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DNA Replication DNA helix unwinds from histones Helicase untwists the double helix and exposes complementary strands The site of replication = replication bubble Each nucleotide strand serves as a template for building a new complementary strand DNA Replication RNA primers begin DNA synthesis DNA polymerase III continues from the primer and adds complementary nucleotides to the template 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 lagging strand DNA Replication Figure 3.31 Cell Division Essential for body growth and tissue repair Mitosis – nuclear division Cytokinesis – division of the cytoplasm Mitosis The phases of mitosis are: Prophase Metaphase Anaphase Telophase Cytokinesis A cleavage furrow is formed in late anaphase by contractile ring Cytoplasm is pinched into two parts after mitosis ends Early and Late Prophase Asters are seen as chromatin condenses into chromosomes Nucleoli and nuclear membrane disappear Centriole pairs separate and the mitotic spindle is formed 2 microtubule types: polar and kinetichore Early Prophase Figure 3.32.2 Late Prophase Figure 3.32.3 Metaphase Chromosomes arrange themselves with their centromeres aligned at the middle of the cell This arrangement of chromosomes = metaphase plate Metaphase Figure 3.32.4 Anaphase Centromeres of the chromosomes split Motor proteins in kinetochores pull chromosomes toward poles Anaphase Figure 3.32.5 Telophase and Cytokinesis New sets of chromosomes uncoil into chromatin New nuclear membrane is formed from the rough ER Nucleoli reappear Generally cytokinesis completes cell division Telophase and Cytokinesis Figure 3.32.6 Control of Cell Division Surface-to-volume ratio of cells Chemical signals such as growth factors and hormones Contact inhibition Cyclins and cyclin-dependent kinases (Cdks) complexes Protein Synthesis DNA serves as master blueprint for protein synthesis Genes are segments of DNA carrying instructions for a polypeptide chain Triplets of nucleotide bases = codon Each triplet specifies coding for an amino acid From DNA to Protein Nuclear envelope Transcription DNA Pre-mRNA RNA Processing mRNA Ribosome Translation Polypeptide Figure 3.33 Roles of the Three Types of RNA 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 assemble proteins Ribosomal RNA (rRNA) – a structural component of ribosomes Transcription Creation of a copy of mRNA from DNA 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 Transcription: RNA Polymerase An enzyme that oversees the synthesis of RNA Unwinds the DNA double helix Adds complementary nucleotides on the DNA template Joins these RNA nucleotides together Stops transcription at a termination signal Coding strand Termination signal Promoter Template strand Transcription unit 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 polymerase bound to promoter RNA nucleotides mRNA RNA nucleotides RNA polymerase mRNA synthesis begins RNA polymerase moves down DNA; mRNA elongates mRNA synthesis is terminated DNA (a) mRNA transcript Coding strand RNA polymerase Unwinding of DNA Rewinding of DNA Template strand RNA nucleotides mRNA RNA-DNA hybrid region (b) Figure 3.34 Initiation of Translation mRNA attaches to the small subunit of the ribosome Initiator tRNA binds to the small subunit The large ribosomal unit now binds to this complex forming a functional ribosome Genetic Code RNA codons code for amino acids according to a genetic code Figure 3.35 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 bonded to form polypeptide chains Start and stop codons are used in initiating and ending translation Nucleus Nuclear membrane RNA polymerase Nuclear pore mRNA Template strand of DNA Amino acids Released mRNA 1 After mRNA processing, mRNA leaves nucleus and attaches to ribosome, and translation begins. tRNA Aminoacyl-tRNA synthetase Small ribosomal subunit Codon 15 Codon 16 Codon 17 Direction of ribosome advance Portion of mRNA already translated tRNA “head” bearing anticodon Large ribosomal subunit 2 4 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. 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 Information Transfer from DNA to RNA Figure 3.38