Cambridge AS level Biology - Cell and nuclear division

1. 5 CELL AND NUCLEAR
DIVISION

2. LO:
• explain the importance of mitosis in the
production of genetically identical cells,
growth, repair and asexual reproduction;
• describe, with the aid of diagrams the
behaviour of chromosomes during the mitotic
cell cycle and the associated behaviour of the
nuclear envelope, cell membrane, centrioles
and spindle (names of the main stages are
expected)

3. Historic video from 1960

4. Importance of mitosis
• Growth
– Allows a zygote to produce more cell in order to
grow; daughter cells are genetically identical
• Repair and replacement
– Allow the multicellular organism maintain its
tissues, example skin cells and blood
• Asexual reproduction
– clone

5. CHROMOSOMES
• CHROMOSOMES

6. • Thread like structures in the nucleus
• Human cells – 46 chromosomes
• Karyogram
– all the chromosomes rearranged so that there are
matching pairs of chromosomes = homologous
pairs (on of each pair comes from the mother and
the other one from the father)
– last pair = sex chromosomes – determine the sex
of the individual (XX – female, XY – male)
– All the other chromosomes = autosomes

8. • Each chromosome has a characteristic set of
genes which code for different features
• Down syndrome – trisomy of the 21st pair
• Cystic fibrosis – chromosome 7

9. • Diploid cells (2n) = contain two sets of
chromosomes; somatic cells (46 in humans)
• Haploid cells (n) = contain only one set of
chromosomes; gametes (23 in humans)

10. • Chromosome before cell division is a double
structure – made of two identical chromatids
joined together by the narrow region =
centromere
– Each chromatid contains one DNA molecule
– DNA is made up of genes; each gene is a unit of
inheritance = coding for one polypeptide
– The gene for one characteristic is always found at
the same position = locus

12. Homologous chromosomes

13. • Homologous pairs of chromosomes
– Similar in structure and composition
– Each member comes from one of the parents
– Each homologous pair possesses genes controlling
the same characteristics (may exist in different
forms = alleles); a change in allele is called
mutation

14. TYPES OF NUCLEAR DIVISION
• GROWTH
– New cells are genetically identical, with the same
number of chromosomes = MITOSIS
• SEXUAL REPRODUCTION
– Number of chromosomes is halved = MEIOSIS;
gametes are always haploid; reduction division

16. LO:
• outline the cell cycle, including interphase
(growth and DNA replication), mitosis and
cytokinesis
• outline the significance of telomeres in
permitting continued replication and preventing
the loss of genes
• outline the significance of mitosis in cell
replacement and tissue repair by stem cells and
state that uncontrolled cell division can result in
the formation of a tumour

17. • describe, with the aid of photomicrographs and
diagrams, the behaviour of chromosomes in plant
and animal cells during the mitotic cell cycle and
the associated behaviour of the nuclear
envelope, cell surface membrane and the spindle
(names of the main stages of mitosis are
expected)
• observe and draw the mitotic stages visible in
temporary root tip squash preparations and in
prepared slides of root tips of species such as
those of Vicia faba and Allium cepa

18. MITOSIS IN AN ANIMAL CELL
• = Nuclear division that produces two
genetically identical daughter nuclei, each
containing the same number of chromosomes
as the parent nucleus

19. CELL CYCLE
Cell grows
• Cell cycle - The period
that extends from the
time a new cell is
produced until the time
the cell completes a cell
division.
• Mitosis: nuclear division
• Cytokinesis: cytoplasmic
division
• The cell cycle is divided
into two major phases:
1. Interphase
2. Mitosis

20. Interphase
The period between division, divided into 3 sub
phases (G1, S and G2):
i.G1- cells grow rapidly and new organelle are
synthetized
ii.S - synthesis of DNA and chromosomes are
replicated (2 chromatids)
iii.G2 - cells prepare for mitosis, synthesis
protein and mitotic spindle begin to form; DNA
is checked and errors are erased

