D2.1 Cell and nuclear division Flashcards
D2.1.1—Generation of new cells in living organisms by cell division
In all living organisms, a parent cell—often referred to as a mother cell—divides to produce two daughter
cells.
New cells are produced by division of pre-existing cells. The cell that divides is the mother cell and the two cells produced when it divides are daughter cells.
D2.1.2—Cytokinesis as splitting of cytoplasm in a parent cell between daughter cells
Students should appreciate that in an animal cell a ring of contractile actin and myosin proteins pinches a
cell membrane together to split the cytoplasm, whereas in a plant cell vesicles assemble sections of
membrane and cell wall to achieve splitting.
Cytokinesis is the division of a cell’s cytoplasm to form two cells. It occurs after mitosis and happens differently in plant and animal cells.
* Plants make a new cell wall across the cell’s equator, with plasma membrane on both sides. This divides the cell in two.
cell wall of mother cell
plasma membrane of mother cell
vesicles containing pectin are linked up across the equator to form a middle lamella. Cellulose is then added on either side to form two walls, one for each daughter cell
* Animals divide the cytoplasm of cells by moving the plasma membrane. Movement is due to actin and myosin proteins adjacent to the membrane. Before cytokinesis they are randomly arranged, but some are reorientated so they run in parallel in a ring around the equator of the cell, where they exert tension to form a cleavage furrow, with the membrane pulled inwards so it eventually splits the cell.
cleavage
furrow at the equator with circular actin
actin filaments randomly arranged
filaments
Adapted from: Spira, F. et al. (2017). Cytokinesis in vertebrate cells initiates by contraction of an equatorial actomyosin network composed of randomly oriented filaments. eLife, 6. doi:10.7554/elife.3086/
D2.1.3—Equal and unequal cytokinesis
Include the idea that division of cytoplasm is usually, but not in all cases, equal and that both daughter
cells must receive at least one mitochondrion and any other organelle that can only be made by dividing a
pre-existing structure. Include oogenesis in humans and budding in yeast as examples of unequal
cytokinesis.
In many cases, cytokinesis divides the cytoplasm of the mother cell into equal halves. This happens for example when a human zygote divides to form a two-cell embryo.
Cytoplasm is sometimes divided unequally. Small cells produced by unequal division can survive and grow if they receive a nucleus plus at least one mitochondrion and other organelle that cannot be assembled from components in the cell. Oogenesis (egg production) in humans and budding in yeast are two examples.
the two polar bodies are small cells that soon die
Ist polar body
2nd polar body /
large diploid ® mother cell
arge haploid e daughter cell
large haploid egg ® cell
lst division of meiosis
2nd division of meiosis
the egg cell is a female gamete
mother cell develops an outgrowth of its cell wall containing only a small part of the cell’s cytoplasm
the outgrowth receives a nucleus and then buds off to form a separate cell
budding may occur repeatedly
D2.1.4—Roles of mitosis and meiosis in eukaryotes
Emphasize that nuclear division is needed before cell division to avoid production of anucleate cells.
Mitosis maintains the chromosome number and genome of cells, whereas meiosis halves the chromosome
number and generates genetic diversity.
If a mother cell divides before it has divided its nucleus, one daughter cell will receive a nucleus, but the other
Mitosis-for continuity
Meiosis-for change
one will not. Cells without a nucleus cannot synthesize polypeptides, so they do not grow or maintain themselves and have limited lifespans. Red blood cells which are anucleate only survive for about 120 days.
Usually nuclear division does happen before cytokinesis, so each daughter cell can receive a nucleus. There are two types of nuclear division: mitosis or meiosis. They have different roles, so most organisms use both during their life cycle.
With mitosis, daughter cells receive all the chromosomes and genes of the mother cell.
The chromosome number is unchanged.
Mitosis is used in asexual reproduction to produce genetically identical offspring. It is also used in multicellular organisms to produce genetically identical body cells.
In meiosis a diploid nucleus divides into haploid nuclei, halving the chromosome number. This allows haploid gametes to be produced in sexual life cycles.
Meiosis generates genetic diversity because every haploid cell produced from a diploid mother cell has a different combination of alleles.
D2.1.5—DNA replication as a prerequisite for both mitosis and meiosis
Students should understand that, after replication, each chromosome consists of two elongated DNA
molecules (chromatids) held together until anaphase.
Cells replicate all their DNA before the start of both
mitosis and meiosis.
DNA replication ensures that two daughter cells
produced by mitosis will receive the entire genome.
2 n
mitosis
1st division
of meiosis
2nd division
of meiosis
mitosis
The DNA is in an elongated state when it is replicated
and is then packed up tightly (condensed) during the
early phases of mitosis or meiosis. Condensation makes
the two DNA molecules visible as separate structures.
DNA replication provides enough DNA for a mother cell
to divide twice in meiosis, producing four haploid cells.
It also provides enough DNA for recombination by the
process of crossing over.
They are called sister chromatids. Throughout the early
phases of mitosis and meiosis, each chromosome
consists of two sister chromatids. The chromatids only
separate in the penultimate phase.
