1. MEIOSIS

2. Introduction Asexual reproduction: offspring are genetically identical
Living organisms reproduce. Reproduction may be:
asexual (without sex)
sexual (with sex)
Asexual reproduction:
offspring are genetically identical
involves:
mitosis (in eukaryotes)
binary fission (in prokaryotes)
Sexual reproduction:
offspring are genetically different
parents produce gametes or sex cells
fuse during fertilisation:
bring characteristics from each parent 2

3. Q. What do the terms haploid and diploid mean? haploid diploid
Q. Which of the cells are haploid?
Q. Which of the cells are diploid?  3

4. Mitosis leading to growth Mitosis leading to growth
Meiosis
Egg n=23
Sperm n=23
Fusion of gametes
Zygote n=46
Mitosis leading to growth
Mitosis leading to growth
Mitosis 4

5. Q. What do the terms haploid and diploid mean? haploid
contains a single set of chromosomes
in humans, n=23
diploid
contains two sets of chromosomes
in humans, 2n = 46
Q. Which of the cells are haploid?
the sex cells or gametes
sperm and egg
Q. Which of the cells are diploid? 
all of the cells that are not gametes!
zygote, embryo and body cells 5

6. Meiosis halves the chromosome number in the gametes so that when fertilisation happens, the normal diploid number is restored.
KEY POINT Without meiosis the chromosome number would double in successive generations.
Meiosis also has a key role introducing genetic variation into gametes and so into the offspring of sexually reproducing organisms.
Variation is also introduced by the mixing of genetic information from the two parents and from mutation. 6

7. Q. Why is genetic variation so important?
Natural selection operates on variation.
Some variants are adapted and so survive.
Without variation, evolution would not occur.
Changing conditions might result in extinctions of poorly adapted organisms. 7

8. Mitosis
In the Cells Exchange and Transport Unit you studied cell division (AS)
There are two types of cell division:
mitosis and meiosis
However, you will only have studied mitosis in any depth.
You may still need to know mitosis for your A2 exam! 8

9. Mitosis is a continuous sequence, but is divided into four stages:
prophase
metaphase
anaphase
telophase
Essentially:
chromatids are separated by contraction of spindle fibres
chromatids are pulled to opposite poles of the cell
the cell then divides
Each chromatid contains identical genetic information so each daughter cell also contains identical information. 9

10. During prophase the chromosomes become more distinct:
they coil up
shorten
thicken
take up stain more intensely
condense
The centriole divides.
Nucleolus becomes less prominent
Early Prophase 10

11. Late Prophase
The chromosomes have become more distinct and are seen to consist of two chromatids joined by a centromere.
The centrioles migrate to opposite poles of the cell.
The nucleolus continues to shrink and disappears.
The nuclear envelope disintegrates. 11

12. Each centriole is at a pole. Centrioles grow/produce spindle fibres.
Spindle fibres attach to the centromere of the chromosomes.
Each centromere is attached to both poles.
Chromosomes pulled to the metaphase plate or equator.
Metaphase 12

13. Spindle fibres contract: the centromere divides
chromatids (daughter chromosomes) are pulled to opposite poles of the cell
Pulled centromere first.
Each half of the cell receives one chromatid from each chromosome.
Anaphase 13

14. Chromatids reach the poles of the spindle: they begin to uncoil
they become less distinct
Nuclear envelope starts to reform.
Q. What are the chromatids known as when they reach the poles of the spindle?
Daughter chromosomes
Telophase 14

15. Cytokinesis The cell divides! In animal cells:
starts by constriction from the edges of the cell (invagination)
In plant cells:
a cell wall is laid down
Daughter cells have the same chromosome number and genetic make-up as each other and the parent cell – DNA replication precedes mitosis.
Cytokinesis 15

16. Q. Why is mitosis biologically important?
During mitosis two genetically identical daughter cells are produced whose nuclei contain the same number of chromosomes as the parent cell. Allowing:
growth
All cells of the organism are genetically identical.
repair
Replacement cells are the same as those they are replacing (genuine parts capable of doing an identical job vs cheap imitation that gives out quickly!).
asexual reproduction
Offspring are identical and able to colonise quickly. 16

17. Mitosis – The following slides show stages in Allium
Prophase (early)
Chromosomes coil and condense.
Nuclear envelope is present.
Nucleolus is evident.
Prophase (late)
Chromosome clearly visible as two chromatids joined at the centromere.
Nuclear envelope disappears.
Nucleolus disappears. 17

18. Metaphase
Spindle forms – some fibres attach to the centromeres, others run from pole to pole.
Chromosomes are pulled to the equator of the cell (metaphase plate) by contraction of the fibres.
Centromere splits and cell then enters anaphase. 18

19. Anaphase Chromatids move to opposite poles of the cell.
They are pulled centromere first by the contracting spindle fibres. 19

20. Telophase
Chromatids (now often called daughter chromosomes) reach the poles of the spindle.
Nuclear envelope reforms.
Nucleolus reforms.
Cell moves into cytokinesis or cell division. 20

21. Cytokinesis
As shown here, in plant cells a cell wall is laid down in the position of the metaphase plate. 21

22. Interphase
This stage comes between successive cell divisions. It is not really part of mitosis, but mitosis couldn’t happen without it.
DNA replication occurs (allowing for the double-stranded chromosome which later divides).
Cellular structures are made (subsequently divided between the two daughter cells).
A significant proportion of time is spent checking genetic information. 22

23. Mitosis in Allium
Prophase
Metaphase
Interphase
Anaphase
Telophase 23

24. Mitosis – The following slides show stages in whitefish
Prophase
Chromosomes coil and condense.
Nuclear envelope is present.
Nucleolus is evident.
The centriole replicates/separates.
In late prophase
Chromosome seen as two chromatids joined at the centromere.
Nuclear envelope disappears.
Nucleolus disappears. 24

