1. UNIT
Cell life Cycle
MWEMBA F. MUTALIFE

2. Terminologies
 DNA- Deoxyribonucleic Acid
 Genome- all the genes an organism has
 Chromosomes- structures made of DNA
and proteins
 Chromatids- “half-chromosomes”
 Chromatin- DNA/protein complex that
makes up chromosomes
 Synthesis – growth
 Mitosis - division

3. Cell cycle
 is the sequence of events that result into
new cell
◦ growth and division.
 cell cycle has two phases:
◦ Interphase longest phase
◦ Mitosis short phase

5. Interphase
 generally lasts at least 12 to 24 hours in
mammalian tissue.
 during this period, the cell is constantly
◦ synthesizing RNA,
◦ producing protein
◦ growing in size.
 interphase can be divided into 3 steps:
 G1 (the time gap between mitosis and DNA
replication),
 S (the period of DNA synthesis), and
 G2 (the gap between DNA duplication and the next

7. Cont…
 G1-Organelle duplication
 S -DNA replication
 G2-Proteins needed for mitosis are
produced
 A 4th phase is described G0
 DNA stays loosely packed in the nucleus
as chromatin

8. Cont…
Gap 0 (G0):
 There are times when a cell will leave the
cycle and quit dividing.
 This may be a temporary resting period
or more permanent.
 An example of the latter is a cell that has
reached an end stage of development and
will no longer divide (neuron, muscle).
 Temporary resting cells example liver
cells

9. Cont…
Gap 1 (G1)
 longest and most variable phase
 Cells increase in size
 produce RNA and synthesize protein.
 An important cell cycle control mechanism
activated during this period (G1
Checkpoint)
◦ ensures that everything is ready for DNA
synthesis

10. checkpoints

11. Cont..
S Phase:
 To produce two similar daughter cells, the
complete DNA instructions in the cell must
be duplicated.
 DNA replication occurs during this S
(synthesis) phase

12. Cont…
Gap 2 (G2)
 cell continue to grow and produce new
proteins.
 At the end of this gap is another control
checkpoint (G2 Checkpoint)
◦ to determine if the cell can now proceed to
enter M (mitosis) and divide.

13. Cont…
Mitosis or M Phase:
 Cell growth and protein production stop at
this stage in the cell cycle.
 All of the cell's energy is focused on the
complex and orderly division into two similar
daughter cells.
 Mitosis is much shorter than interphase,
 lasting perhaps only one to two hours.
 As in both G1 and G2, there is a Checkpoint
in the middle of mitosis (Metaphase
Checkpoint) that
◦ ensures the cell is ready to complete cell division

14. Cont…
 Rates of cell division varies from tissue
to tissue
 Epithelial tissue subject to loss of cell
division is rapid
 In healing of wounds division is also rapid

15. Cont…
 Progression through the cell cycle is
halted by a variety of adverse conditions
such as;
◦ inadequate nutrition (nutrient stress)
◦ inappropriate cellular microenvironments
◦ DNA damage
 Damage here can arrest the cell cycle not
only at the G1 restriction point but also
during S or at a checkpoint in G2

16. Cont…
 G1 arrest may permit repair of the
damage before the cell enters S phase,
 so that the damaged DNA does not
reproduce gene defects during
replication.
 If the problem encountered at any
checkpoint cannot be corrected fairly
quickly while cycling is halted,
 proteins encoded by tumor suppressor
genes are activated and that cell’s activity
is redirected toward cell suicide or

17. Purpose of cell division
 Why cell division?
◦ Growth of an organism
◦ Repair
◦ Repletion of cells lost
◦ Reproduction

18. Mitosis
 Occurs in Somatic cells
 Identical cells produced with same
genome
 In adult specialized cells
 embryonic division may lead to
specialization

19. Mitosis
 Mitosis produces two daughter cells that
are identical to the parent cell.
 If the parent cell is haploid (N), then the
daughter cells will be haploid.
 If the parent cell is diploid, the daughter
cells will also be diploid.
 N ----------------N
 2N --------------2N
 cell division allows multicellular organisms
to grow and repair damaged tissue

20. Cont..
 4 phases recognized in mitosis
1.prophase
2.metaphase
3.anaphase
4.telophase

21. Prophase
 The chromosomes coil (condense).
 The nuclear membrane disintegrates.
 Spindle fibers (microtubules) form,
Metaphase
 Chromosomes further condense
 Attach to mitotic spindles at centromere
 become aligned at the equator

23. Anaphase
 Sister chromatids separate at the
centromere
 Move to the opposite spindle poles

24. Telophase
 cell divides into two.
 chromosomes uncoil.
 nuclear envelope reforms.
 spindle apparatus disassembles

25. Cytokinesis
 happens at the end of telophase
 A belt-like contractile ring of actin filaments
and myosins develops in the peripheral
cytoplasm.
 constriction produces a cleavage furrow
and progresses until the cytoplasm and its
organelles are divided into two daughter
cells, each with one nucleus.

26. Cont...
 Most tissues undergo cell turnover
with slow cell division and cell death.
 Nerve tissue and cardiac muscle are
exceptions because their differentiated
cells cannot undergo mitosis.

