1. Meiosis , crossing over and linked genes
Dr.Aida Fadhel Biawi
2013

2. The Mitotic Process

3. Figure :Cell division cycle. A
dividing cell with 46 chromosomes passes
through the G1, DNA synthesis (S), and
G2 phases before entering mitosis, which
consists, in succession, of prophase (P),
metaphase (M), anaphase (A), and
telophase (T). The end result of the
cell division cycle is the formation of
two daughter cells, each with 46
chromosomes.

4. Figure : Essential features of the
phases of the cell division cycle of a cell
with two pairs of chromosomes.

5. Figure : Metaphase chromosome.
The centromere (primary constriction) is
marked by the downward diagonal
pattern. The kinetochore is the region of
attachment of spindle fibers that forms on
the outward portion of the centromere. At
metaphase the chromosome is duplicated,
and each complete chromosome is a
chromatid.

6. The Meiotic Process

8. Figure : Meiotic process. Meiosis I
consists of G1, DNA synthesis (S), and G2
phases; prophase I (PI), which includes
leptotene (L), zygotene (Z), pachytene
(Pa), diplotene (D), and diakinesis (Di);
metaphase I (MI), anaphase I (AI), and
telophase I (TI). The products of meiosis
I carry 23 duplicated chromosomes (23d)
from cells that entered meiosis I with 46
chromosomes. Meiosis II consists of a
combined G1/G2 phase followed by
prophase II (PII), metaphase II (MII),
and telophase II (TII). The final products
of the meiotic process are cells that
contain 23 chromosomes that comprise
one of each of the original set of 23 pairs
of chromosomes. In human females, each
meiotic process produces one functional
gamete and three nonfunctional cells
called polar bodies. In human males, all
meiotic products are functional.

13. Figure : Essential features of chromosome
distribution during the phases of
the meiotic process of a cell with two
pairs of chromosomes.

15. Figure : Overview of meiosis
Figure : Overview of meiosis. Meiosis is a form of cell division in which certain cells are set aside and give rise to haploid gametes.
This simplified illustration follows the fate of two chromosome pairs rather than the true 23 pairs. In actuality, the first meiotic division
reduces the number of chromosomes to 23, all in the replicated form. In the second meiotic division, the cells essentially undergo mitosis.
The result of the two meiotic divisions (in this illustration and in reality) is four haploid cells. In this illustration, homologous pairs of
chromosomes are indicated by size, and parental origin of chromosomes by color.

16. Meiosis – A Source of Distinction
Why do you share some but not all characters of each parent?
What are the rules of this sharing game?
At one level, the answers lie in meiosis.

17. Meiosis does two things -
1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid).
This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number.
In meiosis, one diploid cells produces four haploid cells.

18. Why do we need meiosis?
Meiosis is necessary to halve the number of chromosomes going into the sex cells
Why halve the chromosomes in gametes?
At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents

19. This makes for a lot of genetic diversity
This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over.
Genetic diversity is important for the evolution of populations and species.

20. Meiosis Parent cell – chromosome pair Chromosomes copied
1st division - pairs split
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

21. Meiosis I : Separates Homologous Chromosomes
Interphase
Each of the chromosomes replicate
The result is two genetically identical sister chromatids which remain attached at their centromeres

22. Prophase I This is a crucial phase for mitosis.
During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad.
Extremely IMPORTANT!!! It is during this phase that crossing over can occur.
Crossing Over is the exchange of segments during synapsis.

23. Metaphase I
The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles.
Still in homologous pairs

24. Anaphase I
The spindle guides the movement of the chromosomes toward the poles
Sister chromatids remain attached
Move as a unit towards the same pole
The homologous chromosome moves toward the opposite pole
Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

25. Telophase I This is the end of the first meiotic cell division.
The cytoplasm divides, forming two new daughter cells.
Each of the newly formed cells has half the number of the parent cell’s chromosomes, but each chromosome is already replicated ready for the second meiotic cell division

26. Cytokinesis Occurs simultaneously with telophase I
Forms 2 daughter cells
Plant cells – cell plate
Animal cells – cleavage furrows
NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS

27. Figure :The stages of meiotic cell division: Meiosis I

28. Meiosis II : Separates sister chromatids
Proceeds similar to mitosis
THERE IS NO INTERPHASE II !

29. Prophase II
Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator

30. Metaphase II
The chromosomes are positioned on the metaphase plate in a mitosis-like fashion

31. Anaphase II The centromeres of sister chromatids finally separate
The sister chromatids of each pair move toward opposite poles
Now individual chromosomes

32. Telophase II and Cytokinesis
Nuclei form at opposite poles of the cell and cytokinesis occurs
After completion of cytokinesis there are four daughter cells
All are haploid (n)

33. Figure 13.7 The stages of meiotic cell division: Meiosis II

34. One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment
Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.
In humans, n = 23 and 223 = ???!
That’s a lot of diversity by this mechanism alone.

36. Figure : Overview of meiosis
Figure : Overview of meiosis. Meiosis is a form of cell division in which certain cells are set aside and give rise to haploid gametes.
This simplified illustration follows the fate of two chromosome pairs rather than the true 23 pairs. In actuality, the first meiotic division
reduces the number of chromosomes to 23, all in the replicated form. In the second meiotic division, the cells essentially undergo mitosis.
The result of the two meiotic divisions (in this illustration and in reality) is four haploid cells. In this illustration, homologous pairs of
chromosomes are indicated by size, and parental origin of chromosomes by color.

37. Figure :Crossing over recombines genes. Crossing over
generates genetic diversity by recombining genes and thereby
mixing parental traits. The capital and lowercase forms of the same
letter represent different variants (alleles) of the same gene. A
chromosome actually has hundreds to thousands of genes.

38. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

40. - Genes close on a chromosome are packaged into the same gametes and are said to be “linked”
- Linkage has this very precise meaning in genetics. .
- Linkage refers to the transmission of genes on the same chromosome. Linked genes do not assort independently .

44. Boy or Girl? The Y Chromosome “Decides”
X chromosome
Y chromosome

45. Boy or Girl? The Y Chromosome “Decides”

46. Meiosis – division error
Chromosome pair

47. Meiosis error - fertilization
Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’.
In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

48. Thank You