Cell Cycle, Division, Diversity & Organisation • State that mitosis occupies only a small percentage of the cell cycle. • Describe, with the aid of diagrams and photographs, the main stages of mitosis (behaviour of the chromosomes, nuclear envelope, cell membrane and centrioles). • Explain the meaning of the term homologous pair of chromosomes. • Explain the significance of mitosis for growth, repair and asexual reproduction in plants and animals. • Outline, with aid of diagrams and photographs, the process of cell division by budding in yeast. • State that cells produced as a result of meiosis are not genetically identical (details of meiosis are not required). • Define the term stem cell.
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Cell Cycle, Division, Diversity & Organisation · 2019-05-12 · Cell Cycle, Division, Diversity & Organisation •State that mitosis occupies only a small percentage of the cell
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Cell Cycle, Division, Diversity
& Organisation• State that mitosis occupies only a small percentage of the cell
cycle.
• Describe, with the aid of diagrams and photographs, the main stages of mitosis (behaviour of the chromosomes, nuclear envelope, cell membrane and centrioles).
• Explain the meaning of the term homologous pair of chromosomes.
• Explain the significance of mitosis for growth, repair and asexual reproduction in plants and animals.
• Outline, with aid of diagrams and photographs, the process of cell division by budding in yeast.
• State that cells produced as a result of meiosis are not genetically identical (details of meiosis are not required).
• Define the term stem cell.
The Cell Cycle
• The events that take place as one parent
cell divides to produce two daughter cells
which then grow to full size.
– The daughter cells are genetically identical to
each other and to the original parent cell.
– In some cases the daughter cells are new
organisms
ChromosomesChromosome
arrangement
Stages of
the Cell
Cycle
Mitosis
• Nuclear division where 2 genetically identical nuclei are formed from one parent nucleus.
• Divided into 4 named phases:
– Prophase
– Metaphase
– Anaphase
– Telophase
Interphase
• The cell grows to full size.
• DNA replicates ready for the next round of cell division.
• The cell carries out its normal biochemical functions.
• Chromatin is not coiled & chromosomes are not visible.
Prophase
• The replicated chromosomes become supercoiled (short & thick).
• Now visible with a light microscope.
• Centriole divides and two daughters move to opposite poles.
• Spindle fibres form.
• Nuclear envelope disappears.
Metaphase
• The chromosomes line up
along the equator of the
cell.
• Each becomes attached to
a spindle thread by it’s
centromere.
Anaphase
• The centromere splits &
chromatids are pulled
apart.
• Spindle fibres shorten &
pull the chromatids towards
the poles.
Telophase
• A new nuclear envelope
forms around the
separated chromatids (now
called chromosomes).
• Spindle fibres break down.
• Chromosomes uncoil.
• Cytokinesis occurs
Timescales
• Cell cycle times vary from one species to the next.
– Some bacteria complete a whole cycle in 20 -30 minutes (given ideal conditions).
– Yeasts can take up to 4 hours.
– The fastest mammalian cell cycle is 9 – 10 hours for some intestine epithelial cells.
– Some mammalian cells can have a cycle time of over 200 hours.
– Fully differentiated cells rarely divide.
Stage G0
• G0 is the name given to the phase where a cell leaves
the cell cycle (either permanently or temporarily).
• Reasons for this:
– Differentiation
• Specialised cells no longer divide. They just do the
job they have specialised to do.
– DNA Damage
• Damaged cells do not divide.
– Aging
• Most cells only divide a certain number of times
then become senescent. Aging is the increase in
number of senescent cells.
Controlling the Cell Cycle
• Cells need to ensure they only divide
when they are ready to.
– When they have grown large enough.
– When replicated DNA is error free.
– When chromosomes are in their correct
position during mitosis.
• There are 3 checkpoints within the cell
cycle which check these process have
taken place correctly:
Checkpoints:
• Mitosis cannot proceed
unless this checkpoint is
passed
Plants v Animals v Yeasts
• Only certain plant cells (meristem cells) can divide.
• Plant cells do not have centrioles.– Spindle fibres are made in the cytoplasm.
• Plant cytokinesis starts with the formation of a cell plate across the equator of the cell.– A new cell membrane & cell wall is laid down along
this cell plate.
• Yeast cytokinesis by producing a small bud which pinches off from the cell.
Animals
Plants
Yeasts
Cleavage
Furrow
Buds
Cell plate
How long is each phase?
Why do we need mitosis?
• All organisms need to produce genetically identical cells for:– Growth
• Multicellular organisms grow by producing extra cells. Each cell must be able to carry out the same functions as existing ones.
– Repair• Damaged cells need to be replaced by new ones that are
able to perform the same functions.
– Asexual reproduction• Single celled organisms divide to produce two identical
organisms. Some multicellular organisms produce genetically identical offspring by mitosis.
– Replacement• RBCs & skin cells are constantly replaced.