2.1.6 Cell division, cell diversity and cellular organisation Flashcards
what is the cell cycle?
The cell cycle is the repeating sequence of events whereby a cell’s genetic information is replicated (during a stage called interphase) and then the cell undergoes division, via mitosis followed by cytokinesis, to produce two genetically identical daughter cells. Each of these cells then itself goes through the cell cycle.
what is the longest pahse in the cell cycle
interphase
what is included in interphase?
G1, S and G2
Can the cell cycle ever go in reverse?
cell cycle cannot ever go into reverse, e.g if damaged DNA is detected
at the G2 checkpoint, the cell cycle is halted and the cell tries to repair the damage. The cell
cannot return to an earlier stage, if it cannot be repaired, the cell will enter the G0
What is G0, why does it enter this phase?
The name given to the stage in the cell cycle, where the cell leave the cycle temporarily or permenatnly, and stops divinding
Cell differentiation
Failed G1 checkpoint: DNA may be damaged, or cell’s nutrients/size were not sufficient. The cell is not allowed to go into S phase
What occurs at the G1 checkpoint? 4
checks for: Adequate cell volume/size nutrients Absence of DNA damage Growth factors
What occurs at the S checkpoint? 2
checks for:
Absence of DNA damage
All DNA has been replicated
What occurs at the G2 checkpoint? 4
checks for: Adequate cell volume/size nutrients Absence of DNA damage Growth factors
What is interphase?
The first (and by far the longest) stage in the cell cycle is called interphase. Interphase is made up of three phases: G1, S and G2.
is interphase a stage of mitosis?
NO! It is the stage which prepares a cell so it can then divide by mitosis.
What occurs in S phase of interphase? 3
- Chromosomes are replicated via semi‐conservative DNA replication: following this process, each chromosome will now have two chromatids (of identical DNA base sequence) that are held together by a centromere.
- Aerobic repsiration to produce ATP
- Checking of the newly‐replicated DNA, i.e. proofreading: some mutations can be
corrected at this stage, e.g. if a base is identified as being wrongly paired.
Draw a chromose before and after semi conservative replication.
look at notes :)
What occurs in G1 phase of interphase? 6
- Production of more organelles, e.g ribosomes and mitochodnria
- Replication of the centrioles (Animal cells only, made of microtubules), they are replicated during interphase and then used later in the cell cycle to organise the spindle fibres during mitosis (or meiosis).
- Protein synthesis via transcription and translation
- Aerobic repsiration to produce ATP
- Increase in cell volume due to increased surface area of plasma membrane (cell surface
membrane) and increased volume of cytoplasm, more phospholipids are inserted into the plasma membrane, enabling an increase in cell volume to occur as the membrane surface area expands; the increase in cell volume is necessary so that when the cell eventually
divides, the two daughter cells will receive enough cytoplasm and organelles. - Checking of the newly‐replicated DNA, i.e. proofreading: some mutations can be
corrected at this stage, e.g. if a base is identified as being wrongly paired.
What occurs in G2 phase of interphase? 6
- Production of more organelles, e.g ribosomes and mitochodnria
- Replication of the centrioles (Animal cells only, made of microtubules), they are replicated during interphase and then used later in the cell cycle to organise the spindle fibres during mitosis (or meiosis).
- Protein synthesis via transcription and translation
- Aerobic repsiration to produce ATP
- Increase in cell volume due to increased surface area of plasma membrane (cell surface
membrane) and increased volume of cytoplasm, more phospholipids are inserted into the plasma membrane, enabling an increase in cell volume to occur as the membrane surface area expands; the increase in cell volume is necessary so that when the cell eventually
divides, the two daughter cells will receive enough cytoplasm and organelles. - Checking of the newly‐replicated DNA, i.e. proofreading: some mutations can be
corrected at this stage, e.g. if a base is identified as being wrongly paired.
what is the definition of mitosis?
nuclear division stage in the mitotic phase of the cell cycle, producing two genetically identical daughter cells
When does mitosis occur?
