MEIOSIS

Formation of gametes, division of the sex cell (egg and sperm)

(Readapted from Slideshare): 1. Jay Swan

2. Karl Pointer3. Mbrown

Chromosome Matching

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:• Similar length,• Centromere (attaches sister chromatids together)

position • gene locations (locus).

• A locus (plural, loci) is the position of a gene.• Different versions or variations (alleles) of a

gene may be found at the same locus on maternal and paternal chromosomes.

MEIOSISThe process to make cells with half the number of chromosomes for

sexual reproduction

Usually humans and most animals and some plants have diploid (2n) body cells. Meaning that they have two sets of chromosomes (one from each parent)

Meiosis occurs in our germ cells that produce gametes (Sperm & egg)

• Meiosis results in four cells which are genetically different from parent cell and from each other.

• The end products of Meiosis are 4 Haploid (n) cells

Meiosis is a process that converts diploid nuclei to haploid nuclei.• Diploid cells have 2 sets of chromosomes.• Haploid cells have 1 set of chromosomes.• Meiosis occurs in the sex organs,

producing gametes—sperm and eggs.

Fertilization is the fusion of a sperm and egg cell.

The zygote has a diploid chromosome number, one set from each parent.

Why do we need Meiosis?

It is the fundamental basis of sexual reproduction

Two haploid (n) gametes are brought together through fertilization to form a diploid (2n) zygote

If egg and sperm had the same number of chromosomes as other body cells then the offspring would have too many chromosomes.

Meiosis must reduce the chromosome number by half (n)

Fertilization then restores the 2n number

Summary of the Meiotic process

MEIOSIS has two distinct 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 1:Interphase

Cell build up energy

DNA Replication (to make duplicated chromosomes

Cell doesn’t change structurally.

MEIOSIS 1:Prophase 1

Prophase 1in detail

Events occurring in the nucleus:• Chromosomes coil and become individual

chromosomes, nucleolus and nuclear envelope disappear.

• Homologous chromosomes come together as pairs by synapsis forming a tetrad (Each pair, with four chromatids)

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

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

PROPHASE 1

• Early prophase 1Homologous pair.Crossing over

occurs.

Late Prophase 1Chromosomes

condense.Spindle forms.Nuclear envelope

fragments.

Prophase 1: Crossing over

• Synapsis – the pairing of homologous chromosomes• Group of 4 chromatids

Homologous chromosomes(each with sister chromatids)

Join to form a TETRAD

Prophase 1: Crossing overHomologous chromosomes in a tetrad cross over each other- Genes are exchanged

Crossing over

Homologous pairs of chromosomes align along the equator of the cell

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

Metaphase ISpindle fibre attached to a kinetochore

Metaphase plate

Anaphase 1

Spindle fibers contact.Homologous chromosomes separate and move to opposite poles.

Sister chromatids remain attached at their centromeres.

Telophase 1

Nuclear envelopes reappear

Spindle fibres disappear.

Cytokinesis (when the cytoplasm divides) divides cell into two.

Telophase 1 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.

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

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

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

Meiosis ii: prophase 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.

Meiosis ii: metaphase II

Sister chromatids separate and move to opposite poles.

- Spindle fibers contract.

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

Anaphase 2

• 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.

Meiosis 2: Telophase 2

MEOITIC DIVISION 2: SUMMARY

Prophase II Metaphase II Anaphase II Telophase IIand Cytokinesis

Gametes (egg & sperm) form

Four haploid cells (n) with one copy of each chromosome

One allele of each gene

Different combinations of alleles for different genes along the chromosome

Results of Meiosis

When Chromosome number is altered

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.

Trisomy 21 (Down Syndrome) produces a characteristic set of symptoms, which include:

1. mental retardation,

characteristic facial features,

2. short stature,

3. heart defects,

4. susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and

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

mother.

Bibliography• Jackson, R., & Jackson, J. M. (2011). Henderson's Dictionary

of BIOLOGY (15th ed.). (E. Lawrence, Ed.) (pp. 181-183). London: Pearson Education Limited.• Livingstone, C. D., & Nobbe, M. E. (1998). The Molecules of

Life. Oxford: Biochemistry Department, University of Oxford.• Reece, J., Urry, L. A., Cane, M. L., Wasserman, S. A.,

Minrsky, P. V., & Jackson, R. B. (2011). Campbell Biology (9th ed.). San Francisco: Pearson Benjamin Cummings.• Urry, L. A., & Cane, M. L. (2011). The Molecular Basis of

Inheritance. In J. B. Reece, Campbell Biology (pp. 351-370). San Francisco: Pearson Benjamin Cummings.