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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 Module 1 Section 1.7 Continuity of Cells
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
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!
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
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.
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
Cytokinesis • Once cells have divided they are both stuck together • To separate the cells they undergo a process called CYTOKINESIS
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
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
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!
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
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
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
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
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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
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
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