Cell Division Flashcards
Cell Division
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Why must cell division occur in multicellular organisms?
So they can grow and to replace damaged or worn out tissue
What cells only go through the cell cycle once and what replaced them?
Specialised cells only go through it once
Are replaced by stem cells
What are the two types of cell divisions eukaryotic cells go though?
Mitosis and meiosis
What does Mitosis produce?
2 genetically identical diploid daughter cells
What does Meiosis produce?
4 genetically non-identical haploid daughter cells (gametes)
What is mitosis?
- asexual reproduction
- process for growth tissues
- replacement of lost cells
- repairing damaged tissue
- formation of clones of white blood cells in an immune response
- how cancerous tumours form
What is meiosis?
- takes place in reproductive organs
- used to produce haploid sex cells for sexual reproduction
What is a cell cycle?
A process cells that can divide multiple times go through. It starts with a cell that has been produced by mitosis and ends when the cell is ready to divide again.
Specialised cells and stem cells move from G1 into a resting phase known as G0, this can be temporary.
Where is the majority of the cell cycle spent?
In interphase
What happens during interphase?
This is when the cell is growing and DNA is replicated.
The DNA is uncondensed and replicated so the amount of DNA doubles so each cell will have the same amount. Organelles are also replicated so there will be enough for both new cells.
The amount of ATP being produced is increased as energy is required for cell division.
What are the three stages in interphase?
G1 (gap 1)
S (synthesis)
G2 (gap 2)
What happens during each stage of interphase?
G1- phase where the cell elongates and new organelles and proteins are made
S - phase where the cell replicates its DNA - this is essential before the cell can
G2 - phase where the cell keeps elongating and proteins needed for cell division are made
What happens during mitosis?
It is the formation of two new identical daughter cells from an original cell
It involves nuclear division
It is a continuous process but it can be described in stages
What happens during cytokinesis?
It is the final stage of the cell cycle.
It is where the cytoplasm divides and two separate genetically identical cells are formed
Each cell will now start interphase to prepare for the next round of mitosis
Stages of Mitosis
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What does a chromosome consist of?
Two strands called sister chromatids joined in the middle by a centromere
When are chromosomes visible?
Only visible during cell division - they are formed during interphase when DNA is replicated
What do each sister chromatid contain?
Contains identical genetic information so that each daughter cell also contains identical information
Are the chromosomes visible in interphase?
The chromosomes are not visible as -
DNA is uncondensed
Nucleolus is still visible
Nuclear envelope is still intact
Centrioles have replicated and may be visible
What are the different stages of mitosis?
Prophase
Metaphase
Anaphase
Telophase
What occurs in prophase?
The chromosomes condense (they thicken and shorten as DNA is coiled tightly around proteins called histones) and become visible
The nucleolus disappears nuclear envelope begins to break down
The centrioles move to the poles of the cell
The centrioles (bundles of protein) begin to produce spindle fibres which start to extend towards the chromosomes
What occurs in metaphase?
The nuclear membrane has disappeared
Spindle fibres have attached to the centromere or the chromosomes
Each centromere has a spindle fibre from each pole of the cell
Chromosomes are pulled to the middle/equator of the cell where they line up
What occurs in anaphase?
The spindle fibres contract
The centromere of each chromosome splits in half so that one chromatid from each chromosome can be pulled to opposite poles of the cell
This stage makes sure that each half of the cell receives one chromatid from each chromosome
This stage can be recognised by the V-shape of the chromatids as they are dragged across
What occurs in telophase?
Nuclear envelopes begin to reform around each new group of chromosomes
The spindle fibres disappear
The chromosomes begin to uncoil/unravel and become less distinct
The cytoplasm begins to divide to form two new genetically identical daughter cells
Each new cell will have half the amount of DNA compared to the cell in interphase, until interphase begins again and DNA is replicate
Calculating time taken for stages of mitosis
The time it takes for cells to go through mitosis depends on the type of cell and the conditions during cell division.
You can calculate the time spent in each stage if given the right information e.g:
The number of cells out that are observed to be in the stage of mitosis
The total number of cells observed
How long a complete cell cycle lasts (from interphase to cytokinesis).