D2.1.6—Condensation and movement of chromosomes as shared features of mitosis and meiosis
Include the role of histones in the condensation of DNA by supercoiling and the use of microtubules and
microtubule motors to move chromosomes.
During mitosis and meiosis, chromatids are separated and moved to
opposite ends (poles) of the mother cell. This could not be done if the
DNA was still in an elongated state, so it is packed up (condensed) to form
much shorter and fatter chromosomes. Initial stages of condensation are
carried out by wrapping the double helix of DNA around groups of histone
proteins. The structures produced (shown right) are then linked together.
Further details of this are given (for HL) in A1.2.13.
Later stages of condensation are accomplished by supercoiling, ending
up with 10,000 micrometres of DNA packed into each micrometre of
chromatid.
Chromosomes are moved by microtubules and microtubule motors.
Microtubules are narrow tubular structures that are assembled from many
molecules of tubulin (a globular protein). This happens at the poles of the
cell. As more tubulins are added, the microtubules extend further towards
the equator of the cell. The microtubules form a spindle shape, tapering at
both ends, so are called spindle microtubules.
(8 in total)
Microtubule motors cause movement by removing tubulin subunits from the end of microtubules, shortening them.
Each chromatid has a microtubule motor, called the kinetochore, which is anchored to the chromatid’s centromere.
Microtubules that have grown from one of the poles become attached to the kinetochores. During anaphase in
mitosis and meiosis, kinetochores remove tubulins from the end of the microtubules, pulling chromosomes to
opposite poles.
sister chromatids
histone proteins
movement towards the pole
centriole adds tubulins
. . to make microtubules
grow to the equator
. kinetochore removes
tubulins to make microtubules pull
chromatids to the poles
D2.1.7—Phases of mitosis
Students should know the names of the phases and how the process as a whole produces two genetically
identical daughter cells.
microtubules
are growing
from the
centrioles to
2 Late prophase 3 Metaphase
spindle microtubules
e x t e n d from e a c h
pole to the equator
form a spindle
s h a p e
chromosomes are becoming
shorter and fatter by supercoiling
(condensation)
each c h r o m o s o m e consists of two identical sister chromatids formed by DNA
replication in interphase, each
with a centromere a n d a kinetochore
4 Anaphase 5 Early telophase
all c h r o m o s o m e s .
have reached the poles
and nuclear m e m b r a n e s
form a r o u n d them
nuclear membrane has broken
down and chromosomes
have moved to the equator
spindle microtubules
have attached
to kinetochores,
with sister
chromatids
attached to
opposite poles
6 Late telophase
sister
chromatids
have
separated,
so each is
n o w a
separate
c h r o m o s o m e
kinetochores shorten spindle
microtubules, pulling genetically
identical chromosomes to opposite poles
c h r o m o s o m e s uncoil
and are no longer
individually visible
spindle microtubules break down
the cell divides (cytokinesis)
to form two cells with genetically
identical nuclei
D2.1.8—Identification of phases of mitosis
Application of skills: Students should do this using diagrams as well as with cells viewed with a
microscope or in a micrograph.
Interphase-no sign of condensation
Prophase-condensing inside the nucleus
Anaphase-V-shaped and moving to poles
Telophase-decondensing in nuclei at poles
Metaphase-aligned on the equato
D2.1.9—Meiosis as a reduction division
Students should understand the terms “diploid” and “haploid” and how the two divisions of meiosis
produce four haploid nuclei from one diploid nucleus. They should also understand the need for meiosis
in a sexual life cycle. Students should be able to outline the two rounds of segregation in meiosis.
Meiosis is described as a reduction division because it halves the number of chromosomes. The mother cell that divides has a diploid nucleus and the four cells produced all have haploid nuclei. A diploid nucleus has two sets of chromosomes, whereas a haploid nucleus has only one set. A diploid nucleus contains pairs of homologous chromosomes. Homologous chromosomes carry the same genes, in the same sequence, but they may have different alleles of any of the genes.
Prophase !
chromosomes pair up; the chromosomes in each pair
are
homologous
nuclear membrane will soon break down
spindle microtubules grow from the poles
spindle microtubules
from the two poles attach to different chromosomes in each pair, ensuring that they segregate
Prophase Il
Anaphase Il
kinetochores pull chromatids to the poles
The haploid number of chromosomes of a species is represented by the letter n so the diploid number is 2n. Most body cells in plants and animals are diploid. Gametes such as the sperm and eggs are haploid. Two haploid gametes fuse during fertilization to produce one diploid cell-the zygote.
This divides by mitosis to produce more diploid body cells with the same number of chromosomes. If meiosis did not occur at some stage during a sexual life cycle, the chromosome number would double every generation.
