2.3 (cell structure) Flashcards
within multicellular organisms, not all cells retain the ability to
divide
eukaryotic cells that do retain the ability to divide show a
cell cycle
mitosis is part of a precisely controlled process known as the
cell cycle
the cell cycle is the
- regulated sequence of events
- that occurs between one cell division and the next
the cell cycle has three phases
- interphase
- nuclear division (mitosis)
- cell division (cytokinesis)
nuclear division is also known as
- mitosis
- mitosis is a process of nuclear division in eukaryotic cells that occurs when a parent cell divides to produce two identical daughter cells, during cell division, mitosis refers specifically to the separation of the duplicated genetic material carried in the nucleus
the physical process of cell division is
- cytokinesis
- which divides the cytoplasm of a parental cell into two daughter cells
the length of the cell cycle is very variable depending on
- environmental conditions
- the cell type
- and the organism
- for example, onion root tip cells divide once every 20 hours (roughly) but human intestine epithelial cells divide once every 10 hours (roughly)
the movement from one phase to another is triggered by chemical signals called
cyclins
the cell cycle diagram
- S: synthesis (of DNA)
- G: gap
- M: mitosis
during Interphase the cell increases in
mass and size
during Interphase the cell increases in mass and size and
- carries out its normal cellular functions
- eg. synthesising proteins and replicating its DNA ready for mitosis
interphase consists of three phases
- G1 phase
- S phase
- G2 phase
at is at some point during the G1 phase a
signal is received telling the cell to divide again
it is at some point during the G1 phase a signal is received telling the cell to divide again, the DNA
- in the nucleus replicates
- (resulting in each chromosome consisting of two identical sister chromatids)
this phase of the interphase stage of the cell cycle, when DNA in the nucleus replicates is called the
- S phase: S stands for synthesis (of DNA)
- the S phase is relatively short
the gap between the previous cell division and the S phase is called the
- G1 phase : G stands for growth or gap
- cells make the RNA, enzymes and other proteins required for growth during the G1 phase
during the G1 phase cells make the
- RNA
- enzymes
- and other proteins required for growth
between the S phase and the next cell division event the
G2 phase occurs
during the G2 phase, the
- cell continues to grow
- and the new DNA that has been synthesised is checked
- and any errors are usually repaired
- other preparations for cell division are made (eg. the production of tubulin protein, which is used to make microtubules for the mitotic spindle)
interphase =
G1 + S + G2
main events of the G1 stage of interphase
- cell grows
- and receives a signal to divide
main events of the S stage of interphase
synthesis of new DNA
main events of the G2 stage of interphase
- further cell growth
- error checking of newly synthesised DNA
events of interphase table
nuclear division (mitosis) follows
interphase
nuclear division (mitosis) referred to as the
M phase: M stands for mitosis
during the M phase what stops
cell growth stops
cell growth stops during the
M phase
cytokinesis follows
M phase
during cytokinesis, once the nucleus has
- divided into two genetically identical nuclei
- the whole cell divides
- and one nucleus moves into each cell
- to create two genetically identical daughter cells
during cytokinesis, once the nucleus has divided into two genetically identical nuclei the whole cell divides and
- one nucleus moves into each cell
- to create two genetically identical daughter cells
during cytokinesis, once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create
two genetically identical daughter cells
in animal cells, cytokinesis involves
constriction of the cytoplasm between the two nuclei
in plant cells, cytokinesis involves
a new cell wall is formed
mitosis is the process of
- nuclear division by which
- two genetically identical daughter nuclei are produced
- that are also genetically identical to the parent nucleus
- (they have the same number of chromosomes as the parent cell)
the process of mitosis is fundamental to three main biological processes
- growth of multicellular organisms
- replacement of cells & repair of tissues
- asexual reproduction
with growth of multicellular organisms the two daughter cells produced are
- genetically identical to one another (clones)
- and have the same number of chromosomes as the parent cell
growth of multicellular organisms by mitosis enables
unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms
multicellular organisms growth may occur across the whole body of the organism or be
- confined to certain regions
- such as in the meristems (growing points) of plants
damaged tissues can be repaired by
mitosis followed by cell division
as cells are constantly dying they need to be
continually replaced by genetically identical cells
in humans, for example, cell replacement occurs particularly rapidly in
- the skin
- and the lining of the gut
some animals can regenerate body parts, for example
- zebrafish can regenerate fins and axolotls regenerate legs
- and their tail amongst other parts
asexual reproduction is
- the production of new individuals of a species by a single parent organism
- the offspring are genetically identical to the parent
for unicellular organisms such as Amoeba, cell division results in
the reproduction of a genetically identical offspring
gor multicellular organisms (as seen with many plant species) new individuals grow from
- the parent organism (by cell division) and then detach (‘bud off’) from the parent in different ways.
