Cyto Report: Mitosis and Meiosis Chromosomes

7/27/2019 Cyto Report: Mitosis and Meiosis Chromosomes

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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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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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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.



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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.



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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.



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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.



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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.html


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Cyto Report: Mitosis and Meiosis Chromosomes

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