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Reproduction in the cells
Mitosis and Meiosis
Definitions
• Genome is the entirety of an organism's hereditary
information. It is encoded either in DNA or, for many types
of virus, in RNA.
• Diploid (indicated by 2x) cells have two homologous copies
of each chromosome, usually one from the mother and one
from the father.
• Haploid (n) is the number of chromosomes in a gamete of
an individual, and this is distinct from the monoploid
number (x) which is the number of unique chromosomes in
a single complete set.
• Gametes is a cell that fuses with another gamete during
fertilization (conception)—called an ovum (or egg)—and a
male produces the smaller tadpole-like type—called a
sperm.
Mitosis and Meiosis
• Mitosis and meiosis are an integral part of
cell division.
• Meiosis The resultant number of cells is four
times the number of original cells. This
results in cells with half the number of
chromosomes present in the parent cell.
• A diploid cell duplicates itself, then
undergoes two divisions (tetraploid to diploid
to haploid), in the process forming four
haploid cells. This process occurs in two
phases, meiosis I and meiosis II.
Mitosis vs Meiosis
• Mitosis The resultant number of cells in mitosis
is twice the number of original cells. The number
of chromosomes in the daughter cells is the
same as that of the parent cell.
• Mitosis occurs in somatic cells, while meiosis
occurs in gametes.
Prokaryotes Have a Simple Cell Cycle
•
Cell division in prokaryotes takes place in two
stages, which together make up a simple cell
cycle
1. copy the DNA

this process is called replication
2. split the cell in two to form daughter cells

this process is called binary fission
Prokaryotes Have a Simple Cell Cycle
• The hereditary information in a prokaryote is
stored in DNA
▫ the prokaryotic chromosome is a single circle of DNA
▫ DNA replication begins with the unzipping of the
double-stranded DNA at a point called the origin of
replication
▫ a new double helix is formed by adding
complementary nucleotides to the exposed DNA
strands that have been unzipped
▫ the end result of replication is that the cell possess two
complete copies of the hereditary information
Eukaryotes Have a Complex Cell Cycle
• Eukaryotic cells undergo two different
mechanisms to divide up the DNA
▫ mitosis is a cell division mechanism that occurs
in nonreproductive cells
 these cells are called somatic cells
▫ meiosis is a cell division mechanism that occurs
in cells that participate in sexual reproduction
 these cells are called germ cells
Eukaryotes Have a Complex Cell Cycle
• Mitosis (M phase)
▫ a microtubular apparatus binds to the
chromosomes and moves them apart
• Cytokinesis (C phase)
▫ the cytoplasm divides, creating two daughter cells
How the cell cycle works
Mitosis and Meiosis
• Mitosis:
-division of somatic (body) cells
• Meiosis
-division of gametes (sex cells)
Chromosomes
• Chromosome number varies among organisms
▫ most eukaryotes have between 10 and 50
chromosomes in their somatic cells
• Chromosomes are paired in somatic cells
▫ these pairs are called homologous chromosomes,
or homologues
▫ homologues contain information about the same traits
but the information may vary
▫ cells that have two of each type of chromosome are
called diploid cells
 one chromosome of each pair is inherited from the mother and
the other is inherited from the father
Chromosomes
• Prior to cell division, each of the homologous
chromosomes replicates, forming two identical
copies called sister chromatids
▫ the sister chromatids are joined together by a
structure called a centromere
▫ humans have 23 pairs of homologous
chromosomes
 when each chromosome in the pair is replicated, this
makes for a total of 92 chromatids
The difference between homologous
chromosomes and sister chromatids
Chromosomes
• A karyotype is an
arrangement of
chromosomes
• Chromosomes can be
compared based on
size, shape, and
centromere location
• The karyotype at right
shows the 23 pairs of
human chromosomes
The chromosomes of a
human
Chromosomes
• Chromosomes are comprised of chromatin, a
complex of DNA and protein
▫ there is also some RNA associated with
chromosomes
▫ the DNA in a chromosome is one very long
double-stranded fiber that extends unbroken for
the length of the chromosome
▫ the DNA is coiled in order to allow it to fit into a
small space despite being very long
Chromosomes
• DNA is coiled around proteins called histones
▫ the histones have positive charges to counteract the
negative charges associated with the phosphate groups
of the DNA
• The DNA coils around a core of eight histone
proteins to form a complex called a
nucleosome
▫ the nucleosomes in turn can be coiled together further
to ultimately form a compact chromosome
WHY MEIOSIS?
• MITOSIS – RESULTS IN GENETICALLY IDENTICAL
OFFSPRING – INCLUDING THE # CHROMOSOMES
• WHAT WOULD HAPPEN IF THE EGG AND SPERM
HAD THE SAME # OF CHROMOSOMES AS THE
BODY CELLS?
EGG = 46 CHROMOSOMES
SPERM = 46 CHROM.
