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Mitosis and Meiosis
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Is the process of nuclear cell division.
It is the process of chromosome segregation
and nuclear division that follows replication
of the genetic material in eukaryotic cells.This process assures that each daughter
nucleus receives a complete copy of the
organism's genome.
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During division, the nucleus of the cell
divides, resulting in two sets of identical
chromosomes, or organized DNA proteins.
This process is almost always accompaniedby a process called cytokinesis, in which the
rest of the cell divides, leading to two
completely separate cells, called daughter
cells.
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Stages of Mitosis:
Chromosomes are in the dyad form before
mitosis, and in the monad form after mitosis.
Prophase:
--chromosomes condense--centrioles move to opposite ends of the cell
--spindle forms
--nuclear envelope disappears
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Metaphase:
--chromosomes are lined up on cell
equator, attached to the spindle at thecentromeres
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Anaphase:--centromeres divide. Now
chromosomes are monads--the monad chromosomes are pulledto opposite poles by the spindle.
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Telophase:
--chromosomes de-condense
--nuclear envelope re-forms
--spindle vanishes
--cytoplasm divided into 2 separate cells
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Sexual reproduction occurs only in eukaryotes.
During the formation of gametes, the number
of chromosomes is reduced by half, and returned to
the full amount when the two gametes fuse
during fertilization.
For comparison purposes, mitosis is the process of
cell division in eukaryotes, in which the parental
chromosome number is conserved in each of the
daughter cells, while meiosis is a two-cell-divisionprocess in sexually reproducing eukaryotes that
results in cells (typically gametes) with one-half the
chromosome number of the original parental cell.
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It is the form of eukaryotic cell division that
produces haploid sex cells or gametes (which
contain a single copy of each chromosome)
from diploid cells (which contain two copies of
each chromosome). The process takes the form
of one DNA replication followed by two
successive nuclear and cellular divisions
(Meiosis I and Meiosis II). As in mitosis,
meiosis is preceded by a process of DNAreplication that converts each chromosome into
two sister chromatids.
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WHY???
Without meiosis, there would be no sex because
every fusion of gametes would increase the
number of chromosomes in the progeny.
Sexual reproduction is an important survival
strategy.
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Before meiosis begins genetic
material is duplicated.
Followed by first division ofmeiosis.
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PROPHASE I
METAPHASE I
ANAPHASE I
TELOPHASE I
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Duplicated chromosomes condense and become
visible.
Spindle forms and synapsis occurs.
A mixture of RNA and proteins holds the
chromosome pairs together. At this time, the
homologs exchange parts.
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1.Longer duration
2.Cytogenetic eventsThe prophase first has been subdivided into five
consecutive stages: leptonema, zygonema,
pachynema, diplonema, diakinesis.
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During leptonema stage the chromosomes
appear as long single threads.
The centrioles move towards the oppositepoles of the cell and a definite type of
orientation and polarization of chromosomes
towards the centrioles take place.
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Homolog pairing initiated during leptotene
becomes much more intimate during
synapsis as the chromosomes become
linked by transverse fibers to form thesynaptonemal complex.
The SC is responsible for the genetic
variation and differentiation of species.
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During the zygonema stage the homologous
chromosomes pair with one another, gene by
gene, over the entire length of the chromosomes.
Paired homologous chromosomes.
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During the pachynema stage each paired
chromosome becomes shorter and thicker than in
earlier substages and splits into two sister
chromatids except at the region of the
centromere.
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During diplonema stage, chiasmata appear to
move towards the ends of the synapsed
chromosomes in the process of terminalization
and ultimately to slip off the ends.
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During diakinesis, the chromosomes begin to coil
and so become shorter and thicker.
The nucleolus detaches from the nucleolarorganizer and disappears completely
The nuclear envelope starts to degenerate and
spindle formation is well under the way.
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Paired homologous chromosomes align along the
equator of cell.
Homologous pair of chromosomes separate with
sister chromatids remaining together.
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Two daughter cells are formed with each
daughter containing only one chromosome of the
homologous pair.
Cell are divided into two.
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PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
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Presence of nuclear envelope fragments.
Spindle forms and fibers attach to both
chromosomes.
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Chromosomes align along equator of the cell.
Centromeres divide and sister chromatids migrate
separately to each pole.
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Nuclear envelopes assemble around two
daughter nuclei.
Chromosomes decondense.Spindle disappears.
Cytokinesis divides cells.
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A chromosome is an organized structure of DNAand protein that is found in cells. It is a single
piece of coiled DNA containing many genes,
regulatory elements and other nucleotide
sequences.Chromosomes also contain DNA-bound proteins,
which serve to package the DNA and control its
functions.