Once interphase is complete
what are the 4 stages of mitosis, and how to remember order
Prophase, Metapahse, Anaphase, Telophase
PMAT
How can we easily view the stage of mitosis? What stain do we use? How do we prepare it?
A stained onion root tip squash, viewed by light microscopy, is often used to study mitosis
because the root tip contains meristem tissue, a site of active cell division. All the stages of the mitotic cell cycle are therefore likely to be visible (though most cells will appear to be in interphase since it is the longest phase).
Methylene blue
Preparing the root tip as a squash means that the cells are spread into a single layer, giving an
image without overlapping cells, straightforward to interpret and easily allowing light to pass
through.
what is the name of the first stage of mitosis and what occurs? 3 parts
Prophase
Chromosomes condense as DNA becomes more tightly coiled; the chromosomes become shorter and fatter and hence become visible as distinct structures. (Nucleolus dissapears)
Centrioles move to opposite poles of the cell (in animal cells only); their role is to organise the spindle fibres.
Spindle fibres (made of microtubules) begin to form; these are considered to be a component of the cytoskeleton.
what is the name of the second stage of mitosis and what occurs?
Metaphase
The nuclear envelope breaks down, such thatthe chromosomes are released into the cytoplasm.
The spindle fibres are completed; each spindle fibre attaches to one individual chromosome at its centromere.
Each spindle fibre then pulls its attached chromosome to the equator of the cell; this results in the chromosomes lining up on the equator in a single row (in a random order, which has no significance).
what is the name of the third stage of mitosis and what occurs?
Anaphase
The centromeres divide, releasing the twochromatids of each chromosome so that they are now separate from each other; each of these will now become a chromosome in its own right.
Spindles fibres shorten and so pull each of the two chromatids to opposite poles of the cell.
what is the name of the fourth and final stage of mitosis and what occurs?
Telophase
IMPORTANT: A full (diploid) set of chromatids (now called chromosomes again) has reached each pole ofthe cell.
New nuclear envelope forms around the chromosomes at each pole, giving the cell has two identical nuclei (though this is usually
temporary, as the cell will go on to divide by cytokinesis).
Chromosomes decondense (disappearing as individual structures) and a new nucleolus forms in each nucleus.
What is the step after Mitosis?
Cytokinesis
What is cytokinesis?
It is the division of the cell itself, into two daughter
cells, each of which receives one nucleus plus a share of cytoplasm containing the other
organelles.
What are the steps of cytokinesis in an animal cell?
- In a plane corresponding to the cell equator, microtubules pull the plasma membrane
inwards until it fuses with itself in the centre of the cell. - This causes the cell to be divided into two daughter cells (usually of similar sizes), each with one nucleus.
- The cytoplasm (including organelles, e.g. ribosomes and mitochondria) has been shared out and divided to form the two daughter cells.
Why are the steps in cytokinesis in plant cells different to animal cells?
Because a plant cell must for a new cell wall
What are the steps of cytokinesis in an plant cell?
- Along the plane of the cell equator, vesicles containing cellulose line up (moved into
position using microtubules as tracks). - These vesicles start merging together; the structure that forms is called the cell plate,
and it eventually divides the cell in two, by forming a complete new cellulose cell wall
to separate the two daughter cells. - However, narrow threads of cytoplasm usually remain in connecting pores that link the two cells; these are called plasmodesmata.
Are you able to recognise photos of each stage of mitosis?
look at photos :)
What is significant about mitosis?
generates genetically identical daughter cells
chromosome number is maintained (original cell is diploid i.e. has two sets of chromosomes then both daughter cells formed will also be diploid.)
eukaryotic cells
how do prokaryotes divide?
binary fission
What are 3 general roles of mitosis?
- Growth: mitosis increases cell numbers and hence causes growth (the increase in size
of the organism’s body) in multicellular eukaryotes.
