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Intro to Cell Biology: Nucleotides, DNA Structure, and DNA Replication

Learn about nucleotides, DNA structure, and the process of DNA replication. Understand the cell cycle stages and the importance of DNA replication for genetic heredity. Get ready for your Cytology exam and Osmosis Lab Write-up.

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Intro to Cell Biology: Nucleotides, DNA Structure, and DNA Replication

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  1. REMEMBER: Cell Analogy Project (assigned Fri 10/19) is due this Thursday by 4pm. 2. Osmosis Lab Write up is Due Tuesday 11/6 Your Cytology exam is next Tuesday (MC/Grid-In only, due to time constraints….I know you’re sad.) - I’ll give you a study guide tomorrow and - Recorded CRAM will also be available tomorrow

  2. Section 11.1 Summary – pages 281 - 287 Phosphate group Nitrogenous base Sugar • Nucleotides have three parts: a simple pentose (5C) sugar, a phosphate group, and a nitrogenous base.

  3. 5' C 1' 4' C C 3' 2' C C Numbered carbons

  4. DNA STRUCTURE Section 11.1 Summary – pages 281 - 287 • These paired bases are called complementary base pairs. • In DNA, adenine always pairs with thymine, A --- T • and guanine always pairs with cytosine. G --- C

  5. Directionality • Anti-Parallel • Flip flopped arrangement • The numbered carbon atoms in the nucleotide sugar-ring gives rise to a 5'-end and a 3'-end (pronounced "five prime end" and "three prime end"). 5’ TGCA 3’ 3’ ACGT 5’

  6. At this moment, cells in your body are growing, dividing, and dying. Section 8.2 Summary – pages 201 - 210 • Old cells on your scalp, palms of your hands, and soles of your feet are shedding and being replaced. • Cuts and bruises are healing • Your intestines are producing millions of new cells each second.

  7. THE CELL CYCLE THE GROWTH AND DIVISION OF A CELL INTERPHASE: The longest/busiest stage in a cell’s life 1. Cell grows bigger 2. DNA replicates 3. Organelle duplication NUCLEAR DIVISION: Mitosis (if body cell) or Meiosis (if sex cell) CYTOKINESIS: Cell division/separation

  8. The Cell Cycle Section 8.2 Summary – pages 201 - 210 • If a cell is in the process of dividing, the majority of time is spent in the growth period known as interphase. • This is a preparation for division.

  9. G1 S G2 Interphase itself is divided into three sub-phases

  10. Interphase: G1 (gap 1) • Growth period • The cell grows bigger • Continues its normal functions Interphase

  11. During this time DNA is in a long stringy form called ‘Chromatin’.

  12. Interphase: S • DNA Synthesis Stage • DNA replicates • (a copy is made of each chromatin strand) S Interphase

  13. Still in chromatin form, this has to happen before a cell can divide.

  14. The two halves of the doubled structure are called sister chromatids. • Sister chromatids are identical to each other. Sister chromatids

  15. Sister chromatids (two identical replicated strands of DNA) are closely held together by a structure called a centromere. Centromere

  16. Interphase: G2 (gap 2) G2 • 2nd Growth period • The cell grows even bigger • Organelles duplicate Interphase

  17. Before a cell can divide, it must first make a copy of its DNA. WHY? Section 11.1 Summary – pages 281 - 287 To make sure each cell at the end of the cell cycle contains its own set of genetic material. DNA replication allows for heredity. We are able to pass our genes to every cell in our body, and pass them to our offspring because of this process.

  18. When exactly does DNA replication occur? Section 11.1 Summary – pages 281 - 287 This is when the cell PREPARES for division. G₁ S G₂

  19. DNA REPLICATION Section 11.1 Summary – pages 281 - 287 is the process by which a DNA molecule is copied. It begins with one DNA molecule, and ends with 2 identical DNA molecules.

  20. DNA REPLICATION Section 11.1 Summary – pages 281 - 287 This process is also known as ‘semi-conservative’. ‘semi-’means… partial / some ‘to conserve’means… save / keep Daughter molecules half from original, half new Original ‘unzipping’ for replication Original At the end of DNA replication, in each product molecule- one strand is newly synthesized and the other is from the original template.

  21. There are particular sequences found along the length of a DNA molecule that we call ‘origins of replication’ Step 1: Initiation proteins recognize the origin sequences and attach to the DNA, separating the strands and creating replication ‘bubbles’ Step 2?

  22. Replication fork – is the Y shaped corners of the replication bubbles. As DNA replication continues, this is where untwisting and unzipping of the DNA occurs. Helicase – Enzyme that unzips DNA during replication (breaks the hydrogen bonds)

  23. Step 3? There is also an enzyme called Topoisomerase that is found ahead of each helicase and is there to rotate and stretch the DNA to decrease tension / supercoiling.

  24. Then an enzyme, DNA Polymerase, attaches to each pulled apart strand (where a small primer has been laid) and begins build a new complementary strand by attaching free nucleotides to the original unwound templates. Section 11.1 Summary – pages 281 - 287

  25. DNA replication occurs bi-directionally. In other words, it is building in both directions at the same time (at either replication fork.)

  26. Section 11.1 Summary – pages 281 - 287 DNA polymerase builds the new strand from 5’ to 3’ This causes a ‘leading’/ continuous side AND a ‘lagging’ side. 3’ 5’

  27. The leading side gets to build continuously from 5’ to 3’ as helicase unzips the DNA

  28. The lagging side is discontinuous. It has to wait for the helicase to unzip, then builds from 5’ to 3’ until it hits its last built segment.

  29. These discontinuous small segments of DNA that are not linked are called Okazaki fragments.

  30. Step 4?

  31. Step 4: Ligase is an enzyme that helps to join the fragments on the lagging side and wind the strands back up into a double helix.

  32. When the process is complete, two DNA molecules have been formed identical to each other and to the parent molecule What About errors? Errors during replication are rare, especially since the human body contains more than one hundred and thirty types of enzymes to ensure there are no mutations. DNA polymerase makes very few errors, and most of those that are made are quickly corrected by another enzyme which "proofreads" the nucleotides added into the new DNA strand.  The error rate is equal to a person copying 100 large dictionaries word for word, and symbol for symbol, with only one error for the whole process!

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