25. Metaphase The centrioles reach poles and produce spindle fibres.
Spindle forms – some fibres attach to the centromeres, others run from pole to pole.
Chromosomes are pulled to the equator of the cell (metaphase plate) by contraction of the fibres.
So-called asters are visible
Centromere splits and cell then enters anaphase. 25

26. Anaphase Chromatids move to opposite poles of the cell.
They are pulled centromere first by the contracting spindle fibres. 26

27. Telophase
Chromatids (now often called daughter chromosomes) reach the poles of the spindle.
Nuclear envelope reforms.
Nucleolus reforms.
Cell moves into cytokinesis or cell division. 27

28. Cytokinesis The cell membrane invaginates or constricts.
This eventually pinches the cell into two. 28

29. …Mitosis
Interphase
This stage comes between successive cell divisions. It is not really part of mitosis, but mitosis couldn’t happen without it.
DNA replication occurs.
Cellular structures are made.
A significant proportion of time is spent checking genetic information.
As seen here cytokinesis merges into interphase with chromosomes disappearing. 29

30. Mitosis in Whitefish Prophase Metaphase Cytokinesis – Interphase
Anaphase
Telophase –Cytokinesis 30

31. Meiosis Meiosis involves two divisions: During Meiosis I:
genetic variation is introduced
‘crossing over’
‘independent assortment’
chromosome number is halved
During Meiosis II:
chromosomes (pairs of chromatids) are split
four haploid cells are produced 31

32. Homologous chromosomes pair up:
pairing is called synapsis
paired chromosomes are called bivalents or tetrads
The centriole divides and starts migrating to opposite poles of the cell.
Nucleolus is present, but less prominent.
Early Prophase I 32

33. Crossing over (chiasma formation) may occur:
especially in the longer chromosomes
adjacent chromatids can break and reconnect with another chromatid
introduces genetic variation
The nuclear envelope and the nucleolus disappear.
At the end of Prophase I the spindle is formed.
Late Prophase I 33

34. Bivalents (tetrads) line up at the metaphase plate.
Each bivalent is attached to a spindle fibre from opposite poles of the cell:
like in a tug of war, the paired chromosomes are pulled to the metaphase plate or equator
Metaphase I 34

35. Contraction of spindle fibres separate whole chromosomes.
Centromeres do not divide.
Note:
orientation of one bivalent is independent of the orientation of other bivalents
during anaphase either the maternal or paternal chromosome can pass into either cell
2n possible combinations (where n = haploid number)
Anaphase I 35

36. Chromosomes reach the poles of the spindle.
Following telophase:
animal cells:
usually divide
nucleolus and nuclear envelope reform
cytokinesis follows
plant cells:
often go straight into the second meiotic division
Telophase I 36

37. Prophase II and Metaphase II
The centriole divides, nucleolus and nuclear envelope disappear.
Spindle fibres attach to the centromeres:
one to each pole
pull chromosome to the metaphase plate or equator of the cell 37

38. The centromere divides:
Anaphase II
The centromere divides:
contraction of spindle fibres pull the chromatids to opposite poles of the cell
the chromatids are now sometimes called daughter chromosomes 38

39. Telophase II The daughter chromosomes:
reach the opposite poles of the spindle
start to decondense
nuclear envelope and nucleolus reform
Cytokinesis follows and results in the formation of four haploid daughter cells.
Note:
half the chromosome number
variation due to chiasma/cross-overs 39

40. What are the key markers for each of the stages of meiosis?
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II 40

41. Chromosomes arrive at poles. De-condense, nucleus reforms (detail)
Prophase I
Homologous chromosomes pair (bivalents present); chiasma (cross-overs) present; nucleolus disappears; envelope disintegrates. Centrioles.
Metaphase I
Bivalents line up at equator; chromosomes attached to only one pole; behaviour is independent.
Anaphase I
Homologous chromosomes separate; centromeres intact; chromosome number halved.
Telophase I
Chromosomes arrive at poles. De-condense, nucleus reforms (detail)
Prophase II
Chromosomes condense, envelope disappears.
Metaphase II
Chromosomes at equator, attached to each pole.
Anaphase II
Chromatids separate; centromeres break
Telophase II
Four haploid cells produces by cytokinesis. Chromosomes de-condense, nucleus reforms (detail) 41

42. What are the differences between mitosis and meiosis?
Prophase:
Chromosome pairing
Chiasma/cross-over
Attachment of spindle fibres
Number of divisions
Cells produced
Role 42

43. Mitosis Meiosis Prophase: Chromosomes unpaired
Chromosome pairing
Chromosomes unpaired
Pairing to form bivalents or tetrads
Chiasma/cross-over No
Yes, exchanges DNA between chromosomes
Attachment of spindle fibres
Each centromere attached to each pole
Meiosis I: centromere attached to one pole.
Meiosis II: centromere attached to both poles.
Number of nuclear divisions
One
Two
Cells produced
Two diploid (2n) cells
Four haploid (n) cells
Role
Growth, repair, asexual reproduction. Daughter cells identical.
Gamete production. Daughter cells different – variation and half chromosome number. 43

44. Meiosis in a Nutshell
During meiosis dissimilar daughter cells are produced. The nuclei contain half the number of chromosomes as the parent cell.
This halving prevents the chromosome number doubling in each generation of organisms during sexual reproduction.
Thus in humans:
somatic (body) cells have 46 chromosomes
gametes have 23 chromosomes
Fertilisation:
restores the diploid number in the zygote
introduces variation (genes from both mother and father!) 44