What are the stages for calculating time taken for stages of mitosis
Step 1: calculate the proportion of cells are in prophase
Step 2: convert cell cycle time to unit required for the answer
Step 3: calculate the proportion of time spent in prophase
Observing Mitosis
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Why are light microscopes used to observe mitosis?
They are most commonly used because they are relatively cheap, easy to use and they can magnify living cells or small organisms so their activity and behaviour can be studied
How does a light microscope work?
The light has to pass through the specimens to be seen - so they must be thin
A stain is often used to colour cell structures so that they can be seen more easily as most cells are transparent and don’t contain pigments
What is magnification?
It is making an image larger than the objects actual size
What does using a higher magnification allow?
Allows us to see in more detail because it increases the size of the image of small details
How is total magnification calculated?
Total magnification = eyepiece lens magnification x objection lens magnification
Why would you make an image more focused on a microscope?
To make the image become clear, sharp and easy to see
How do you focus an image on a microscope?
Using the coarse or fine focus knobs which move the slide closer of further away from the objective lens
What do you adjust at a low power magnification?
You would use the coarse focus
What do you adjust at a high power magnification?
You would need to use the fine focus so that we do not move the stage by too much
The Root Tip Squash Practical
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What is the equipment list for the root squash tip practical?
- Optical microscope
- Microscope slides and cover slips
- Water bath
- Hydrochloric acid
- Toluidine blue O stain
- Distilled water
- Scalpel
- Forceps
- 100 ml beaker
- Root tip
What is step 1 of the root squash tip practical?
Heat 1 mol dm HCI at 60°C in a water bath
What is step 2 of the root squash tip practical?
Cut a small sample of the root tip using a scalpel
What is step 3 of the root squash tip practical?
Transfer root tip to HCI and incubate for 5 minutes
What is step 4 of the root squash tip practical?
Remove from HCl and wash sample in cold distilled water and remove the very tip using a scalpel
What is step 5 of the root squash tip practical?
Place tip on a microscope slide and add a few drops of stain (e.g. toluidine blue O). This makes the chromosomes visible and will therefore show which cells are undergoing mitosis
What is step 6 of the root squash tip practical?
Lower the cover slip down carefully onto the slide. Make sure there are no air bubbles in the slide which may distort the image, and that the coverslip doesn’t slide sideways which could damage the chromosomes
What is step 7 of the root squash tip practical?
Place under a microscope and set the objective lens on the lowest magnification
What is step 8 of the root squash tip practical?
Use the coarse adjustment knob to move the lens down to just above the slide
What is step 9 of the root squash tip practical?
Use the fine adjustment knob to carefully re-adjust the focus until the image is clear (you can use a higher magnification if needed)
What is step 10 of the root squash tip practical?
To calculate mitotic index, cells undergoing mitosis must be counted (cells with chromosomes visible), as well as the total number of cells
Why is hydrochloric acid a hazard?
There is a risk of causing harm/irritation to eyes or in cuts
What are the safety precautions for using hydrochloric acid?
Wear eye protection; avoid contact with skin, tie up long hair
Why is toluidine blue O stain a hazard?
There is a risk of causing harm/irritation to eyes or in cuts
What are the safety precautions for using toluidine blue O stain?
Wear eye protection; avoid contact with skin
Why is using a scalpel a hazard?
There is a risk of there being cuts from sharp object
What are the safety precautions for using a scalpel?
Cut away from fingers; use forceps to hold sample whilst cutting, keep away from the edge of the desk
Why is possible broken glass a hazard?
There is a risk of there being cuts from sharp object
What are the safety precautions for having broken glass?
Take care when handling slides and coverslips; keep glassware away from edge of desk
What part of the root tip does mitosis occur in?
In plants, cell division by mitosis is restricted to specific tissues, called meristems, that are found at the growing root and shoot tips
The cells in the meristems are undifferentiated
Stem cells in plant meristems divide to produce cells that increase the height of the plant, length of the roots and girth of the stem
Why is toluene blue stain used?
It binds to chromatin DNA, staining it deep blue and making the chromosomes visible.
Staining the chromosomes to be clearly visible.
Why are the root tips suspended in water?
The root tips are suspended in a drop of distilled/pure water on the slide, the water can help to elongate the cells as it is absorbed by osmosis causing the cells to swell. This can separate the chromosomes and make them easier to see.
