Cell Division Flashcards
Microtubules
organise movement of chromosomes
during mitosis
Interphase
chromosomes are threadlike
structures dispersed throughout the nucleus.
S Phase
DNA is replicated.
For the replicated DNA to be segregated between the two
daughter cells, it needs to be packed.
DNA undergoes a dramatic reorganization during the
M phase.
During M phase, DNA achieves a highly degree of compaction and
organization to form the compact ‘mitotic chromosomes‘
How is DNA packed to form chromosomes in M phase?
DNA undergoes a dramatic reorganization during the
M phase.
First, DNA is is packed as chromatin – with the help of histone proteins.
hen further reorganized by a family of proteins called
’structural maintenance of chromosomes (SMC)’ proteins – which hold
the chromosome structure intact
Condensin Cohesin
Cohesin glues replicated sister
chromatids together.
It is partially removed after prophase,
(only stays at the centromere).
It is removed totally during anaphase.
Condensin reorganizes
chromosomes into their
highly compact mitotic
structure.
It coats DNA and make it
compact
Prophase
Prophase prepares the cell for division by
condensing the chromosomes.
DNA molecules are ‘packed’ into chromosomes.
Packing is achieved by histones - proteins with positive charges that
attract negative phosphate groups of DNA.
Interactions result in the formation of bead-like units called
nucleosomes.
interphase prophase
DNA is packed into chromosomes by the help of histones
Sister chromatids are held together by Cohesin molecules
Cohesin
At the beginning of Prophase,
Cohesin holds sister
chromatids together.
As prophase proceeds, cohesin proteins
are degraded, except at the centromere,
chromatids become visible
Separase enzyme separates
sister chromatids by
degrading cohesin.
Kinetochores
Kinetochores are protein
complexes which attach
to the sides of the
centromere
Kinetochores are the
anchor points for the
mitotic spindles.
During prophase, cells prepare for the segregation of
chromosomes
Most animal cells have only 1 centrosome.
During S phase, the centrosome number also double.
Mitotic cells have 2 centrosomes.
During prophase, each centrosomes move to one cell pole.
Each centrosome contains 2 centrioles.
A high concentration of tubulin dimers
surround the centrosomes.
centrioles
During prophase, centrosomes initiate the assembly
of microtubules and form MITOTIC SPINDLES.
During prophase, centrosomes initiate the assembly
of microtubules and form MITOTIC SPINDLES
Prometaphase
During prometaphase, the
nuclear envelope breaks
down.
Chromosomes consisting of two chromatids attach to the
mictotubules called KINETOCHORE MICROTUBULES.
Microtubules grow and shrink from the centrosome to
‘capture’ chromosomes at the kinetochore.
3 Types of Microtubules
- Kinetochore microtubules - attach to kinetochores on
the chromatids. Sister chromatids attach to
opposite halves of the spindle. - Polar microtubules - form spindle & overlap in centre.
- Astral microtubules – Bind cell membrane to keep the
spindle in place.
bipolar kinesin
Motor protein bipolar kinesin on the polar microtubules push the
spindle poles apart and hold them together
Metaphase
During Metaphase, the chromosomes align in
the centre of the cell at the metaphase plate.
The Spindle Assembly Checkpoint (SAC)
prevents the cell from entering anaphase
until all chromosome kinetochores are
attached to microtubules, and correctly
aligned in the centre of the cell
Anaphase
Anaphase
During Anaphase, the
sister chromatids are
separated by the
removal of cohesin at
the centromere by the
enzyme separase
A motor protein at the kinetochores - dynein
- hydrolyzes ATP for energy to move
chromosomes along the microtubules towards
the poles.
Microtubules also shorten, drawing
chromosomes toward poles
Telophase
Telophase occurs after chromosomes have
separated:
Spindle breaks down
Chromosomes uncoil
Nuclear envelope and nucleoli appear
Two daughter nuclei are formed with
identical genetic information.
Cytokinesis
Cytokinesis: Division of the cytoplasm differs in plant
and animals.
Animal cells: Plasma membrane pinches between the
nuclei because of a contractile ring of microfilaments
of actin and myosin.
Plant cells: Vesicles from the Golgi apparatus
appear along the plane of cell division and fuse to
form a new plasma membrane
End of Mitosis in ANIMALS
Telophase
Nuclear membrane reassembly
Assembly of contractile ring
Cytokinesis
Reformation of interphase microtubule array
Contractile ring forms cleavage furrow
Cytokinesis: Division of the cytoplasm differs in plant
and animals.
Animal cells: Plasma membrane pinches between the
nuclei because of a contractile ring of microfilaments
of actin and myosin.
Plant cells: Vesicles from the Golgi apparatus
appear along the plane of cell division and fuse to
form a new plasma membrane.
