Download Cell Divison Mitosis and Meiosis

Cell Divison
Mitosis and Meiosis
Cell Division
The ability of an organism to procreate and survive
is dependent upon cell division. Some organisms
are exact copies of their parents, while others are
unique and different from their parents. There are 2
types of cell division to accomplish this, each with
different functions.
Types of Cell Division


Mitosis – production of identical daughter cells,
usually for the purpose of growth or replacement
of body cells. This form of cell division is also
found in asexual reproduction.
Meiosis – production of the gametes (sex cells).
These cells (sperm and egg) contain only one
half of the total genetic information from the
parent. The combination of gametes during
sexual reproduction gives rise to individuals with
unique characteristics.
Some required terms:



Chromatin – threadlike network in a
cells nucleus that is the combination of
all DNA (genes) and proteins (not visible
with a light microscope)
Chromosome – is a single thread-like
structure in the nucleus of a dividing cell
that consists of chromatin coiled up.
Humans have 46 chromosomes (23
pairs) in every cell except the gametes,
which contain only 23 chromosomes.
(We count chromosomes by counting
centromeres.)
Chromatids – the two identical parts of
a chromosome following replication of
DNA. Usually two sister chromatids are
held together by a centromere.

Gene – is the basic unit of heredity that is
composed of a specific sequence of nucleotide
bases in DNA. This code is translated into
specific proteins and structures within the cell.

Haploid – (N) the number of distinct
chromosomes an organism possesses in the
gametes (sperm or egg cells)
Diploid – (2N) having twice the number of
chromosomes that are found in the gametes
Homologous chromosomes – are
chromosomes that are the same size and shape
and carry the genes for the same traits, but have
different details
Autosomes – are chromosomes that do not
influence biological sex
Sex Chromosomes – X and Y chromosomes that
determine biological sex in humans




The Cell Cycle

Most cells only spend a short percentage of their time (10%)
dividing. Most of the time (90%), cells are busy carrying on
with other cell activities, and preparing to divide.
The cell cycle can be divided up into
phases:





Interphase – refers to the time when cells are not actively
dividing. This accounts for about 90% of the cell’s time. This
phase involves a number of sub-phases:
G1 – gap phase - the cell manufactures proteins and amino
acids needed for both cell processes and cell division. (8 hours
of the cell cycle)
S – synthesis phase – all nuclear DNA replicates. (6-8 hours
of the cell cycle) This stage is critical for cell division to occur.
G2 – gap phase – the cell increases the rate of protein
synthesis and completes all processes necessary for cell
division. The cell also grows larger. (4.5 hours of the cell cycle)
Mitosis/ Meiosis – refers to the time when cells are actively
dividing. This process takes up about 10% of the cell cycle.
This can be divided up into a series of sub-stages.



Cells do not remain functional for an indefinite length
of time. Cells replace themselves on a regular basis,
though cell division. There is a limit on the number of
times a cell can undergo cell division. On average it is
believed that a cell can divide only about 50 times on
average.
To ensure that the cells in a tissue are healthy, cells
will undergo a form of cell suicide called apoptosis.
This programmed cell death ensures that the cells
remaining in a tissue are functional, healthy and
active. The cell cycle really consists of a pattern of
development of cells that involves both cell division
and cell death.
Cell Death
Mitosis – Asexual cell division

Mitosis involves the replication of daughter cells
for the purpose of growth and replacement of
dead cells. This process produces diploid
daughter cells that are genetically identical to
the parent (original cell). Mitosis is usually
divided up into 4 major stages: prophase,
metaphase, anaphase and telophase (PMAT).
Prophase

chromatin condenses into chromosomes, nucleolus
and cell membrane begin to break down, microtubules
form from spindle fibres in plants, centrioles
separate to form asters in animals and migrate to the
poles
Metaphase

chromosomes move towards the middle of the cell
(equatorial plate), each chromosome is attached to a
separate spindle fibre
Anaphase