Why is the acid used?
It helps to break down the cell walls and make them more permeable to allow the stain to penetrate the cell and reach the chromosomes.
Why does the coverslip need to be pressed down?
The coverslip on the prepared slide is pressed gently to squash the cells and to produce a layer that is one cell thick this ensures light can travel through the slide.
Why do you have to press the coverslip down firmly and not slide it?
Pressing down firmly and not sliding the coverslip prevents one layer of cells from rolling on top of another obscuring the view of the chromosomes. It also prevents chromosomes from breaking or being damaged. The root tip squash is performed gently so that the cells are not ruptured and to prevent the microscope slide and coverslip from cracking
What will the results of the practical vary on?
The root tips used, different results may be seen if different types of plant were used and depending on the age of the root tip or time of day the roots were cut.
It could also depend on the conditions the plant was grown in (e.g temperature and water or nutrient availability)
How could you make the practical more reliable?
In order for results to be reliable and representative of the plant at the time a large number of fields of view should be observed and an average taken.
Repeat counting the cells in different areas of the cell to get a mean to get a more accurate result
How do you calculate the mitotic index?
Number of cells in the field of view undergoing cell division -divided by- total number of cells in the field of view -x100
What factors can affect the mitotic index?
Nutrients available
Amount of water
Type of plant root tip
Age of root
Temperature
Cancer
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What can the rate of cell division depend on?
Environment - nutrients and oxygen, hormones known as growth factors but also genes
What are risk factors for cancer?
People can inherit genes that make them more likely to suffer with some cancers
Lifestyle factors - e.g smoking, drinking lost of alcohol, diet and exercise
How do mutations in genes sometimes lead to cancer?
Mutation in genes that control cell division can cause cells to grow out of control
What are mutations?
Mutations are changes to the base sequence of DNA - can be caused by mutagens
How are cancer cells different other normal cells?
Structure and function, they will either die from apoptosis (kill itself) or they can be destroyed by the immune system which recognises the as ‘foreign’
What happens if the cancer cells pass the checkpoints?
If they pass the checkpoints despite having damaged DNA and do not get destroyed by the immune system they can continue to divide by mitosis
What does uncontrollable cell division cause?
Cancer or uncontrolled cell division causes cells to to keep dividing and then layer on top of each other forming a tumour
What can tumours do (damage wise)?
Can damage the organs they grow in, can cause blockages or they can cause damage to other organs
What do tumours need to survive and what do they suffer from?
Need their own blood supply
Often suffer from low oxygen as the blood cannot satisfy the tumour oxygen demand as cell division rate it too high
What are the two types of tumours?
Malignant and benign
What is a benign tumour?
The tumour grows slowly and is kept in one place
They do not invade other tissues and start tumours in other organs
These are not usually life threatening.
What is a malignant tumour?
A malignant tumour grows quickly, will invade other tissues and can break off and spread around the body causing tumours in other organs (metastasis)
These are more likely to be life threatening
What can cancer treatments control?
Cancer treatments can control the rate of cell division in cancer cells by targeting the cell cycle to try and stop it, this will kill the tumour cells and prevent them from dividing further.
What can cancer treatments not do?
However, the treatments are not able to distinguish between cancer cells and other normal body cells that divide a lot e.g. hair cells, stem cells in bone marrow and cells that line the small intestine
Cancer cells have a faster rate of division so these cells are more likely to be affected by cancer treatment
What are examples of cancer treatment?
Chemotherapy
Radiotherapy and chemotherapy drugs
Drugs preventing spindle fibres
How does chemotherapy work?
Prevents the synthesis of enzymes required for DNA replication. This stops the cell cycle before the S phase as it cannot take place. If DNA cannot be replicated cells will not be able to divide.
How does the combination of radiotherapy and chemotherapy drugs work?
Can damage DNA, if DNA is damaged the cell will not pass the check points in S phase. This will force the cell to kill itself (apoptosis). Cells will not divide/mitosis is prevented
How do drugs that prevent spindle fibres work?
Prevent mitosis (anaphase) as one cell will contain double the DNA and one cell will contain no DNA. Prevents cells from functioning or dividing again
Cancer and the cell cycle
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