Cell Division, Diversity & Organisation

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Cell Division, Diversity & Organisation
The Cell Cycle and Mitosis
Cells divide for growth and repair. 2 identical cells are produced. Mitosis only takes up a small percentage of the cell cycle.
Interphase ­ Cells carry out normal functions but prepare to divide. The DNA is unravelled and replicated (organelles also
replicate). ATP content is increase as it's needed for cell division. It also checks for any genetic mutations, if so the cell
may kill its self.
Chromosomes ­ Joined in the middle by a centromere, separate strands are called chromatids, 2 - called sister
o Prophase ­ Chromosomes get shorter and fatter centrioles move to opposite ends of the cell (forming the
spindle) nuclear envelope breaks down and frees chromosomes into the cytoplasm.
o Metaphase ­ Chromosomes line up at the spindle equator and become attracted to the spindle by their
o Anaphase ­ Centromeres divide separating each pair of sister chromatids spindles contact pulling
chromatids to opposite ends of the cell, centromere first.
o Telophase ­ Chromatids reach opposite poles on the spindle uncoil and become long and thin again
nuclear envelope forms around a group of chromosomes, now 2 nuclei cytokinesis then occurs: the cell
membrane constricts, pinches the cell in to 2 daughter cells.
How is mitosis different in plants?
o Only cells in meristem can divide by mitosis.
o Plants don't have centrioles so for spindles without them.
o Due to the cell wall cytokinesis is different ­ It begins with a cell plate in the cell centre. This is a double
membrane which secrets material to form 2 cell walls. They begin in the centre and move outward until they
meet the perimeter.
Reproduction and Meiosis
Asexual reproduction: Budding in yeast
o Parent cell swells on one side forming a bud on the cell surface.
o Cell undergoes interphase and mitosis and the replicated DNA, cytoplasm and organelle move into the bud.
o Cytokinesis occurs and the bud pinches off to for a genetically identical cell.
Meiosis ­ 2 gametes to a zygote
o DNA replicates and coils up to form chromosomes, these arrange themselves into homologous pairs.
o The chromosome pairs then swap bits with each other.
o First division ­ Homologous pairs split up (any one chromosome can go into either cell as long as each cell gets
a number 1, a number 2 ect.)
o Second division ­ Each chromosome splits in half, any half can go into any cell.
o 4 genetically different cells are produced.
Stem Cells and Differentiation
Stem cell ­ An unspecialized cell that can differentiate into any type of cell.
Undifferentiated stem cells in bone marrow can differentiate into neutrophils and erythrocyte.
In plants stem cells are found in the cambium ­ Cambium cells divide and begin to differentiate with xylem and phloem on
opposite sides.
Specialised cells...
o Erythrocytes ­ Biconcave shape so a larger surface area for gas exchange. No nucleus, more room for
o Neutrophils ­ Flexible shape allows them to engulf foreign particles or pathogens. Contain many lysosomes to
break down the engulfed particles.
o Epithelial cells ­ Joined by interlinking cell membranes at their base. Some have cilia to beat and move particles;
others have microvilli (folds in the cell membrane that increase cells surface area.
o Sperm cells ­ Flagellum for swimming to the egg. Lots of mitochondria to provide energy for swimming. The
acrosome contains digestive enzymes to enable the sperm to penetrate the surface of the egg.
o Palisade mesophyll cells ­ Contain many chloroplast to absorb sunlight. Walls are thin so CO can easily diffuse
into the cell.

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Root hair cells ­ Large surface area for absorption and a thin, permeable cell wall. Cytoplasm contains extra
mitochondria to provide energy needed for active transport.
o Guard cells ­ In light they open and take water into their vacuoles and become turgid. Their thin outer walls and
thickened inner walls bend outwards open the stomata. This allows the leaf to exchange gasses for
photosynthesis.…read more


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