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Cell Cycle How do baby elephants grow up to be big elephants? Why do organisms need more cells? Why do organisms need new cells? • When you are growing! • When you need to replace damaged cells! How do cells make new cells? • All cells are created from other cells. The only way to get more cells is for cells to divide. • Cell division is the process by smaller which a parent cell divides into two or more daughter cells (IDENTICAL CELLS!). nutrients grow • The parent cell provides the genetic code to each daughter cell. Identical Daughter Cells Two identical daughter cells Parent Cell Cell Cycle • In order for new somatic (body) cells to be made, they must go interphase through the cell cycle: • Interphase • Mitosis Interphase • Before dividing, cells spend most of their time finding nutrients and growing. • This phase is called interphase. interphase • During interphase, there are two growth phases. These are known as G1 and G2. Interphase - DNA Replication DNA replication occurs during the synthesis (S) phase of interphase. Why Replicate? Remember: ALL cells need genetic material….DNA! So….before a cell can divide, the DNA must be replicated (copied). If a cell doesn’t get a copy of DNA, it is USELESS and essentially DEAD! DNA Replication DNA → DNA (copy) DNA is a HUGE molecule, in fact it is so HUGE it cannot exit the nucleus EVER! Sooo… DNA Replication occurs in the NUCLEUS! 3 EASY STEPS STEP 1: ◦DNA Unzips 5’ 3’ ◦DNA Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds 3’ 5’ 3 EASY STEPS STEP 1: ◦DNA Unzips ◦DNA Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds 3 EASY STEPS STEP 1: ◦DNA Unzips ◦DNA Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds 3 EASY STEPS STEP 1: ◦DNA Unzips ◦DNA Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds 3 EASY STEPS STEP 1: ◦DNA Unzips ◦DNA Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds Replication Fork The areas where the double helix separates are called replication forks because of their Y shape. Once the 2 strands are separated, additional proteins attach to each strand, holding them apart. 16 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 2: ◦ Complimentary Base Pairing 5’ 3’ 5’ 3’ 3’ 5’ 3’ 5’ ◦ DNA Polymerase adds matching nucleotides 3 EASY STEPS STEP 3: ◦ DNA Ligase Rezips DNA 5’ 3’ 5’ 3’ 3’ 5’ 3’ 5’ DNA REPLICATED! Two IDENTICAL copies of DNA; one copy goes into each new cell Blue – Original DNA Green – New DNA DNA replication is a semi-conservative process; there is not 1 new set and 1 old set. Each set of DNA contains 1 old strand and 1 new strand. 5’ 3’ 5’ 3’ 3’ 5’ 3’ 5’ Checking for Errors Errors sometimes occur and the wrong nucleotide is added to the new strand. An important feature of DNA replication is that DNA polymerases have a “proofreading” role. It can backtrack to remove the incorrect nucleotide. Reduces errors in DNA replication to about one error per 1 billion nucleotides. 31 Let’s recap Interphase… G1: The cell grows – makes more cytoplasm & organelles; cell carries on normal metabolic activity (respiration, cell transport, etc). S: DNA is replicated. G2: The cell continues to grow, preparing for mitosis. Mitosis • • Once the cell is ready to begin dividing, it enters MITOSIS. The purpose of mitosis is to separate the two sets of DNA into TWO DIFFERENT NUCLEI – each having it’s own complete set of chromosomes. Cells are diploid (2n) Mitosis Mitosis has four phases: • • • • Prophase Metaphase Anaphase Telophase The DNA coils to form chromosomes, the chromosomes condense and become visible. The nuclear membrane dissolves. Special proteins called spindle fibers line the chromosomes up in a straight. line. The chromosomes (which are doubled due to replication) divide in half so that each complete and identical set of chromosomes move to opposite sides of the cell. Cytokinesis Once two nuclei have been formed, the cell can divide its cytoplasm, which includes all its organelles, into two new daughter cells. This process is known as cytokinesis. Cellular Division in Prokaryotes Fission • Mitosis occurs in Eukaryotes. • Why? • Because they have a nucleus! • Binary Fission occurs in Prokaryotes. • Asexual reproduction CheckpointsFissin Ignoring Checkpointsn Cancer