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Meiosis: Stages, Assortment, and Significance in Eukaryotic Life, Summaries of Cell Biology

An in-depth exploration of meiosis, a type of cell division used by eukaryotic cells to produce haploid cells. It covers the stages of meiosis, the processes of independent assortment and crossing over, and the importance of meiosis in transmitting genetic information during eukaryotic life cycles.

Typology: Summaries

2021/2022

Uploaded on 09/12/2022

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Meiosis and Sexual Life Cycles
In this topic we will examine a second
type of cell division used by eukaryotic
cells: meiosis.
In addition, we will see how the 2
types of eukaryotic cell division,
mitosis and meiosis, are involved in
transmitting genetic information from
one generation to the next during
eukaryotic life cycles. 2
Objective # 1
List, describe, diagram, and
identify the stages of meiosis.
3
Objective 1
Only diploid cells can divide by meiosis.
We will examine the stages of meiosis in
a diploid cell where 2N = 6
Meiosis involves 2 consecutive cell
divisions. Since the DNA is duplicated
only prior to the first division, the final
result is 4 haploid cells:
4
Objective 1
DNA duplication
during interphase
Meiosis I
Meiosis II
Overview of meiosis in a cell where 2N = 6
After meiosis I the cells are haploid.
5
Objective 1, Stages of Meiosis
Prophase I:
¾Chromosomes condense. Because of
replication during interphase, each
chromosome consists of 2 sister
chromatids joined by a centromere.
¾Synapsis the 2 members of each
homologous pair of chromosomes line
up side-by-side to form a tetrad
consisting of 4 chromatids: 6
Objective 1, Stages of Meiosis
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Meiosis and Sexual Life Cycles „ In this topic we will examine a second type of cell division used by eukaryotic cells: meiosis. „ In addition, we will see how the 2 types of eukaryotic cell division, mitosis and meiosis, are involved in transmitting genetic information from one generation to the next during eukaryotic life cycles. (^2)

Objective # 1

List, describe, diagram, and

identify the stages of meiosis.

3

Objective 1

„ Only diploid cells can divide by meiosis.

„ We will examine the stages of meiosis in a diploid cell where 2N = 6

„ Meiosis involves 2 consecutive cell divisions. Since the DNA is duplicated only prior to the first division, the final result is 4 haploid cells:

4

Objective 1

DNA duplication during interphase

Meiosis I

Meiosis II

Overview of meiosis in a cell where 2N = 6

After meiosis I the cells are haploid.

Objective 1, Stages of Meiosis Prophase I: ¾ Chromosomes condense. Because of replication during interphase, each chromosome consists of 2 sister chromatids joined by a centromere. ¾ Synapsis – the 2 members of each homologous pair of chromosomes line up side-by-side to form a tetrad consisting of 4 chromatids: (^6)

Objective 1, Stages of Meiosis

7

Objective 1, Stages of Meiosis Prophase I: ¾ During synapsis, sometimes there is an exchange of homologous parts between non-sister chromatids. This exchange is called crossing over.

8

Objective 1, Stages of Meiosis

9

Objective 1, Stages of Meiosis Prophase I: ¾ the spindle apparatus begins to form. ¾ the nuclear membrane breaks down:

10

Objective 1, Stages of Meiosis (2N=6)

Prophase I

11

Objective 1, Stages of Meiosis

Metaphase I:

¾ chromosomes line up along the equatorial plate in pairs, i.e. homologous chromosomes remain paired.

¾ spindle microtubules attach to the kinetochores on each centromere

¾ orientation of each pair of chromosomes is random and independent of the other pairs (independent assortment): 12

Objective 1, 4 Possible Metaphase I Arrangements:

Metaphase I

*We will follow this arrangement

19

Objective 1, Stages of Meiosis

Metaphase II:

¾ chromosomes line up along the equatorial plate

¾ spindle microtubules attach to the kinetochores on each centromere:

20

Objective 1, Stages of Meiosis (2N=6)

Metaphase II

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Objective 1 Stages of Meiosis

Anaphase II:

¾ centromeres split and each former chromosome becomes 2 separate chromosomes

¾ spindle microtubules contract pulling the 2 members of each pair of chromosomes (that were formerly sister chromatids) to opposite poles of the cell: (^22)

Objective 1, Stages of Meiosis (2N=6)

Anaphase II

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Objective 1, Stages of Meiosis

Telophase II:

¾ nuclear envelope reforms around the 4 sets of daughter chromosomes

¾ chromosomes uncoil

¾ cytokinesis divides the 2 daughter cells into 4, genetically unique, haploid cells:

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Objective 1, Stages of Meiosis (2N=6)

Telophase II

25

Objective # 2

Describe the processes of

independent assortment and

crossing over and explain their

biological significance.

26

Objective 2 „ Independent assortment and crossing over increase genetic variability among the daughter cells produced during meiosis. „ If there are n pairs of chromosomes in the original cell, independent assortment produces 2n^ possible chromosome combinations in the daughter cells:

27

Objective 2

Independent assortment produces 2n

possible chromosome combinations :

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Objective 2 „ In addition, because of crossing over, each chromosome involved in meiosis winds up with a different combination of genes. „ Therefore, an almost limitless number of gene combinations are possible in the daughter cells produced during meiosis:

Objective 2

With just 1 crossover, the number of possible gene combinations increases from 8 to 16!

Objective # 3

Compare mitosis with meiosis and explain the importance of each.