Cell Divison Mitosis and Meiosis

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Transcript Cell Divison Mitosis and Meiosis

Cell Division
Mitosis and Meiosis
Types of Cell Division
occurs in all body cells (somatic cells) in
animals, plants, and humans
production of 2 identical daughter
cells that are diploid
growth or replacement of body cells.
asexual reproduction. (offspring and
parents are identical)
Types of Cell Division
2) Meiosis
 occurs only is sex cells (sperm and
 production
of the 4 non-identical
gametes (sex cells) that are haploid
 Sperm
and eggs have half the genetic
 sexual
The Cell Cycle
10% time dividing (mitotic phase/meiotic phase)
90% time interphase (G1, S, G2 phases)
1) G1 (gap phase) -manufactures proteins and
amino acids needed for both cell processes
and cell division, carries out metabolic duties
Cell grows
2) S phase (synthesis phase) – DNA
-longest stage of the cell cycle
3) G2 (gap phase)
 increases the rate of protein synthesis
and prepares to divide.
 grows larger
Mitosis or Meiosis
cell can divide only about 50 times on
To ensure that the cells in a tissue are
healthy, cells will undergo a form of cell
suicide called apoptosis.
Cell Death
Chromatin – complex long threads made
of DNA and protein that makes up
Uncondensed chromosome – long, thin
strands not visible with light microscope
Condensed chromosome – short and
Chromosome vs. Chromatid
Chromosomes replicate before division to
form 2 matched sister chromatids
Humans have 46 chromosomes (diploid or
Sex gametes have 23 chromosomes (haploid
or N
Some organisms have polyploidy
Ex) corn – 4n (tetraploidy)
Homologous chromosomes
–same size and shape and carry the genes for the
same traits, but have different details
 Humans have 23 homologous chromosomes
for a total of 46
basic unit of heredity
 sequence of nucleotide bases in DNA.
 codes for a specific proteins
Autosomes – chromosomes that do not
influence gender (humans have 22 pairs of
Sex Chromosomes – X and Y
chromosomes that determine gender in
humans (humans have 1 pair of these, the
23rd chromosome pair)
(longest phase)
Early: chromatin condenses to form
chromosomes, centrioles move to poles
and attach to spindle fibres
Late: spindle fibres attach to centromere,
nuclear membrane dissolves
chromosomes line up on the equatorial
plate (center of the nucleus)
Spindle fibres contract from the centrioles
and sister chromatids begin to separate
apart, immediately chromosomes being to
nucleolus re-appears, nuclear membrane
reforms, the chromosomes unravel to form
a loose mass of chromatin
cells undergo cytokinesis (divide
Plant vs Animal Cells
a cell plate forms
between the new
daughter cells
the cell membrane
pinches in between
the two daughter cells
Mitosis Overall
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
Meiosis – Sexual Cell Division
Occurs in sexual
The end result is 4
gamates that are
genetically different
4 sperms
Oogonium 4 eggs
Meiosis I – Reduction Division
Prophase I – homologous chromosomes
undergo synapsis (pair up) and crossing
over occurs.
-also, as in mitosis prophase:
Chromos condense
Nuclear membrane disappears
Centrioles move to opposite
Metaphase I – homologous pairs line up
along the equatorial plate, spindles insert
into the centromeres.
Anaphase I – homologous chromosomes
separate and are pulled to opposite poles
= segregation
Chromosomes undergo independent assortment
in during Anaphase I
-increases genetic diversity
Telophase I – daughter cells separate,
each has one chromosome from each
homologous pair.
-reduction has occurred so that the new cells
will be haploid (n)
 -cytokinesis occurs
Meiosis II – like mitosis but no initial
replication of DNA
-very brief, not as long as meiosis I
Prophase II –spindles form
Metaphase II –chromosomes align at the
equatorial plate
Anaphase II – the daughter chromatids
separate and move towards separate
Telophase II – spindle fibres disappear,
nuclei reform and cytokinesis takes place.
(All 4 daughter cells are haploid.)
How many genetic combinations
of gamates?
The number of possible chromosome
combination in a gamate is 2^n
 Where ‘n’ is the haploid number
 Ex. Humans n=23, thus 2^23 =8, 388, 608
different combinations!!
 Now
THAT’S diversity!!
NOVA Online | 18 Ways to Make a Baby |
How Cells Divide: Mitosis vs. Meiosis
Human Life Cycle
Fern Life Cycle
Oogenesis vs Spermatogenesis
Oogenesis (female)
meiosis II completes only
if egg is fertilized
Secondary oocyte
receives most of the
cytoplasm and becomes
the egg
polar bodies have less
cytoplasm and
one egg cell is produced
meiosis I and II are both
completed before
each sperm cell produced
receives an equal share
of cytoplasm
flagellum forms from one
of the centrioles
four spermatids are
Human Karyotype Chart
 Female
is XX
 Male is XY
Chromosomal abnormalities can be
detected by doing a karyotype chart.
Fetal cells are collected through
amniocentesis or chorionic villi
Successful cell division relies on 2
1. accurate replication of the
2. exact separation / distribution of the
Trisomy –three chromosomes
replace a normal pair (47
chromosomes in humans)
Abnormal Meiosis
Non-disjunction – occurs during
meiosis when two homologous
chromosomes move to the same pole.
Chromosomes do not separate properly
Normal Division
Some Common Genetic Disorders
Down’s syndrome – trisomy 21
 Extra 21 chromosome
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.
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, some have increase
XYY male
 taller than normal, recurrent acne
–one chromosome
replaces a normal pair (45
chromosomes in humans)
 Monosomy
Turner’s syndrome – XO female
 females with short stature, broad chest,
heart defects, lack of breasts and absence
of sexual maturation and menstruation
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
 identical offspring are formed from a single
cell or tissue of the parent. (similar to
when plants send out runners, when
bacteria divide and in identical twins.
How to Clone: (the quick and easy, yet not
so successful way)
Take an 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
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
Click and Clone
nucleus from udder cell put into egg cell
Problems with cloning:
some think it is wrong!!! (ethical issue)
artificially cloned organisms seem to
age faster
it is very time and resource consuming
and is not guaranteed to work
Cell Ageing
the ends of the chromosomes, called
telomeres, shorten each time a cell divides,
and when they become critically short, the cell
Totipotent: when cells
reach a certain number
of divisions, or age,
DNA begins to altered
abnormal, uncontrolled cell division.
May be caused due to mutation of genes
Tumour suppressor genes – suppress cell
division. (mutation turns these off)
 Proto-oncogenes – stimulate cell division.
(mutation turns these on)
Telomeres do not shrink in cancerous cells
Why is cancer bad
Cancer cells do not participate in helping
the body to function.
Cancer cells require and take away
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