3.2.10 Cells Divide by Binary Fission and Mitosis Flashcards
Cells that keep their ability to divide follow the ____ ____
cell cycle
What is interphase?
Period of cell growth and DNA replication
Name the 2 phases that the cell cycle consists of
- Interphase
- Mitosis
Name the 3 growth stages that interphase is subdivided into
- Gap phase 1
- Synthesis
- Gap phase 2
Where does the cell cycle start and end?
At mitosis
Draw the cell cycle
What happens in gap phase 1?
Cells grows and new organelles and proteins are made
What happens in synthesis?
Cell replicates its DNA, ready to divide by mitosis
What happens in gap phase 2?
Cell keeps growing and proteins needed for cell division are made
What is mitosis?
When a parent cell divides to produce 2 genetically identical daughter cells
(Contain exact copy of DNA of parent cell)
Why is mitosis needed?
For growth of multicellular organisms and repairing damaged tissues
Describe what occurs before mitosis, during interphase
- Caries out its normal functions but prepares to divide
- DNA unravels and replicates, organelles are replicated
- ATP content increased
Name the 4 main stages in mitosis
- Prophase
- Metaphase
- Anaphase
- Telophase
As mitosis begins why are chromosomes made of 2 strands (chromatids) joined by a centromere?
∵ each chromosome has already made an identical copy of itself during interphase
When mitosis is over, what do the chromatids end up as?
One-strand chromosomes in the daughter cells
Describe Prophase
- Chromosomes condense and super coil & become visible
- Centrioles (tiny bundles of protein) move towards opposite end of cell, forming the spindle (network of protein fibres)
- Nuclear envelope breaks down allows chromosomes to move
Describe Metaphase
- Chromosomes line up along centre of cell
- Become attached to the spindle by their centromere
Describe Anaphase
- Centromeres divide, separating each pair of sister chromatids
- Spindles contract, pull chromatids to opposite poles of the spindle, centromere first (are separated)
- Makes chromatids appear v-shaped
Describe Telophase
- Chromatids reach opposite poles on spindle
- They uncoil & become long and thin again = chromosomes
- Nuclear envelope reforms preventing chromosomes moving away
- Cytoplasm divides (cytokinesis) so they’re now two daughter cells that are genetically identical to original cell
- (Each daughter cells starts interphase)
Describe how you use root tips to observe mitosis
- Cut 1 cm from tip of growing root (e.g. onion)
- Hydrochloric acid in boiling tube, put in water bath at 60°C
- Transfer root tip into boiling tube & leave for 5 mins
- Use pipette to rinse root tip with cold water
- Leave tip to dry on paper towel
- Place root tip on microscopic slide & cut 2 mm from very tip of it
- Discard the rest
- Use mounted needle to break tip open and spread cells thinly
- Add few drops of stain and leave it for few minutes
- Place cover slip over cells and push down firmly to squash tissue
- Look at all stages of mitosis under optical microscope
Investigating Mitosis
Why do you have cut the from the tip of a growing root?
Has to be tip ∵ it’s where growth occurs (i.e. mitosis)
Investigating Mitosis
Why do you have add stain to the sample?
Stain makes it easier for chromosomes to be seen
Investigating Mitosis
Why do you squash the tissue (by pushing down the cover slip firmly over the cells)?
This makes tissue thinner = allows light to pass through it
Investigating Mitosis
Why shouldn’t you smear the cover slip sideways?
∵ you’ll damage the chromosomes
Label the stages of mitosis and interphase
Describe how you would use an optical microscope to observe cells
- Clip prepared slide onto the stage
- Select lowest-powered objective lens
- Use coarse adjustment knob to bring stage up to just below objective lens
- Look down eyepiece & use coarse adjustment knob to move stage downwards until image is roughly in focus
- Adjust focus with fine adjustment knob until you get clear image of slide
- If you need a greater magnification, swap to higher-powered objective lens and refocus
What is mitotic index?
Proportion of cells undergoing mitosis
Why do we work out the mitotic index?
To find out how quickly a tissue is growing
State the formula for the mitotic index
Why does a plant root tip have a high mitotic index (i.e. lots of cells in mitosis)?
∵ it’s constantly growing
Why may other tissues have a high mitotic index?
∵ tissue repair is taking place or there’s cancerous growth
What is an artefact?
Things that you can see down microscope that aren’t part of cell or specimen you’re looking at
e.g. bits of dust, air bubbles and fingerprints, to inaccuracies caused by squashing and staining your sample
When are artefacts made?
During preparation of your slides
What is binary fission?
When a prokaryotic cell replicates its genetic material before splitting into 2 daughter cells
Describe Binary Fission
- Circular DNA and plasmid(s) replicate
- Main DNA loop is replicated once but plasmids can be replicated loads of time
- Cells get bigger and DNA loops move to opposite ‘poles’ (ends) of cell
- Cytoplasm begins to divide (and new cell walls begin to form)
- Cytoplasm divides and 2 daughter cells produced
- Each daughter cells has 1 copy of variable DNA but can have variable no. of copies of plasmid(s)
What can occur during conjugation?
DNA may be passed from one species of bacterium to another (species)
“DNA may be passed from one species of bacterium to another (species) during conjugation”
What is this known as?
Horizontal gene transmission
What is vertical gene transmission?
When genes are passed down from one generation (of a species) to next generation (of same species)
Describe how DNA transfers by conjugation in bacteria
