Life Sciences Term 1

Where does meiosis take place in animals?

Meiosis usually takes place in the reproductive organs of animals. The following diagrams illustrate where it takes place in males and females:

In men meiosis takes place in the testis and in women in the ovary as well as the fallopian tube.

Where does meiosis take place in plants?

In plants meiosis takes place during the production of spores. It usually takes place in the anther and ovule in flowering plants. The following diagram shows where meiosis takes place in plants.

Structure of a chromosome

It is important to know what a chromosome is and what the difference between a replicated and unreplicated chromosome. The diagram below illustrates the structure of chromosomes. It starts at DNA level and shows how DNA is supercoiled to form a chromosome.

DNA replication

Replication takes place during interphase where two copies of the chromosome is made. Each copy is now called a chromatid which is joined by a centromere. This is illustrated by the diagram below:

Chromosome number

• Every species has a specific number of chromosomes in the nucleus
• Somatic cells (body cells) have the diploid number (2n) (equal amount of chromosomes)
• There are two chromosomes of each kind, one from the mother and one from the father
• Sex cells (gametes) contain only half the number of chromosomes (n)
• When a female (n) gamete and a male gamete (n) fuse the resultant zygote is diploid (2n)
• Human somatic cells have 46 chromosomes in the nuclei and gametes have 23 chromosomes

The process of meiosis

Stages of meiosis

Meisosis l

Prophase 1

Chromosomes shorten and become visible as two chromatids joined by a centromere
Homologous pairs of chromosomes are now visible
The nuclear membrane and nucleolus disappear
The spindle starts to form

Chromatids from each homologous pair touch. The point where they touch is called a chiasma
DNA is crossed over (swopped) at the chiasma

1. The spindle extends across the whole cell
2. The homologous chromosomes line up along the equator of the spindle in their homologous pairs
3. One chromosome of each pair lies on either side of the equator.  Random arrangement takes place
   
4. The centromere of each chromosome attaches to the spindle fibres

Anaphase 1

1. Chromosomes- move to opposite sides of the cell, spindle fibers shorten
2. Cell begins to split:  a furrow forms in animal cells; a cell plate forms in plant cells

Telophase 1

1. The nuclear membrane re-forms around the chromosomes
2. The nucleolus reforms
3. Each pole has half the number of chromosomes present in the original cell

4. The cell membrane constricts and divides the cytoplasm in half to form two cells

Meiosis ll

Prophase 2

1. Centrioles move to opposite sides in an animal cell
2. Nuclear membrane and nucleolus disintegrate
3. Chromosomes formed with 2 chromatids
4. At the very end of prophase, the spindle forms

Metaphase 2

1. Centromeres, which hold the chromatids together, attach to spindle fibers
   Chromatids line up across the equator of the cell
   
2. Chromatids line up across the equator of the cell

Anaphase 2

1. Centromeres split and chromatids move to opposite sides of the cell
   
2. Cell begins to split:  a furrow forms in animal cells; a cell plate forms in plant cells

Telophase 2

1. The nuclear membrane re-forms around the single stranded chromosomes
2. The nucleolus reforms
3. The cell splits into four haploid sister cells
   
   

The difference between meiosis and mitosis:

	MITOSIS	MEIOSIS
Site where it occurs	Somatic cells	Ovaries and testis (Animals) Ovules & anthers (Plants)
Purpose of process	Growth	Formation of haploid gametes
No. of daughter cells produced	Two identical cells	4 haploid cells genetically different
No. of divisions	One nuclear division	Two nuclear divisions