21. Mitosis
• Mitosis is a division of the nucleus to produce
two new daughter cells containing
chromosomes identical to the parent cell.
• Mitosis is a continuous process and divided
into 4 main phases based on the appearance
and behavior of the chromosomes.
1. Prophase, P
2. Metaphase, M
3. Anaphase, A
4. Telophase, T

25. Prophase
Early prophase
• Chromosome condense and appear
shorter and thicker and become visible
in a light microscope
• Each chromosome now consists of a
pair of sister chromatids joined
together at centromere.
• Nucleolus disappears & nuclear
membrane disintegrates.
• Paired centrioles move to opposite
ends of the cell
Late prophase
• Nuclear membrane disappears
• Spindle form
Centrioles
Centromere

26. Metaphase
• Spindle fibres are fully
form
• Sister cromatids line up at
the spindle equator/
metaphase plate.
• Two sister chromatids are
still attached to one
another at the
centromere.
• At the end of metaphase,
the centromers divide.

27. Anaphase
• Anaphase begin with the
separation of the centromers
• The sister cromatids are drawn
to opposite poles of the cell by
shortening of spindle fibres.
• Once separated, chromatids
are referred to as daughter
chromosome.
• End anaphase: the poles of the
cell have complete and
equivalent sets of
chromosome.

28. Telophase
• Telophase begin when the two
sets of daughter chromosomes
have reached the two poles of the
cell.
• The spindle fibres disintegrate, the
nuclear membrane forms around
each set of daughter
chromosomes and the nucleoli
reappear.
• The nucleolus also re-forms in
each nucleus.
• The chromosomes uncoil and
become less visible under the light
microscope.
• Mitosis process is now completed.

29. Cytokinesis
 process of cytoplasmic division
to form two daughter cells.
• Different in animals and plants.
• Animal cells, the cytoplasm
contracts to pull the plasma
membrane inwards, forming
groove called a cleavage furrow
• In plant cells, cytokinesis in cells is
markedly different.
• A cleavage furrow does not form.
Instead, membrane-enclosed
vesicle gather at a plant cell’s
equator between the two nuclei.
• Vesicle fuse to form a cell plate

31. outline the significance of telomeres in permitting
continued replication and preventing the loss of genes

33. The importance of controlled
mitosis
• Controlled mitosis: the ability of the cell to divide at
its own rate and time.
• Crucial to normal growth, development and
maintenance for the perpetuity of living things.
• Eg: in plants  controlled mitosis occurs at shoots
results in growth of plants.
in human Malphigian layer of skin divides actively
to replace dead and damaged cells.

34. The effects of uncontrolled mitosis in
living things
• Mutation is the change in the DNA structure of
the cell.
• This change in the DNA corrupts the coded
genetic instructions for mitosis control.
• This leads to uncontrolled mitosis, which is the
non-stop division of cells, producing a mass of
new daughter cells, called tumour.

35. Uncontrolled mitosis

36. Causes of Cancer
 Genetic- some forms of cancer like prostate, colon, breast,
skin, ovary are suspected to be inherited from the parents
 Carcinogens- these are chemicals which affect genetic
activity and cause cancer, e.g. of carcinogen a diesel
exhaust, cigarette smoke, hair dyes, soot, arsenic, benzene
and formaldehyde.
 Radiation- excess exposure to x-ray, gamma-rays and ultra
violet rays lead to increase cancer risk.
 Viruses- some viruses (such as the HPV and HIV-1) cause
cancer.

37. Application of Mitosis in Cloning
• Mitosis is used to improve the quality, to
produce new species and to ensure
uniformity in the traits.
• Cloning: process of artificially creating a new
individual that is genetically identical to an
existing individual.
• Technique in application of mitosis in cloning 
tissue culture.

38. Application on mitosis in cloning
Plant tissue
Skin tissue

39. HW:
• End of chapter questions