Mitosis and meiosis

1. Chapter 5 Cell Division
Ong Yee Sing
14/06/2017

2. Objectives
• Identify the phases in a cell cycle
• Understanding the processes of mitosis and meiosis
• State the importance of mitosis and meiosis
• Explain the importance of controlled mitosis
• Understanding the process of cloning and its characteristics

3. Chromosomes
• DNA are stored as chromosomes in the nucleus.
• Chromosomal number is the characteristics number
of chromosomes of a species.
• In some cases, chromosomes can be found in pair,
which share the same structure, referred as
homologous chromosomes.
• Somatic cells of a diploid organisms contain two
pair of chromosome (2n).
• Gametes or haploid organisms contain only one set
of chromosome (n).
染色体
染色体数
同源染色体
二倍体体细胞
配子 单倍体

4. Karyotype
• A karyotype is the number and appearance of chromosomes in the
nucleus of a eukaryotic cell.
A female genome visualized with fluorescent in situ
hybridization (FISH).
Karyotype variability in different cultivars of maize demonstrated
using FISH.
核型

5. The cell cycle
• The cell cycle is a cyclic
sequences of events that a cell
undergoes, from the time it is
formed until it divides
completely into two.
• Two main phrases in the cell
cycle is:
• Interphase 间期 – cell grows and
prepares for cell division
• Cell division (M phase) – cell
divides its nucleus and
cytoplasm
细胞周期
循环

6. Interphase
• Interphase can be divides into G1, S and G2 phases.
间期

7. G1 phase
• The first gap phase where the chromosomes are
not very visible and appeared as chromatin.
• The cell grows larger.
• If the environmental condition is flavourable, the
cell will decide to proceed to cell division.
• The centrosome duplication process starts.
Left: Fluorescence microscopic images of nuclei and
chromosomes of BY-2 cells
染色质
中心体

8. S phase
• Synthesis of DNA occurs in this phase.
• DNA is replicated and each chromosome contain two identical sister
chromatids that are joined together by a centromere.

9. G2 phase
• The cell continue to grow in the second
gap phase.
• Organelles are synthesis and metabolism
is active.
• The DNA are present as chromatin.
• The pair of centrosomes has matured.
Fluorescence microscopic images of nuclei and chromosomes
of BY-2 cells

10. Cell division 细胞分裂
• Cell division is the process of forming of new cell(s) from pre-existing
cell.
• Cell division can be achieved by the process of mitosis or meiosis,
followed by cytokinesis.
• Mitosis is the division of the cell nucleus which results in the
production of two identical daughter nuclei.
• Meiosis is a the division of the cell nucleus which results in the
production of four unidentical daughter nuclei with half of the
chromosome number of the parent cell.
• Cytokinesis is the division of the cytoplasm into two.
有丝分裂 减数分裂
胞质分裂 细胞质

11. 5.1 Mitosis

12. Significance of mitosis 有丝分裂
• Increase number of cells for growth
• Replacing dead or worn out cells, such as blood cells, epithelial cells
• Repair injured organs and regenerate lost parts, such as tail of a lizard
• Asexual reproduction
• Binary fission in Amoeba
• Budding in yeast
• Vegetative reproduction in plants using rhizomes, tubers and bulbs
无性繁殖

13. The M phase (mitosis and cytokinesis)
• Mitosis can be subdivided
into four continuous phases:
prophase, metaphase,
anaphase and telophase.
• Following telophase,
cytokinesis occurs.