- some examples of these are budding in Hydra and yeast and runners from strawberries
although mitosis is, in reality, one continuous process, it can be divided into four main stages
- prophase
- metaphase
- anaphase
- telophase
during prophase
chromosomes
condense and are now visible when stained
during prophase the chromosomes consist of
- two identical chromatids called sister chromatids (each containing one DNA molecule)
- that are joined together at the centromere
during prophase the two centrosomes (replicated in the G2 phase just before prophase)
move towards opposite poles (opposite ends of the nucleus)
during prophase spindle fibres (protein microtubules)
begin to emerge from the centrosomes (consists of two centrioles in animal cells)
during prophase the nuclear envelope (nuclear membrane)
breaks down into small vesicles
prophase process
- chromosomes condense and are now visible when stained
- the chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere (a constricted region of a chromosome)
- the two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)
- spindle fibres (protein microtubules) begin to emerge from the centrosomes (a cellular structure involved in the process of cell division) (consists of two centrioles in animal cells)
- the nuclear envelope (nuclear membrane) breaks down into small vesicles
centromere diagram
centrosome diagram
prophase diagram
duirng metaphase centrosomes
reach opposite poles
during metaphase
spindle fibres (protein microtubules) continue to extend from centrosomes
during metaphase chromosomes
line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles
during metaphase spindle fibres (protein microtubules)
reach the chromosomes and attach to the centromeres
during metaphase each sister chromatid
is attached to a spindle fibre originating from opposite poles
metaphase diagram
during anaphase the sister chromatids
separate at the centromere (the centromere divides in two)
during anaphase the spindle fibres (protein microtubules)
begin to shorten
during anaphase the separated sister chromatids (now called chromosomes)
are pulled to opposite poles by the spindle fibres (protein microtubules)
anaphase diagram
during telophase chromosomes
arrive at opposite poles and begin to decondense
during telophase the nuclear envelopes (nuclear membranes)
begin to reform around each set of chromosomes
during telophase the spindle fibres
break down
telophase diagram
during cytokinesis
although not regarded as a phase of mitosis, cytokinesis is an important
- final step in the cell cycle
- because during this stage the cytoplasm is divides forming two genetically identical cells
- and it takes place after the telophase stage of mitosis
cytokinesis is
- this is the physical separation of the parent cell into two genetically identical daughter cells
- and occurs once a new nucleus has completely re-formed at each pole of the parent cell at the end of telophase
during cytokinesis the process differs slightly in animal and plant cells: in animal cells, a
cleavage furrow forms and separates the daughter cells
during cytokinesis the process differs slightly in animal and plant cells: in plants, a
- cell plate (the precursor to a new cell wall) forms at the site of the metaphase plate
- once the cell plate reaches the cell walls of the parent cell, new cell walls are produced, separating the new daughter cells
cytokinesis in an animal cell diagram
cytokinesis in a plant cell diagram
recognising the stages of mitosis from images: prophase
- chromosomes are visible
- the nuclear envelope is breaking down
recognising the stages of mitosis from images: metaphase
chromosomes are lined up along the middle of the cell
recognising the stages of mitosis from images: anaphase
chromosomes are moving away from the middle of the cell, towards opposite poles
recognising the stages of mitosis from images: telophase
- chromosomes have arrived at opposite poles of the cell
- chromosomes begin to decondense
- the nuclear envelope is reforming
recognising the stages of mitosis from images: cytokinesis
- animal cells: a cleavage furrow forms and separates the daughter cells
- plant cells: a cell plate forms at the site of the metaphase plate and expands towards the cell wall of the parent cell, separating the daughter cells
micrograph showing a cell undergoing prophase (P)
micrograph showing cells undergoing metaphase (M) and anaphase (A)
micrograph showing cells undergoing metaphase (M) and anaphase (A)
micrograph showing a cell undergoing anaphase (A)
growth in plants occurs in specific regions called
meristems
the root tip meristem can be used to study
mitosis
the root tip meristem can be found
just behind the protective root cap
in the root tip meristem, there is a
zone of cell division that contains cells undergoing mitosis
pre-prepared slides of root tips can be studied or temporary slides can be prepared using the
- squash technique (root tips are stained and then gently squashed,
- spreading the cells out into a thin sheet and allowing individual cells undergoing mitosis to be clearly seen)
micrograph showing a stained root tip
Required Practical: Identifying Mitotic Stages method
- garlic or onion (Allium cepa) root tips are most commonly used (the bulbs can be encouraged to grow roots by suspending them over water for a week or two)
- remove the tips of the roots (about 1cm) and place them in ethanoic alcohol
- this helps to “fix” the tissue and prevent mitosis from continuing in the cells
- place the root tips in warm (60oC) dilute hydrochloric acid
- this separates the cells
- transfer the root tip to a microscope slide
- using a pipette, add a suitable stain (eg. acetic orcein, which stains chromosomes a deep purple)
- place a coverslip on top of the root tip and press down firmly using a paper towel
- this spreads out the cells so they are only one layer thick
- view the cells under a microscope
Required Practical: Identifying Mitotic Stage limitations
- the preparation of tissue for microscope slides can damage cells and alter their appearance
- during preparation, a sample is often squashed or stained, which can generate artefacts
- the occurrence of artefacts can be decreased by more careful preparation of samples
- the size of cells or structures of tissues may appear inconsistent in different specimen slides
- cell structures are 3D and the different tissue samples will have been cut at different planes resulting in inconsistencies when viewed on a 2D slide
- optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that can not be seen
the mitotic index is
the proportion of cells (in a group of cells or a sample of tissue) that are undergoing mitosis
the mitotic index can be calculated using the formula
mitotic index = number of cells with visible chromosomes ÷ total number of cells
if you need to give the mitotic index as a percentage you can multiply the answer by
100
a student who wanted to observe mitosis prepared a sample of cells, they counted a total of 42 cells in their sample, 32 of which had visible chromosomes, calculate the mitotic index for this sample of cells (give your answer to 2 decimal places)
the images (micrographs) taken of slides containing root tip cells can be used to calculate the actual size of root tip cells, the magnification equation:
actual size = size of image ÷ magnification
there are 1000 nanometers (nm) in a
micrometre (µm)
there are 1000 micrometres (µm) in a
millimetre (mm)
there are 1000 millimetres (mm) in a
metre (m)