(Note that the term growth refers here to the increase in number of cells NOT to an
increase in the size of each cell.) - Tissue repair and cell replacement: mitosis produces more (genetically identical) cells,
which can be used to repair a tissue (e.g. if injury has caused damage) or replace worn
out cells (e.g. stem cells in bone marrow must constantly divide by mitosis to produce
new red blood cells, as this cell type only remains functional for around 100 days). - Asexual reproduction: in unicellular eukaryotes, and some multicellular eukaryotes, cell
division by mitosis generates new individuals who are clones of a single parent
organism, with genetically identical cells.
Asexual reproduction does not involve meiosis or the fusion (fertilisation) of gametes;
only one parent is required; there will be no genetic variation amongst the offspring
(except for rare, random mutations).
What are 4 specific roles of mitosis?
- Development of the body plan: mitosis produces new cells to contribute to the
formation of a specific body plan during the development of an organism (though
selected cells are then removed by apoptosis (programmed cell death). - Clonal expansion of B and T lymphocytes: during a specific immune response, selected
B and T lymphocytes are stimulated to repeatedly divide by mitosis in order to increase
the number of cells able to specifically attack the pathogen that has been detected. - Producing gametes from haploid cells: in a minority of species (including male ants and
honeybees), the adult body cells are haploid (i.e. contain only one set of
chromosomes); this means that haploid gametes can be produced by mitosis in this case
(to maintain the same chromosome number, rather than using meiosis to halve it). - Renewal of stem cells: stem cells are unspecialised, however when a stem cell divides,
usually one daughter cell differentiates and the other divides again, maintaining a
population of stem cells at the appropriate level.
Specialised
What is a sepecialised cell?
Specialised cells develop structures and produce (only) the proteins relevant to that cell’s specific role.
How do sepecialised cells arise?
differentiation
is differentiation reversable?
Generally no
What are the 5 specialised animals cells you must know?
- Erythrocytes
- Neutrophils
- Squamous epithilial cells
- Ciliated epithelial cells
- Sperm cell
What are the 3 specialised plant cells you must know?
- Palisade mesophyll cell
- Root hair cell
- Guard cells
What is an Erythrocytes, what is its functions and how is it specialised?
What:
Red blood cells
Function:
Transport oxygen from the lung alveoli to body cells, for use in respiration
Specialisations:
No organelles, e.g no nucleus, no ribosome and no mitochonsria (Organelles present in immature erythrocytes, but are broken down once enough haemoglobin is produced, it is then mature. This is good as there is more space for heamoglobin so more oxygen can be carried
Biconcave disk shape, increases SA and therfore rate of diffusion increases, plus allows the cell to be flexible
Small, 7 um, therefore is able to fit through narrow blood capillaries, also creates a large SA:V ratio, increasing the rate of diffusion
What is a Neutrophil, what is its functions and how is it specialised?
What:
Type of white blood cell, and hence is part of the immune system.
Function:
Its specific role is to engulf pathogens by phagocytosis
Specialisations:
Multi-lobed nucleus, allos for more flexibility during phagocytosis, and can squeeze through small gaps
plasma membrane contains many receptors (which antibodies on the antigens of the pathogens) can fit into, triggering phagocytosis
Granular cytoplasm, contains many lysososmes, to break down pathogens , mitochondria to provide energy, and RER + Golgi apparatus to produce hydrolytic enzymes and packaging them into lysosomes
What is a Squamous epithilial cell, what is its functions and how is it specialised?
What:
Squamous epithelial cells are flattened, smooth cells.
Function:
Forms the protective lining of the inner cheek, and gives a short diffusion distance to gases by forming the thin walls of the alveoli in the lungs.
Specialisations:
Smooth surface, (in blood vesssels, it reduces friction to blood cells)
Flat and thin, (in the alveoli walls it allows for a short diffusion diustance for gases)
Basement membrane, made of collagen, holds the cells in a single layer without any gaps between them creating a continuous surface of cells
What is a Ciliated epithelial cell, what is its functions and how is it specialised? (Plus goblet cells)
What:
Ciliated epithelial cells are found in the lining of the trachea, bronchi and bronchioles
Function:
The hair‐like projections called cilia carry out beating or wafting movements in order to propel
mucus (which contains trapped dust, pollen and pathogens).