ZYGOTE = 46 + 46 = 92 CHROMOSOMES =
NOT HUMAN
MEIOSIS
• A TYPE OF CELL DIVISION WHICH PRODUCES
GAMETES CONTAING HALF THE NUMBER OF
CHROMOSOMES AS THE BODY CELLS
• 2 STAGES – MEIOSIS I & MEIOSIS II
• START W/ 1 DIPLOID CELL, END UP W/ 4
HAPLOID CELLS (GAMETES)
• 4 DAUGHTER CELLS ARE GENETICALLY
DIFFERENT FROM EACH OTHER AND MOTHER
CELL
MEIOSIS
•
•
•
•
SPERM – MALE GAMETE (n)
EGG – FEMALE GAMETE (n)
FERTILIZATION PRODUCES A ZYGOTE (2n)
THIS TYPE OF REPRODUCTION IS CALLED
SEXUAL REPRODUCTION
STAGES OF MEIOSIS
• MEIOSIS I
▫ PROPHASE I, METAPHASE I, ANAPHASE I,
TELOPHASE I (PMAT)
• MEIOSIS II
▫ PROPHASE II, METAPHASE II, ANAPHASE II,
TELOPHASE II (PMAT)
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Interphase
• Interesting things happen!
1. Cell preparing to divide
2. Genetic material doubles
Prophase
• Chromosome pair up!
• Chromosomes thicken and shorten
•
-become visible
•
-2 chromatids joined by a centromere
• Centrioles move to the opposite sides of the
nucleus
• Nucleolus disappears
• Nuclear membrane disintegrate
Metaphase
• Chromosomes meet in the middle!
• Chromosomes arrange at equator of cell
• Become attached to spindle fibres by
centromeres
• Homologous chromosomes do not associate
Anaphase
• Chromosomes get pulled apart
• Spindle fibres contract pulling chromatids to the
opposite poles of the cell
Telophase
•
•
•
•
•
•
Now there are two!
Chromosomes uncoil
Spindle fibres disintegrate
Centrioles replicate
Nucleur membrane forms
Cell divides
Before Meiosis begins…
• As with mitosis, before meiosis begins, the DNA
in the original cell is replicated during S-phase
of the cell cycle.
• Two cell divisions separate the replicated
chromosomes into four haploid gametes or
spores.
Meiosis
• During meiosis, the genome of a diploid germ
cell, which is composed of long segments of DNA
packaged into chromosomes, undergoes DNA
replication followed by two rounds of division,
resulting in four haploid cells.
Meiosis
• Because the chromosomes of
each parent undergo
homologous recombination
during meiosis, each gamete,
and thus each zygote, will have
a unique genetic blueprint
encoded in its DNA. Together,
meiosis and fertilization
constitute sexuality in the
eukaryotes, and generate
genetically distinct individuals
in populations.
Meiosis
• 4 daughter cells produced
• Each daughter cell has half the chromosomes of
the parent
• 2 sets of cell division involved
Problems with Meiosis
• The normal separation of chromosomes in meiosis I
or sister chromatids in meiosis II is termed
disjunction.
• When the separation is not normal, it is called
nondisjunction.
• This results in the production of gametes which have
either too many or too few of a particular
chromosome, and is a cause genetic problems.
• Nondisjunction can occur in the meiosis I or meiosis
II, phases of cellular reproduction, or during
mitosis.
Problems with Meiosis
• Down Syndrome - trisomy of chromosome 21
(abnormalities)
• Patau Syndrome - trisomy of chromosome 13
(abnormalities)
• Edward Syndrome - trisomy of chromosome 18 (95% die
in utero) less than 1% live to the age of 10. On average they
only live 5-10 days.
• Klinefelter Syndrome - extra X chromosomes in males - ie
XXY, XXXY, XXXXY (infertile)
• Turner Syndrome - lacking of one X chromosome in
females - ie XO (Occurring in 1 out of every 2500 girls)
• Triple X syndrome - an extra X chromosome in females
(once in every 1,000 births) can be mistaken for behavior
problems
• XYY Syndrome - an extra Y chromosome in males (once in
every 1,000 births)
Stages of Mitosis
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•
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Interphase
Preprophase
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis
Interphase
• The mitotic phase is a relatively
short period of the cell cycle. It
alternates with the much longer
interphase, where the cell
prepares itself for cell division.
• Interphase is therefore not part
of mitosis. Interphase is divided
into three phases, G1 (first gap),
S (synthesis), and G2 (second
gap).
• During all three phases, the cell
grows by producing proteins
and cytoplasmic organelles.
Phases of Meiosis
• Meiosis I
• Meiosis I separates homologous chromosomes,
producing two haploid cells (23 chromosomes, N
in humans), so meiosis I is referred to as a
reductional division.
Phases of Meiosis
• Prophase I
• During prophase I, DNA is exchanged between
homologous chromosomes in a process called
homologous recombination. This often results in
chromosomal crossover.
Phases of Meiosis
• Metaphase I
• Homologous pairs move together along the
metaphase plate: As kinetochore microtubules
from both centrioles attach to their respective
kinetochores, the homologous chromosomes
align along an equatorial plane that bisects the
spindle, due to continuous counterbalancing
forces exerted on the bivalents by the
microtubules emanating from the two
kinetochores of homologous chromosomes
Phases of Meiosis
• Anaphase I
• Kinetochore microtubules shorten, severing
the recombination nodules and pulling
homologous chromosomes apart. Since each
chromosome has only one functional unit of a
pair of kinetochores[3], whole chromosomes are
pulled toward opposing poles, forming two
haploid sets.