Asexual Reproduction
Asexual reproduction is based on mitotic division of
the nucleus.
A unicellular organism can reproduce itself, e.g.
budding yeast.
Cells of multicellular organisms break off to form a
new individual.
The offspring are clones - genetically identical to the
parent.
Sexual Reproduction
Sexual reproduction: The offspring are not identical to
the parents.
It requires gametes created by meiosis; two parents
each contribute one gamete to an offspring.
Gametes, and therefore offspring, differ genetically
from each other and from the parents.
Somatic cells – Diploid - 2n
Somatic cells - body cells not specialized for
reproduction.
Each somatic cell contains homologous pairs of
chromosomes. (2n)
Each chromosome is from one parent.
So, for every gene there are 2 alleles – each from
one parent.
Gametes – Haploid - n
Gametes are haploid: Number of chromosomes = n
Gametes contain only one set of chromosomes.
Fertilization: Two haploid
gametes (female egg and male
sperm) fuse to form a diploid
zygote.
Zygote’s chromosome number =
2n
sexual life cycles.
- Haplontic life cycle: Most of the life cycle is in haploid
stage. In protists, fungi, and some algae - zygote is the
only diploid stage. - Haplodiplontic life cycle (alternation): In Life cycle is
50/50 – haploid/diploid. Most plants, some protists -
meiosis gives rise to haploid spores. - Diplontic life cycle: Most of the life cycle is in diploid
stage. In animals and some plants; gametes are the only
haploid stage.
What role does cell division play in a sexual life
cycle?
Sexual reproduction generates diversity among individual
organisms.
It allows the random selection of half the diploid
chromosome set - this forms a haploid gamete that fuses
with another to make a diploid cell.
Thus, no two individuals have exactly the same genetic
makeup.
Meiosis Overview
Meiosis is nuclear division in cells involved in sexual
reproduction – germ cells.
The cells resulting from meiosis are not identical to the parent cells
Meiosis consists of two nuclear divisions, but DNA is
replicated only once.
Meiosis has two main functions:
Reduce the chromosome number from diploid (2n)
to haploid (n).
Generate diversity in the genetic products.
What happens during Meiosis?
Two meiotic divisions:
Meiosis 1 introduces genetic diversity.
Meiosis 2 separates the individual chromatids.
DNA replication takes place at S phase before Meiosis I.
Similar to mitosis, at the beginning stages of Meiosis I, each
chromosome consists of two sister chromatids, held together
by cohesin proteins.
Prophase I and metaphase I: The chromatin continues to coil and
compact.
During prophase I, the homologous chromosomes pair: synapsis
(= maternal chromosomes pair with paternal homologs)
The four chromatids of each homologous pair form a tetrad or
bivalent.
The homologs are held together at chiasmata that form between
non-sister chromatids.
Synapsis
During prophase I, the homologous
chromosomes align closely side by side.
The aligned homologous chromosomes are
called bivalents (or tetrads)
Crossover- Chiasmata
Once the chromosomes are paired, crossover takes
place.
This is where sections of chromatids from
homologous chromosomes exchange genetic material,
leading to genetic recombination.
The homologs are held together at chiasmata that
form between non-sister chromatids.
Chiasmata: Evidence of Genetic Exchange
between Chromatids
Exchange of genetic
material occurs at the
chiasmata - called
crossing over.
Crossing over results in
recombinant chromatids
and increases genetic
variability of the
products.
Anaphase 1
Homologous chromosomes separate; daughter nuclei
contain only one set of chromosomes. Each chromosome consists of
two chromatids.
Random distribution of different pairs of chromosomes to the
gametes (sperm or egg cells) takes place - this is called random
assortment.
Telophase 1
The nuclear envelope reaggregates, sometimes
followed by an interphase called interkinesis.
In other organisms, meiosis II begins immediately – this process has
some similarity to mitosis.
Differences Between Meiosis II and Mitosis
DNA does not replicate before meiosis II.
In meiosis II the sister chromatids may not be identical
because crossing over has already happened in meiosis I.
The number of chromosomes at the equatorial plate in meiosis
II is half the number of those in mitosis.
Aneuploidy
Aneuploidy may be caused by lack of cohesins that hold the
homologous pairs together.
Without cohesins both homologs may go to the same pole.
The resulting gamete will have two of the same chromosome, or
none.
Example: In humans, if both chromosome 21 homologs go to
the same pole and the resulting egg is fertilized, it will be
trisomic for chromosome 21.
This results in the condition known as Down syndrome.
A fertilized egg that did not receive a copy of chromosome 21
will be monosomic, which is lethal
Trisomies and monosomies are common in human zygotes.
Most embryos from these zygotes do not survive.
Trisomies and monosomies for chromosomes other than 21
are lethal - many miscarriages are due to this