Specialiasations of Cilitaed epithelial cells:
Cilia, hair like projection which make a wafting movement to propel mucus
many mitochondra, to supply ATP for the movement of the cilia
Basement membrane, made of collagen, holds the cells in a single layer without any gaps between them creating a continuous surface of cells
Mucus, provided by the goblet cells, it sticky and traps bacteria, dust, pollen etc
Specialiasations of goblet cells:
Mucus, provided by the goblet cells, it sticky and traps bacteria, dust, pollen etc
many mitochondra, to supply ATP for the movement of the production of glycoproteins/mucus
Many RER (with ribosomes) and golgi apparatus for the production of glycoproteins (to make mucus)
What is a Sperm Cell, what is its functions and how is it specialised?
What:
Male gamete
Function:
To fertilise the female gamete (egg) in order to
produce a zygote (Sexual reproduction)
Specialisations:
Acrosome, contains lysosomes with hydrolytic enzymes to break the outer coating of the egg
Many mitochndria to provide energy for the movement of the flagellum so the sperm can move
Haploid nucleus, contains one set of chromosome, which can then fuse with an egg to create a diploid cell.
Flagellum, for propulsion towards an egg (cell movement), contains a 9+2 arrangement of microtubules to enable a whip-like movement
What is a Palisade mesophyll Cell, what is its functions and how is it specialised?
What:
A cell present in the leaf of a plant
Function:
photosynthesis
Specialisations:
Thin and fully permeable cellulose cell wall - provides a short diffusion distance for CO2 and give the cell its shape
Large sap vacuole, pushes the chloroplast to the outer edges of the cell where more light can be absorbed
Many chlorplasts for light absobtion enabling a high rate of phtosynthesis
What is a Root hair Cell, what is its functions and how is it specialised?
What:
cells are found in the outer epidermis of roots;
Function:
Adapted for uptake of water (by osmosis) and mineral ions (mainly by active transport) from the surrounding soil
Specialisation:
Long root hair, long thin extension of the cytoplasm, plasma membrane and cell wall; provides a huge surface area for uptake of water by osmosis and mineral ions by active transport
thin, fully permeable cellulose cell wall: short diffusion distance; NOT lignified (as lignin would cause impermeability to water)
plasma membrane, containing many carrier proteins for active transport of mineral ions; also present are aquaporins (channel proteins allowing increased water uptake by osmosis)
Sap vacuole, can store mineral ions such that water
potential in the cell is lower than in the soil (hence a steep water potential gradient is maintained and water will enter by osmosis).
What is a Guard Cell, what is its functions and how is it specialised? How do they open and close?
What:
The cells found in pairs in the lower (and/or upper) epidermis of leaves
Fucntion:
they are adapted control stomatal opening, and hence control the uptake of carbon dioxide from the atmosphere into the leaf (for photosynthesis) and the excretion of waste oxygen from the leaf. The stomata are also significant for being the principal route through which water vapour is lost from the leaf during transpiration.
Specialisation:
plasma membrane contains, carrier proteins for active transport of potassium ions to control the water potential gradient.
cytoplasm contains many mitochondria providing ATP for K+ ion active transport and also contains chloroplasts (to enable detection of light intensity)
stoma
extra thick cellulose in the part of the cell wall adjacent to the stoma: results in the guard cells bowing/bending
outwards when they gain turgor due to water entry.
Open: When the guard cells are turgid
Closed: When the guard cells are flaccid.
What is a tissue
a group of specialised cells working together for a specific function
what is the correct level of organisation?
Organelles → Cells → Tissues → Organs → Organ systems → Organism
What is a cartilage, what is its functions and how is it specialised?
What:
Cartilage is a tough but flexible tissue
Function:
Structural support to the nose and outer ear
Some fish have an entire skeleton made of cartilage
A coating of cartilage on the ends of most bones, where its smooth surface reduces friction, enabling smoother movements, so the bonds dont wear away
Shock absorber
Specialisation:
contains cells called chondrocytes These cells produce and secrete structural, fibrous proteins including collagen and elastin. These proteins form an extracellular matrix, into which the chondrocytes are embedded.