chromatids begin to separate with one member of each
pair being pulled to opposite poles
Telophase

nucleolus re-appears and the nuclear membrane reforms
around each set of chromosomes, the chromosomes
unravel to form a loose mass of chromatin, cells undergo
cytokinesis.
Plant vs Animal Cells

a cell plate forms
between the new
daughter cells


the cell membrane
pinches in between the
two daughter cells
This is called cytokinesis
Mitosis Overall

http://www.loci.wisc.edu/outreach/bioclips/CDBio.html
Some Methods of Asexual Reproduction
1. Binary fission - equal division
of both the organism
cytoplasm and nucleus to form
two identical organisms ex:
Protist - amoeba
2. Budding - one parent
dividing its nucleus (genetic
material) equally, but cytoplasm
unequally ex: Fungi- yeast
3. Sporulation (spore
formation) --is reproduction
involving specialized single
cells coming from one parent
ex. Fungi -mold spores
Meiosis – Sexual Cell Division

Meiotic cell division only occurs in the sex cells
and is responsible for the production of haploid
gametes (sperm and eggs). Unlike mitosis, the
goal of meiosis is to produce variability. Half of
the DNA from the parent cell is passed down to
one gamete. Each gamete will be different
depending what combination of the 23
chromosomes it receives. To increase
variability, crossing over occurs, which causes
a mixing of genetic information on one
chromosome. The stages of meiosis are very
similar to the stages of mitosis, except that there
are 2 cell divisions, with only one replication of
the DNA.
Meiosis I – Reduction Division




Prophase I – homologous chromosomes
undergo synapsis (the pairing of homologous
chromosomes) so that information can be
exchanged by crossing over.
Metaphase I – homologous pairs line up along
the equatorial plate.
Anaphase I – homologous chromosomes
separate and are pulled to opposite poles by
centromeric spindle fibers.
Telophase I – daughter cells separate, each has
one chromosome from each homologous pair.
Synapsis / Crossing Over
Meiosis II – no replication of DNA





Prophase II – cells have one chromosome from each
homologous pair
Metaphase II – double stranded chromosomes align at
the equatorial plate
Anaphase II – the daughter chromatids separate and
move towards separate poles
Telophase II – spindle fibers disappear, nuclei reform
and cytokinesis takes place. All 4 daughter cells are
haploid.
NOVA Online | 18 Ways to Make a Baby | How Cells
Divide: Mitosis vs. Meiosis (Flash)
Successful cell division relies on 2
things:
1. accurate replication of the chromosomes
2. exact separation / distribution of the
chromosomes
Oogenesis vs Spermatogenesis

Oogenesis

Spermatogenesis

meiosis II completes only
if egg is fertilized
ootid receives most of the
cytoplasm and becomes
the egg
polar bodies have less
cytoplasm and
degenerate
one egg cell is produced

meiosis I and II are both
completed before
fertilization
each sperm cell produced
receives an equal share
of cytoplasm
flagellum forms from one
of the centrioles
four spermatids are
produced






Oogenesis
Spermatogenesis
Abnormal Meiosis

Non-disjunction – occurs during meiosis
when two homologous chromosomes move to
the same pole during either meiosis I or
meiosis II.
Normal Division
Non-disjunction
Trisomic Zygote

When the daughter cell with too many or too few
chromosomes is fertilized, the chromosome
number becomes either one too many or one
too few.

Trisomy – a condition where three
chromosomes replace a normal pair (47
chromosomes in humans)
Monosomy – a condition where one
chromosome replaces a normal pair (45
chromosomes in humans)

Biological Sex
– in
humans is determined
by the presence / absence
of the Y chromosome.
 Female is XX
 Male is XY
(This varies with other
organisms)
The process of crossing over during meiosis can also
result in a number of mutations (alterations to the
genetic code).








