Cell Division

NUCLEUS Properties Function

Nucleus  Found in all eukaryotic cells.  Essential for cell
 Except mature phloem sieve division
tube element cells and red  Contains hereditary
blood cells(erythrocytes) material(chromosome
 Usually one nucleus per s) of an organism.
cell(uninucleated)  Directs protein
 Cells can be synthesis by
binucleated(paramecium) or synthesising mRNA
multinucleated(skeletal and sending to
muscle fibres) cytoplasm via nuclear
 Shapes are typically pores.
spherical.

Nuclear  Composed of two  to allow exchange of

Envelope membranes – outer and substances between
inner membrane separated the nucleus and
by a fluid filled space. cytoplasm
 The outer membrane of the
nuclear envelope is
continuous with the
endoplasmic reticulum.
 Nuclear envelope is
perforated by nuclear
pores.

Nucleolus  most visible structure  site of synthesis on

within a nucleus that is not rRNA -> component
undergoing division. of ribosomes
 composed of a network of
densely stained granules
and fibres
 one or more nucleoli may
be found in nucleoplasm.
 no. of nucleoli depends on
species and stage in cell
cycle.
Nucleoplasm  semi-fluid matric that fills  serves as a
the nucleus suspension liquid of
metabolites and
large
macromolecules

Importance of cell division

 unicellular organisms
o cell division of one cell reproduces another organism(asexual
reproduction)
 multi-cellular organisms
o cell division of germ cells in testes and ovaries produces gametes.
o cell division enables organisms to grow and develop from a single zygote.
o enables organisms to repair and replace damaged and worn-out body
cells.
Cell cycle
 sequence of events from the
time a cell is formed until its
division into two daughter cells.
 3 stages
o Interphase(resting phase)
o Mitosis(nuclear division)
o Cytokinesis(division of
cytoplasm)

Chromatin vs Chromatid vs
Chromosome
 chromatin thread replicated to produce
two identical chromatin threads -> sister
chromatids.
 chromatin thread will coil and shorten and
condense to become chromosome.

Chromatids
 at the end of replication, there are 2
identical DNA molecules held together at a point along their length called the
centromere.
 correspond to the 2 identical parts of the replicated chromosome => sister
chromatids
 after separation – individual chromosome

Chromosome
 most important structure during cell division
 responsible for transmission of the hereditary information from one generation
to the next
 contain DNA – molecule of inheritance.
 during interphase, long, thin threads that are loosely coiled are seen spread
throughout the nucleus.
 these fibers are known as chromatin.
 just before nuclear division, chromatin fibers condense(coil up) into much more
compact structure, appearing shorter, thicker.
 after replication => double structures
 compact structure = chromosome

Centromere and Kinetochore

 centralised region joining 2 sister chromatids.
  occurs anywhere along the length of the chromosome.
 directs the formation of a protein complex – kinetochore.
 kinetochore: provides the major attachment point for spindle microtubules
during nuclear division.

Chromatids Chromosomes
1  replicated form of chromatin  collectively refers to the pair of
. joined at sister chromatids in a replicated
centromere(decondensed) chromosome, or unreplicated
 replicated form of chromosomes DNA molecule, or separated
joined at centromere(condensed) chromatids after anaphase
2  each exists in duplicate, as 2  exists as a single entity
genetically identical copies(sister
chromatids) held by a centromere
3  each is separated from the other  at mitotic anaphase,
. copy(sister chromatid partner) chromosomes are separated
during mitotic anaphase chromatids(individual
chromosome)
4  sister chromatids exist as pairs in  homologous chromosomes not
. mitosis paired in mitosis
5  sister chromatids are genetically  no two chromosomes are
. identical genetically identical(unless they
are duplicates, separated sister
chromatids after anaphase)
6  sister chromatids contain  Only homologous chromosomes
. identical gene loci contain identical gene loci
7  1 chromatid is one single DNA  1 chromosome can consist of 1
. molecule or 2

Interphase
Phas Function Checkpoint
e
G1 Cellular contents, excluding the Ensures that the cell is ready for DNA
chromosomes are duplicated. synthesis.
Metabolic activity and growth  Cell contents duplicated.
 Protoplasm sufficient
 Cell size
 Growth factors
S Each of the 46 chromosomes is –
duplicated(DNA replication)
G2 Metabolic activity, growth, and Ensures that the cell is ready for
preparation for mitosis mitosis and cytokinesis.
 DNA is duplicated correctly.
 Repair any errors in DNA.
 Cell is large enough
Microtubul Properties Image

es
Microtubules  Tubular polymers
made of
tubulin(globular
protein)
 Forms cytoskeleton
in cytoplasm
 Made via
polymerisation of
monomers.
 Monomer = 1
tubulin dimer(α and
β tubulin)
Centrosomes  Microtubule
Organising
Centre(MTOC) =
centrosome
 Centrosome = 2
centriole
 Centriole
perpendicular to
each other
 During cell division,
duplicated
centrosomes
assemble and
organise
microtubules to form
mitotic spindle.
Mitotic  Kinetochore
Spindle microtubule
attaches to
kinetochore and to
spindle poles

Interphase
 “resting”/non-dividing stage.
 During these stages, the cell
o Absorbs nutrients.
o Build up protoplasm.
o Synthesise new organelles.
o Replicate DNA
Phase Description Diagram Pictomicrograph
Prophase  Chromatin fibers condense by supercoiling to
become tightly coiled and compact discrete
structures called chromosomes.
 Centriole pairs and their associated centrosomes
migrate towards opposite poles in the cell.
 Mitotic spindle begins to form as microtubules are
reorganised by centrosomes which act as MTOC.
 Nucleolus gradually disappears.
 Nuclear envelope disappears