What are some types of muscular tissue, what are their functions and how are they specialised?
- Skeletal muscle - attached to the bones, contraction of the muscles pulls on the bones, causing them to move. Described as Voluntary, meanining it can be consciously controlled, and straiated, meaning it has a striped appearance
- Muscle fibres - individual cells in muscular tissue, long and cylindrical in shape, multinucleate (i.e. each fibre contains many nuclei) and the cytoplasm (known as sarcoplasm) contains multiple cylindrical structures called myofibrils. myofibrils are made up of protein filaments (specifically, thin actin filaments and thick
myosin filaments); the regular, repeating pattern of overlap between these two types of
filaments gives the skeletal muscle fibres their striated appearance
What is the xylem, and what is its functions?
The xylem is a vascular tissue that is adapted to transport water and dissolved minerals from
roots to leaves
What is the phloem, and what is its functions?
The phloem is a vascular tissue that is adapted to translocate sucrose (and other assimilates)
from a source to a sink.
What is a stem cell?
Stem cells are undifferentiated cells, which are capable of repeated division by mitosis and which have the potential to differentiate into many (or any) cell type.
They can be described as a renewing source of undifferentiated cells
how can stem cells be described?
They can be described as a renewing source of undifferentiated cells
What are the three types of stem cells?
- Totipotent
- Pluripotent
- Multipotent
What are totipotent stem cells?
Totipotent stem cells are those which could differentiate into any type of cell. In animals, only the zygote (fertilised egg) and the cells in the very early embryo
(embryonic stem cells) are totipotent; there are no totipotent stem cells in an adult’s body. These cells can go onto become a whole organism (with all 200 different cell types present) plus they can also produce the tissues that make up the umbilical cord, placenta and amniotic sac (i.e. the extra‐embryonic tissues).
What are pluripotent stem cells?
Pluripotent stem cells can form any of the tissue types in the organism’s body but not any extra‐embryonic tissues; whole organisms cannot be produced from pluripotent cells. In an animal, only cells in the early embryo are pluripotent; there are no pluripotent stem cells in an adult’s body.
What are multipotent stem cells?
Multipotent stem cells can only form a limited range of different tissue types, for example, bone marrow stem cells can only differentiate to become the various type of blood cell. Stem cells with such properties may be referred to as tissue stem cells. Since, multipotent stem cells are the only type of stem cell in an adult’s body, they may also be referred to as adult stem cells.
How do stem cells differentate?
When a cell becomes specialised, i.e. when it differentiates, there are changes in gene expression, but no change in which genes are present.
what is a change in gene expression?
A change in gene expression means that different genes are switched on (meaning they are active and are being transcribed and translated) and switched off (meaning they are inactive and will not be transcribed and translated). These changes occur because, as a cell specialises in terms of its structure and function, it only needs to make the structural protein, enzymes etc. that are appropriate for the cell type it is becoming
why is differentation irreversible?
when a cell differentiates, some genes become inactive permanently. Moreover, the cell may have become a specific shape, with the cytoskeleton fully committed to maintaining this and unable to take part in spindle fibre assembly or cytokinesis. These reasons mean that differentiation is irreversible.
Where do stem cells go before they differentate?
The cell leaves the cell cycle, as it no longer needs to go through interphase as it is not preparing for division; instead, the cell may enter a state called G0.
Animal example of a stem cell
all blood cell types including erythrocytes (red blood cells) and neutrophils (a type of white blood cell) are derived from multipotent (adult) stem cells in the bone marrow, which continuously divide by mitosis.
Plant examples of stem cells
pluripotent cells in meristem tissues divide by mitosis and then differentiate: this can lead to the production of more xylem vessel elements (to form xylem vessels in xylem tissue) and more phloem sieve tube elements (to form sieve tubes in phloem tissue). Meristem tissue that can form more xylem and phloem is found in the cambium tissue in stems and in the pericycle tissue in roots. There is further meristem tissue in shoot tips and root tips (apical meristems) and in flower or leaf buds; all meristem cells in a plant are pluripotent, meaning that they can differentiate into any cell type needed by the growing plant.