27. Summary of Mitosis

28. MEIOSIS
THE BASIS OF SEXUAL
REPRODUCTION

30. Meiosis
 process by which two successive cell
divisions produce cells called gametes
 containing half the number of
chromosomes found in somatic cells.
fate of meiosis
 Production of gametes
 Chromosomes are halved
 Reassortment of genes
 Genetic variations

31. Cont…
 There are 22 pairs of matching chromosomes, the
autosomes, and one pair of sex chromosomes.
 If the sex pair is XX, the individual is genetically
female
 if the pair is XY, the individual is genetically male
 One chromosome of each pair is derived from the
maternal and paternal gametes.
 Thus each gamete contains a haploid number of
23 chromosomes
 the union of the gametes at fertilization restores
the diploid number of 46.

32. Meiosis
 cell division that takes place in the germ
cells
 also known as reductional division.
 One diploid cell produces 4 haploid cells
 Has 2 process:-
◦ Meiosis I
◦ Meiosis II

33. Meiosis I
 Meiosis results in the formation of gametes
(sex cells).
 In meiosis I:
◦ Homologous chromosomes join together=
sypapsis
◦ homologous chromosomes are separated
after synapsis
◦ crossing over = genetic material exchange
(genetic recombination) occurs.
 In meiosis II:
◦ sister chromatids are separated
◦ sequence of phases resembles mitosis.
3-
33

34. Meiosis
Ensures genetic diversity
Accomplished through
independent assortment and
crossing-over
Genetic diversity is important
for the evolution of
populations and species

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

37. Meiotic Prophase I
 a long and complex phase that differs
considerably from mitotic prophase
and
 Homologous, double-stranded
chromosomes in the parent cell form
pairs (synapsis).
 Tetrad:
◦ Pair of homologous chromosomes
 Crossing over
◦ occurs between the maternal and paternal
chromosomes.

39. Cont...
 each paired chromosomes has two
chromatids,
 Or tetrads four copies of each genetic
sequence.
 During synapsis recombination or
crossing over , four chromatids mixes up
the genes inherited from each parent
 yields a new and different set of genes to
be passed on to the next generation.

40. Metaphase-I
 Has a short duration,
 chromosomes move towards the
equator of the cell and come to lie in
two parallel metaphase plates.
 The 2 parallel plates are formed by
one set each of the homologous
chromosomes.

41. Anaphase 1
 similar to anaphase in mitosis except
that each chromosome consists of 2
chromatids that remain held together.
 2 Chromosomes migrate to the poles.
 actual reduction in the chromosome
number (from 2n to n) occurs
 However, each chromosome found at
the poles consists of two chromatids.

43. Telophase-I
 chromosomes at each pole uncoil and
elongate to form the chromatin.
 nucleolus reappears at each pole.
 Spindle fibres and asters disappear
and centrioles split.
 nuclear membrane is formed at each
pole resulting in the formation of two
daughter nuclei.

44. Cytokinesis - I
 Simultaneously with the formation of 2
daughter nuclei,
 a cleavage furrow appears in the
middle of the cell.
 furrows gradually deepen and divide
the cell into 2 daughter cells.

45. 45

46. Meiosis Prophase II
 Resembles the prophase stage of
mitosis.
 In each of the two new cells:
◦ nuclear membrane breaks down
◦ chromosomes collect together.
 Crossing over does not occur in this
phase.
3-
46

47. Metaphase II
 The double-stranded chromosomes
form a single line in the middle of the
cell.
 Spindle fibers extend from the
centrioles at the poles to the
centromere of each double-stranded
chromosome.
3-
47

48. Anaphase II
 The sister chromatids of each double-
stranded chromosome are pulled
apart at the centromere.
 Each chromatid (single strand) is
pulled to the opposite pole of the cell.
3-
48

49. 49

50. Telophase II and Cytokinesis
 The single-stranded chromosomes arrive at
opposite ends of the cell.
 A cleavage furrow forms
 Cytoplasm in both cells divides
 Produces a total of four haploid daughter
cells.
 These daughter cells mature:
◦ sperm in males
◦ oocytes in females.
3-
50

52. Errors in Meiosis
 It is estimated that from 10–25% of all
human fertilized eggs contain
chromosome abnormalities
 these are the most common cause of
pregnancy failure (35% of the cases).
 These chromosome abnormalities arise
from errors in meiosis,
 usually meiosis I; occur more often
(90%) during egg formation than during
sperm formation; become more frequent
as a woman ages.

53. Cont…
 Euploid – any exact multiple of n (23)
 Aneuploid— the gain or loss of whole
chromosomes
 Chromosomal abnormalities account for 7%
of major birth defects
 gene mutations account for an additional
8%.
 most common chromosomal abnormality
caused by nondisjunction,
◦ the failure of chromosomes to correctly separate

54. Cont..
 Zygotes missing one chromosome
("monosomy") cannot develop to birth
(except for females with a single X
chromosome).
 An extra chromosome ("trisomy") is also
lethal except for chromosomes 13, 18,
and 21
 trisomy 21 is the cause of Down
syndrome.

55. Activity
 Describe the cell cycle
 Describe the 2 processes of cell
division
 State the importance of cell division
 Explain the fate of errors in meiosis
cell division.

56. Thank you