Meiosis

NEREA LÓPEZ PEIRONA 4ºB

Significance of Meiosis

(1) Crossing over in bivalent- Produces new combinations of

genes in both chromosomes

Chiasma and Crossing Over

Significance of Meiosis

(2) Reduction and fusion of gametes- Meiosis produces haploid gametes- In sexual reproduction, a male

gamete fertilizes a haploid female gamete to produce a normal diploid zygote

Significance in Mitosis

(3) Independent (random assortment)- During metaphase I, homologous

pairs of chromosomes align at the equator

- It is by chance which “way round” each pair lies, before these homologous pairs of chromosomes separate into two different daughter cells.

Cell Cycle

Stages of Meiosis

Meiosis I

Prophase IMetaphase IAnaphase ITelophase I

• Meiosis II

Prophase IIMetaphase IIAnaphase IITelophase II

Interphase

Interphase is an important stage preceding meiosis. Without this stage

meiosis would not occur. During this stage, each individual

chromatid replicates, similar to mitosis. B replicates B and b replicates b

At this stage, the chromosomes are long and stringy and are not visible.

**Remember: All somatic cells are diploid in number (2n), therefore for each

chromatid there also exists its homolog, which also replicates during interphase.

Early prophase I

During early prophase it is the same as in mitosis

Chromosomes

Nuclear envelope

Spindle pole

Prophase I

Now, during prophase, also occurs the crossing over, where the homologous chromosomes

exchange information.

Spindle fiber

Metaphase I

The chromosomes align in the middle

Telophase I

Each chromosome separates from the

other, making two news.

Meiosis I Flowchart

After telophase I After telophase I

the second meiotic division the second meiotic division occurs occurs (without DNA duplication (without DNA duplication before), before),

as a final result we have 4 as a final result we have 4 haploid cells)haploid cells)

Prophase II

The nuclear membranes of the daughter cells disintegrate again. The spindle fibres re-form in each daughter cell

Metaphase II

The chromosomes, each still made up of sister chromatids, are positioned randomly on the metaphase plate with the sister chromatids of each chromosome pointing towards the opposite poles.

Metaphase II

Each sister chromatid is attached to the spindle fibres at the centromere

Anaphase II

The centromeres of the sister chromatids finally separate, and the sister chromatids of each chromosome are now individual chromosomes.

The chromosomes move towards the opposite poles of the cell.

Telophase II

Finally, the nucleoli and nuclear membranes re-form. The spindle fibres break down.

Cytokinesis follows and four haploid daughter cells are formed, each containing half the number of chromosomes and is genetically different from the parent diploid cell.

Telophase II

These haploid cells will develop into gametes.

Males: all 4 hapoid cells become sperm

Females: in oogenesis, only 1 of the haploid cells becomes an egg, and the other 3 are reabsorbed by the body.

Human genetics = 46 chromosome (2n)23 pairs of chromosome

Mutation

Mutation is a change in structure, arrangement or quantity of the DNA in the chromosome

May be caused by: Mistakes in the replication of DNA Damage to the DNA by radioactive

and carcinogenic substance Disruption to the orderly movement

of chromosomes during cell division

In Mitosis

If the functions of these genes are disrupted due to mutation, cancers may form.

Somatic mutations are not transmitted to the offspring, but may cause body cells to malfunction

Cancers are caused by somatic mutation

In Meiosis

Meiosis involves an orderly movement and reduction (in meiosis I) of a diploid cell to two haploid cells that subsequently divide (in meiosis II) to form four haploid gametes

Since these are gametes, so any mistakes – caused by disorderly movement of chromosomes during meiosis --- are inherited by the offspring.

Example: non-disjunction or improper segregation (separation) of chromosome

During anaphase I, certain homologous chromosomes fail to segregate, resulting in the production of gametes with either an extra chromosome (n+1) or a missing chromosome (n-1)

If this abnormal gametes unites with a normal gamete, an abnormal zygote will be produced.

Down’s syndrome

3 copies of chromosomes number 21, instead of the normal 2 chromosomes

This means a down syndrome patient has (2n+1 = 47) 47 chromosomes instead of the normal (2n=46) chromosomes

Trisomy 21/ Down Syndrome

References

TRabajo de clase para Biología MEC-BC. IES Pedro de Luna- 2011- Zaragoza SPAIN

by imkaelah on Jul 24, 2013

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Rebecca Choong

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by Anesh Jeyakumar on Oct 31, 2011

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