The word ''chromosome'' comes from the Greek(''chroma'', color) and (''soma'', body) due to their
property of being very strongly stained by
particular dyes.
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Chromosomes come in pairs. Normally, each cell
in the human body has 23 pairs of chromosomes
(46 total chromosomes). Half come from the
mother; the other half come from the father.
Two of the chromosomes (the X and the Y
chromosome) determine if you are born a boy or
a girl (your gender). They are called sex
chromosomes:
Females have 2 X chromosomes.
Males have 1 X and 1 Y chromosome.
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The mother gives an X chromosome to the child.
The father may contribute an X or a Y. It is thechromosome from the father that determines
if the baby is a girl or a boy.
The remaining chromosomes are called
autosomal chromosomes. They are known aschromosome pairs 1 through 22.
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Each chromosome has a constriction point calledthe centromere, which divides the chromosome into
two sections, or arms. The short arm of the
chromosome is labeled the p arm. The long arm of
the chromosome is labeled the q arm. Thelocation of the centromere on each chromosome
gives the
chromosome its characteristic shape, and can be
used to help describe the location of specific genes.
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Chromosomes vary widely between different organisms.
The DNA molecule may be circular or linear, and can be
composed of 10,000 to 1,000,000,000 nucleotides in a
long chain.
Typically eukaryotic cells (cells with nuclei) have largelinear chromosomes and prokaryotic cells (cells without
defined nuclei) have smaller circular chromosomes,
although there are many exceptions to this rule.
Cells may contain more than one type of chromosome;
for example, mitochondria in most eukaryotes and
chloroplasts in plants have their own small
chromosomes.
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FUNCTIONS:
Cell Division Process
Gene Packaging
Packaging of Regulatory Elements
DNA Packaging
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Every organism is defined by a blueprint
consisting of information stored on itsCHROMOSOMES
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Composed of enormously long circular or linear
molecules of DNA.
Contains the genetic information that defines
each organism- its GENOME.
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3 Classes of specialized DNA sequences areneeded to make a fully functional chromosome.
1. A Centromere
2. Two Telomeres3. An origin of DNA replication .
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Chromosomal locus that regulates the
movements of the chromosomes during mitosis
and meiosis.
Defined by specific DNA sequences plus proteins
that bind to them.
The heart
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Has an important role in monitoring the
attachment of the chromosomes to the spindle
and controlling the progress of the cells through
mitosis.STRUCTURE: waist-like structure or primary
constriction where two sister chromatids are most
intimately paired.
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Protects the ends of the chromosomal DNA
molecules and ensure their complete replication.
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A. Chromosomes found in Human
B. Chromosome can be categorized on the basis of
following criteria.
1. Number of centromeres
2. Position of centromeres
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Chromosomes are highly dispersed and difficult
to locate before cell division. When the cell begins
to divide, the chromosomes are seen clearly and
can be photographed as well as their morphology,size and shape can be studied.
The chromosome types are divided into two
categories
Autosomes
Sex Chromosome.
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A. Autosomes
Autosomes are structures that contain the hereditary
information.
They do not contain information related to reproductionand sex determination.
They are identical in both sexes, i.e., male and female
species of humans.
There are 46 (2n) chromosomes in humans. Of these 46
chromosomes,there are 44 pairs of autosomes and contain information
related to the phenotypic characters.
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B. Allosomes/ Heterosomes
The allosomes are sex chromosomes that are different from
autosomes in form, behavior and size.
There are a pair of allosomes in humans.
The X chromosomes are present in the ovum and either the Xor Y chromosome can be present in the sperm.
These chromosomes help in determination of sex of the
progeny. If the offspring receives X chromosome from the
mother as well as father, it results in a female child (XX). If the
offspring receives one X and one Y chromosome from theparents, it results in a male child (XY).
In simple words, it is the donation of X or Y chromosome by
the father that helps in determination of the sex of the child.
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Apart from these two categories, chromosomes can further be
divided according:
TO THE LOCATION OF THE
CENTROMERE
Metacentric Chromosome
Submetacentric Chromosome
Telocentric Chromosome
Subtelocentric Chromosome
Acrocentric Chromosome
NUMBER OF CENTROMERES
Acentric Chromosome
Monocentric ChromosomeDicentric Chromosome
Polycentric Chromosome
Holocentric Chromosome
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Based On The Centromere Position
Metacentric Chromosome
The metacentric chromosome has its centromere
centrally located between the two arms.
