Cell Division 2 ( Meiosis ) Flashcards
What is meiosis?
- the division of a cell into four daughter cells with half the number of chromosomes
- It produces haploid cells
- it occurs in gametes ( reproductive/ sex cells)
- It results in four genetically different cells.
( In meiosis two cell divisions occurs ( meiosis 1 and meiosis 2)
Meiosis occurs in …..
Homologous chromosomes- they carry genes with the same characteristics
(in the same position)
Homologous pairs are..
Paired Chromosomes that seperate during meiosis and the gametes formed recieve only one chromosome of the pair.
These are called haploid (N) - containing half the number of chromosomes of a cell.
what happens when two gametes join
When two gametes join they grow a new individual.
This new individual will have the same
number of chromosomes as the adult.
Why are gametes haploid rather than diploid?
As new individuals were reproduced, they would contain more and more chromosomes. An excessive number of chromosomes in a cell causes abnormalities in the organism - ( E.g. Down’s syndrome in humans)
Crossing over
When two chromosomes exchange genes.
( occurs only in prophase I )
( Explain fully what occurs in prophase 1 )
- Chromosomes condense ( they become shorter and thicker ) and produce repica of themselves.
( CROSSING OVER )
* Chromatids of homologous chromosomes become entangled and exchange corresponding sections of DNA
** ( crossing over )
**
* The exchange of genetic material between the pairs of chromosomes creates gametes with a different genetic makeup from
the parent cell
** ( provides variatian amongst the species)
**
* Chromosomes are no longer homologous when they cross over.
WHAT OCCURS
* The centrosomes produce two star shaped asters on opposite ends of the cell.
- Nucleuar membrane and nucleolus diappears
What occurs in metaphase 1
- Homologous pairs line up at the center of the cell
- Orienation of each pair is random
What occurs in Anaphase 1
- Homologous pairs are pulled apart to opposite ends of the cell.
- Chromatids remain attached and don’t come apart
What occurs in Telophase 1
Two new cells form, each with half the number chromatids of the orginial (haploid)
Chromosomes arrive at opposite poles of the cell
Nuclear membrane re-forms and chromoses condense
In meiosis ll ….
Daughter cells undergo a second division like mitosis ( no additional DNA Replication occurs )
Teleophase ll ( Remember : every stage remain the same as Mitosis )
-The cell constricts at the center
- Four haploid daughter cells results where each chromosome has just one chromatid
- nuclear membrane reforms.
cytokenesis occurs: splitting of the cell into two new cells.
Importance of meiosis
- one diploid cell (2N) produced 4 haploid (N) cells. Known as
**” reduction division” **. - Reduction division makes it possible for off springs to have the same number of chromosomes as their parents when fertilization occurs.