Inversions – occur when a segment of a chromosome is
turned 180 degrees and reconnected in the wrong orientation
Ex. Toothpaste - Tophtoaste
Translocations – occur when a chromosomal segment breaks
off and re-attaches to another chromosome
Ex. Swimming Elephants - Swimm Ingelephants
Deletions - occur when a chromosomal segment breaks off
and does not re-attach anywhere else
Ex. Superstitious - Suitiious
Duplications – occur when a chromosomal segment is present
more than once on the chromosome because extra copies of
that segment were made.
Ex. Bubblegum - Bubblebleblebleblegum

Chromosomal abnormalities can be detected by
doing a karyotype chart. Fetal cells are collected
through amniocentesis or chorionic villi
sampling, and the chromosomes are arranged in
homologous pairs according to size and banding
patterns. Chromosomes can be counted to
determine whether a non-disjunction has occurred.
Human Karyotype Chart
Some Common Genetic Disorders
Down’s syndrome – trisomy
21
 more common in children
born to women over 40
 characterized by short stature,
folds to the eyelids, stubby
fingers, wide gap between 1st
and 3rd toes, large fissured
tongue, round head, palm
creases, mild to severe
mental retardation.
Edward’s syndrome –
trisomy 18
 results in severe overall
defects with a life
expectance of only 10
weeks
Patau’s syndrome –
trisomy 13
 results in non-functioning
eyes, severe deficits and
limited life expectancy.
Turner’s syndrome – XO
female
 females with short
stature, broad chest,
heart defects, lack of
breasts and absence of
sexual maturation and
menstruation
Klinefelter’s syndrome –
XXY male
 sterile males with
underdeveloped testes,
overdeveloped breast
tissue and sub-normal
intelligence



Metafemale – XXX female
no obvious deficits however menstrual
irregularities and early menopause are
common
XYY male
taller than normal, recurrent acne, barely
normal intelligence
Cri du Chat syndrome
deletion of a portion of one copy of
chromosome number 5, malformed face
and head, short life
Societal Issues of Cell Division

Cloning – is a process in which identical
offspring are formed from a single cell or tissue
of the parent. Cloning occurs in nature when
plants send out runners, when bacteria divide
and in identical twins. Cloning as a technology
is more complicated.
How to Clone: (the quick and easy, yet not
so successful way)






Take a haploid egg cell from an adult female and
remove the nucleus (enucleation).
Replace the nucleus with the nucleus from a body
cell of the individual to be cloned. (the cell must
be toti-potent, or have its identity genes turned
off) (you can use stem cells here)
Zap it!!! (a little magic is required here)
Implant the zygote or blastula into the mother’s
uterus.
Wait until the gestational period is up and voila!
Click and Clone
Problems with cloning:



some think it is wrong!!!
artificially cloned organisms seem to age
faster
it is very time and resource consuming and
is not guaranteed to work
Ageing

is a normal part of life. Many believe that we are preprogrammed to age, since repeated divisions and a
long cellular life could lead to more genetic mutations.
There is a limited number of times (about 50) that cells
divide before they undergo a form of cell suicide called
apoptosis. Scientists believe that the ends of the
chromosomal arms, called telomeres, shorten each
time a cell divides, and when they become critically
short, the cell dies.
Cancer

involves abnormal, uncontrolled cell division.
Each time a cell divides, start and stop genes
are activated to tell the cell when it is appropriate
to replicate, and when to stop. In cancer, one of
these genetic switches quits working. Cancer is
dangerous for a number of reasons:




Cancer cells do not differentiate, so they are
not effective and do not participate in helping
the body to function.
Cancer cells require lots of nutrients, and take
nutrients away from functional cells preventing
other cells from doing their jobs.
Cancer cells can break away (metastasis) from
the tumour mass and spread to other parts of
the body.
NOVA Online | Cancer Warrior | How Cancer
Grows



Since many substances that are known to cause
mutations also cause cancer, it is believed that
cancer is a mutation of specific genes.
Tumour suppressor genes – suppress cell
division. (mutation turns these off)
Proto-oncogenes – stimulate cell division.
(mutation turns these on)