of the cytoplasm Meiosis produces ova and sperm w/ only ½ of the number of genes found in the body cells PLAY Figure 3.31 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Prophase Metaphase Anaphase Telophase 2 daughter cells Figure 3.32 in Text Mitosis Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 1 Early Prophase Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Late Prophase Figure 3.32.2 Metaphase Chromosomes cluster at the middle of the cell with their centromeres aligned at the exact center, or equator, of the cell This arrangement of chromosomes along a plane midway between the poles is called the metaphase plate Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Anaphase Centromeres of the chromosomes split Motor proteins in kinetochores pull chromosomes toward poles Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 3.32.3 Metaphase Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 3.32.4 Anaphase Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 3.32.5 2 Control of Cell Division Telophase and Cytokinesis Surface-to-volume ratio of cells (e.g. cells outgrow their membranes…cost effective to divide rather than synthesize more membrane Chemical signals such as growth factors and hormones Contact inhibition Cyclins and cyclin-dependent kinases (Cdks) complexes Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings New nuclear membrane is formed from the rough ER Nucleoli reappear Generally cytokinesis completes cell division Cytokinesis Figure 3.32.6 Protein Synthesis DNA serves as master blueprint for protein synthesis Genes are segments of DNA carrying instructions for a polypeptide chain (or a variety of RNAs) 4 nucleotides: A,C,G,T Triplets of nucleotide bases form the genetic library Each triplet specifies coding for an amino acid or a stop codon directing transcription to stop Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings New sets of chromosomes extend into chromatin Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Telophase and Cytokinesis Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Cleavage furrow formed in late anaphase by contractile ring Cytoplasm is pinched into two parts after mitosis ends Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Protein Synthesis Many genes contain exons, regions encoding for a polypeptide sequence, and Introns, noncoding intervening sequences. We are still not sure why introns exist Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 3 From DNA to Protein Roles of the Three Types of RNA Nuclear envelope Transcription Messenger RNA (mRNA) – carries the genetic information from DNA in the nucleus to the ribosomes in the cytoplasm Transfer RNAs (tRNAs) – bound to amino acids base pair with the codons of mRNA at the ribosome to begin the process of protein synthesis Ribosomal RNA (rRNA) – a structural component of ribosomes DNA Pre-mRNA RNA Processing mRNA Ribosome Translation Polypeptide Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 3.33 Genetic Code Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Transcription Rules by which base sequences of a gene are translated into an amino acid sequence. 2 Major steps: Transcription Translation Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Transfer of information from the sense strand of DNA to RNA Transcription factor Loosens histones from DNA in the area to be transcribed Binds to promoter, a DNA sequence specifying the start site of RNA synthesis Mediates the binding of RNA polymerase to promoter Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Transcription: RNA Polymerase Coding strand Termination signal Promoter Template strand An enzyme that oversees the synthesis of RNA Unwinds the DNA template Adds complementary ribonucleoside triphosphates on the DNA template Transcription unit RNA polymerase bound to promoter RNA nucleotides mRNA Joins these RNA nucleotides together In a process mediated by a transcription factor, RNA polymerase binds to promoter and unwinds 16–18 base pairs of the DNA template strand RNA polymerase Unwound DNA RNA nucleotides RNA polymerase mRNA synthesis begins RNA polymerase moves down DNA; mRNA elongates mRNA synthesis is terminated DNA Encodes a termination signal to stop transcription (a) mRNA transcript Coding strand RNA polymerase Unwinding of DNA Rewinding of DNA Template strand RNA nucleotides mRNA RNA-DNA hybrid region (b) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 3.34 4 Transcription Codon: 3 base sequence on mRNA corresponding to a specific amino acid 4 nucleotides (A,C,G,U) in RNA so there are: 43 = 64 codons 3 are stop codons (termination of the polypeptide chain) 61 code for amino acids There are only 20 amino acids, so more than 1 codon codes for a specific amino acid Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Transcription Pre mRNA contains introns and exons Pre mRNA is processed whereby the introns are spliced out and the exons are spliced together. This is done by the splicesome mRNA complex proteins then associate w/ the processed mature mRNA and guide its export from the nucleu Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Genetic Code RNA codons code for amino acids according to a genetic code Figure 3.35 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Translation Nucleic acid sequences are “translated” into amino acid sequences (polypeptides) Occurs in the cytoplasm and involves mRNA, tRNA, rRNA A leader sequence on mRNA attaches to the small subunit of the ribosome by base pairing to rRNA (ribosomal RNA) tRNA (transfer RNA)loads a single amino acid, migrates to the ribosome, and maneuvers the amino acid into position as specified by the mRNA tRNA has 2 active sites: 1) binding of amino acid at one end, and 2) a 3-base complementary to the mRNA codon (anticodon) calling for the amino acid carried by the particular tRNA Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Translation Nucleus Nuclear membrane RNA polymerase Nuclear pore Anticodons form hydrogen bonds w/ complementary codons (base pairing) mRNA Template strand of DNA tRNA is the link (translator) between nucleic acids and amino acids 1 After mRNA processing, mRNA leaves nucleus and attaches to ribosome, and translation begins. tRNA Aminoacyl-tRNA synthetase Small ribosomal subunit There are 45 different tRNAs each capable of binding to a specific amino acid Codon 15 Codon 16 Codon 17 Direction of ribosome advance Portion of mRNA already translated tRNA “head” bearing anticodon Large ribosomal subunit 2 4 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Amino acids Released mRNA Once its amino acid is released, tRNA is ratcheted to the E site and then released to reenter the cytoplasmic pool, ready to be recharged with a new amino acid. 3 As the ribosome moves along the mRNA, a new amino acid is added to the growing protein chain and the tRNA in the A site is translocated to the P site. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Incoming aminoacyltRNA hydrogen bonds via its anticodon to complementary mRNA sequence (codon) at the A site on the ribosome. Energized by ATP, the correct amino acid is attached to each species of tRNA by aminoacyl-tRNA synthetase enzyme. Figure 3.36 5 Information Transfer from DNA to RNA DNA triplets are transcribed into mRNA codons by RNA polymerase Codons base pair with tRNA anticodons at the ribosomes Amino acids are peptide bonded at the ribosomes to form polypeptide chains Start and stop codons are used in initiating and ending translation Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Information Transfer from DNA to RNA Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 3.38 KU Game Day! Home Games Tuesday Volleyball vs. WPU @ 7 pm and Field Hockey vs. MSU @ 7:30 pm Wednesday Saturday Men’s Soccer vs. RutgersCamden @ 7pm and Football vs. MSC @ 1pm Men’s Soccer vs. Rutgers-Newark @ 7:30 pm Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 6