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AS Biology

AS Biology. Module 1 Section 1.7 Continuity of Cells. Specification. The cell cycle. The need to divide. All cells have to divide as a means to pass on their genetic material before they die Bacterial cells can divide every 30 minutes if conditions are right!

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AS Biology

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  1. AS Biology Module 1 Section 1.7 Continuity of Cells

  2. Specification

  3. The cell cycle

  4. The need to divide • All cells have to divide as a means to pass on their genetic material before they die • Bacterial cells can divide every 30 minutes if conditions are right! • Human cells can divide every 2 hours (the liver), months or years (nerve cells) or less than 1 hour (stomach cells) • The cycle that leads to cell division has FOUR main stages

  5. G1-phase • The first gap phase • Here the cell is just growing and living as normal • During this stage protein synthesis occurs and the cell grows to almost double its size • More organelles are produced, cytoplasmic volume is increased • You need more cytoplasm and organelles if you are to make an identical new cell afterall!

  6. S-phase • The synthesis phase • Here the cell replicates its DNA (by semi-conservative replication) • One set of the DNA will remain in the parent cell and the other set will end up in the newly synthesized daughter cell

  7. G2-phase • The second gap phase • Here the cell resumes growing and getting ready to divide • These previous three stages (G1, S and G2) are all collectively called INTERPHASE • Of a total 24 hour cell cycle, interphase lasts for approximately 20 hours! • This means that it only takes 4 hours for M-phase to occur.

  8. M-phase • Mitosis • Has itself FOUR steps; • Prophase (Pre) • Metaphase (Molars) • Anaphase (Are) • Telophase (Teeth) • Results in formation of TWO daughter cells that are genetically identical to each other

  9. Cytokinesis • Once cells have divided they are both stuck together • To separate the cells they undergo a process called CYTOKINESIS

  10. Cytokinesis • In animals the cells are divided by a sealing of the plasma membrane • In plants vesicles form along the middle of the cell and seal to split it into two cells

  11. Mitosis as a means of growth • Mitosis is required for growth of an organism • More cells means you have got bigger! • Some organisms that reproduce ASEXUALLY carry out mitosis only as a means to procreate • Mitosis is vitally important in maintaining genetic stability • DNA must be accurately copied and present in all cells

  12. The “extra” stage • Cells that do not divide very often, or at all, enter a stage called the G0 phase which can be thought of as permanent G1 phase • This means that the cell is essentially out of the cell cycle!

  13. DNA and chromosome structure

  14. Histones • The DNA double helix is massive in length! • To fit it in the cell it is tightly wrapped around proteins called HISTONES • These allow the DNA to be compressed and folded up

  15. 23 chromosomes • Cells contain 23 PAIRS of chromosomes • 22 autosomal pairs and 1 sex pair • This means we have 46 chromosomes total in all of our cells • Having all 46 means that a cell is DIPLOID KARYOTYPE Micrograph showing chromosome arrangements

  16. The stages of mitosis

  17. The stages in a bit more detail • Prophase • The chromatin begins to condense into chromosomes • The nuclear membrane disintegrates • Metaphase • The chromosomes arrange along the middle of the cell • Anaphase • The chromosome pairs are pulled to either end of the cell • Telophase • Nuclear membranes reform around the separating chromosomes • Cytokinesis begins

  18. Haploid cells • Sex cells (eggs and sperm) are HAPLOID • That is, they only have 23 chromosomes • So when a sperm fertilizes an egg and they both combine, the zygote has 46 and is a viable diploid cell • How haploid cells are formed is through a process called MEIOSIS • This happens only in sperm and egg cells

  19. A) B) C) D) E)

  20. Meiosis • Meiosis is a process that results in formation of FOUR daughter cells from a single cell • It is a major source of genetic variation due to two steps throughout it that lead to chromosome rearrangements • This is how eggs and sperm cells are produced and explains why they are HAPLOID

  21. The initial round of division

  22. The second round of division

  23. The sources of genetic variation • Prophase 1 • When the chromosomes have replicated and there are two pairs, the pieces of DNA can swap regions of DNA • This process is called CHIASMA formation or CROSSING-OVER • The chromosomes swap regions of DNA at random, leading to variable DNA sequences

  24. The sources of genetic variation • Independent chromosome assortment • When the chromosomes align along the centre spindle plate during metaphase, the chromosome that is pulled into each cell is random • This means that the chromosome will have a 50% chance of being in either cell • During prophase II this means that the chromosomes will cross-over and generate more diversity

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