DNA Replication Replication begins simultaneously on several chromatin threads & continues until all DNA has been replicated. Steps in DNA Replication

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1 DNA Replication Replication begins simultaneously on several chromatin threads & continues until all DNA has been replicated Steps in DNA Replication 1) 2) 3) 4) 5) 6) DNA helices unwind from the nucleosomes Helicase untwists the double helix into 2 complementary nucleotide chains exposing the nitrogenous bases Each nucleotide strand serves as a template for building a new complementary strand. Occurs at the replication fork The replisome uses primers to begin DNA synthesis of 2 nd strand DNA polymerase III continues from the primer and covalently adds complementary nucleotides to the template DNA ligase splices the short segments together Steps in DNA Replication (cont.) 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 DNA Replication Figure 3.31 Cell Division Essential for body growth and tissue repair Mitosis nuclear division Cytokinesis division of the cytoplasm Meiosis produces ova and sperm w/ only ½ of the number of genes found in the body cells Mitosis DNA from the mother cell 2 daughter cells The phases of mitosis are: Prophase Metaphase Figure 3.32 in Text Anaphase Telophase PLAY Mitosis 1

2 Early Prophase Late Prophase Figure 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 Anaphase Centromeres of the chromosomes split Motor proteins in kinetochores pull chromosomes toward poles Figure Metaphase Figure Anaphase Figure

3 Control of Cell Division 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 Telophase and Cytokinesis New sets of chromosomes extend into chromatin New nuclear membrane is formed from the rough ER Nucleoli reappear Generally cytokinesis completes cell division Telophase and Cytokinesis Cytokinesis Cleavage furrow formed in late anaphase by contractile ring Cytoplasm is pinched into two parts after mitosis ends Figure 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 s) 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 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 3

4 From DNA to Protein Processing m Nuclear envelope DNA Pre-m Ribosome Polypeptide Roles of the Three Types of Messenger (m) carries the genetic information from DNA in the nucleus to the ribosomes in the cytoplasm Transfer s (ts) bound to amino acids base pair with the codons of m at the ribosome to begin the process of protein synthesis Ribosomal (r) a structural component of ribosomes Figure 3.33 Genetic Code Rules by which base sequences of a gene are translated into an amino acid sequence. 2 Major steps: Transfer of information from the sense strand of DNA to factor Loosens histones from DNA in the area to be transcribed Binds to promoter, a DNA sequence specifying the start site of synthesis Mediates the binding of polymerase to promoter : Polymerase An enzyme that oversees the synthesis of Unwinds the DNA template Adds complementary ribonucleoside triphosphates on the DNA template Joins these nucleotides together Encodes a termination signal to stop transcription (a) Coding Termination signal strand Promoter Template unit strand In a process mediated by a transcription Unwound DNA polymerase factor, polymerase binds to promoter and unwinds base pairs of the DNA template strand polymerase bound to promoter nucleotides m synthesis begins m nucleotides polymerase moves down DNA; m elongates polymerase m synthesis is terminated DNA m transcript Coding strand polymerase Rewinding of DNA Unwinding of DNA (b) Template strand m -DNA hybrid region nucleotides Figure

5 Codon: 3 base sequence on m corresponding to a specific amino acid 4 nucleotides (A,C,G,U) in so there are: 4 3 = 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 Genetic Code codons code for amino acids according to a genetic code Figure 3.35 Pre m contains introns and exons Pre m is processed whereby the introns are spliced out and the exons are spliced together. This is done by the splicesome m complex proteins then associate w/ the processed mature m and guide its export from the nucleu Nucleic acid sequences are translated into amino acid sequences (polypeptides) Occurs in the cytoplasm and involves m, t, r A leader sequence on m attaches to the small subunit of the ribosome by base pairing to r (ribosomal ) t (transfer )loads a single amino acid, migrates to the ribosome, and maneuvers the amino acid into position as specified by the m t has 2 active sites: 1) binding of amino acid at one end, and 2) a 3-base complementary to the m codon (anticodon) calling for the amino acid carried by the particular t Nuclear membrane Nucleus polymerase Anticodons form hydrogen bonds w/ complementary codons (base pairing) t is the link (translator) between nucleic acids and amino acids There are 45 different ts each capable of binding to a specific amino acid Portion of m already translated Nuclear pore m Released m 1 After m processing, m leaves nucleus and attaches to ribosome, and translation begins. Small ribosomal subunit Codon 15 Codon 16 Codon 17 Direction of ribosome advance Template strand of DNA t head bearing anticodon Amino acids t Aminoacyl-t synthetase 4 Once its amino acid is released, t is ratcheted to the E site and then released to reenter the cytoplasmic pool, ready to be recharged with a new amino acid. Large ribosomal subunit 3 As the ribosome moves along the m, a new amino acid is added to the growing protein chain and the t in the A site is translocated to the P site. 2 Incoming aminoacylt hydrogen bonds via its anticodon to complementary m sequence (codon) at the A site on the ribosome. Energized by ATP, the correct amino acid is attached to each species of t by aminoacyl-t synthetase enzyme. Figure

6 Information Transfer from DNA to DNA triplets are transcribed into m codons by polymerase Codons base pair with t 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 Information Transfer from DNA to Figure 3.38 KU Game Day! Home Games Tuesday Wednesday Volleyball vs. 7 pm and Field Hockey vs. 7:30 pm Men s Soccer vs. 7:30 pm Saturday Men s Soccer vs. Rutgers- 7pm and Football vs. 1pm 6