Metaphase I
orientation of pairs of homologous chromosomes on the equator is random
Anaphase I
each chromosome still consists of two
chromatids
homologous chromosomes are pulled to opposite poles.
this halves the
chromosome number
Telophase I|
two haploid cells
produced by the first
division of meiosis
DNA does not have to be replicated between the lst and 2nd divisions because each chromosome still has two chromatids
every nucleus produced by meiosis is genetically different
, chromosomes decondense inside the reformed nuclear membranes.
new spindle microtubules
grow from the poles to the equator
sister chromatids are not identical because of exchange of alleles due to crossing over
both haploid cells produced by the Ist division have divided again
four haploid cells produced
D2.1.10—Down syndrome and non-disjunction
Use Down syndrome as an example of an error in meiosis.
Down syndrome is due to an error in meiosis. A pair of homologous chromosomes (pair 21) fails to separate in anaphase 1 (non-disjunction) so both move to the same pole. The result is a sperm or egg with two copies of chromosome 21 and a zygote with three copies. A child developing from such a zygote has Down syndrome.
Having three chromosomes instead of two is trisomy.
Most trisomies have more serious consequences and cause death of the gamete or early-stage embryo, for example trisomy of chromosome 1.
D2.1.11—Meiosis as a source of variation
Students should understand how meiosis generates genetic diversity by random orientation of bivalents
and by crossing over.
Meiosis generates genetic diversity (different combinations of alleles) in two ways:
1. Random orientation of bivalents
A bivalent is a pair of homologous chromosomes, one inherited from the male and one from the female parent.
Orientation of each bivalent in metaphase I determines which pole each chromosome moves to. Orientation is random and does not influence other bivalents, so many different combinations can be produced when homologous chromosomes separate in anaphase I. There are 2ª possible combinations-over 8 million in humans where n is 23.
2. Crossing over
Homologous chromosomes pair up in the very early stages of meiosis and non-sister chromatids exchange lengths of DNA by “crossing over”. This produces chromatids with new combinations of alleles. It is a significant source of genetic variation because where along the length of the chromosomes the exchange occurs is random.
chiasma
D2.1.12—Cell proliferation for growth, cell replacement and tissue repair
Include proliferation for growth within plant meristems and early-stage animal embryos as examples.
Include skin as an example of cell proliferation during routine cell replacement and during wound healing.
Students are not required to know details of the structure of skin.
Cell proliferation is repeated division of cells. It is
required for three reasons in plants and animals:
1. Growth-to increase the size of the body
Examples: plants have groups of dividing cells
(meristems) that are retained throughout the plant’s life,
to allow growth to continue (indeterminate growth).
There are meristems in the apices (tips) of both stems
and roots. The root apical meristem lengthens the root.
The shoot apical meristem lengthens the stem and
produces cells for growing leaves or flowers. All cells in
96
early-stage animal embryos are dividing, so the entire
embryo grows rapidly.
2. Cell replacement-routine production of cells to
replace those with a limited lifespan
Example: a layer of dividing cells in the skin replaces cells
that are abraded from the skin surface.
3. Tissue repair-healing after loss or damage of tissues
Example: skin cells divide to produce the cells needed to
heal a cut or other wound.
D2.1.13—Phases of the cell cycle
Students should understand that cell proliferation is achieved using the cell cycle. Students should
understand the sequence of events including G1, S and G2 as the stages of interphase, followed by mitosis
and then cytokinesis.
ell proliferation is achieved by a repeated sequence
of events called the cell cycle. The phases in the cell
cycle are shown in the diagram.
cyclin E
cyclin D S phase
when DNA is replicated
phase
cell
a lo prepare ior mitos en
INTERPHASE
(with three
growth
phase
phases)
cytokinesis
telophase
MITOSIS
(with four phases)
anaphase
metaphase
cyclin A
prophase
cyclin B
D2.1.14—Cell growth during interphase
Students should appreciate that interphase is a metabolically active period and that growth involves
biosynthesis of cell components including proteins and DNA. Numbers of mitochondria and chloroplasts
are increased by growth and division of these organelles.
Interphase is a very active phase in the life of a cell when many metabolic reactions occur.
Some of these, such as the reactions of cell respiration, also occur during mitosis, but DNA replication in the nucleus and protein synthesis in the cytoplasm only happen during interphase.
During interphase, mitochondria in the cytoplasm grow and divide, so they increase in number. In plant cells chloroplast numbers increase in the same way.
D2.1.15—Control of the cell cycle using cyclins
Limit to the concentration of different cyclins increasing and decreasing during the cell cycle and a
threshold level of a specific cyclin required to pass each checkpoint in the cycle. Students are not required
to know details of the roles of specific cyclins.
There are checkpoints between one phase of the cell cycle and the next. Cyclins are proteins used to control progression though these checkpoints. Their concentrations rise and fall during the cell cycle. At each checkpoint a threshold concentration of one specific cyclin is required for the cell to progress to the next phase. The concentration of that cyclin then falls.
Cyclins bind to kinase enzymes, activating them.
The kinases phosphorylate other proteins in the cell, activating them. The phosphorylated proteins perform tasks specific to the phase of the cell cycle that has been entered.
There are four main types of cyclin in human cells.
The checkpoints that each cyclin controls are shown in the cell cycle diagram in Section D2.1.13.