Uses of stem cells in medicine and research
Repair of damaged tissues in the human body
Treatment of neurological conditions such as Alzheimer’s and Parkinson’s
Drug testing
Research into developmental biology
What is the definition of meiosis?
A form of cell division where the nucleus divides twice (meiosis I and meiosis II) resulting in a halving of the chromsome number and producing four genetically different haploid daughter cells from one diploid cell
How can meiosis be described?
reductive division
What cells are produced by meiosis
gametes (i.e. sex cells) for use in sexual
reproduction
what is the main role of meiosis
production of gametes for sexual
reproduction
where in animals, is meiosis only possible?
in the ovaries or testies, hence gametes can only be produced here.
What does haploid mean? what represents it?
HAPLOID means that there is only one set of chromosomes present in a nucleus/cell. Haploid cells have no homologous pairs, since there is only one copy of each type of chromosome present. The gametes produced by meiosis are haploid.
Represented by n = 23
what does n = 23 represent?
The haploid number of chromosomes is given the symbol ‘n’. For humans, n = 23. This means
that a single set of chromosomes (as is found in a gamete) is made up of 23 different
chromosomes. Other species have different haploid numbers.
What does diploid mean? what represents it?
DIPLOID means that there are two sets of chromosomes present in a nucleus/cell. Diploid cells
contain homologous pairs of chromosomes (one set inherited from each gamete/parent), i.e.
there are two copies of each type of chromosome.
Represented by 2n = 46
what does 2n = 46 represent?
The diploid number of chromosomes is denoted as ‘2n’. For humans, 2n = 46. This means that
in diploid body cells, there are two sets of 23 chromosomes present, or 23 homologous pairs,
making 46 chromosomes in total.
what is a homologous chromosome?
HOMOLOGOUS CHROMOSOMES are chromosomes of the same size, structure and centromere position; they contain the same genes at the same specific positions (though may have different versions of those genes, i.e. different alleles); two chromosomes that are homologous can pair up to form a bivalent during prophase 1 of meiosis; one chromosome in the pair originates from each parent (i.e. there is a maternal copy of the chromosome plus a paternal copy).
Draw a homologous pair of chromosomes
Check notes
What is the significance of meiosis in the life cycle?
Meiosis produces genetically varied, haploid cells, for use in sexual reproduction.
In a species that reproduces sexually, the body cells of an individual will be diploid. If an offspring is to also have the diploid number of chromosomes, the gametes produced by their parents must both be haploid. This would mean that when two gametes fuse (when one sperm fertilises one egg), two haploid sets of chromosomes are combined (one set from each gamete/parent); the zygote (fertilised egg) will therefore have two sets of chromosomes and
so be diploid.
In this way, the chromosome number is maintained throughout the generations. Meiosis, which halves chromosome number, is effectively compensating for (or balancing out) fertilisation, in which two gametes fuse. Without meiosis, the chromosome number would tend to double in each generation.
Once a diploid zygote has been produced by fertilisation, that zygote divides repeatedly by mitosis to form an embryo, which continues to grow by mitosis to form a foetus, baby, child, and eventually adult.
Unlike meiosis, mitosis maintains the same chromosome number, so if the zygote was diploid, the body cells produced by mitosis in the new individual will also be diploid.
What happens before meiosis?
Interphase
Interphase is NOT part of meiosis, but a cell must go through interphase of the cell cycle in order
to prepare for meiosis.
The processes taking place are the same as those previously noted for interphase in the mitotic cell cycle:
increase in cell volume
production of more organelles
Replication of centrioles (in animal cells only)#
Respiration to produce ATP
Protein synthesis (transcription and translation)
semi‐conservative DNA replication (producing chromosomes that now have two chromatids)
Checking of the DNA for errors (proof‐reading).
What are the names of the stages in Meiosis?
Meiosis I: Prophase I, Metaphase I, Anaphase I and Telophase I
Meiosis II: Prophase II, Metaphase II, Anaphase II and Telophase II
What occurs in Prophase I.