14. Prophase
• In the nucleus, chromosomes condense,
appear shorter and thicker and are
visible under the light microscope.
• In the cytoplasm, spindle fibres begin to
form from the centrioles. Each pair of
centrioles migrates to the opposite site
poles of the cell.
• The chromatids are attached to the
spindle fibres by their centromeres.
• The nucleolus disappears and the
nuclear membrane disintegrates at the
end of prophase.
纺锤体
着丝粒
核仁
核膜
前期

15. Metaphase
• Centromeres of all the chromosome are
lined up on the metaphase plate.
• The mitotic spindle is now fully formed.
• Each sister chromatid is attached to a
spindle fibre originated from opposite
pole.
赤道板
中期

16. Anaphase
• The two sister chromatids of each
chromosome separate at the centrode.
• The sister chromatids are pulled apart to
the opposite poles by the shortening of the
spindle fibres.
• Once separated, the chromatids are
referred as daughter chromosomes.
• By the end of anaphase, two poles of the
cell have complete and equivalent set of
chromosomes.
后期
姐妹染色单体

17. Telophase
• The chromosomes uncoil and revert back
to extended chromatin.
• The chromosomes become less visible
under the microscope.
• The spindle fibres disappear.
• New nuclear membrane forms.
• Nucleolus re-forms in each nucleus.
末期

18. Cytokinesis in animal
• In animal cell, actin filaments in the
cytoplasm contracts to pull a ring of the
plasma membrane inwards, forming a
groove called a cleavage furrow.
• Eventually the cell breaks at the
constricted region and two daughter
cells are form.
胞质分裂
槽 卵裂沟

19. Cytokinesis in plant
• In the plant cells, vesicles gather at a
plant cells equator to form a cell plate.
• The cell plate grow until it fuses to
from new cell walls and plasma
membrane.
胞质分裂
细胞板

20. Mitosis Summary

21. Comparison of mitosis in plant and animal
• Similarities
• Involves interphase, prophase, metaphase, anaphase, telophase, cytokinesis
• Nucleus divides once and cytoplasm divide once
• Daughter cells are genetically identical to the parent cells.
• Differences
Animal cell Plant cell
Spindle threads emerge from
centrioles.
Spindle threads do not emerge from
centrioles.
Cytokinesis occurs by constriction of
the cytoplasm – cleavage furrow is
formed.
Cytokinesis occurs by the formation of
cell plate.

24. Importance of controlled mitosis
• Correct number of chromosomes is
obtained by the daughter cell
• Correct growth, development and
maintenance of tissues
• To avoid presence of cancerous cells that
divide uncontrollably and form tumour
Left: Atomic-force microscopy
(AFM) maps of adhesion of
the AFM probe to the cell
surface of (a) normal, (b)
immortal (premalignant), and
(c) cancer cells. SEM images of
(d) normal, (e) immortal, and
(f) cancer cells. AFM provided
a higher 3D resolution
compared to SEM. [Sokolov et
al, New Journal of Physics]
Top, Spectral karyogram of normal human male chromosomes.
Image by Nallasivam Palanisamy, Ph.D., Michigan Center for
Translational Pathology. Bottom, Spectral karyogram of a
breast cancer cell line. Image by Mira Grigorova and Paul
Edwards

25. Application of mitosis: cloning
• Cloning is the process of producing
genetically identical organisms through
asexual reproducing, such as budding,
binary fission, grafting, stem cuttings and
tissue culture.
• Tissue culture is the growth of tissues and
cells in sterile culture medium.
• Plant cells will form undifferentiated
callus, but later can developed into
embryo, plantlets and adult plant.
• Genetic engendering can altered the genes
of the plant to enhance crop performance.
组织培养
愈合组织
胎
苗

26. Application of mitosis: animal cloning
• Animal cloning
involves the transfer of
the nucleus from a
somatic cell to an
ovum or embryonic
cell with the nucleus
removed.

27. 5.2 Meiosis

28. Meiosis 减数分裂
• Meiosis is a type of cell division that reduces the number of
chromosomes in the parent cell by half and produces four gamete
cells.
• This process is required to produce egg and sperm cells for sexual
reproduction.
• Sexual reproduction involves the fusion of two haploid gametes (n)
during fertilization to produce a diploid zygote (2n).
• Meiosis is divided into meiosis I and meiosis II.
二倍体
单倍体
有性生殖

29. Meiosis I
• Meiosis I takes place after the interphase.
• Here, gene recombination occurs here to generate diversity in the
offspring.
• The chromosomal number is reduced in the daughter cells produced
in meiosis I.