Phases of Meiosis
• Telophase I
• The last meiotic division effectively ends when
the chromosomes arrive at the poles. Each
daughter cell now has half the number of
chromosomes but each chromosome consists of
a pair of chromatids.
Phases of Meiosis
• Meiosis II
• Meiosis II is the second part of the meiotic
process. Much of the process is similar to
mitosis. The end result is production of four
haploid cells (23 chromosomes, 1N in humans)
from the two haploid cells (23 chromosomes, 1N
* each of the chromosomes consisting of two
sister chromatids) produced in meiosis I. The
four main steps of Meiosis II are: Prophase II,
Metaphase II, Anaphase II, and Telophase II.
Phases of Meiosis
• Prophase II takes an inversely proportional time
compared to prophase I. In this prophase we see
the disappearance of the nucleoli and the
nuclear envelope again as well as the shortening
and thickening of the chromatids. Centrioles
move to the polar regions and arrange spindle
fibers for the second meiotic division.
• In metaphase II, the centromeres contain two
kinetochores that attach to spindle fibers from
the centrosomes (centrioles) at each pole. The
new equatorial metaphase plate is rotated by 90
degrees when compared to meiosis I,
perpendicular to the previous plate.
• This is followed by anaphase II, where the
centromeres are cleaved, allowing microtubules
attached to the kinetochores to pull the sister
chromatids apart. The sister chromatids by
convention are now called sister chromosomes
as they move toward opposing poles.
• The process ends with telophase II, which is
similar to telophase I, and is marked by
uncoiling and lengthening of the chromosomes
and the disappearance of the spindle.
• Nuclear envelopes reform and cleavage or cell
wall formation eventually produces a total of
four daughter cells, each with a haploid set of
chromosomes.
• Meiosis is now complete and ends up with four
new daughter cells.
Stages of Meiosis
Significance of Meiosis
• Meiosis facilitates stable sexual reproduction.
• Without the halving of ploidy, or chromosome count,
fertilization would result in zygotes that have twice the
number of chromosomes as the zygotes from the
previous generation.
• Successive generations would have an exponential
increase in chromosome count. In organisms that are
normally diploid, polyploidy, the state of having three or
more sets of chromosomes, results in extreme
developmental abnormalities
Significance of Meiosis
• Most importantly, recombination and
independent assortment of homologous
chromosomes allow for a greater diversity of
genotypes in the population. This produces
genetic variation in gametes that promote
genetic and phenotypic variation in a population
of offspring.
Cancer
• Cancer is a growth disorder of cells
• It starts when an apparently normal cell begins
to grow in an uncontrolled way.
• The result is a cluster of cells, called tumor.
• Benign tumors are completely enclosed by
normal tissue and are said to be encapsulated.
Cancer
• Tumors that grow in size eventully begins to
shed cells that enter the bloodstream through
the lymph system.
• This can form new tumors at distant sites call
metastases.
US Caner Stats
• One in every two Americans born in 2006 will be
diagnosed with some form of cancer during their
lifetime
• The US top 3 deadliest human cancers
▫ Lung
▫ Colon/Rectum
▫ Breast
How is cancer caused?
• Cancer can be caused by
▫
▫
▫
▫
Cigarette smoke
Environmental factors
UV rays (damage DNA)
Viruses (circumvent the cells normal growth)
How is cancer caused?
• Cancer results from damaged genes failing to
control cell division
• Damage to DNA, such as damage to genes, is
called mutation
• One gene seems to be a KEY regulator of the cell
cycle called the p53.
2 classes of growth factors genes
• Proto-oncogenes: encode proteins that stimulate
cell division. Mutation to these genes can cause
cells to divide excessively.
▫ Mutated proto-oncogenes become cancer-causing
genes call oncogenes
• Tumor-suppressor genes: cell division is turned
off by protiens encoded by tumor-suppressor
genes. Mutations to these genes essentially
“release the breaks” allowing the cell containing
the mutated gene to divide uncontrolled.
• It plays a key role in the G1 checkpoint of the cell
division.
P53 gene
• Monitors the integrity of DNA, checking that it
has been successfully replicated and is
undamaged.
• If the p53 protein detects damaged DNA, it halts
cell division and stimulates the activity of special
enzymes to repair the damages.
• In cases where the DNA cannot be repaired, the
p53 then directs the cell to kill itself, activating
an apoptosis (cell suicide).
P53 gene
• Abnormal p53 does not stop cell division and the
damaged strands is replicated, which results in
multiplied damaged cells---lead to cancer
• In 50% of cancers, the p53 cancer defense
malfunctions because of damage by chemicals or
radiation so that the protein code no longer
functions properly.
• In the other 50% defects like in other genes
because of damage that inhibit the p53 gene
Curing cancer worksheet