Chromosomes condense as DNA becomes more
tightly coiled; the chromosomes become shorter
and fatter and hence become visible as distinct
structures. (Nucleolus dissapears)
Centrioles move to opposite poles of the cell (in
animal cells only); their role is to organise the
spindle fibres.
Spindle fibres (made of microtubules) begin to
form; these are considered to be a component of
the cytoskeleton.
A special thing occurs here which is unique to meiosis chromosomes pair up to form a structure known as a bivalent; the process of bivalent formation is called synapsis. The points of attachment between the non‐sister chromatids within the bivalent are call chiasmata (singular: chiasma).
The consequence of bivalent formation is called crossing over: sections of non‐sister chromatid are exchanged at the chiasmata; this results in the swapping of alleles between the chromatids, leading to the formation of recombinant chromatids.
How does a bivalent form? What is the process called
Chromosomes pair up, the processs is called synapsis
what is the point of attachement between the non-sister chromatids called within the bivalent?
Chiasmata
What is the consequence of the bivalent formation?
The consequence of bivalent formation is called crossing over: sections of non‐sister chromatid are exchanged at the chiasmata; this results in the swapping of alleles between the chromatids, leading to the formation of recombinant chromatids.
what happens when a recombinant chromatid forms?
This generates new allele combinations (i.e. recombination of alleles has occurred), hence
crossing over is a mechanism for the generation of genetic variation.
What is crossing over a mechansism of?
Genetic variation
What occurs in Metaphase I.
The nuclear envelope breaks down, releasing the bivalents into the cytoplasm. Spindle fibre construction is completed, such that spindle fibres now reach from pole‐to‐pole. Each chromosome (in each bivalent) is attached to a spindle fibre by its centromere. The bivalents are then pulled, by the spindle fibres, into a row across the equator of the cell.
The two chromosomes in each homologous pair (bivalent) can line up on the equator either way round, i.e. the maternal and paternal copies of the chromosome could face either pole, with equal probability. This is true for every bivalent on the equator: the two chromosomes in the pair could face either pole.
Hence there are many different possible arrangements of chromosomes at this stage, which will ultimately result in different combinations of maternal and paternal chromosomes in the gametes formed by meiosis. This is called Independant assortment
What is independant assortment
The two chromosomes in each homologous pair (bivalent) can line up on the equator either way round, i.e. the maternal and paternal copies of the chromosome could face either pole, with equal probability. This is true for every bivalent on the equator: the two chromosomes in the pair could face either pole.
Hence there are many different possible arrangements of chromosomes at this stage, which will ultimately result in different combinations of maternal and paternal chromosomes in the gametes formed by meiosis. This is called Independant assortment
How to work out the number of chromosome combinations
2^n
where n is the haploid number of the organism
What occurs in Anaphase I.
At anaphase 1, the chiasmata (which were holding the two chromosomes in each bivalent together) are broken. The two homologous chromosomes (which typically now contain at least one recombinant chromatid each) are now separated from one another. Each of them is now pulled towards one or other pole of the cell, by the contracting spindle fibres.
This step is significant as it will have the consequence that the daughter cells produced by meiosis will only contain one chromosome from each homologous pair: they will be haploid.
Notice that at this stage, centromeres have NOT (yet) divided, and each chromosome therefore
still contains two sister chromatids.
What occurs in Telophase I.
A haploid (single) set of chromosomes reaches each pole of the cell; each chromosome is still comprised of two chromatids. In animal cells, new nuclear envelope forms around each cell of chromosomes; the DNA decondenses (unravels) to some extent. In plant cells, nuclear envelope may not form in this stage (i.e. there may not be a true telophase 1 stage).
A cytokinesis step then occurs, such that the original cell is divided into two. Each of the two cells contains a haploid set of chromosomes, but each chromosome still has two chromatids. These two cells both now progress to meiosis 2 (the second meiotic division).
What occurs in Prophase II.
