1.5

Meisosis

cell division  produce gametes for sexual reproduction

crossing over:

the exchange of genes by homologous chromosomes

 

Homologus

chromosomes that have the same genes in the same order

Meiosis produces haploid cells -

These cells only have one copy of each autosome and one sex chromosomes

is important that the gametes have half the number of chromosomes so that the zygote (new cell formed) has the correct number .

Mums egg   +  Dad’s sperm   = Human zygote

     (23)                  (23)                  (46)

meiosis vs meitosis

P	MITOSIS	MEIOSIS
Where does it occur ?	All cells except sex cells. (somatic cells)	Ovaries and Testes
How many divisions occurs?	1	2
How many cells are produced?	2	4
Number of chromosomes in daughter the cells:	Same as parent cell	Half of parent cell
What types of organisms do this type of cell division?	Multicellular for growth and repair. Unicellular organisms for reproduction	Sexually reproducing organisms
Genetic variability of the cells produced.	Identical to each other and parent cell	Different to each other and parent cells

Purpose mitosis : growth and repair , meisosis: profuce gamtes for reprotudction

Whats a karyotype and how many homologus inchromosomes in karyotype human somatic cells:

Complete set of chromosomes , theirs 22 hologus in chromosmes and one pair of sex chromosomes so that equals 23

Crossing over in prophase 1

Note in Prophase 1 homologous chromosomes pair up.

Crossing over can occur here, where small segments of DNA can be exchanged between chromosomes. This increases genetic variation.

Theirs 8 phases in meiosis start with 1

Meiosis stage 1 : first division of meisos called "reduction " division cause it halves number of chromosomes ,

homologous , what are they and how they work

 

• Homologous chromosomes(pairs of chromosomes) l

 

Prophase 1

: nuclear membrane dissolves and DNA coils to become visible as pairs of homologous chromosomes. Crossing over occurs here as non-identical chromatids swap bits of information. This increases genetic variation.

Metaphase I:

• spindle fibers attach to the centromeres and pairs of homologous chromosomes line the equator of the cell

 

Anaphase I:

• Spindle fibres contract and pull pairs of homologous chromosomes apart, moving them to the opposite ends of the cell (poles)

 

• Telophase I

chromosomes uncoil, and new nuclear membranes form around the two sets of chromosomes. 2 haploid cells produced

 

Chromosomes rearrange during meiosis  , whats the process (cro’’)

 

• When the homologous pairs are aligned the DNA can become entangled.

 

• Pieces from one non-sister chromatid can exchange with another non-sister chromatid.

 

• This is called crossing over and helps to mix the maternal and paternal genes. This increases variation in the gametes

 

Meiosis stage 2 :

 

• At the end of this first division one chromosome from each homologous pair has been donated to each intermediate cell.

 

• The second division of meiosis is called a ‘mitotic’ division, because it is similar to mitosis.

 

• Prophase II:

 very similar to prophase I but now pairs of chromatids become visible

Metaphase II:

 Now single chromosomes (called a pair of chromatids) line up along the equator of the cell. This contrasts with Metaphase I where pairs of homologous chromosomes lined up

Anaphase II:

pairs of chromatids are pulled apart by spindle fibres

Telophase II:

 4 new nuclear membranes form around the separated chromosomes.

Cytokinesis II

•  Cytoplasm divides to produce 4 non identical daughter cells (gametes).

 

Each daughter cell is haploid.

Fertilisation

• fertilition introduces further variation because:

  • A male and female gamete fuse randomly.

  • Any sperm can fertilise any egg, increasing genetic diversity.