This gives the chromosome a typical 'V' shape
that is seen during the anaphase.
The arms of this chromosome are roughly equal
in length. In certain cells, fusion of two acrocentric
chromosomes leads to formation of metacentric
chromosome.
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Based On The Centromere Position
Submetacentric Chromosome
The arms of the submetacentric chromosome are
said to be unequal in length.
This is because the kinetochore is present in the
sub median position.
This gives rise to the 'L' shape of the
submetacentric chromosome.
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Based On The Centromere Position
Telocentric Chromosome
Also known as the monarchial type of
chromosomes,
they have a centromere that is located towards
the end of the chromosome.
telocentric chromosomes have a 'rod' shaped
appearance.
In some cases, the telomeres extend from boththe chromosome ends. The telocentric
chromosome is not present in humans. (ex.
rodents)
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Subtelocentric Chromosome
Chromosomes that have a centromere that is
located closer to the end than the center, are
called subtelocentric chromosomes.
In human Karyotype Y chromosome is
subtelocentric.
Based On The Centromere Position
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Acrocentric Chromosome
The location of the centromere in the acrocentric
chromosome is subterminal. This causes the
short arm of the chromosome to become really
short making it very difficult to observe.
Based On The Centromere Position
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Based On The Number Of Centromeres
Acentric Chromosome
Acentric chromosomes are those that lack
centromeres,
the centromere is totally absent on the
chromosome.
These chromosomes are observed due to effects
of chromosome-breaking process like irradiation.
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Monocentric chromosomes
contain a single centromere.
This type of chromosome is present in most of
the organisms.
The monocentric chromosomes can be called
acrocentric, metacentric , telocentric
chromosomes.
Based On The Number Of Centromeres
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Based On The Number Of Centromeres
Dicentric chromosomes
have two centromeres that are present on its
arms. These chromosomes are formed after two
chromosomal segments with a centromere each,
are fused end to end. This causes them to losetheir acentric fragments, leading to formation of
dicentric chromosome.
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Polycentric chromosomes
contain more than two centromeres.
These chromosomes are very common in plants,
for example, the Adder's-tongue fern has 1262chromosomes.
Based On The Number Of Centromeres
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Chromosomes Diffuse
Holocentric ChromosomeIn holocentric chromosomes, the centromere runs
through the entire length of the chromosome. These
chromosomes are very common in cells belonging to
organisms in the animal and plant kingdom.In this type of Chromosomes Diffuse the centromere
is non-localized.
the most well known example being the
nematode Caenorhabditis elegans
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Found in Human
AutosomesAllosomes
(Sex Cells)
22 pairs of chromosomes 1 pair of chromosome
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Based on the position of centromeres
TypePosition of
CentromereShape Details Examples
MetacentricCentre
(median)v-shaped equal arms
tradescantia,
amphibians
Sub-metacentricNear centre
(sub-median)J or L shaped Two unequal
armshuman being
Acrocentric
Near at one
end (sub-
terminal)
One arm very short and
another longgrasshopper
Telocentric Terminal Rod like drosophila
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Based on the Number of CentromeresType Number of centromeres
Acentric noneMonocentric OneDicentric TwoTricentric ThreePolycentric Many
Holocentric Diffused (Here, microtubules
are attached along the lengthof chromosome)
R f
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References:
For chromosomes:
http://www.buzzle.com/articles/types-of-
chromosomes.html
http://theagricos.com/genetics/chromosome/types
-of-chromosome/
http://www.buzzle.com/articles/types-of-chromosomes.htmlhttp://www.buzzle.com/articles/types-of-chromosomes.htmlhttp://theagricos.com/genetics/chromosome/types-of-chromosome/http://theagricos.com/genetics/chromosome/types-of-chromosome/http://theagricos.com/genetics/chromosome/types-of-chromosome/http://theagricos.com/genetics/chromosome/types-of-chromosome/http://theagricos.com/genetics/chromosome/types-of-chromosome/http://theagricos.com/genetics/chromosome/types-of-chromosome/http://theagricos.com/genetics/chromosome/types-of-chromosome/http://www.buzzle.com/articles/types-of-chromosomes.htmlhttp://www.buzzle.com/articles/types-of-chromosomes.htmlhttp://www.buzzle.com/articles/types-of-chromosomes.htmlhttp://www.buzzle.com/articles/types-of-chromosomes.htmlhttp://www.buzzle.com/articles/types-of-chromosomes.htmlhttp://www.buzzle.com/articles/types-of-chromosomes.html7/27/2019 Cyto Report: Mitosis and Meiosis Chromosomes
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