Meiosis

CHROMOSOMES ARE MATCHED IN HOMOLOGOUS PAIRS

In humans, somatic cells (body cells) have:

• 23 pairs of homologous chromosomes and

• one member of each pair from each parent. The human sex chromosomes (Gonosomes)

X and Y differ in size and genetic composition.

The other 22 pairs of chromosomes are autosomes with the same size and genetic composition.

HOMOLOGOUS CHROMOSOMES ARE MATCHED IN:

• length,

• centromere position, and

• gene locations (locus). A locus (plural, loci) is the position of a gene. Different versions (alleles) of a gene may be found

at the same locus on maternal and paternal chromosomes.

HOMOLOGOUS CHROMOSOME PAIR

GAMETES HAVE A SINGLE SET OF CHROMOSOMES

• All organisms have different numbers of chromosomes.

• A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent.

•These two sets of chromosomes are homologous.

•Each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent.

• A cell that contains both sets of homologous chromosomes is said to be diploid.

• The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.

• For Drosophila, the diploid number is 8, which can be written as 2N=8.

DIPLOID

• Di= two sets• Cell that contains both sets of homologus chromosomes• Cell contains

• 2 complete sets of chromosome• 2 complete sets of genes

• Number of chrms in diploid cell represented by 2N• For Drosophilia (fruit fly) 2N=8• Mendel said:• Each adult cell contains two copies of each gene

• The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes.

• These cells are haploid. Haploid cells are represented by the symbol N.

• For Drosophila, the haploid number is 4, which can be written as N=4.

HAPLOID

• Means “one set”

• Refers to cells that contain only one set of chromosomes

• Gametes (sex cells)

• Represented by N

• Drosophilia fruit fly• N=4

WHAT IS MEIOSIS?

• Process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell

WHAT HAPENS DURING MEIOSIS?

• Each organism must inherit a single copy of every gene from each of its “parents.”

• Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set.

SUMMARY OF MEIOSIS

MEIOSIS HAS TWO STAGES

MEIOSIS I consisting of 5 phases: Interphase I, Prophase I, Metaphase I,

Anaphase I, Telophase I.MEIOSIS II consisting of 4 phases

Prophase II, Metaphase II, Anaphase II, Telophase II.

MEIOSIS I

Interphase I Prophase I Metaphase I Anaphase I Telophase I and

Cytokinesis

INTERPHASE I

Cell build up energyDNA Replication (to make duplicated chromosomes

Cell doesn’t change structurally.

PROPHASE I

• Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

• There are 4 chromatids in a tetrad.

• Non-sister chromatids exchange genetic material through the process of crossing over to ensure genetic variation.

• Centrioli move to opposite poles with spindle fibres between them.

CROSSING OVER

• When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over.

• Crossing-over produces new combinations of alleles.

METAPHASE I

Centrioli has reached the poles.

Homologous pairs align at the cell equator.

The two chromosomes attach to one spindle fiber by means of the kinetochore of the centromere.

.

ANAPHASE I

• The spindles pull homologous chromosomes apart to opposite poles/ends

TELOPHASE I AND CYTOKINESIS

• Duplicated chromosomes have reached the poles.

• A nuclear envelope and nucleolus re-forms around chromosomes.

• Each nucleus now has the haploid number of chromosomes.

• Cell invaginates forming a cleavage furrow, which extends to for 2 separate haploid cells.

MEIOSIS II

Meiosis II• The two cells produced by meiosis I now enter a

second meiotic division.

• Unlike meiosis I, neither cell goes through chromosome replication.

• Each of the cell’s chromosomes has 2 chromatids.

MEIOSIS II

Telophase I and Cytokinesis I Prophase II Metaphase II Anaphase II Telophase II

and Cytokinesis

PROPHASE II

• Chromosomes coil and become compact (if uncoiled after telophase I).

• Nuclear envelope and nucleolus, if re-formed, disappears again.

• Centrioli move to opposite poles, forming spindle fibers between them.

METAPHASE II

• Individual duplicated chromosomes align on the equator.

• One chromosome per spindle fiber attached by means of kinetochore of centromere.

• Centrioli has reached the poles.

ANAPHASE II

• Spindle fibers contract.

• Duplicated chromosomes split in half (centromere dividing in 2)

• Daughter chromosomes move to opposite poles.

TELOPHASE II AND CYTOKINESIS

• Daughter chromosomes has reached the poles.

• Two cells invaginate and form 4 daughter haploid cells (gametes)

• They uncoil and form chromatin.

• Nuclear envelope and nucleolus for around chromatin again.

• Centrioli for centrosome.

GAMETE FORMATION

DIFFERENCES BETWEEN MITOSIS ANS MEIOSIS

MITOSIS MEIOSIS• Cells produced by mitosis have

the same number of chromosomes and alleles as the original cell.

• Mitosis allows an organism to grow and replace cells.

• Some organisms reproduce asexually by mitosis.

ex: coral

• Cells produced by meiosis have half the number of chromosomes as the parent cell.

• These cells are genetically different from the diploid cell and from each other.

• Meiosis is how sexually-reproducing organisms produce gametes.

ALTERATION IN CHROMOSOME NUMBER

An extra copy of chromosome 21 causes Down syndrome or also known as TRISOMY 21.

A. Trisomy 21

• involves the inheritance of three copies of chromosome 21 and

• is the most common human chromosome abnormality.

DOWN SYNDROME

Trisomy 21 produces a characteristic set of symptoms, which include:

• mental retardation,

• characteristic facial features,

• short stature,

• heart defects,

• susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and

• shortened life span. The incidence increases with the age of

the mother.

Accidents during meiosis can alter chromosome number

Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during:

• meiosis I, if both members of a homologous pair go to one pole or

• meiosis II if both sister chromatids go to one pole.

Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes.

Alterations of chromosome structure

Chromosome breakage can lead to rearrangements that can produce:

• genetic disorders or,

• if changes occur in somatic cells, cancer.

These rearrangements may include:

• a deletion, the loss of a chromosome segment,

• a duplication, the repeat of a chromosome segment,

• an inversion, the reversal of a chromosome segment, or

• a translocation, the attachment of a segment to a non-homologous chromosome that can be reciprocal.

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REFERENCES

• http://www.slideshare.net/MissMabena/savedfiles?s_title=meiosis-9964891&user_login=anesh95

• Anesh Jeyakumar

• http://www.slideshare.net/MissMabena/savedfiles?s_title=meiosis-31958724&user_login=mikeu74

• mikeu74

• http://www.slideshare.net/MissMabena/savedfiles?s_title=meiosis-422302&user_login=itamarita1984

• itamarita1984

• http://www.slideshare.net/MissMabena/savedfiles?s_title=10-1-meiosis&user_login=cspavao

• cspavao•