The DNA condenses fully once again. Centrioles move to opposite poles and spindle fibres begin to form. This step closely resembles prophase of mitosis: unlike in prophase 1 of meiosis, in prophase 2 there is NO pairing up of homologous chromosomes to form bivalents, since there are no longer any homologous pairs in the cells (which are already haploid). No genetic variation is generated in this stage, since no crossing over can take place.
What occurs in Metaphase II
The nuclear envelope breaks down (if applicable). Spindle fibre construction is completed, such that spindle fibres reach from pole‐to‐pole. A spindle fibre attaches to each individual chromosome at its centromere, and pulls it to the equator of the cell. The chromosomes thereby line up on the equator in a single row.
This step closely resembles metaphase of mitosis, however in metaphase 2 of meiosis there are fewer chromosomes on the equator than is the case for mitosis (because the cell now only has the haploid number of chromosomes).
However, there is a phenomenon occurring in metaphase 2 that is unique to meiosis: the
independent assortment of chromatids. This is the idea that the two sister chromatids within each chromosome can face either pole when the chromosomes line up on the equator, with equal probabilities. This is significant because the two chromatids may now contain different allele combinations, due to the crossing over process that occurred during prophase 1. This independent assortment of chromatids generates further genetic variation.
What occurs in Anaphase II
At this stage, the centromere of each chromosome finally divides, releasing the two sister chromatids from each other. Each chromatid is pulled to one pole of the cell or the other, by the contracting spindle fibres.
This step closely resembles anaphase of mitosis, though there will only be the haploid chromosome number present in anaphase 2, i.e. fewer chromosomes present compared to mitosis. Once the chromatids have separated from each other, each may now be referred to a chromosome in its own right.
What happens in Telophase II
New nuclear envelopes reform around each haploid set of chromosomes. The DNA starts todecondense (unravel).
A cytokinesis step then follows, dividing each of the two cells into two. This is the second cytokinesis (remembering that a cytokinesis also occurred after telophase 1); hence the original cell, having been through cytokinesis twice has produced four daughter cells. Each is haploid, and genetically unique. (Some of) these genetically varied, haploid cells may subsequently undergo differentiation in order to form gametes.
What are the causes of variation in meiosis and what stages do they occur?
- Crossing over of non‐sister chromatids in PROPHASE I - alleles are exchanged between non‐sister chromatids within a bivalent, as a consequence of chiasma formation; this generates recombinant chromatids and hence new allele combinations.
- Independent assortment of chromosomes in METAPHASE I - the two homologous chromosomes within each bivalent can line up either way round (facing either pole) on the equator; this leads to many different possible combinations of chromosomes and
hence of alleles in the gametes produced by meiosis. - Independent assortment of chromatids in METAPHASE II - the two chromatids within each chromosome can face either pole when the chromosomes line up on the equator; this leads to many different possible allele combinations in the gametes produced.
- Chromosome mutations: random errors during meiosis can lead to changes in the number or structure of chromosomes in the gametes produced.
What are the further causes of genetic variation in fertilisation of an egg by a sperm cell?
- Random mating: in many populations, any male can mate with any female, with equal probability.
- Random fusion of gametes: any egg could be released and any sperm could fertilise this egg.
3 .Gametes all genetically different: every gamete ever produced, from any individual, is likely to be genetically unique (due to the huge amount of genetic variation generated by meiosis).
- The male and female gametes come from genetically different parents: especially in the case of out‐breeding (i.e. parents are not closely related), each parent contains different alleles, hence many different, new allele combinations are formed in the offspring.
Why is it thought that meiosis and sexual reproduction are advantageous in changing environmental conditions?
Because a genetically varied population can evolve by natural selection and thus adapt to change.
Why is it thought that meiosis and asexual reproduction are advantageous in stable environmental conditions?
This strategy requires less energy expenditure: no gametes have to be produced, no mate has to be found or attracted; hence it is possible for a greater number of offspring (who are all clones of their one parent) to be formed in a shorter time period.
Compare the features of both mitosis and meiosis
Look on sheet
mitosis is what to what?
diploid to diploid
meiosis is what to what?
diploid to haploid