30. Prophase I
• The chromosomes condensed.
• The homologous chromosomes come
together to form bivalents (tetrads) through a
process called synapsis.
• Non-sister chromatids exchange segments of
DNA in a process called crossing over.
• The point at which the chromatids cross over
is called chiasmata.
• At the end of prophase I, the nucleolus and
nuclear membrane disappear, centrioles
migrate to the opposite poles of the cell and
spindle fibre form.
交點
「二價體」或「四分體」聯會
染色體的交換
前期 I

31. Crossing over of homologous chromosome
同源染色体联会

32. Metaphase I
• The chromosome are lined up side by side
as tetrads on the metaphase plate.
• Each tetrad is attached with two spindle
fibre from the opposite pole.
• The centromere does not divide.
中期 I

33. Anaphase I
• The spindle fibres pull the homologous
chromosome away.
• Each chromosome still consists of two
sister chromatids.
后期 I

34. Telophase I
• Each pole has a haploid daughter nucleus
because it contains only one set of
chromosome.
• The spindle fibres dissapear.
• The nuclear membrane and nucleolus
reappear.
• Cytokinesis usually occurs simultaneously
with telophase I.
末期 I

35. Meiosis II
• Meiosis II generally occurs immediately after cytokinesis, usually with
no interphase between them.
• The chromatids still have two sister chromatids.
• In meiosis II, the sister chromatids will be separated.

36. Prophase II
• Chromosome condensed.
• The nuclear membranes disintegrate.
• Spindle fibres reformed.
前期 II

37. Metaphase II
• The chromosome are rearranged in the
metaphase plate with the sister
chromatids of each chromosome pointing
towards the opposite poles.
• Each sister chromatid is attached to the
spindle fibres at the centromere.
中期 II

38. Anaphase II
• The centromeres of the sister chromatids
separate.
• The sister chromatids become individual
chromosome.
• The chromosomes movet owards the
opposite poles of the cell.
后期 II

39. Telophase II
• Nucleolus and nuclear membrane reform.
• Spindle fibre break down.
• Cytokinesis occurs after telophase II.
• The daughter cells are haploid.
末期 II

41. Meiosis I summary
The cells are haploid.

42. Meiosis II summary

43. Comparison between mitosis and meiosis
Mitosis Meiosis
One nuclear division Two nuclear division
Two diploid daughter cell Four haploid daughter cell
Daughter cells identical to parent cell Daughter cells unlike parent cell
No genetic variation in daughter cells Genetic variation in daughter cells

44. 5.3 Appreciating
the movement
of
chromosomes
during mitosis
and meiosis

45. Mutation
• Mutation is the permanent alteration of the nucleotide sequence of
the genome of an organism, virus, or extrachromosomal DNA or other
genetic elements.
• Radiation, carcinogenic chemicals such as nicotine, and food
preservations (sodium nitrite 亚硝酸钠, benzene 苯, formaldehyde
甲醛) are known to cause changes to the DNA molecule or disturb
mitosis and meiosis.
• Mitosis and meiosis gone awry can lead to mutation.
突变
致癌

46. Down’s Syndrome
• Down syndrome is usually caused by an error in
cell division called "nondisjunction."
• Nondisjunction is the failure of one or more pairs
of homologous chromosomes or sister chromatids
to separate normally during anaphase I or II.
• Nondisjunction results in an embryo with three
copies of chromosome 21 instead of the usual
two.
• Characteristic of affected individuals have slanted
eyes, short stature and mental retardation.
染色体不分离
唐氏综合症

47. Summary
• Two main phrases in the cell cycle is:
• Interphase – cell grows and prepares for cell division
• Cell division (M phase) – cell divides its nucleus and cytoplasm
• Mitosis is the division of the cell nucleus which results in the
production of two identical daughter nuclei.
• Meiosis is a the division of the cell nucleus which results in the
production of four unidentical daughter nuclei with half of the
chromosome number of the parent cell.
• Down’s syndrome is usually caused by an error in meiosis.
Characteristic of affected individuals have slanted eyes, short stature
and mental retardation.