Eukaryotic Cell Division- Mitosis Flashcards
What is mitosis?
The process by which a cell divides to produce two genetically identical daughter cells
What is asexual reproduction?
The production of genetically identical offspring from a single parent cell or organism
What is mitosis used for?
- growth, repair and replacement
* asexual reproduction
What is a chromosome made up of?
A mass of coiled threads of DNA and proteins
What is the difference in the chromosomes in a cell that is not actively dividing and one that is?
- in a cell that is not actively dividing the chromosomes are translucent to both light and electrons so we cannot see them easily or identify them as individual structures
- In a cell that is dividing the chromosomes condense - they become much shorter and denser. They then take up stains very readily and at this stage we can identify individual chromosomes
How are DNA molecules packaged when they condense?
- achieved with the help of positively charged basic proteins called histones
- the DNA winds around the histones to form dense clusters known as nucleosomes
- they interact to produce more coiling and then supercoiling to form the dense chromosome structures you see in the nulceus of a dividing cell
- in the supercoiled areas the genes are not available to be copied to make proteins
What is a karyotype?
A way of displaying an image of the chromosomes of a cell to show the pairs of autosomes and sex chromosomes
How many chromosomes are in the human cells and how are they presented?
There are 46 chromosomes which occur in matching pairs, one of each pair from each parent
Why must a cell duplicate the original set of chromosomes before it divides?
Because in mitosis the two cells that result from the division must both recieve a full set of chromosomes
What is the cell cycle?
A regulated process of three stages in which cells divide into two genetically identical daughter cells
What are the three phases of the cell cycle and what happens in each stage?
- interphase: a period of non-division where the cells increase in mass and size, carry out normal cellular activities and replicate their DNA ready for division
- Mitosis: a period of active division
- cytokinesis: the new cells seperate
Describe the phases of the cell cycle with interphase in detail
• interphase
- G1 (gap 1) is the time between the end of the previous round of mitotic cell division and the start of chromosome duplication. The cell assimates material, grows and develops. This is the time that is most variable. In actively dividing cells G1 is very short but in other cells it can last months
- S is the stage when the chromosomes replicate and become double stranded chromatids ready for the next cell division
- G2 is the time that the organelles and other materials needed for cell division are synthesised. Before a cell can divide it needs two of everything
• mitosis is when the cells are actively dividing
• cytokinesis is the final stage of cell division when the new cells seperate
How is the cell cycle controlled?
- by a number of chemical signals made in response to different genes
- this control is brought about at a number of checkpoints where the cell cycle moves from one phase to the next
- the control chemicals are small proteins called cyclins. These build up and attach to enzymes called cyclin- dependent kinases (CDKs)
- the cyclin/ CDK complex that is formed phosphorylates other proteins changing their shape and bringing about the next stage in the cell cycle. E.g. the phosphorylation of the chromatin in the nucleus results in the chromosomes becoming denser
What are the different phases in mitosis?
Prophase, metaphase, anaphase and telophase. PMAT
What happens in prophase?
- by the beginning of prophase both the originals and the copies of the chromosomes are referred to as chromatids
- in prophase the chromosomes coil up, can take stains and become visible
- at this point each chromosome consists of two daughter chromatids that are attached to eachother in a region known as the centromere
- the nucleolus breaks down and the centrioles begin to pull apart to form the spindle
What happens in Metaphase?
- the nuclear membrane has broken brown
- the centrioles have moved to opposite poles of the cell, forming a set of microtubules between them that is known as the spindle
- the chromatuds jostle about for a place on the metaphase plate/equator of the spindle and eventually line up along the plate, with each centromere associated with a microtubule of the spindle
What happens in anaphase?
- the centromeres that have linked the two identical chromatids split and from then on the chromatids act as completelt seperate entities
- the chromatids from each pair are drawn centromere first towards opposite poles of the cell
- at the end of anaphase the two sets of chromatids have been separated to opposite ends of the cell
- the chromatids can’t move on their own and rely on the microtubules of the spindle to allow them to move.
- the spindle is made up of overlapping microtubules containing contractile fibres and it is the contraction of the overlapping fibres that causes the movement of the chromatids
- this is an energy requiring process which uses the energy supplied from respiration
What happens in telophase?
- the spindle fibres break down and the nuclear envelopes form around the two sets of chromosomes
- the nucleoli and centrioles are also re-formed
- the chromosomes begin to unravel and become less dense and harder to see
What happens in cytokinesis?
- In animals a ring of contractile fibres tightens around the centre of the cell like a belt tightening around a sack of flour. These fibres are the same as those found in animal muscles
- in plant cells a cellulose cell wall builds up from the inside of the cell outwards.
- in both cases two identical daughter cells are formed
What are the advantages and disadvantages of asexual reproduction?
• Advantages:
- it does not rely on finding a mate
- it can give rise to large numbers of offspring very rapidly
• disadvantages:
- the offspring are all genetically identical to the parent organism. This can be a problem if the enviroment changes in any way because none of them would be able to survive
Producing spores is a strategy for asexual reproduction. What is this?
- Sporulation involves mitosis and the production of asexual spores that are capable of growing into new individuals.