Metaphas  All chromosomes are attached by kinetochore

e microtubules from opposite poles at their paired
kinetochores.
 Centriole pairs and centrosomes are positioned at
opposite poles in the cell.
 Chromosomes line up along the metaphase plate
and centromeres lie at plate.
 Polar microtubules overlap at metaphase plate with
those from opposite poles.
Anaphase  Centromeres divide, allowing sister chromatid to
split and separate as kinetochore microtubules
shorten.
 Shortening kinetochore microtubules pull the
daughter chromosomes towards the opposite poles.
 Polar/non-kinetochore microtubules from opposite
poles elongate, overlap and slide pass on another.
This causes the opposite poles to be pushed further
apart and the cell elongates.
Telophase  Daughter chromosomes reach the opposite poles of
the cell. Each pole has an identical, complete diploid
set of chromosomes.
 Daughter chromosomes decondense to form
chromatin fibers.
 Spindle fibers/microtubules disassemble.
 Nucleolus reforms
 Nuclear envelope reforms around the daughter
chromosomes
 Two daughter nuclei form in the cell
Cytokinesis
 Division of cytoplasm of parent to form 2 daughter cells.
 Equal distribution of organelles, cytoplasm, and chromosomes into daughter
cells
Animal cells
 Formation of cleavage furrow in cell surface
 Contracting ring of microfilaments (made of actin)
 Pinches parent cell into 2 completely separated daughter cells.
Plant cells
 Cell plate formation
 Golgi vesicles coalesce to form cell plate.
 Forms new cell wall and cell surface membrane
 Cell plate enlarges and extends across parent cell to completely separate
daughter cells.

Ploidy
 Ploidy: no of complete sets of chromosomes in a cell
 Haploid(n): cell only contains one set of cells (cells produced via meiosis)
 Diploid(2n): cell contains two sets of chromosomes (cells produced via mitosis)
Homologous chromosomes
 also known as bivalents/tetrads
 criteria for pairing up.
o size
o position of centromere
o gene loci – position of gene
Meiosis
 a form of nuclear division in which chromosome number is halved from 2n to n -
> reduction division.
 involves one round of DNA replication in parent cell, followed by two cycles of
nuclear and cell divisions.
 Significance(reduction division)
o Formation of haploid gametes with half the number of chromosomes
o Enables the diploid condition to be restored upon the fusion of
gametes(fertilisation)
o Ensures that the chromosome number characteristic of the species is
maintained after fertilisation
Genetic Variation
Source Description Diagrams
Crossing  Occurs during prophase I – random process.
over  Pairing up of homologous chromosomes -> synapsis
 Crossing over is the equivalent exchange of genetic
material between sister chromatids of the homologs ->
results in homologous recombination.
 Chiasma: point where two non-sister chromatids have
criss-crossed to exchange genetic material

Independ  During metaphase I, pairs of homologous chromosomes line

ent up along metaphase plate  random and independent
assortme orientation of bivalents
nt

Random  A human ovum and sperm represent ~8.4 million possible

fertilisatio chromosome combination each which will produce a zygote
n with 70 trillion diploid combinations. Adding in the variation
brought about by crossing over, the possibilities is endless
Mitosis vs Meiosis
Mitosis Meiosis
1. Refers to a nuclear division which separates Refers to a nuclear division in which meiosis I
sister chromatids separates homologous chromosomes, and the
sister chromatids separate at meiosis II
2. One division per cell(one cytoplasmic Two divisions per cell
division/cytokinesis)
3. Homologous chromosomes don’t pair up Homologous chromosomes pair up during
during prophase. Chiasmata is never formed prophase I. Chiasmata and crossing over may occur
and there is no crossing over
4. Chromosomes form a new single row at the Homologous chromosomes form two rows at the
equator of the spindle during metaphase equator of the spindle during metaphase I and a
single row at metaphase II.
5. Each of the sister chromatids move to the Each of the homologous chromosomes moves to
opposite pole of the spindle during anaphase the opposite pole of the spindle during anaphase I.
and become independent chromosomes. in anaphase II, sister chromatids separate and
move to the opposite ends on the spindle
6. Does not introduce genetic variability. Introduces genetic variability among the gametes.
Genetic exchange does not occur between Genetic exchange occurs between homologous
homologous chromosomes in prophase chromosomes in prophase I.
7. Dividing cells can be haploid or diploid Dividing cells are diploid
8. 2 daughter cells are produced per mitotic 4 daughter cells , each haploid, called gametes are
cycle produced per meiotic cycle
9. Genetic content of daughter cells re identical Genetic content of daughter cells are not identical
to the parental cell in terms of chromosome as chromosome number has been halved and the
number and genetic sequence(in absence of genetic sequence is not due to crossing over and
mutation) independent assortment
10. Occurs in almost all somatic cells Occurs only in specialised cells of the germ line
11. Mitotic products are usually capable of Meiotic products cannot undergo additional
undergoing additions mitotic divisions meiotic divisions although they may undergo
mitotic division
12. Occurs at zygote stage and continues Occurs only after a higher organism has begun to
throughout the lifecycle of an organism mature, occurs in zygote of many algae and fungi