11ኛ ክፍል ባዮሎጂ በአማርኛ Grade 11 Biology Unit 4 Part 6

Описание: 11ኛ ክፍል ባዮሎጂ
11ኛ ክፍል ባዮሎጂ
በአማርኛ
በአዲሱ ትምህርት ፖሊሲ
Meiosis
A. Meiosis I
Prophase–I
Metaphase-I
Anaphase-I
Telophase-I
B Meiosis II
Prophase–II
Metaphase-II
Anaphase-II
Telophase-II
Meiosis
Meiosis is the division that produces sex cells (gametes).
It has two phases,
Meiosis I
Meiosis II
During Meiosis I, a cell duplicates all of its contents and divides into two daughter cells
Meiosis
it divides into four different daughter cells with haploid number of chromosomes in Meiosis II.
Meiosis reduces the number of chromosome by half and produces genetic variation through a process of crossing over and independent assortment
Meiosis cell division has eight stages (four stages for each meiosis)
. Meiosis I
In diploid organisms, chromosome exists in pairs
each members of the pair are called homologous chromosomes.
In meiosis I, homologous chromosomes are separated into two cells consisting of two chromatids (chromosome pair) in each daughter cell.
Meiosis I is also called reduction division. Why?
Prophase–I
Prophase I is the first step
chromosomes replicate to form two sister chromatids.
nuclear envelope(Nuclear Membrane) disintegrates,
The chromosomes begin to condense and spindle fibers appear.
Spindle fibers are attached to the chromosomes at centromeres.
A diploid cell contains two copies of every chromosome,
one derived from male gamete and the other from the female gamete.
These pairs of chromosomes are called homologous chromosomes.
Sister chromatids are the two chromatids of a replicated chromosome that are connected by the centromere.
A non-sister chromatid is one of the two chromatids of two homologous chromosomes
Prophase–I
Non-sister chromatids of homologous chromosomes form chiasmata to exchange genetic material during prophase I of meiosis I.
In this phase(Prophase–I), homologous chromosomes pair each other and crossing over takes place.
During crossing over,
homologous chromosomes exchange small parts to each other so that one chromosome contains parts of male and female DNA.
This results in an increase in genetic variation (Figure 4.14).
Crossing over increases the variability of a population.
Metaphase I
Metaphase I is the second step
The pairs of chromosome align next to each other along the center (equator) of the cell.
When the pairs of chromosomes line up randomly, they align themselves on either side of the equator.
The meiotic spindles extend from centrioles at opposites poles of the cell and attach to one chromosome of each pair (figure 4.15).
. Anaphase I
Anaphase I is the third step in part one of meiosis I
the pair of chromosomes are then pulled apart by the meiotic spindle.
Each of the homologous chromosomes get pulled towards opposite poles of the cell as the spindle fibres retract.
This equally divides the DNA between the two cells to be formed.
Unlike what happens in mitosis and meiosis II, the sister chromatids stay together (Figure 4.16).
Telophase I
Telophase I is the fourth step
The chromosomes complete their move to the opposite poles of the cell.
During this step,(Telophase I)
The spindle fibers disappear,
The full set of chromosomes gather together,
The nuclear envelope reforms and
A membrane forms around each set of chromosomes.
Cytokinesis
is the final phenomenon of Meiosis I
the single cell pinches in the middle to form two separate daughter cells
Daughter cells containing a half set of the parent chromosomes within a nucleus (Figure 4.17).????