- These spores can usually survive adverse conditions and are also easily spread over great distances
- This form of asexual reproduction is most common in fungi and plants such as mosses and ferns
Regeneration is a strategy for asexual reproduction. What is this?
- when organisms replace parts of the body that have been lost
- some lizards shed their tail when they are attacked and then grow another
What is fragmentation?
- when organisms reproduce themselves asexually from fragments of their original body
- starfish can regenrate after being chopped up
Producing buds is a strategy for asexual reproduction. What is this?
- in reproductive budding there is an outgrowth from the parent organism that produces a smaller but identical individual, produced purely by miotic cell division
- this bud eventually becomes detatched from the parent organism and lives independently
- yeast cells reproduce by budding
- it is rare in the animal kingdom but hydra reproduce by budding
What is vegatative propagation?
- a plant forms a structure that develops into a fully differentiated new plant, which is identical to the parent and eventually becomes independent.
- the new plant may be propagated from the stem, leaf, bud or root of the parent, depending on the thpe of plant.
- it only involves miotic cell division
- it often involves perennating organs that contain stored food from photosynthesis and can remain dormant in the soil to survive adverse conditions
- They are often also a means of surviving from one season to the next
- examples incule bulbs, corms, runners, suckers, rhizomes, stem tubes and root tubers
How do human gardeners exploit vegetative propagation?
- splitting daffodil bulbs, removing new strawberry plants from their runners and cutting up rhizomes are all easy ways of increasing plant numbers cheaply
- an added advantage is that the new plants are all clones so will have exactly the same characteristics of their parents, so will be just as good
What is taking cuttings?
They induce fragmentation artificially. This involves taking a small piece of a plant - often parts of the shoot ane planting it to grow on and develop by mitosis into another entire identical plant
Why is asexual reproduction common in plants?
Because they maintain areas of unspecialised dividing cells throughout their life
What is growth in a biological sense?
A permanent increase in the number of cells or in the mass or size of an organism
What are the three distinct aspects of growth?
Cell division, assimilation and cell expansion
What is the baisis of growth and why?
Mitosis because once cells have divided they usually get larger before dividing again
What is assimilation?
Getting the resources needed to produce new cell material from photosynthesis in plant cells, from feeding in animal cells and from nutrient absorption in fungi
What is cell expansion?
When the materials from assimilation are incorporated into the cells
What may growth be affected by?
- availability of food, temperature and light intensity
* genetic makeup of the organism
Why it it hard to measure growth?
- linear dimensions such as height can be deceptive
- measuring mass is problematic as the water contents of cells may vary and more complex animals will have varying qualities of faecal material and urine held in their body
What is the most accurate way of measuring growth?
Measuring dry mass
What is dry mass?
The mass of the body of an organism with all the water removed from it
What must you do to get useful results from dry mass measurements?
You need to grow large samples of genetically identical organisms under similar conditions, then take random samples and dry them to a constant dry mass. This method is very useful for plants, fungi and bacteria
What are the drawbacks of using dry mass?
- if you remove all the water from an organism you kill it so further growth cannot be measured
- it is not easy or ethical to maintain large colonies of genetically identical vertebrates and then kill and dry them
What does a growth curve show?
Growth throughout the life of an organism?
What does the growth curve show in many animals?
After an initial relatively slow start there is a rapid period of growth until maturity is reached when growth slows down and may stop
In land animals why does the growth curve stop completely with maturity?
Because size is limited by the weight of the animal and the ability of it’s muscles to move it against gravity
What does the growth curve look like in plants and marine animals?
Growth continues throughout life. In marine animals the mass of the body is supported by the water
How do insects grow?
- in a series of moults
- they shed one exoskeleton and then while the new exoskeleton is soft they expand the body by taking in air or water and grow
- once the new skeleton has hardened the air or water can be released and there is room for the tissues of the insect to increase in size and mass
- this is known as discontinuous growth
- if length is measured the insect seems to grow in a series of steps
When does the largest amount of growth occur in a human and why?
- in the development of the embryo
- different parts of the organism can grow at different rates. In the human embryo the nervous system and the head grow much faster than some other areas.
- right ar the beginning of life, mitosis takes place at a very rapid rate
Where does growth take place in plants?
- in regions known as the meristems
- after the cells have divided they absorb water into their vacuoles and elongate rapidly before the cellulose cell wall becomes more rigid
- these areas of plant growth are particularly sensitive to a variety of stimuli such as light and gravity
What is the meristem?
The region of mitosis and growth just behind the tip of a plant shoot or root where the cells continue to divide actively throughout the life of the plant
Where does mitosis continually take place often and why?
- the surface of our skin (replaced every 28 days)
* in the red bone marrow of the flat bones such as your ribs because the red blood cell only lasts 28 days
Where does mitosis take place rapdily and why?
- the surface of our skin (replaced every 28 days)
* in the red bone marrow of the flat bones such as your ribs because the red blood cell only lasts 28 days
What happens in old age?
Mitosis occurs less frequently and the cells dying begin to outnumber the new cells being formed. When this process reaches a certain point, death of the whole organism will occur
What happens if the skin is damaged?
- rapid mitosis is triggered in the cells around the world
* they regenerate the lost skin tissue and heal the wound
How do plants repair damage to their bark?
They produce scar tissue to seal damage to their bark
